Owner's Manual - HydroCAD Stormwater Modeling Owners Manual.pdf · Sharp-Crested Rectangular Weir ... V-Notch Weir ... Rectangular, vee & trapezoidal weirs Broad-crested weirs Custom

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H y d r o C A D ®

Stormwater Modeling System

Version 10

Owner's Manual

Copyright © 2011 HydroCAD Software Solutions LLC. All rights reserved.

HydroCAD® is a registered trademark of HydroCAD Software Solutions LLC.Other trademarks are the property of their respective owners.

HydroCAD Software Solutions LLCP.O. Box 477

Chocorua, NH 03817

1-800-927-7246Tel: (603) 323-8666Fax: (603) 323-7467

www.hydrocad.net

ISBN 978-0-913633-15-1

6182 rev. 7/27/11

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Copyright

This publication and the associated software are copyrighted, withall rights reserved to HydroCAD Software Solutions LLC. (“HSS”).Your rights are subject to the limitations and restrictions imposedby international and U.S. copyright laws. No part of this publicationmay be reproduced in any form or by any means without writtenpermission from HSS.

Trademarks

This publication incorporates trademarks which are the property ofHSS. You may use these trademarks only for the purpose ofidentifying the products of HSS, in accordance with acceptedtrademark practice. Such use of any trademark does not give youany rights of ownership in that trademark.

Other Trademarks

AutoCAD® is a registered trademark of Autodesk, Inc. Windows,Windows 7, and Vista are registered trademarks of Microsoft Corp.

License Agreement

The accompanying computer software is licensed, not sold, to you byHSS, under the terms of the license agreement shown in thesoftware’s installation program. By installing or using the softwareyou agree that you have read the license, and that you accept itsterms.

Disclaimer of Warranty

Although HSS has used its best efforts in the compilation andpreparation of this publication, it is provided “as-is”, with nowarranties, express or implied, that the publication or associatedsoftware are error-free.

HSS MAKES NO WARRANTY, EXPRESS OR IMPLIED,REGARDING THE PERFORMANCE OF THIS PUBLICATION ORTHE ASSOCIATED SOFTWARE, OR ITS MERCHANTABILITY ORFITNESS FOR A PARTICULAR PURPOSE. HSS SHALL NOT BELIABLE FOR ANY DAMAGES ALLEGED TO ARISE FROM THEUSE OF THIS PUBLICATION OR THE ASSOCIATEDSOFTWARE, INCLUDING LOSS OF REVENUES OR DAMAGE TOPROPERTY, PERSONS, OR INTERESTS, INCLUDING BUSINESSINTERRUPTION OR LOSS OF BUSINESS PROFITS, EVEN IFHSS IS ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

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T a b l e o f C o n t e n t s

Introduction to HydroCAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Section 1 - What is HydroCAD? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Section 2 - HydroCAD Features and Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . 15

Current Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Features added in HydroCAD-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Features added in HydroCAD-9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Features added in HydroCAD-8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Features added in HydroCAD-7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

HydroCAD User's Guide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Section 3 - About this Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Finding the Information You Need . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Conventions Used in This Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Section 4 - Installing HydroCAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25License Pooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Section 5 - Using HydroCAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Operating Sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27What is a “Project”? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Starting HydroCAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28The HydroCAD Screens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29The Routing Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Working With Nodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Automatic Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Calculation Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33The Message Window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Printing Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Units of Measure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Section 6 - HydroCAD Project Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Project Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Predefined Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Default Project Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Read-Only Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Editing a Project File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Creating Project Files with Other Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

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Section 7 - Data Import . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Tabular Data Import . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Automated Tabular Import . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Importing a TR-20 Data File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37Importing Data from AutoCAD® . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

HydroCAD Technical Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Section 8 - Understanding Hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41What is Stormwater and why does it need to be modeled? . . . . . . . . . . . . . . . . 41Understanding HydroCAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Section 9 - Units of Measure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43Section 10 - Rainfall Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Intensity-Duration-Frequency Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45IDF Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Synthetic Rainfall Distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46Rainfall Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Custom Synthetic Rainfall Distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Importing TR-20 Rainfall Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Rainfall Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47Unit Hydrographs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48Unit Hydrograph Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Section 11 - SCS Curve Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Curve Number Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Curve Number Lookup Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Composite Curve Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Unconnected Impervious Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Separate Pervious/Impervious Runoff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Adjustments for Antecedent Moisture Condition . . . . . . . . . . . . . . . . . . . . . . . . 51

Section 12 - Time of Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Lag/Curve Number Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Sheet Flow Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Shallow Concentrated Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Channel Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55Travel Time Through Lakes and Reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Other Tc Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56Tc Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

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Section 13 - SCS Unit Hydrograph Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Data Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Runoff Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61TR-55 and the Tabular Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Section 14 - Santa Barbara Urban Hydrograph . . . . . . . . . . . . . . . . . . . . . . . . . . . 63Runoff Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Section 15 - Rational Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Runoff Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Special Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Frequency Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Section 16 - Reach Routing Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Reach Routing Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67Reach Routing Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69Reach Routing Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70Reach Routing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Storage-Indication Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Muskingum-Cunge Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Simultaneous Reach Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75Effects of Reach Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

Section 17 - Pond Storage Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77Prismatoid Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79Vertical Conic Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80Round Pipe Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81Box Pipe Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Elliptical and Arch Pipe Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Parabolic Arch Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83Prefab Chamber Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84Custom Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

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Section 18 - Pond Hydraulics Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87Sharp-Crested Rectangular Weir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88Broad-Crested Rectangular Weir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89V-Notch Weir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90Trapezoidal Weir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91Weir Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92Custom Weir/Orifice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92Asymmetrical Weir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93Submerged Weirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94Dam Breach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95Rectangular Orifice in a Vertical Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96Rectangular Orifice in a Horizontal Plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Orifice Discharge Coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Circular Orifice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98Orifices Under Low-Head Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99Modeling a Grate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99Culvert Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100Tube & Siphon Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102Constant-Flow Outlet Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104Special Outlet Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105Pump Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106Exfiltration Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108Tips for Using Exfiltration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110Discharge Multiplier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111Discharge Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

Section 19 - Pond Routing Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113Stage-Storage Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113Stage-Discharge Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113Compound Outlet Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114Pond Routing Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115Storage-Indication Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115Additional Routing Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117Dynamic Storage-Indication Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117Simultaneous Pond Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Tailwater Capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119Reverse Flows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Section 20 - Detention Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121Section 21 - Hydrograph Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

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Section 22 - Link Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Basic Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Advanced Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Elevation Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126Link Routing Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126Using a Link to Model a Large Watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127External Hydrograph Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128Hydrograph Export Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

Section 23 - Land-Use Analysis & Pollutant Loading . . . . . . . . . . . . . . . . . . . . . 129Section 24 - Calculation Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131Section 25 - Frequently Asked Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143Section 26 - References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

Appendix A1: Hydrologic Soil Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149Appendix A2: Runoff Curve Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150Appendix A3: Curve Number Adjustment for AMC . . . . . . . . . . . . . . . . . . . . . . . . . . . 154Appendix B1: HydroCAD Rainfall Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155Appendix B2: SCS Synthetic Rainfall Distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . 157Appendix B4: Rainfall Depth Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159Appendix C: Manning's Number Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162Appendix D1: Broad-Crested Weir Coefficients for Sharp-Edged Crests . . . . . . . . . . . 164Appendix D2: Broad-Crested Weir Coefficients for Assorted Profiles . . . . . . . . . . . . . 165Appendix E: Culvert Entrance Loss Coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166Appendix F: Sheet Flow Roughness Coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167Appendix G: Velocity Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168Appendix H: Cross-Sectional Area & Perimeter Equations . . . . . . . . . . . . . . . . . . . . . 169

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

10

Introduction to HydroCAD 11

Introduction to HydroCAD

This section contains general informationabout HydroCAD and the capabilities itprovides.

Introduction to HydroCAD12


Section 1 - What is HydroCAD?

HydroCAD is a Computer Aided Design program for modeling the hydrology and hydraulics ofstormwater runoff, commonly referred to as H&H. HydroCAD uses procedures developed by theSoil Conservation Service (now the Natural Resources Conservation Service), plus a wide range ofother standard H&H calculations, to produce a fully-integrated, interactive stormwater modelingsystem. Although HydroCAD was initially developed for use in the United States, it has globalapplication due to its ability to incorporate local rainfall and soil data.

HydroCAD is commonly used to generate runoff hydrographs for a given watershed and study theirflow through a drainage system consisting of natural and/or artificial components. This allows thedesigner to verify the adequacy of the drainage system, or to predict where flooding or erosionproblems are likely to occur. These studies are often performed under a number of different rainfallconditions, to verify the behavior of the system under various environmental conditions.

HydroCAD takes this capability one step further by maintaining a complete database for thewatershed and drainage system. This allows HydroCAD to provide an interactive working modelfor the entire system where changes can easily be made and their effects viewed. With HydroCADthis takes just seconds, not hours, so the engineer can interact with the watershed model in a waynot previously possible. This lets the engineer evaluate multiple design alternatives and choosethe most suitable, based on a range of safety, environmental, and financial considerations.

The advent of interactive design tools, like HydroCAD, frees the engineer to concentrate on creativedesign, a goal which is often sacrificed when analysis of each alternative requires hours or days oftedious calculations. No program can substitute for human creativity, but it can greatly aid thatcreativity by assisting with the critical analysis of each idea or design. This is the goal ofHydroCAD.

The following pages provide a detailed list of HydroCAD features,plus a summary of the latest changes.



Section 2 - HydroCAD Features and Capabilities

Current Features

A basic summary of HydroCAD features appears below. For a complete, up-to-datelist please visit www.hydrocad.net.

Runoff hydrograph generationSCS unit hydrograph procedureSanta Barbara Urban Hydrograph (SBUH)Separate pervious/impervious runoffCurve number lookup & weightingUnconnected impervious areasAMC/ARC adjustmentRational methodModified Rational methodUnlimited hydrograph span/points

Time-of-concentration calculationsUnrestricted Tc valuesSheet flow methodShallow concentrated flowChannel flowUpland methodCurve Number methodReservoir travel timeDirect entry

Rainfall ManagementOver 100 predefined distributionsSCS Type I, IA, II, III stormsCustom synthetic rainfall distributionsUser-defined rainfallsRainfall editor, reports, & graphicsUnlimited rainfall eventsAutomatic back-to-back stormsAutomatic IDF curvesIDF curve editor, reports, & graphicsDownload local IDF dataUse local PFD data from NOAA, et al.

Unit HydrographsIncludes common tables (SCS, Delmarva, etc)Predefined gamma UH tablesCustom UH tablesUH curve editor, reports & graphics

Reach RoutingStorage-Indication methodLong-reach translationMuskingum-Cunge routingBase flowCommon geometriesCustom cross-sectionsDirect storage entryManning’s lookup tables

Pond RoutingStorage-Indication routingDynamic Storage-Indication routingSimultaneous pond routingMultiple outletsAutomatic diversionsCompound outlet devicesExfiltrationTidal tailwater conditionsDraw-down simulations

Pond Outlet HydraulicsRectangular, vee & trapezoidal weirsBroad-crested weirsCustom weirsSubmerged weirsOrifices & gratesLow-head weir flowCulvert flowTubes & siphonsFloat-operated valvesDam breachSkimmersCompound devicesStand-pipesCustom devicesPumpsExfiltration calculations


Pond StorageCustom stage-storage dataPrefabricated chamber definitionsCommon storage shapesEmbedded storage volumesAdjustable voids (for stone fill)Complex storage arrangements

Underground StorageExtensive library of prefab chambersChamber reportsChamber layout wizardAutomatic end-cap handling

Water Quality CalculationsCenter-of-Mass detention timePlug-Flow detention timeLand-Use reportingPollutant loading

Special OperationsLinked projectsFlow thresholds & limitsAutomatic flow diversions

Data ExchangeTabular watershed import and exportHydrograph import and exportLinkage to Carlson HydrologyImport sub-area data from AutoCAD®

ReportingInstant on-screen reports & graphsMultiple report formatsMulti-node & pre/post comparisonsMetric, English, & custom unitsIndependent units for input & reportsExport reports in multiple formatsAutomatic data import/export

GeneralFully automatic calculationsUnlimited hydrograph pointsAutomatic hydrograph summationOn-screen routing diagramFull drag-and-drop operationDiagram snap-to-gridDiagram background imagesAutomatic hints and warningsComplete on-line helpAutomatic timed backupDefault project settingsMulti-project operation

International UseMetric units (SI) or English (US Customary)Hard-Metric or English calculationsAccepts local rainfall dataCustomizable ground-cover tables


Features added in HydroCAD-10

HydroCAD-10 adds a wide range of new engineering, reporting, and operational capabilities.

New Engineering Features• Direct support for Precipitation Frequency Data from NOAA and compatible sites.• PFD files from NOAA, NRCC, and other sites can be used directly as IDF data.• Creates custom synthetic rainfall distributions from any IDF or PFD data file.• Rainfall distributions and events can be imported from TR-20 and WinTR-20 files.• Additional rainfall distributions.• New unit hydrograph definitions, including gamma unit hydrographs.• Pipes and culverts with internal fill.*• Expanded library of prefabricated storage chambers.• Automatic handling of chamber end-caps and row length adjustments.• Multi-span stormwater chambers.*• Constant-flow outlet (for floating skimmers & similar devices).• Tube/Siphon outlet (also used to model float valves).• Dam breach outlet (for simulation of a progressive dam breach).• Asymmetrical weir.

Key Operating Features• Import and export of watershed data in tabular format.• Configure active ground covers and/or data import on new Settings|Watershed screen.• Direct import of sub-area data from AutoCAD®.• Automatic watershed import from Carlson Hydrology.*• Automatic definition of rainfall events.*• Event-specific links.*• Built-in lookup table for standard arch and elliptical pipe sizes.• New projects automatically set to user’s default units (English or Metric/SI).• Improved compatibility with Windows® 7 and Vista®

• HydroCAD-10 can open projects from any previous version of HydroCAD.

New Reporting Capabilities• Project reports screen can remain open while editing with automatic updates.• Ground Cover report added to project reports screen.• New “Text/Image” node allows placement of images and annotations on the diagram.• Support for additional graphics formats for background images.

* New features added in HydroCAD 9.1.



New Engineering Features• Pump modeling, including friction losses, headwater/tailwater sensitivity, and separate on/off

points (hysteresis).• Automatic Curve Number adjustment for unconnected impervious areas.• Chamber wizard provides automatic layout and modeling of underground storage systems, plus

cost estimating.• Updated chamber definitions include overall dimensions, plus recommended bedding, cover,

and spacing.• Over 100 new chamber definitions added to library.• Automatic storage adjustment for wall thickness of embedded chambers. *• Enhanced exfiltration options, including hydraulic conductivity and Darcy’s Law.• Expanded library of rainfall distributions and unit hydrographs.• Elliptical and pipe-arch culverts are now supported.• “Horizontal cylinder” storage upgraded to “pipe storage”, with support for box, elliptical, arch,

and round geometries (flat or sloped).• Rational method “frequency factor” may be set manually or defined for each event in IDF file.*

Key Operating Features• Automatic import and conversion of TR-20 data files. *• Expanded hydrograph import capabilities, including uneven time steps. *• Expanded message functionality, including direct node selection, reporting, & editing. *• Tree view for selection of chambers, rainfall tables, IDF curves, and other items.• Updated installation program for Windows Vista. *• HydroCAD-9 can open projects from any previous version of HydroCAD.

New Reporting Capabilities• Land-use reporting and pollutant loading.• Enhanced chamber report, with tree view for chamber selection.• Project-wide reports, including Curve Number usage and soil groups. *• Multi-node comparisons. *• Pre/post comparison reports. *• Multi-event reports. *• Outflow volume vs. time now available on tabular hydrographs. *• Percentage impervious area can be reported for each inflow hydrograph. *




New Engineering Features• Option for independent evaluation of runoff from pervious and impervious surfaces.• A reach can be defined with custom cross-section data.• Reach cross-sections can have variable Manning’s values.• Muskingum-Cunge reach routing procedure added.• Weir rise parameter allows modeling of compound weirs, such as a notch in a spillway. *• Custom orifice/weir device allows modeling of arbitrary openings. *• A link can be used to introduce a specified time lag or a constant flow.• A link can be used to define an arbitrary tailwater elevation vs. time. *• Expanded library of rainfall distributions.• Minimum allowed Tc may be specified within each project. *• Default Ia/S ratio may be changed for specialized runoff situations. *• Pipe storage can now be sloped as well as level. *• A single orifice can be used to model an array of vertical openings. *• Library of rating tables added for Hydro International vortex valves. *

Key Operating Features• Individual units (including decimal places) can be customized within each project.• New curve editor simplifies creation of custom rainfall, UH, IDF, and chamber definitions.• Built-in Manning’s value lookup table. *• Implemented polynomial-based IDF curves. *• Automatic timed backup. *• HydroCAD-8 can open projects from any previous version of HydroCAD.

New Reporting Capabilities• New reports added for IDF curves, rainfall tables, unit hydrographs, and storage chambers.• New “Area Listing” report summarizes Curve Number usage for an entire project.• Separate reporting of pervious and impervious runoff areas.• Report time span can be adjusted independently of calculation span. *• Context-sensitive help available on summary report. *• Project notes can be entered to create a report cover page or narrative. *• Expanded support for JPEG import and export. *




New Engineering Features• User-defined rainfall events allow each project to automatically calculate, print, export, and

link data for multiple events (10-year, 25-year, etc).• Dynamic Storage-Indication method provides enhanced tailwater-sensitive routing.• Pond storage may be defined with any combination of common shapes, such as a pipe, arched

chamber, vault, cylinder, cone, prism, or custom stage-storage data.• Automatic storage calculations are provided for chambers embedded in a stone bed.• Predefined storage definitions are supplied for CULTEC storage chambers.• Catch basins may be modeled as “zero-storage ponds,” with no storage information required.• A link may be used to model a fixed or tidal tailwater elevation.• Rational method can use multi-event IDF curves, with automatic intensity lookup.• With Rational method, the critical duration can be automatically calculated for each node.• The Center-of Mass and Plug-Flow detention times are now calculated for all ponds.• Pipes & culverts can be automatically sized for pipe-full conditions or user-defined headwater.

Key Operating Features• Routing diagram can display individual node names, as well as a user-defined grid.• Routing diagram supports snap-to-grid, plus pan and zoom with the mouse wheel.• A background image (or logo) can be displayed and/or printed with the routing diagram.• Most data entry tables can be loaded from a CSV (spreadsheet) file.

New Reporting Capabilities• Automatic multi-event reports - Just pick the storms to include.• Fast hydrograph plots with detailed annotations. (For more concise reports.)• Individual inflow hydrographs may be tabulated. (To show hydrograph summation.)• Many new values are calculated and reported, such as the inflow area and depth for each node.• Flow and discharge velocity are calculated and reported for individual pond outlets.• Each node may have user-defined notes, for more complete, self documenting reports.• Reports can be exported in multiple text, graphics, and spreadsheet formats.• Automatic data export allows creation of custom spreadsheets and reports.

HydroCAD-6 was the first native Windows release. It provided all the capabilities of earlierversions, plus many new features including:• Complete support for English, metric, mixed, or custom units.• New tailwater-sensitive routing procedures.• Ability to work on multiple projects at the same time.• Enhanced data entry, reporting, and data export.• Calculations speed increased by approximately fifty times.

HydroCAD User's Guide 21

HydroCAD User's Guide

This section contains information on theinstallation and operation of HydroCAD.It’s a hands-on guide for users of theprogram which supplements theinformation contained in the HydroCADhelp system.

HydroCAD User's Guide22


Section 3 - About this Manual

Finding the Information You Need

This manual is intended to supplement the information contained in the HydroCAD help system,which should be consulted for complete information on most topics, including step-by-stepoperating instructions. Together they provide the basic information needed by qualified engineersto install and use HydroCAD.

For assistance while using the program, click the Help button onany screen, or select one of the Help items on the HydroCAD menu.The help system includes hints, definitions, equations, andbackground information for each field on all HydroCAD screens, aswell as detailed information on all program operations. In manyareas, the help system includes considerably more detail than theprinted documentation.

A comprehensive Tutorial is included in the HydroCAD helpsystem. The Tutorial is the fastest and most complete way tobecome familiar with HydroCAD, and should be reviewed by allusers. To run the Tutorial, select Tutorial on the HydroCAD Helpmenu. The tutorial offers several lessons covering most aspects ofHydroCAD operation, and will significantly boost your HydroCADproductivity.

For new, updated, and expanded material visit the HydroCAD website at www.hydrocad.net and click on “Support.” The web siteis updated regularly in response to new questions and issues thatmay not be covered in this Manual, and includes contact informationin case you need personal assistance.

Conventions Used in This Manual

! Small bold type indicates a menu selection (such as Project|Open) or a keystroke (such asEnter).

! Underlined text indicates a user entry, such as the numeric value 12.40.

! The Tab key is often the most convenient way to step from one data field to another. Shift-Tab can be used to step backwards through fields.

! Clicking the left (or primary) mouse button is indicated by Click. The right (or secondary)mouse button is indicated by Right-Click, and is used to activate the context menu for manyitems.



Section 4 - Installing HydroCAD

Installation

Follow these steps to install HydroCAD:

1a) To install from a CD, insert the disk in your CD drive and wait for the SETUP programto appear. If setup doesn’t appear in a few seconds, open the CD (in My Computer) andselect the SETUP program.

1b) To install from the web, download and run the SETUP program.

2) Follow the instructions given by the SETUP program.

3) The setup program will create a group of shortcuts under Start|Programs|HydroCAD. AHydroCAD icon will also be created on the desktop.

Installation Notes

For detailed installation instructions, click the “Read Me” button in the setup program.

Always install HydroCAD on a local hard drive, even if you are using HydroCAD on a LAN orsharing your data over a network The default location of \ProgramFiles\HydroCAD is recommendedunless you have a specific need to install elsewhere.

For a network installation, install each HydroCAD license on one workstation as described above.You may also elect to share your HydroCAD license(s) within your office by using License Pooling,as described below. In either case, you must run the installation program on each computer whereHydroCAD will be run.

After installation, each HydroCAD program can access projects on any local or network folder,subject to the access rights assigned by the network administrator.

Installing an Update

A HydroCAD update is installed in the same manner as an initial installation. In order to preserveall existing data and settings, the update should be installed in the same folder as your previousversion of HydroCAD. The installation program should detect the previous installation and suggestthe same directory.

If you decide to install an update in a different directory, it is strongly advised that you firstuninstall the previous program. (This is the only situation where it is necessary to uninstall theprogram.)

Each version of HydroCAD can directly read projects created with any earlier version. However,once a project has been modified, it may contain new features that make it incompatible withearlier versions. If in doubt, make a backup copy of your project files before using them with a newversion.

A detailed list of recent software changes is available on the Start menu under Programs|HydroCAD.


Uninstalling HydroCAD

If you ever need to remove HydroCAD from your computer, use the Windows Add/Remove programfeature, or the “Uninstall” option under Start|Programs|HydroCAD. In either case, your existing dataand program settings will be preserved and remain available should you reinstall the program ata later time.

Note: You do not normally need to uninstall HydroCAD before installing an update.

License Pooling

The HydroCAD License Agreement allows “License Pooling.” This technique allows you topurchase a given number of HydroCAD licenses, and share them among multiple computers at thesame site. (See your software License Agreement for details.)

For example, if you buy 3 licenses, you would have the ability to run HydroCAD on any threecomputers in your office at the same time. When properly configured, HydroCAD keeps track ofthe number of licenses and users, allowing only the licensed number of copies to run at one time.

To implement License Pooling:

1) Do a standard HydroCAD installation on any one of the computers that will be usingHydroCAD. Start HydroCAD and enter all your assigned serial numbers underSettings|Serial Number. Press the Network button on the Serial Number form, and browse to a sharednetwork location to store the serial numbers. Click OK and shut down HydroCAD.

2) Install and run HydroCAD on each additional computer. On the Settings|Serial Number form,press Network to browse to the same shared folder and click OK. The shared serial number(s) willtake effect as soon as you select the correct folder, and the main HydroCAD screen will appear.(You do not have to reenter the serial numbers.) When the main screen appears, shut downHydroCAD and repeat this step for any additional computers.

If you have multiple licenses that are already installed separately, you can enable license poolingat any time. On each station select Settings|Serial Number and press Network to browse to the sameshared folder. As each station is configured, its serial number(s) will be automatically merged withthe shared list.

To disable License Pooling:

If you ever need to disable license pooling, start each copy of HydroCAD, selectSettings|Serial Number, and use the Network button to browse to a private local folder. (TheHydroCAD installation directory is recommended.) This will remove all serial numbers fromshared use. Delete all but one serial number from the list. Repeat the process with each additionalcopy of HydroCAD, entering a single unique serial number on each computer, and selecting aprivate local folder.

1Although the node position does not affect any of HydroCAD’s calculations, the nodes can be positioned at actualstructure locations if desired.


Section 5 - Using HydroCAD

Operating Sequence

Although HydroCAD's capabilities can be used in any sequence, its power is most easily understoodby viewing it in five basic phases.

Phase I - Construction of Routing Diagram

A diagram is constructed showing the functional components, or nodes, that make up thewatershed. The diagram shows the relative location1 of each node and how water is routed fromone node to another.

Phase II - Description of each Node

Each node is described in detail so that HydroCAD can calculate the outflow from each node oncethe inflow is known.

Phase III - Setting Rainfall Data & Calculation Options

Enter basic information necessary for runoff and routing calculations, such as the rainfallparameters.

Phase IV- Calculation of flow through each Node

Calculations occur automatically whenever a report is selected. Starting at the upstream end ofthe diagram and working downstream, HydroCAD calculates the outflow and other results for eachnode. Multiple inflows are summed automatically. A minimal recalculation feature automaticallyreuses the results of previous calculations where no changes have occurred.

Phase V - Display and Examination of Results

Opening one or more report windows lets the user verify the behavior of the watershed. If anychanges are required, the user may modify the watershed, causing the calculations and reports tobe automatically updated.

In practice, it is generally recommended that these phases becompleted for each node as it is added to the routing diagram. Thisallows the model to be fine-tuned at an early stage, while thecalculations are relatively easy to understand. As the modelbecomes more complex, a single modeling error can have widespreadconsequences, making it more difficult to locate.


What is a “Project”?

Each HydroCAD project file includes a routing diagram, associated node data, and all the relatedproject settings, such as the rainfall, runoff, and routing parameters necessary to model thehydrology and hydraulics of a given area. It is common to model the existing conditions first, andthen use a separate project file to model the proposed conditions. For further information readabout HydroCAD project files on page 35.

Starting HydroCAD

To start HydroCAD without opening a project:

# Click (or double-click) the HydroCAD icon on the desktop -or-# Click the HydroCAD icon located under Start|Programs|HydroCAD.

To open an existing project from Windows:

# Click (or double-click) a HydroCAD project on the desktop or in any folder -or-# Click a recently used HydroCAD project listed under Start|Documents.

You can open a project regardless of whether or not HydroCAD is already running. If HydroCADis already running, the project is opened in the current HydroCAD session in addition to anyprojects that are already open.

To open a project from within HydroCAD:

# Select Project|Open from the HydroCAD menu.

To import and open a project created with HydroCAD-5 (or earlier):

# Select Project|Import|HydroCAD 5 from the HydroCAD menu.

To create and open a new project:

# Select Project|Open from the HydroCAD menu.# Type a name for the new project and click Open.

Other items on the Project menu can be used to close, rename, save, delete, combine, and importprojects.

For details on any menu item, move the mouse over the item(without clicking on it) and press F1.

See page 35 for further details on default projects and projectstorage, or select Help|Index and type Project.


The HydroCAD Screens

Most HydroCAD activities utilize the main screen, containing the routing diagram, plus one ormore report windows, used to view the runoff and routing results. To get more information on anyitem, hold the mouse pointer over the item until a pop-up “tool tip” appears.

Main window

Create new nodesby dragging themfrom the paletteonto the routing

diagram

Title bar showsname of current

project

Main menu providesmost program operations

Event Selector givesinstant access to any

rainfall

Main tool bar givesquick access to

common operations Window buttons let youminimize, maximize, or

close the window

Routing diagram showsinterconnected nodes for

current project

Status line givesinformation aboutanything you point

to

Use the Projectselector to switch

between activeprojects, or to drag

nodes betweenprojects

Multiple Report windowsshow details for each

node and updateautomatically as you work

Right-click anynode to edit orview a report

To change therouting, drag anyoutflow arrow

Settings tool bargives quickaccess to

common settings

Click a button toselect a report,

change the layout,or edit the node

Drag across anygraph to zoom orright-click to select

curves

2 To sum multiple flows without performing a hydrograph routing, use an undescribed reach, pond, or link.

3 To model a pipe under other flow conditions, including headwater and tailwater effects, use a catch basin or pond witha culvert outlet. This applies to most culverted road crossings, manholes, and other impoundments that feed a pipe.

4 When a reach drains a subcatchment along its length, it may be best modeled as a component of the subcatchment'sTc calculation, rather than as an independent reach.


The Routing Diagram

The routing diagram shows the individual nodes that make up each project. The nodes are usuallyconnected by arrows that indicate how their outflows are routed. Multiple inflows are summedautomatically as required.2

Based on the routing diagram, HydroCAD is able to determine the correct sequence of calculations,and then calculate the flows throughout the project. Routing calculations are automaticallyupdated as required. You can manipulate the diagram display with the main scroll bars, the toolbar, the main menu, the palette, and the mouse.

Watershed components

Each drainage system is represented by a network of the following types of nodes:

! Subcatchment: A relatively homogenous area of land that typically drains into a reachor pond. Each subcatchment generates a runoff hydrograph. A subcatchment may also beused to account for the rain falling directly on the surface of a pond. A subcatchment cannotbe used to route an inflow hydrograph. Instead, use a subcatchment to calculate the runoffand a separate reach to perform the routing.

! Pond: A pond, swamp, dam, catch basin, manhole, drywell, or other impoundment thatfills with water from one or more sources and empties in a manner determined by a weir,culvert, or other outlet device(s). The outflow of each pond is determined by a hydrographrouting calculation which attenuates and delays the peak flow. A pond may empty into areach or into another pond. An optional secondary outflow may be used to divert thedischarge from specific outlet devices and route them separately. A discarded outflow isalso available for outflows that are not subject to further routing, such as exfiltration.

! Catch Basin: A special type of pond that provides an insignificant amount of storage, butotherwise has all the properties and capabilities of a pond. Since a catch basin has nostorage capability, it cannot detain or attenuate its inflow. However, the routingcalculations will determine the water surface level (headwater) at each point in time.

! Reach: A uniform stream, channel, or pipe that conveys water from one point to anotherand operates under open channel flow.3 A reach may also be used to route an upstreamhydrograph through a subcatchment.4 The outflow of each reach is determined by ahydrograph routing calculation. This generally delays and attenuates the peak flow. Areach may be routed into a pond or into another reach.

! Link: A link may be used to 1) enter a hydrograph generated outside HydroCAD,2) interconnect several routing diagrams, 3) scale a hydrograph, 4) split a hydrograph intotwo components for independent routing, or 5) define a fixed or tidal tailwater elevation.


! Text/Image: A text/image node may be used to place annotations or images on therouting diagram. These nodes have no effect on the calculations, but can be used toenhance printed reports.

Creating a Node

The easiest way to create a node is to drag the desired item from the palette at the left side of thediagram. (See illustration on page 29.) You can also create a clone of an existing node by draggingthe node while holding down the Ctrl key.

Node Numbering

Each node on the routing diagram must have a unique number in order to distinguish it from othernodes in the same project. The “number” may also contain non-numeric characters includingletters and punctuation. Although the length of numbers is unrestricted, shorter numbers arerecommended for readability.

Different types of nodes cannot share the same number. If you need to use the same number, youcan distinguish them by adding a suffix, such as 4P (for pond 4) or 4S (for subcatchment 4). Thisnotation is automatically applied when importing projects from HydroCAD-5, which allowed thesame numbers to be used with each type of node.

Default Node Numbers

Whenever a new node is created, a default node number is automatically assigned that is uniquewithin the project. A default number is also assigned whenever a node is moved to a project thatcontains a conflicting number.

The default value consists of one or more digits, followed by the first letter of the node type. Thenumeric portion will normally be the lowest possible value that does not conflict with any existingnode. For example, the first default number for a subcatchment will be 1S. When a secondsubcatchment is created, its number will be 2S, unless that value is already in use, in which casethe next available value will be used.

HydroCAD can also be configured to assign sequential node numbers. When this mode is selected(on the Settings|General screen) HydroCAD will use the next available number that is greater thanthe last number assigned, even if a lower number is available.

Node numbers, including the default value, may be changed at any time. When doing so, note thatany unique node number may be used. The default digit-letter format is not required.

X,Y Coordinates

Each node on the routing diagram is located at a specific X and Y coordinate. Since the routingdiagram is a schematic representation of a project, the position of the nodes has no effect on thecalculations. However, specific X,Y coordinates may be used if desired.

The current X-Y position of the cursor is displayed in the status bar.


Node Outflows

Within the routing diagram, the outflow(s) from each node are represented by arrows. A solidarrow indicates the routing of each primary outflow, while a dashed arrow represents the routingof a secondary or tertiary outflow.

A secondary or tertiary outflow is available only for certain types of nodes, and is intended foroutflows that are to be routed separately. Some nodes may also have an unrouted or discardedoutflow, such as the exfiltration from a pond.

When an outflow is not routed, a circular “handle” appears below the node. The outflow can berouted by dragging the handle to the desired node. To change an existing routing, drag the arrowhead to another node. To un-route an outflow, drag the arrow head back to the originating node.Outflow routing can also be modified with Node|Reroute.

Certain nodes can also have a “discarded” outflow (such as the exfiltration from a pond) which isalways discarded and not available for further routing.

Working With the Routing Diagram

For details on working with the routing diagram, please review theHydroCAD Tutorial, which is available under Help|Tutorial.

Working With Nodes

The most common node operations are available on the context menu, which is activated by a Right-click on any node.

# Select Edit to enter or modify specific node information. A separate editing screen isprovided for each type of node, with several categories of information grouped on separatetabs. When editing a node, press F1 or click the Help button for further details. The helpsystem contains extensive information, and should be your primary resource when editingnodes.

# Select Report to open a new report window for the current node. Use the buttons on thereport window to adjust the display and view different reports. There are also several itemson the View menu to help manage report windows.

Shortcut: You can double-click a new node to edit it. Double-clicking an existing node will open areport window.

See page 37 for further details on data entry and import capabilities.

Automatic Calculations

HydroCAD automatically performs runoff and routing calculations as required, such as when youview or print a report. Once a report window is open, calculations are automatically updatedwhenever a change occurs that affects that node or report. You don’t need to close report window(s)when making changes to the project: Just move the report to one side, make the changes, and thereport is automatically updated.

5 The Apply button is used to implement any new settings without closing the window. This lets you see the effects ofdifferent values without having to re-open the window each time. Otherwise you can just use the OK button, which savesthe changes and closes the window. You do not need to click Apply and OK.


Calculation Settings

Each HydroCAD project maintains a number of calculation settings that control all runoff androuting calculations. The most notable values are the rainfall settings, although there are manyother related parameters.

# To change the runoff or routing parameters, select Settings|Calculation or click thecalculator icon on the tool bar.

For ease of use, the calculation settings are grouped into several pages. Click the Help button onany page for full details. After making any changes click OK or Apply.5 Any open report windowswill be automatically updated.

Each project may also define an unlimited number of rainfall events. (See the Rainfall tab of theSettings|Calculation screen.) This allows you to instantly pick any event from the Event Selector onthe main screen, as well as printing reports for multiple events in a single operation.

The Message Window

Whenever calculations are performed, a message window is opened to report the progress of thecalculations. There are three basic types of messages:

Notes provide basic runoff or routing information.Hints indicate conditions that may require your attention.Warnings indicate conditions that you must correct.

Click on any message in the window for additional details. This willlink you to all the related technical information you need tounderstand and resolve the situation. A complete list of messagesis also provided on page 131.

Important: Warning messages indicate that calculations have exceeded acceptable conditions.Runoff and routing results cannot be relied upon while any warning messages are present! Youmust understand and resolve all warning messages.

Right-click any message for other related options. This lets youopen a report or edit a node directly, without returning to therouting diagram.


Printing Reports

You can print individual reports by clicking the Print button on any report window. To print severalreports at once, select Print|Report or click the corresponding button on the main tool bar. This willactivate the report screen, which allows you to design a custom report for your project.

To print the routing diagram, select Print|Diagram, or use the corresponding button on the main toolbar.

Other items on the Print menu can be used to change the page and printer settings for your reports.These values apply to all projects, and are retained from one HydroCAD session to the next.

Units of Measure

To change the units of measure for the current project, select Settings|Units or click thecorresponding button on the settings tool bar. HydroCAD allows independent selection of units foreach of the following purposes:

! Input units are used for all data entry and verification.! Report units are used for all reports and graphs.! File units determine how data is stored in the project file.! Calculation units are used for all internal calculations.

The input and report units include many traditional secondary units, such as rainfall in inches ormillimeters. HydroCAD also supports customized units definitions, including the ability to adjustthe formatting and precision of displayed values. (For details click Help on the units screen.)

For reliable data exchange, project files are always stored in "pure" English or metric units, asdescribed on page 43. You can change this setting if you plan to read the project file with othersoftware that requires specific units.

All internal values are maintained and calculated in the specified calculation units. Since internalvalues are automatically converted to other units as required, this setting normally has no visibleeffect.

New projects are automatically configured for English or metric units depending on yourcomputer’s country setting. To change your default units for new projects, close any openprojects(s) and open the Settings|Units screen. Whenever a project is open, Settings|Units willconfigure the settings only for the current project. By default, HydroCAD uses Large Units for Areas and Volumes. When this option is selected,areas are reported in acres or hectares, and volumes are reported in acre-feet or mega-liters. Forprojects that work on a smaller scale, you can un-check this option to report areas in square-feetor square-meters, and volumes in cubic-feet or cubic-meters.

To change the number of digits or decimal places for anyparameter, click the Custom button on the Settings|Units screen andselect the desired parameter.


Section 6 - HydroCAD Project Files

Project Storage

Each HydroCAD project consists of a master .hcp file that contains all essential project data.Project files are small and easily transmitted as email attachments. You may freely send projectfiles to colleagues or reviewers, who can open them with a free HydroCAD Sampler available fromwww.hydrocad.net You should also send the associated project file with any technical supportinquiries.

When HydroCAD is installed, a HydroCAD\Projects folder is created in “Shared Documents”containing several sample projects. This is the initial folder in which projects are opened orcreated, although projects can be stored anywhere you choose. To find all HydroCAD projects,regardless of location, select Start|Find|Files and search for *.hcp.

Some projects may also employ external hydrograph files, as described on page 128. In order tokeep all related project files together, do not rename projects using normal Windows commands.Instead, open the project in HydroCAD and use Project|Rename.

Predefined Projects

HydroCAD includes a number of predefined projects. These are stored in the HydroCAD\Projectsfolder in Windows “Shared Documents”.

Some predefined projects contain “sample nodes” or templates that you can use in your ownprojects. Simply open your project and the sample project at the same time, and copy the desirednodes to your project.

You can use Copy and Paste to copy nodes between projects, or dragthe nodes via the Project Selector. If the originating project is read-only, dragging a node will make a clone without altering the original.Otherwise, you can use Ctrl-Drag to move a copy of the node.

Default Project Settings

When opening a new project, all project parameters are set to default values. To customize thedefault settings, open a new project called “Default,” configure the desired values, and save theproject.

Whenever a new project is created, HydroCAD automatically loads the contents of any“Default”project that exists in the same folder. If the file isn't found, HydroCAD tries to load itfrom the program's installation directory. You can use this behavior to create default settings forindividual folders, or to create master defaults that will be used for all other folders.

Although default projects normally contain only general project settings, they may also contain arouting diagram and related node data.


Read-Only Projects

A project can be marked Read-Only in order to protect it from accidental changes. To change thissetting use Settings|Read-Only. When a project is Read-Only, it is protected from any major changes.The diagram can still be panned and zoomed, but nodes cannot be moved, added, deleted, or edited.

Read-Only status is also useful when copying nodes from one project to another, in that it allowsnode copy/paste without danger of modifying the Read-Only project. When dragging a node froma Read-Only project via the Project Selector, a clone will be created, as if the Ctrl key were pressed.This makes Read-Only projects behave much like an extended palette. Some of the predefinedprojects supplied with HydroCAD are set to Read-Only for this reason.

Read-Only status can be removed at any time and is not secured in any way. Also note that thisfeature is independent of the read-only file status provided by the operating system.

Editing a Project File

While project files are normally used only by HydroCAD, it’s possible to view or even edit raw filesdirectly.

Warning! Modifying a project file by hand can produce unexpected results or even renderthe file unreadable by HydroCAD. Do not modify a project file unless you're absolutely sureof what you're doing! We cannot provide tech support for problems that result fromimproper modifications to project files.

Before editing a file, make sure it isn't currently open in HydroCAD. Then right-click the file andselect Edit from the context menu. The file is opened in Windows Notepad for examination.

When you're done, close Notepad by clicking the “X” in the upper-right corner. Save your changesonly if you're absolutely sure you want to modify the project!

Creating Project Files with Other Software

HydroCAD project files are stored in an easy-to-read plain-text format, making it relatively easyto generate a usable file with any programming language. If you currently have programmaticaccess to your existing data (via a scripting language, Basic, etc.) then you can export the desireddata to a HydroCAD project file.

Although HydroCAD project files support a large set of capabilities, you only need to provide thespecific data items you want to transfer into HydroCAD. Default values will be supplied formissing values whenever possible, or HydroCAD will prompt the user to supply the missing items.

Files created by this process can be opened directly with HydroCAD by using the Project|Opencommand, or you can use Project|Merge to combine the data with another project. This causes theproject to be selectively updated, retaining most of the original data except when it is beingreplaced by a new value.

6 Original TR-20 files are converted in their entirety. At this writing, support for WinTR-20 files is still underdevelopment, and is limited to the import of rainfall data.


Section 7 - Data Import

Although HydroCAD is generally used as a stand-alone program, it includes a number ofcapabilities for importing data from other sources, as discussed in this section.

Tabular Data Import

HydroCAD-10 includes the ability to import subcatchment (watershed) data from a tabular (CSV)file, which can be readily created with a spreadsheet or database program. To explore thiscapability, open a project and create a set of sample tables using Project|Export|Subcatchments. Thiswill create two files: One containing the subarea (curve number) data, and one containing the Tcdata. Opening these files will generally launch your default spreadsheet program, allowing you toexamine their format and content.

The tabular import capability can also be used to perform tabular editing of any project. First,export the data and open the file(s) with a spreadsheet program as described above. After makingthe desired changes, save the modified spreadsheet(s) to CSV format and import into the originalHydroCAD project using Project|Import|Subcatchments.

You can also transfer sub-area and Tc data separately by using the corresponding items on theProject|Import and Project Export menus. For details press the F1 key on those menu items.

Automated Tabular Import

If tabular data is being imported repeatedly, perhaps as part of an automated transfer, you canconfigure the project for one-click import. On the Settings|Watershed screen, select the Import tab andspecify the import file(s) containing the sub-area and/or Tc data. You can now useProject|Import|Watershed to immediately import the specified files at any time. For details click theHelp button on the Settings|Watershed screen.

Importing a TR-20 Data File

HydroCAD can read an existing TR-20 file and automatically convert the contents into anequivalent HydroCAD project file. To start the process, click Project|Import|TR-20 and select thedesired file.6 For further details click the Help button on the import screen.


Importing Data from AutoCAD®

HydroCAD provides two powerful capabilities for importing data from AutoCAD®, as describedbelow.

Direct import from AutoCAD®

HydroCAD-10 can directly import subarea data from AutoCAD®, without requiring any additionalsoftware. This process is configured on the Import tab of the Settings|Watershed screen, and allowsone-click import of subarea data at any time.

The import process analyzes special drawing layers containing the soil groups, ground covers, andsubcatchment boundaries, and imports each of the intersecting subareas into the appropriatesubcatchment. Subcatchments are automatically created on the HydroCAD routing diagram, withCN values automatically determined from the specified ground cover file. Land use data can alsobe imported for reporting or pollutant loading calculations, as discussed on page 129.

The import process includes multiple options including boundary verification, automatic nodeplacement, and highlighting of drawing areas as the corresponding subcatchment(s) are selectedin HydroCAD. Imported data can also be intermixed with manually created subcatchments,without the risk of manual entries being overwritten by the import process. For details click theHelp button on the Settings|Watershed screen.

Import using Carlson Hydrology

Carlson Hydrology provides an assisted import capability, which is able to extract subarea andtime-of-concentration data from AutoCAD®, and export the data to HydroCAD for calculations andreporting. Although this requires the separate purchase and operation of Carlson Hydrology, thecombination is able to process and export a wider range of data, such as watershed slopeinformation and pond storage contours extracted from Carlson’s terrain model (TIN).

Data transfer from Carlson Hydrology utilizes the Project|Merge capability in HydroCAD, which iscontrolled by various options on the Settings|Merge screen.

HydroCAD Technical Reference 39

HydroCAD Technical Reference

This section contains detailed informationon the calculations performed byHydroCAD. It is intended to help programusers and reviewers fully understand thespecific techniques and formulas employedby HydroCAD.

HydroCAD Technical Reference40


Section 8 - Understanding Hydrology

What is Stormwater and why does it need to be modeled?

During a rainstorm, precipitation reaching the ground is dissipated by several mechanisms: someis lost to evaporation, some infiltrates into the ground, and the remainder appears as stormwaterrunoff. This runoff can cause a variety of undesirable effects, such as erosion and flooding. In orderto prevent such damage, the runoff must be safely conveyed through suitable channels, pipes,ponds, streams, and rivers to a suitable point of disposal, and eventually to the sea. Bydetermining the nature of the runoff and the way it will flow through these channels, it is possibleto predict how, where, and when damage may occur. Steps can then be taken to reduce the chancesof damage, such as enlarging a stream to prevent overflow of its banks, or detaining some of therunoff in a pond to reduce flooding downstream.

Why is there an increased need for stormwater modeling now?

Most new construction involves changes in the usage of the surrounding land. A new shoppingcenter, for example, may remove a stand of trees and replace it with a paved parking lot andseveral buildings. Such a change in the ground cover has a dramatic effect on the runoff, greatlyincreasing its total volume and the rate of runoff. This directly increases the potential for erosionand flooding in all areas downstream of the new construction. To prevent such damage, the runoffmust be predicted before construction so that suitable steps can be taken to handle the runoff ina safe and effective manner.

In the past, such analysis and design was often not performed. As a result, a significant amountof stormwater damage now occurs, ranging from minor flooding of local streams, to erosion offarmland, to flooding of major rivers. The resultant burden on many drainage systems and theincreasing rate of construction means that all new construction and development should includea careful analysis of the effects on stormwater runoff. This not only reduces possible liability forstormwater damage, but is required by many local, state, and federal regulations.

How is stormwater modeled?

Stormwater modeling can be divided into two basic fields: Hydrology, which is the study of runoffand the factors that influence it, and Hydraulics, which is the study of water flow in the channels,pipes, streams, ponds, and rivers that convey it to the sea. In each field there are many techniquesavailable for performing the required analysis. A qualified engineer must choose the best methodsfor each situation.


Understanding HydroCAD

HydroCAD provides a number of techniques for the generation and routing of hydrographs. It alsoprovides many other related calculations, such as time of concentration, weighted curve numbers,pond volumes, stage-discharge curves, pollutant loading, etc. This broad range of capabilitiesallows a large number of studies to be performed entirely within HydroCAD.

Steady-state vs. time-varying flow

There are many different approaches to stormwater modeling and drainage design, which can beroughly divided into two basic groups:

1) Steady-state (constant flow) methods, such as the Rational method, commonly used forstorm sewer (pipe) networks.

2) Hydrograph generation and routing procedures designed to simulate the time-varyingnature of actual runoff, and model volume-sensitive stormwater elements, such as detentionponds.

Although HydroCAD can be used for steady-state designs and does include the Rational method,it is designed primarily as a hydrograph generation and routing program. Certain calculations,such as channel backwater or pressurized pipe networks, are often analyzed under constant flowconditions, and may require steady-state numerical tools, rather than a hydrograph routing systemsuch as HydroCAD. And some projects may require a combined approach: Using HydroCAD tomodel the overall drainage system, combined with a steady-state analysis for specific pipenetworks.

See the Frequently Asked Questions (beginning on page 143) for a discussion of several relatedtopics.

7 Conversion factors are shown to an accuracy of four decimal places. Actual conversions performed by HydroCADutilize a full-precision conversion of at least 10 digits.


Section 9 - Units of Measure

Most equations in this Manual are given in a universal format that may be directly evaluated inmetric (SI) or English (US Customary) units. When evaluating these equations, care must be takento use only primary units as listed in the following table.

Metric (SI) English ConversionFactor7

Time Seconds Seconds 1

Length Meters Feet 3.281

Area Square Meters Square Feet 10.76

Volume Cubic Meters Cubic Feet 35.31

Velocity Meters / Second Feet / Second 3.281

Flow Cubic Meters / Second Cubic Feet / Second 35.31

Weight Kilograms Pounds 2.205

Density Kilograms / Cubic Meter Pounds / Cubic Foot 0.06243

WeirCoefficient

1.811Meters ª Second Feet ª Second

Some empirical equations were developed with specific units (such as inches in the SCS runoffequation), and cannot be readily expressed in a universal form. These equations are marked withthe original units.

HydroCAD also supports many secondary units, such as acres and hectares, which areautomatically converted to and from primary units as required. When evaluating equations byhand, be sure to use the appropriate primary units as listed above.

You can change your default units, as well as the units for individualprojects, as described on page 34.


8 Instead of return period, it is more accurate to think in terms of the exceedence probability (p), where p=1/T. Thus,a “25 year storm” actually designates a rainfall event which has a 4% chance of occurring in any given year.


Section 10 - Rainfall Data

Intensity-Duration-Frequency Data

An IDF curve is one of the most common means of defining the rainfall characteristics at any givenlocation. Each IDF curve defines the rainfall intensity (i) that will occur for a specified rainfallduration (d) at a certain rainfall frequency or return period (T):8

For maximum flexibility, HydroCAD supports a number of different IDF data formats, including:

! Intensity vs. Duration points, using log-log interpolation between points! Coefficient-based curves, allowing direct evaluation for any duration! Local Precipitation Frequency Data

Local Precipitation Frequency Data is available from various web sites, such as those operated bythe National Oceanic and Atmospheric Administration (NOAA) or the Northeast RegionalClimate Center (NRCC). These web sites use a standard data format that is automaticallyrecognized as an IDF file by HydroCAD-10, eliminating the need to manually create an IDF file.For other locations (including international users) check with your weather bureau for theavailability of local data in the standard NOAA format, or create an IDF file manually using oneof the other formats supported by HydroCAD.

9 The SCS is now known as the Natural Resources Conservation Service, or NRCS.

10 In practice, synthetic rainfalls can be generated for any duration using any desired time increment.


IDF Library

Select View|IDF to see the sample IDF curves that are pre-installed in the HydroCAD IDF library.The IDF screen also includes links for downloading additional IDF data, as well as instructions forcreating IDF files by hand. Click More IDF Data to access online data sources. For further detailsclick Help on the IDF report screen, or visit www.hydrocad.net/rainfall

Synthetic Rainfall Distributions

Based on an analysis of nationwide IDF data, the US Soil Conservation Service9 developed a setof four dimensionless synthetic rainfall distributions used to characterize the rainfall patterns forthe entire United States. These are known as the Type I, IA, II, and III distributions. Eachdistribution is expressed as a mass curve indicating what fraction of the total 24-hour precipitationhas fallen at any time. (See page 157 for details.)

Synthetic rainfalls can be developed from standard IDF data discussed on page 45. Using 6 minutesteps, the incremental rainfall depth is calculated for durations of 6 minutes to 24 hours, placingthe highest (6 minute) incremental depth at the center of the storm, and adding the incrementaldepths for successively longer durations on alternating sides of the peak until a complete 24-hourcurve is developed.10

Since these are synthetic rainfall distributions, they are not intended to represent an actual rainfallevent. However, since they contain rainfall data for all durations from 6 minutes up to 24 hours,they can be used to simulate the behavior of a watershed under a wide range of conditions, and arenot limited to the analysis of a single duration like the Rational method. Each curve also providesdepth information for all durations up to 24 hours, making it suitable for volume-sensitivecalculations, such as detention pond simulations.

11 Event definitions are commonly used to model different return periods, such as the 25-year or 100-year storms, butcan also be used to model other rainfall conditions, such as a “water quality” event.


Rainfall Library

HydroCAD provides an extensive library of predefined distributions, including the standard SCSdistributions discussed above. Select View|Storm to see the HydroCAD Storm library and view thepre-installed rainfall distributions. A partial list also appears on page 155.

The storm report screen also includes links for downloading additional rainfall distributions,instructions for creating rainfall files by hand, and the ability to automatically generate a customsynthetic rainfall distribution based on local IDF data. To access these tools click the More Stormsbutton. For further details click Help or visit www.hydrocad.net/rainfall

Custom Synthetic Rainfall Distributions

For situations where none of the pre-installed rainfalls are appropriate, such as locations outsidethe US, or where newer rainfall data is available, HydroCAD-10 can generate custom syntheticrainfall distributions from local IDF data using the process outlined on page 46. To start theconversion process click More Storms on the View|Storm screen.

Importing TR-20 Rainfall Data

HydroCAD-10 also has the ability to import rainfall tables from a TR-20 or WinTR-20 data file.This process creates a native HydroCAD rainfall file, including multiple events when applicable.For details press Help on the Project|Import|TR-20 screen.

Rainfall Events

HydroCAD allows a set of rainfall conditions to be saved as a named rainfall event. Each eventdefinition specifies a rainfall distribution, duration, and depth.11 Events can be defined by handon the Rainfall tab of the Settings|Calculation screen. You can also use the Import Events button toautomatically define rainfall events from a number of different sources:

! Internal lookup table (organized by county)! Multi-event rainfall file! TR-20 data file (including new WinTR-20 format)! IDF file (all formats discussed on page 45)! Another HydroCAD project file

Regardless of the source, rainfall events are always stored in the individual HydroCAD project filewhere they were created.

12 For the traditional SCS UH, K is also equal to twice the fraction of the UH volume that occurs before the peak. OtherUH definitions may not preserve this relationship.


Unit Hydrographs

While a rainfall distribution (see above) specifies how precipitation is distributed over time, a unithydrograph predicts the distribution of runoff over time. More precisely, a unit hydrographrepresents the runoff resulting from a single burst of rainfall with the following characteristics:

! One unit of precipitation excess (expressed as a depth),! Generated uniformly over the watershed,! At a uniform rate,! With a burst duration D.

The hydrograph is made dimensionless by expressing:

! Ordinates as a fraction of the peak discharge qp,! Time axis as a fraction of the time-to-peak Tp.

Most projects in the United States employ the standard SCS unit hydrograph show above, whichis commonly identified by its peak factor of 484. The SCS UH can also be characterized by a shapefactor K=0.75, which is the ratio of the UH peak intensity to the total UH volume.12 For furtherdetails read about the SCS runoff procedure on page 57.

Unit Hydrograph Library

The HydroCAD UH library also includes the Delmarva UH, gamma curves with peak factors of200 to 600, and a number of local tables. Select View|Unit Hydrograph to open the UH report screenand view the pre-installed tables. The UH screen also includes links for downloading additionalUH data, as well as instructions for creating UH files by hand. For further details click Help on theUH screen or visit www.hydrocad.net/rainfall

13 This option was added in HydroCAD 9.0. Earlier versions treat all impervious areas as connected using Eq.1.


CNC 'CN1A1 % CN2A2 . . . CNnAn

A1 % A2 . . . AnEq. 1

Section 11 - SCS Curve Number

Curve Number Effects

The SCS Curve Number (CN) is used to determine the portion of the precipitation depth that willappear as runoff. The CN is a function of the soil type and ground cover. A high CN (such as 98for pavement) indicates low retention and high runoff, while a low CN (such as 30 for certainwooded areas) indicates a high retention capability and low runoff. This relationship is defined bythe SCS runoff equation as shown on page 59.

Curve Number Lookup Table

HydroCAD includes a complete curve number lookup table based on data developed by the SCS(NRCS) and published in TR-55. This table depends on the Hydrologic Soil Group, as discussed inAppendix A1. Also see NEH, which provides additional guidance on curve number selection.

The standard curve number lookup table is based on the relationship Ia=0.2S (see page 59 fordetails.) For other conditions, including international applications, an alternate lookup table canbe specified on the Settings|Watershed screen.

For rainfall on the surface of a pond or lake, a CN value of 98 is commonly used. The HydroCADlookup table contains separate entries for water surfaces, so they can be reported separately frompavement and other impervious surfaces with a comparable CN value.

The lookup table also includes a separate entry for unconnected impervious surfaces, which invokesa special curve number weighting procedure, as described below.13

Composite Curve Number

For subcatchments with multiple CN values, HydroCAD calculates a weighted CN value bysumming the products of each CN multiplied by its fraction of the total area. This composite valueis normally used in subsequent runoff calculations, without reference to the individual CN values.

CNC=Composite CN valueCN1-CNn=Individual CN values

A1-An=Area associated with each CN value

14 This option was added in HydroCAD 7.1. Earlier versions included all CN values in a single composite CN.


CNC ' CNPer %AImp

ATotal(CNImp&CNPer ) (1& AUnc

2AImp) Eq. 2

CNC '

CNPer (APer%AUnc

2) % CNImp (AImp&

AUnc

2)

ATotal

Eq. 3

Unconnected Impervious Surfaces

If runoff from an impervious surface occurs as sheet flow over an adjacent pervious area, theimpervious area is considered to be unconnected, and its runoff may be reduced as it flows over thepervious surface. This effect is considered to be significant only if less than 30% of thesubcatchment is impervious. When these conditions are met, the runoff is reduced by using amodified curve number weighting procedure, as used in TR-55:

CNPer=Composite CN for all pervious surfaces (see Eq.1)CNImp=CN for impervious surfaces (typically 98)ATotal=Total AreaAImp=Impervious Area (including unconnected)AUnc=Unconnected Impervious Area

Restating this in the form of equation Eq.1 shows the underlying basis of this adjustment:

Aper=Pervious Area

Note that the standard weighting for the pervious CN value is increased by half the fraction ofunconnected impervious area, while the weighting for the impervious CN value is decreased by thesame amount. Due to the higher value of the impervious CN, this causes a reduction in the finalcomposite CN value.

Separate Pervious/Impervious Runoff

Since the SCS runoff equation is non-linear, using a composite CN value may yield different resultsthan adding the runoff produced by the individual CN values. The difference is most pronouncedwhen the subcatchment includes both pervious surfaces (CN<98) and impervious surfaces (CN$98).

To account for this difference, HydroCAD provides the option to perform a separate runoffcalculation for the pervious and impervious portions of each subcatchment.14 This causes twocomposite CN values to be calculated for each subcatchment: one that includes all pervioussurfaces, and one for all impervious surfaces. The runoff equation is then evaluated separately forthe two portions, and the combined volume is used to produce the final runoff hydrograph.

Separate pervious/impervious calculations are most often used in conjunction with the SBUHrunoff method (see page 63), while the SCS method is normally used with a single composite curvenumber. The composite CN technique is specified for each project on the Settings|Calculation screen.


Adjustments for Antecedent Moisture Condition

The antecedent moisture condition (a.k.a. antecedent rainfall condition) specifies the moisture levelin the ground immediately prior to the storm. HydroCAD implements four AMC/ARC conditionsas follows:

AMC 1 - DryAMC 2 - NormalAMC 3 - WetAMC 4 - Saturated or frozen

It is common policy to use AMC 2 for most design work. Other values should be used only underspecial circumstances. AMC 1 will produce less runoff, while AMC 3 and 4 can produce dramaticincreases in runoff, and are not normally used for design purposes.

The AMC works by adjusting the Curve Numbers in all subcatchments according to a predefinedtable as shown on page 154. In general terms, AMC 1 reduces all CN values while AMC 3 increasesthe values. AMC 2 uses the original values without adjustment.

The AMC 4 condition increases all CN values up to 98. (Any values above 98 are unchanged.) Thiscapability is provided for saturated or frozen surfaces that are expected to have virtually noretention or infiltration capability. This setting is unique to HydroCAD, and is provided as analternative to manually adjusting multiple CN values.

AMC adjustments are always applied to the composite CN value, rather than to the individual CNvalues. In the case of separate pervious/impervious runoff calculations (see page 49), the AMCadjustment is applied separately to the composite pervious and impervious values.

The AMC is specified for each project on the Rainfall tab of the Settings|Calculation screen.


15 Since the Curve Number Method was designed to evaluate the Tc for an entire subcatchment, it is generally not usedin combination with other Tc procedures in the same subcatchment.


Tc'L.6

where L' l .8 (s%1).7

1900 Y .5and S' 1000

CN&10 Eq. 4

Section 12 - Time of Concentration

One of the key elements required for any runoff calculation is the Time of Concentration, or Tc. TheTc is typically defined as the time required for runoff to travel from the most hydrologically distantpoint of the watershed to the point of collection.

The time of concentration is commonly determined by summing the travel time (Tt) for eachconsecutive flow segment along the subcatchment's hydraulic path. This process requiresidentification of the type of flow occurring in each segment, and application of the appropriatemethod for calculating the Tt. Although these segments will occur in a given physical order, theorder in which they are used in the program has no effect on the total travel time.

HydroCAD provides a variety of techniques for calculating the Tt, plus other procedures (such asthe Lag method) which are designed to directly determine the overall Tc. These procedures arediscussed below. If necessary, the Tc or Tt may also be determined by other procedures and enteredinto HydroCAD directly.

The determination of the time of concentration is one of the mostwidely discussed areas of hydrology. The actual method(s) used onany given project depends upon actual site conditions, regulatoryrequirements, and sound engineering judgement.

Lag/Curve Number Method

The Curve Number Method (a.k.a. Watershed Lag Method, see NEH p.15-5) was developed to allowcalculation of the overall Tc under a wide range of conditions.15 The method is designed for areasof 2000 acres or less. The calculation is quite simple, but requires a proper understanding of theinput requirements:

TC=Time of concentration [hours]L=Lag time [hours]l=Hydraulic length of the watershed [ft]

Y=Average land slope [percent]S=Potential maximum retention [inches]

CN=Weighted Curve Number (See page 149)

Note the use of the average land slope, as described below. (This is distinct from the slope of thehydraulic path, as used in most Tc calculations.) Although some care is required to determine thisvalue, the Curve Number method has the advantage of using a small number of fairly objectiveparameters. This provides more consistent results than some other approaches.


Y ' 100 C IA Eq. 5

Tt '0.007(nL).8

P .52 s .4 Eq. 6

Average Land Slope

The average land slope (or average watershed slope) is a critical factor in the use of the CurveNumber method, as described on page 53. A theoretical determination would require placing a gridover the subcatchment and averaging the slopes for all squares. Other techniques are availablethat have more modest data requirements, such as the following equation from NEH p15-5:

Y=Average land slope [percent]C=Total Contour length [ft] or [m]I=Contour Interval [ft] or [m]

A=Land Area [ft²] or [m²]

C is obtained by adding the length of all contour lines within the subcatchment. The accuracy ofthis technique depends on having a sufficient number of contour lines within the subcatchment.Reducing the contour interval will generally increase the accuracy of the result.

Sheet Flow Procedure

The Sheet Flow procedure is designed for flow over plane surfaces, as usually occurs at theheadwaters of a catchment area. (See NEH p.15-6) The following equation is used for sheet flow:

Tt=Travel time [hours]n=Manning's coefficient for sheet flow (See page 167)L=Flow length [ft]

P2=2-year, 24-hour rainfall [inches] (See map on page 159)s=Land slope (along flow path) [ft/ft]

Determining the actual length of sheet flow is critical to this method. Although the technique wasoriginally intended for lengths up to 300 feet, most agencies now recommend a maximum of 100feet. In any case, the length should not extend past the point where there is evidence ofconcentrated flow on the ground. The length is also critical in that Sheet Flow is often a dominantfactor in a subcatchment's total Tc.

Note: At the point where sheet flow no longer occurs, additional segmentsof shallow concentrated flow and/or channel flow are typically usedto evaluate the remainder of the flow path. The total time for allflow segments is used in the final runoff calculations.


Tt 'L

3600 Vwhere V ' Kv s Eq. 7

Tt'L

3600Vwhere V'

1.486 r 2/3s 1/2

nand r' a

PwEq. 8

Shallow Concentrated Flow

Shallow concentrated flow (aka Upland Method) is designed for conditions that occur in theheadwaters of a watershed, including overland flow, grassed waterways, paved areas, and throughsmall upland gullies. Shallow concentrated flow does not have a well-defined channel, andgenerally has flow depths of 0.1 to 0.5 feet. Although commonly published as a chart of velocity vs.slope for various surfaces (see NEH Ch.15), shallow concentrated flow is based on the followingequations:

Tt=Travel time [hours]L=Flow length [ft] or [m]V=Average velocity [ft/sec] or [m/sec]

KV=Velocity factor [ft/sec] or [m/sec] (See page 168)s=Land slope (along flow path) [ft/ft] or [m/m]

See page 168 for a list of common KV values provided with HydroCAD.

Channel Flow

The Channel Flow procedure (see TR-55 p.3-3) is commonly employed where surveyed cross-sections are available, or anywhere the velocity can be reasonably determined by Manning'sequation.

Tt=Travel time [hours]L=Flow length [ft] or [m]V=Average velocity [ft/sec] or [m/sec]n=Manning's coefficient (See table on page 162)s=Channel slope [ft/ft] or [m/m]r=Hydraulic radius [ft] or [m]a=Cross-sectional flow area [ft²] or [m²]

Pw=Wetted perimeter [ft] or [m]1.486=English factor (use 1 for metric evaluation)

In addition to allowing direct entry of cross-sectional area and wetted perimeter, HydroCADprovides automatic flow analysis of many standard channel and pipe shapes as described onpage 169.

16 When used in combination with a Curve Number of 100, this will produce complete, instantaneous “runoff.”

17 The ability to specify a minimum Tc value was added in HydroCAD 7.1.


Tt'L

3600Vwhere V' gD Eq. 9

Travel Time Through Lakes and Reservoirs

Travel time for a lake or reservoir can be calculated by the following equation: (See NEH p.15-9)

Tt=Travel time [hours]L=Flow length [ft] or [m]V=Wave velocity [ft/sec] or [m/sec]g=Gravitational constant = 32.2 ft/sec2 or 9.81 m/sec2

D=Mean Depth [ft] or [m]

This technique may also be used for swamps with a significant amount of open water. If theamount of open water is less than about 25%, a segment of channel flow (see p.55) will give a betterestimate of travel time.

Note: This procedure is used only if the water body lies within a subcatchment, and is beingmodeled as part of the time of concentration. It does not account for storage effects, which requirea separate pond routing calculation as described on page 113.

Other Tc Procedures

Other Tc procedures can be employed by entering the calculated value directly into HydroCAD.This can be used as the total Tc for a subcatchment, or combined with additional flow segmentscalculated by other means. One situation that calls for direct Tc entry is modeling the “runoff” onthe surface of a pond. This requires the direct entry of a Tc value of zero.16

Tc Restrictions

Although HydroCAD has no inherent limitation on Tc values, some regulations may specify aminimum allowable Tc value. If applicable, this value may be defined on the Settings|Calculationscreen17 and will be automatically applied to each subcatchment except those with an explicit Tcvalue of zero.

18 Now the Natural Resources Conservation Service or NRCS.


Section 13 - SCS Unit Hydrograph Procedure

The US Department of Agriculture Soil Conservation Service18 has developed a number oftechniques for analyzing stormwater runoff. One of the most widely used is the SCS UnitHydrograph procedure (SCS-UH). The SCS-UH procedure is a principal component of SCS/NRCSTechnical Release 20, commonly known as TR-20.

The SCS-UH procedure is the primary runoff technique provided by HydroCAD. AlthoughHydroCAD does not employ any of the actual code from TR-20, it is based on the same SCS-UHprocedure and will produce essentially the same runoff results.

Data Requirements

The following data is required for the SCS unit hydrograph procedure as employed in TR-20 andHydroCAD. Some of these items are provided for each individual subcatchment, while others applyto the entire watershed.

Rainfall Distribution

The SCS unit hydrograph procedure iscommonly used with a syntheticrainfall distribution. This can be oneof the common SCS Type I, IA, II, andIII distributions (show at right), or oneof the other standard distributionsincluded in the HydroCAD rainfalllibrary. (See page 155.)

In addition, HydroCAD can generate acustom synthetic rainfall distributionbased on local rainfall data, asdiscussed on page 46. This makes itpossible to model locations for whichstandard rainfall distributions are notavailable, or where new (updated)rainfall data needs to be employed.

The rainfall distribution is commonly expressed as a dimensionless mass curve, as shown above.Multiplying the vertical axis by the total rainfall depth (see below) gives the actual rainfall depthat any time during the storm.

HydroCAD can also be used with an actual (observed) rainfall distribution, as long as sufficientlydetailed rainfall recordings are available. This is sometimes done to model specific historic stormsthat have produced record flooding.


Rainfall DurationMost storms employ a preset rainfall duration, such as 24 hours, which is used for most studies.The duration may be changed for special applications, such as the Illinois Huff Distributions. Although HydroCAD has the ability to rescale any storm to a different duration, this feature shouldonly be used for distributions (such as the Illinois Huff) that are specifically intended for thepurpose. Otherwise, each distribution should be used only at the duration for which is wasoriginally developed.

Rainfall DepthThe total storm rainfall (in inches or millimeters) must be specified for the project location. Forprojects in the continental United States, see the event lookup table in HydroCAD, or the rainfallmaps starting on page 159. For other locations, consult your local stormwater agency or weatherbureau. Calculations will often be performed for a number of different return periods ( such as 2,5, 10, 25, 50, and 100 years), using the corresponding depth for each return period. Each returnperiod can be defined as a separate rainfall event in HydroCAD, making it easy to change eventsand do a multi-event analysis. See page 47 for details.

Curve NumberThe CN value characterizes the type of soil and ground cover. See page 49 for details.

Time of ConcentrationThe Tc indicates the time required for all parts of the subcatchment to contribute to the runoff. Seepage 53 for details.

Unit HydrographThe UH is a dimensionlesscurve that shows the runoffdistribution resulting fromone unit of precipitationexcess occurring uniformlyover the watershed duringa specified duration. Theuni t hydrograph i scommonly identified by itspeak factor, such as 484 forthe standard SCS UH. Thep e a k f a c t o r i s acharacteristic of the unith y d r o g r a p h c u r v e ,representing the peak-to-volume ratio of the curve.For situations that requirea different peak factor, adifferent UH must be selected, as discussed on page 48.

19 The time base of the UH will vary depending on the exact UH used in this process.

20 Although the UH peak factor does not appear directly in this calculation, the resulting peak is dependent on the UHpeak-to-volume relationship.


Q'(P&Ia )2

(P&Ia )%Sand S'

1000CN

&10 (Q'0 if P#Ia)

if Ia'0.2S then Q'(P&.2S )2

P% .8S

Eq. 10

Tp'5D and Tp'23

Tc ˆ D'Tc

7.5Eq. 11

dQ ' Qt%D & Qt Eq. 12

Runoff Generation

The SCS runoff hydrograph is generated by performing a convolution of the unit hydrograph withthe rainfall excess. (For details see NEH Ch.16.) A brief description of the HydroCADimplementation follows:

1) At any time during the storm, the cumulative precipitation (rainfall depth) can be determinedfrom the selected rainfall distribution and the total rainfall depth. The cumulative precipitationexcess (runoff) can then be determined by the SCS runoff equation. (See TR-55 p.2-1 and NEHCh.10.)

Q=Precipitation excess (runoff) [inches or mm]P=Cumulative precipitation [inches or mm]Ia=Initial abstraction [inches or mm]S=Potential maximum retention [inches]

CN=Curve number

2) The storm is divided into a series of rainfall bursts of equal duration, with the burst durationbased on the unit hydrograph relationships:19

Tp=Time to peakTc=Time of concentrationD=Burst duration

3) The precipitation excess resulting from each burst is calculated by the SCS runoff equation:

4) The unit hydrograph defines how the precipitation excess from each burst will be distributedover time. The volume of the unit hydrograph is given by Eq. 12 and its time-to-peak is given byEq. 11, allowing us to produce a fully dimensioned hydrograph for each burst.20


5) The runoff from the entire storm is determined by summing the hydrographs resulting fromeach rainfall burst. The overall process is illustrated here:

5) The result is a final runoff hydrograph, similar to this:

21 If desired, the HydroCAD runoff procedure is specially designed to permit a Tc of zero. This can be used to model theinstantaneous “runoff” from rain falling on the surface of a pond.

22 This test is for the peak only. If the entire volume is required, you must still determine if the span is sufficient.


Special Considerations

1) The runoff hydrograph consists of a series of ordinates (ft³/sec or m³/sec flows) at evenly spacedintervals “dt.” Each ordinate specifies the average flow during the interval. As a result, if a narrowpeak occurred within one interval, the hydrograph would indicate an average flow that might besignificantly less than the instantaneous peak. This is likely to occur when Tc is less than 2dt, soHydroCAD displays an informative warning in these cases.

When you encounter this situation, keep in mind that the instantaneous peak can exceed theaverage for a time no longer than dt, which is commonly 6 minutes or less. In practice, such a shortinstantaneous peak is usually attenuated to the average value by the storage characteristics of thefirst reach or pond. However, if a true instantaneous peak is required, the runoff interval (dt) maybe reduced to approximately one-half the Tc.

2) The SCS unit hydrograph procedure has no inherent limitations on the time of concentration.As Tc approaches 0, the runoff curve approaches the precipitation excess curve, which is theexpected limiting case.21 Similarly, for a very large Tc, the entire storm becomes a single rainfall“burst” and the runoff approaches the shape of the unit hydrograph.

3) When making comparisons to TR-55, note that the TR-55 tables were produced for a curvenumber of 75 and require a precipitation excess of at least 1.5 inches. As conditions deviate fromthese, an increasing difference of up to 25% can be expected.

4) Runoff hydrographs are generated for a specified time span, such as 10 to 20 hours. You mustensure that this span is suitable for the purposes of your analysis and the rainfall type being used.If you are primarily concerned with peak flows, you can reduce calculation time by using a shortertime span. However, for ponds and other volume-sensitive studies, make sure the time span beginsat or before the earliest runoff, or this early volume won't be included in your calculations.HydroCAD will generate an automatic warning message if the span is not adequate to include theearliest inflow into a pond. Also keep in mind that the volumes displayed by HydroCAD includeonly the specified time span. By increasing the ending time to 25 hours or so, you'll get a completepicture of the storm.

5) As a safeguard, HydroCAD performs an automatic check of runoff peaks in relation to the timespan. A warning message is displayed if the calculated time of the peak doesn't fall within themiddle 90% of the time span. If this warning appears, you should examine the hydrograph andadjust the time span accordingly.22

6) The SCS runoff equation (Eq. 10) normally uses the standard Ia/S ratio of 0.2. which is applicableto most projects. If required, this value may be changed on the Settings|Calculation screen.

Although HydroCAD applies a number of tests to check the accuracyof your model, a visual examination of all hydrographs is highlyrecommended. This will help to detect erroneous input data andensure meaningful results.


TR-55 and the Tabular Method

Because of the enormous computational requirements of the unit hydrograph procedure, the SCSderived a simplified tabular method which it published in Technical Release 55 (TR-55).

The tabular method consists of a number of composite hydrographs produced with TR-20, whichare then scaled and interpolated in order to approximate the results that would have been producedwith TR-20 itself.

In order to keep the number of tables to a minimum, average values were used for several variables.The equations of TR-55 were then designed to reintroduce the dependencies on these parameters.

The two primary assumptions of TR-55 are a Curve Number of 75 and a runoff of 3 inches. TR-20and TR-55 can be expected to deviate as these assumptions become invalid.

A number of other conditions also indicate the use of TR-20:

! Tc<.1 hours or Tc>2 hours! Drainage subareas differ by a factor of 5 or more! The entire hydrograph is required for routing! Accurate volumes are required for routing

The approximations of TR-55 are sufficient to cause the SCS to place the following warnings in thedocumentation:

“This method (TR-55) approximates TR-20, a more detailed hydrograph procedure.... UseTR-20 if the watershed is very complex or a higher degree of accuracy is required.”

This applies particularly to the design of detention basins, since they are very sensitive to changesin the inflow hydrograph. Again quoting from TR-55:

“The procedure (TR-55) should not be used to perform final design if an error in storage of25 percent cannot be tolerated.... More detailed hydrograph development and routing willoften pay for itself through reduced construction costs.”

When evaluating TR-55, keep in mind that it was developed primarily for manual use. Whencomputers are available, the complete TR-20 unit hydrograph methodology is preferred.

Note: The latest Windows TR-55 release now uses the full unit hydrographprocedure, as HydroCAD has done since 1986. However, there arestill limitations in Win-TR-55 that preclude its use in manysituations, and require the use of a more flexible model, such asWin-TR-20 or HydroCAD.


Section 14 - Santa Barbara Urban Hydrograph

The Santa Barbara Urban Hydrograph method (SBUH) was developed by the Santa BarbaraCounty (California) Flood Control and Water Conservation District. The SBUH method has manysimilarities to the SCS Unit Hydrograph procedure discussed in the previous chapter. Bothtechniques employ the same SCS curve numbers, runoff equation, and rainfall distributions.However, the SBUH method does not utilize a unit hydrograph or the convolution process. Instead,an instantaneous hydrograph is generated and then routed through an imaginary reservoir witha time delay equal to the subcatchment's time of concentration.

This calculation is relatively simple in comparison to the SCS-UH procedure, and takes less timeto perform. While the availability of the SCS-UH procedure might appear to eliminate the needfor the SBUH method, some localities prefer the SBUH method for specific situations.

Runoff Procedure

There are two distinct steps involved in generating a runoff hydrograph by the SBUH method:

1) Compute the instantaneous hydrograph: The storm is divided into equal timeincrements (dt). At each increment, the SCS Runoff Equation (see page 59) is used to determinethe precipitation excess. The difference between the successive values represents theinstantaneous runoff at that point in time. A typical instantaneous hydrograph is represented bythe dashed line in the above graph.

2) Compute the runoff hydrograph: The runoff hydrograph is obtained by routing theinstantaneous hydrograph through an imaginary reservoir with a time delay equal to the time ofconcentration. The following equation is used to estimate the routed flow at each point in time:


Qt ' Qt&dt % w [ It&dt % It & 2Qt&dt ]

where w'dt

2Tc%dt

Eq. 13

Qt=Runoff at time t [ft³/sec] or [m³/sec]It=Instantaneous runoff at time t [ft³/sec] or [m³/sec]

dt=Calculation time increment [sec]Tc=Time of concentration [sec]w=Routing Coefficient

A typical runoff hydrograph is shown by a solid line in the graph above. Note the delay andreduction in the peak caused by the routing procedure.


Some implementations of the SBUH method require that the runoff be calculated separately forthe pervious and impervious portions of each subcatchment, rather than using a single compositecurve number. This may be accomplished by modeling the pervious and impervious componentsas separate subcatchments, or by changing the curve number weighting option as described onpage 50.


Q ' CiA Eq. 14

q [cfs] ' 1.01 C i [inches/hr] A [acres] Eq. 15

q [m 3/s] '1

360C i [mm/hr] A [ha] Eq. 16

Section 15 - Rational Method

The Rational method may be used to generate runoff hydrographs. However, since Rationalmethod was developed primarily for predicting peak flow, its use is not advised for volume-sensitiverouting calculations.

Runoff Procedure

The Rational method predicts the peak runoff according to the formula:

Q=Peak RunoffC=Runoff Coefficienti=Rainfall intensity

A=Area

The equation can be evaluated using English or metric units, as long as proper dimensions areobserved. For English use, it is traditional to employ inches-per-hour and acres for the intensityand area, respectively. Converting the units yields the following relationship, in which the factorof 1.01 is often omitted:

For metric use we can substitute mm/hr and hectares for the intensity and area, respectively.Converting the units yields the following relationship:

When using the Rational method, the rainfall intensity can be entered manually, or an IDF curvecan be provided so the intensity can be automatically determined for any specified duration. IDFis available from a number of sources as discussed on page 45.

In order to generate a complete hydrograph (as required by HydroCAD), it is assumed that therunoff begins at the start of the storm and increases linearly to the peak value, which is sustaineduntil the storm duration (D) has elapsed, and then decreases linearly to zero.

The rate at which the hydrograph rises and falls is based on the Tc and a rise/fall factor. For“standard” Rational method, the rise and fall factors are both one. That is, the rise and fall occurover the exact interval Tc. Variations of the Rational method (often called the Modified Rationalmethod), may use different rise and fall factors, which can be set directly using Settings|Calculation.

23 To enable the duration analysis report, you must select an appropriate IDF curve on the Rainfall tab of theSettings|Calculation screen.


Rational method runoff traditionally begins at zero hours, although it can start at whatever timeis specified in the calculation settings. A typical rational method hydrograph is shown below.

Typical Rational Method Hydrograph


Since hydrographs produced by the Rational method do not reflect the total storm runoff volume(or the intensity variation over time), this runoff method is generally not recommended for thedesign and analysis of detention ponds. Whenever possible, the SCS-UH or SBUH runoff methodis preferable in order to produce a complete inflow hydrograph as required for accurate pondrouting.

Proper use of the Rational method also requires that the correct critical duration (and thecorresponding intensity) be used at each point of study. This is typically defined as the durationthat produces the highest peak flow. Depending on the specific watershed, this may occur at anyduration between the shortest and longest Tc. As the study progresses downstream, the criticalduration generally increases, and the determination of the critical duration tends to become morecomplex. Note that as the duration is changed, all upstream subcatchments must be recalculatedfor the new value. (HydroCAD does this automatically.) This is the correct procedure for applyingthe Rational method, despite frequent misuse of the method in which upstream values are heldconstant.

To avoid trial-and-error solutions, HydroCAD provides a duration analysis report that canautomatically determine the critical value.23 When you click the Update button on the durationreport, HydroCAD evaluates the peak flow for a range of durations and automatically determinesthe critical duration. When analyzing a pond, this procedure will determine the duration thatproduces the highest peak water surface elevation.

Frequency Factor

In some applications, the standard intensity value is adjusted by a specific “frequency factor”. Thisvalue can entered directly on the Settings|Calculation screen, or specified within the applicable IDFfile. The latter method is generally preferred, since it allows a different frequency factor to beautomatically applied for each return period.


V'1.486R

23 S

12

o

nEq. 17

also R 'AP

and Q ' VA Eq. 18

thus Q'

1.486 AP

23 S

12 A

n' 1.486 A

53 S

12

n P23

Eq. 19

Section 16 - Reach Routing CalculationsA reach is used to perform an independent hydrograph routing through an open channel, or througha pipe flowing under open-channel conditions. A channel or pipe can alternatively be modeled asa flow segment within a subcatchment, where its travel time will contribute to the Tc. The laterapproach is usually simpler, and may even be necessary in the case of a subcatchment that isdraining along the entire length of the reach. However, for a long reach with a significant inflowat one end, a separate reach routing may be called for. This section details the procedures used toperform an independent reach routing.

Reach Routing Curves

Reach routing requires that the reach first be characterized by two curves: the end-area vs. depth(stage-storage), and the discharge vs. depth (stage-discharge). This information may be determinedby any of the following options:

Option 1 The user may directly specify the end-area and discharge at any number of depths.Values for intermediate depths are interpolated as described on page 70.

Option 2 The user may enter the end-area and wetted perimeter at each depth. Values forintermediate depths are interpolated as described on page 70. Manning's equation is then used tocalculate the discharge at each depth (see Basic Hydraulics p.77):

V=Average velocity of flow [ft/sec] or [m/sec]1.486=English factor (use 1.0 for metric)

R=Hydraulic radius [ft] or [m]S0=Slope of hydraulic grade line [rise/run]

=Slope of channel bottom, assuming normal flown=Manning's number (See table on page 162)

A=Area of flow [ft²] or [m²]P=Wetted perimeter [ft] or [m]Q=Flow [ft³/sec] or [m³/sec]

Option 3 For a rectangular, vee, trapezoidal, parabolic, or circular (pipe) channel, the user mayprovide the appropriate dimensions, and HydroCAD will determine the end-area and dischargecurves using the cross section equations on page 169, and Manning's equation as shown above.


nc 'j (n 3/2

i Pi )

j Pi

2/3

Eq. 20

nc 'j (n 2

i Pi )

j Pi

1/2

Eq. 21

nc 'P R 5/3

j Pi R5/3i

ni

Eq. 22

Option 4a Other shapes, such as natural channels, can be described with a custom cross-section.The section is defined by a table of coordinates that indicate the offset and elevation of points alongthe cross-section. Manning’s equation is then used to calculate the flow at any required depth.

Option 4b A custom cross-section may also be defined in which each segment has a differentManning’s value. The total flow is determined with a composite Manning’s value or segmentedflow, using one of the following methods. (See Open Channel Hydraulics p.136.)

Horton (deep flow) method

nc=Composite Manning’s numberni=Manning’s value for segment iPi=Wetted perimeter for segment i

This technique is commonly used for “deep” channels, where the velocity is relatively constantthroughout the cross-section. If applied to a section with a constant Manning’s value, the resultis identical to the original Manning’s value.

Pavlovskii (shallow flow) method

nc=Composite Manning’s numberni=Manning’s value for segment iPi=Wetted perimeter for segment i

This technique is commonly used for “shallow” channels, where the velocity may vary considerablybetween segments. It is based on the assumption that the total resistant force for the cross-sectionis the sum of the resistant force for each of the segments.

Lotter (subdivided flow) method

nc=Composite Manning’s numberni=Manning’s value for segment iPi=Wetted perimeter for segment iRi=Hydraulics radius for segment i = Areai/PiP=Total wetted perimeter = sum(Pi)R=Hydraulic radius for entire channel = Area/P

24 Water surface profiles are usually calculated under constant-flow conditions, rather than with a hydrograph (time-varying flow). Peak flows (from HydroCAD or another hydrograph model) are often used as input for these calculations.


Q ' j Qi Eq. 23

Q ' j Qn Eq. 24

This technique is based on the assumption that the total flow is equal to the sum of the flows forthe individual segments. If applied to a section with a constant Manning’s value, the result is notthe same as the original Manning’s value.

Subdivision by Segment

Q=Total flow for cross-section [ft³/sec] or [m³/sec]Qi=Flow for segment i (see Eq.19) [ft³/sec] or [m³/sec]

This technique produces exactly the same flow as the Lotter method, described above. It differsonly in the calculation procedure, in which the total flow is the sum of the flows calculatedseparately for each segment, without the use of a composite Manning’s value.

Subdivision by Manning's Value

Q=Total flow for cross-section [ft³/sec] or [m³/sec]Qn=Flow for consecutive segments with same Manning’s value

This technique subdivides the channel only when there is a change (break) in the Manning's value.This produces more consistent results than subdivision by segment, in that the resulting flow isindependent of the number of points along the cross section. When all segments have the sameManning's value, the flow is identical to the traditional solution for a constant Manning's value.This technique is similar (although not identical) to the current procedure used in HEC-RAS.

Other Procedures

If another technique is used to calculate flow through a complex cross-section, the rating curve canbe calculated separately and entered into HydroCAD using option 1, above. However, using adefined geometry or cross-section allows direct evaluation of the channel at any depth, withouthaving to interpolate between a (smaller) number of user-defined stages.

Reach Routing Limitations

The preceding stage-discharge calculations are based solely on Manning's equation, and do notconsider possible inlet, outlet, or tailwater effects. If a complete analysis is desired for a pipe,including entrance losses and possible tailwater effects, it should be modeled as a pond with aculvert outlet. If a detailed water surface profile is required for a channel, you should use aprogram specifically designed for that purpose.24


Reach Routing Table

When performing a reach routing, HydroCAD uses an internal routing table to provide storage anddischarge information at any required depth. The construction of this table depends on the selectedreach option, as listed on page 67. When entering discharge or wetted-perimeter directly (options1 or 2), several interpolation options are available:

Linear: This option creates a routing table containing the same number of depth values (stages)specified by the user. When the routing is performed, intermediate storage and discharge valuesare obtained by linear interpolation between these stages. (This option is provided primarily forcompatibility with HydroCAD 7.1 and earlier.)

Multi-point: This option creates a routing table with a larger number of evenly-spaced depthintervals (normally 100). Each user-specified depth is also included in the table, to ensureaccuracy at these exact depths. Storage is calculated at each tabulated depth using a linearinterpolation between user-defined stages. When entering wetted-perimeter, linear interpolationis used to determine the perimeter at each tabulated depth, and the corresponding discharge iscalculated with Manning's equation. When entering discharge, linear interpolation is used todetermine the discharge at each tabulated depth. Multi-point is the default option, and isrecommended for most situations.

Parabolic: This option creates a routing table with a larger number of evenly-spaced depths, justlike Multi-point, except that intermediate values are determined by parabolic interpolation usingthe three adjacent user-defined stages. This option may provide greater accuracy when theuser-supplied rating table contains a limited number of stages.

Logarithmic: This option creates a routing table with a larger number of evenly-spaced depths,just like Parabolic, except that intermediate storage and discharge values are determined bylogarithmic interpolation. (Since the log of zero is undefined, logarithmic interpolation cannot beused between the first two user stages, so linear interpolation is employed in this range. This effectcan be minimized by including a near-zero stage slightly above the bottom of the channel.)

Note: The Parabolic and Logarithmic options may provide greater accuracywhen the user-supplied rating table contains a limited number ofstages. However, the resulting storage and discharge plots shouldbe examined to be sure the interpolation method is appropriate forthe data.

When using a standard channel geometry (option 3) or custom cross-section (options 4a and 4b) therouting table normally contains 100 evenly-spaced depth values, ranging from zero depth up to theoverall depth of the reach. At each tabulated depth, HydroCAD calculates the perimeter, storage,and discharge based on the channel geometry and Manning’s equation.

For all reach options, the finished routing table is used to perform the actual routing calculations,using a linear interpolation to determine the storage and discharge at intermediate depths. Ifgreater accuracy is required, the number of depth increments can be increased on the Advancedtab of the applicable reach.


Reach Routing Methods

HydroCAD currently provides the following techniques for reach routing, as described in theremainder of this section:

! Storage-Indication Method (Stor-Ind)! Storage-Indication plus Translation (Stor-Ind+Trans)! Dynamic Storage-Indication Method (Dyn-Stor-Ind)! Muskingum-Cunge Method! Dynamic Muskingum-Cunge Method! Simultaneous Routing (Sim-Route)

Storage-Indication Method

The Storage-Indication method (Stor-Ind) is the most basic reach routing technique provided byHydroCAD, and is based on the routing equations developed on page 115. The actual routingprocedure is as follows.

1) The reach's stage-discharge relationship is calculated as described above.

2) The stage-storage relationship is determined from the reach cross-section multiplied by thelength.

3) The stage-discharge and stage-storage curves are used to create a storage-indication curve.

4) Routing is performed using the specified time span and time increment. At each point in time,a storage-indication value is calculated based on the current inflow, plus the previous inflow,outflow, and volume in the reach.

5) The current storage-indication value and the storage-indication curve are used to determine thenew elevation.

6) Using the new elevation, the stage-storage and stage-discharge curves are consulted todetermine the new storage and discharge.

7) This process is repeated for all points in the inflow hydrograph.

In practice, the procedure may incorporate a number of other factors, such as a base flow or inflowloss. Any unusual conditions, such as the channel overtopping, will produce a specific warningmessage as listed on page 131.

Storage-Indication plus Translation

The Stor-Ind+Trans method is identical to the Storage-Indication method described above, exceptthat the storage-routed hydrograph is subject to a further time lag (translation) by the travel time,as defined on page 123. This is an early technique provided by HydroCAD to allow for travel timeon some reaches. For longer reaches, or when peak timing is critical, a full kinematic routingprocedure is recommend, such as the Muskingum-Cunge method, described below.

Dynamic Storage-Indication Method

The Dyn-Stor-Ind reach routing procedure is identical to the Storage-Indication method describedabove, except that the calculations are performed over the entire watershed at each time step.Although the reach routing results are unchanged, this allows the overall watershed to be analyzedin a dynamic manner so that ponds may respond to tailwater effects as described on page 117.


c ' m V Eq. 25

Muskingum-Cunge Method

This routing method is intended to duplicate results that would be obtained with the currentWin-TR-20 software. (See Muskingum-Cunge Flood Routing Procedure in NRCS Hydrologic Modelsby William H. Merkel.) The basic HydroCAD procedure is as follows:

Step 1: Determine m-value vs. Depth

The m value defines the relationship between the flow velocity (V) and the wave velocity or celerity(c). m is the slope of the discharge-area curve in a log-log plot, and is constant at all depths forbasic channel geometries, such as vee and trapezoidal. However, for complex cross sections, andespecially for flood plains, the value can vary considerably with depth. To handle these situations,HydroCAD uses a weighted m value, according to the same procedure developed for TR-20.

When stage data is entered directly, the m value is calculated at each user-specified depth, andthen interpolated to the depths used in the internal routing table. When using a standard channelgeometry or cross-section, m is directly calculated at each depth in the routing table.

Step 2: Select Reference Flow

HydroCAD currently provides a constant-parameter implementation of the Muskingum-Cungeprocedure. This requires the selection of a constant reference flow at which the routing parameterswill be determined. When using a sequential routing procedure, the reference flow is set equal to75% of the peak inflow. When using a dynamic routing procedure, the peak is unknown until therouting is complete, so the reference flow is set equal to 75% of the flow at flood depth or 75% of thechannel-full capacity. In either case, the user may override the default value by entering thepreferred reference flow.

Step 3: Determine Wave Velocity

Using the reference flow, the equivalent flow depth, velocity, and m value are determined by linearinterpolation from the routing table. (The user may override the automatic m value by enteringthe preferred value on the reach edit screen.) The wave velocity (celerity) is then given by:

c=Wave velocity (celerity) [ft/sec] or [m/sec]m=Rating curve exponentV=Flow velocity [ft/sec] or [m/sec]

Step 4: Determine Time and Distance Steps

The time step (∆t) is generally selected in order to provide sufficient definition for the runoff andinflow hydrographs. This selection will generally ensure that the inflow peak occurs after at leastten time steps, as recommended in the TR-20 implementation. If necessary, a smaller time stepcan be specified. The overall reach length may require division into a number of equal sub-reachesof length ∆x, such that ∆x is approximately equal to the distance traveled by the flood wave in asingle time step:


S ' K XI % (1&X)O Eq. 27

K '∆xc Eq. 28

X '12

1& QB So c ∆x

'12

1& V DSo c ∆x

Eq. 29

to obtain ∆x.c∆t where ∆x' LN

ˆ N' round Lc∆t

Eq. 26

∆x=Distance step (length of sub-reach) [ft] or [m]∆t=Time step (dt) [sec]c=Wave velocity (celerity) [ft/sec] or [m/sec]L=Total reach length [ft] or [m]N=Number of sub-reaches (rounded to nearest whole number)

Step 5: Determine Routing Parameters

The following equation defines the Muskingum relationship between reach inflow, outflow, andstorage:

S=Reach storage [ft³] or [m³]K=Storage constant [sec]X=Weighting factor [dimensionless]I=Reach inflow [ft³/sec] or [m³/sec]

O=Reach outflow [ft³/sec] or [m³/sec]

The routing parameters K and X are given by:

K=Wave travel time through sub-reach [sec]∆x=Distance step (length of sub-reach) [ft] or [m]

c=Wave velocity (celerity) [ft/sec] or [m/sec]

X=Routing factor (0.0 to 0.5)Q=Reference flow [ft³/sec] or [m³/sec]B=Average flow width of reference flow [ft] or [m]V=Reference flow velocity [ft/sec] or [m/sec]D=Reference flow depth [ft] or [m]S0=Slope of hydraulic grade line [rise/run]

=Slope of channel bottom, assuming normal flow

Note that the selection of ∆x will produce a wave travel time K that approximates the time step ∆t.The routing factor X controls the peak outflow, with smaller values producing more attenuation.


S2 & S1 'I1%I2

2&

O1%O2

2∆ t Eq. 30

O2 ' C1 I1 % C2 I2 % C3O1

where C1'∆t /K%2X

C0C2'

∆t /K&2XC0

C3'2(1&X)&∆t /K

C0C0'

∆tK

%2(1&X)

Eq. 31

Step 6: Calculate Routing Coefficients

Conservation of mass is assured by use of the continuity equation:

S1,S2=Storage at time t1 and t2 [ft³] or [m³]I1,I2=Inflow at time t1 and t2 [ft³/sec] or [m³/sec]

O1,O2=Outflow at time t1 and t2 [ft³/sec] or [m³/sec]∆t=Time difference between t1 and t2 [sec]

Combining with Eq.27 and simplifying yields the final routing equation:

Step 7: Perform Hydrograph Routing

The routing equation (Eq. 31) is applied at each time step ∆t in the inflow hydrograph. If the reachhas been divided into sub-reaches (N>1), the calculation is repeated for each sub-reach, using theoutflow of the previous sub-reach as the inflow to its successor.

The continuity equation (Eq. 30) is also evaluated at each time step in order to determine the reachstorage, average depth, and average flow velocity over the length of the reach.

As an aid to evaluating the routing results, a detailed list of routing parameters is included in thereach summary report. (Click any part of the report for a detailed discussion of each parameter.)

Dynamic Muskingum-Cunge Method

The Dynamic Muskingum-Cunge procedure is identical to the standard Muskingum-Cungeprocedure described above, except that the calculations are performed over the entire watershedat each time step. Although the reach routing results are unchanged, this allows the overallwatershed to be analyzed in a dynamic manner so that ponds may respond to tailwater effects asdescribed on page 117.

The Dynamic Muskingum-Cunge procedure will produce the same reach routing results as thestandard (sequential) Muskingum-Cunge procedure, as long as the same reference flow is used.


I & O '∆S∆ t

where ∆S ' S2&S1

ˆ S2 ' S1% ( I&O )∆ tEq. 32

Simultaneous Reach Routing

The Sim-Route reach routing procedure is provided for compatibility with simultaneous pondrouting. Although this method does not currently allow reaches to respond to tailwater changes,its does allow the overall watershed to be analyzed in a simultaneous manner so that ponds mayrespond to tailwater effects as described on page 119.

Simultaneous reach routing is based on the basic equation for conservation of mass. (SeeHydrologic Analysis and Design p.545.)

I=Inflow rate [ft³/sec] or [m³/sec]O=Outflow rate [ft³/sec] or [m³/sec]∆t=Time increment (dt) [sec]∆S=Change in storage [ft³] or [m³]S1=Storage at start of time interval [ft³] or [m³]S2=Storage at end of time interval [ft³] or [m³]

The following procedure is used to perform the actual hydrograph routing:

1) Routing is performed using the specified time span and time increment, and begins with nowater stored in the reach.

2) The new reach storage (S2) is calculated using the above equation with the previous rates ofinflow and outflow. (Using the previous value allows non-sequential flows, for which the currentinflow is unknown.)

3) Using the new storage volume, a new flow depth is calculated based on the assumption of normalflow.

4) Using the new flow depth, a new discharge is calculated from Manning's equation or from theuser-defined stage-discharge relationship.

5) Steps 2 through 4 are repeated at each time interval until the entire hydrograph has beendeveloped.

Additional considerations apply to all simultaneous routing procedures as described on page 119.


Effects of Reach Routing

A reach will normally attenuate and delay the hydrograph that is routed through it. The extent ofthis transformation depends on many factors, including the reach dimensions, slope, and Manning'snumber. Short reaches (up toseveral hundred feet) often have aminimal effect on the routedhydrograph. For this reason theyare frequently modeled as a flowsegment within a subcatchment.

On the other hand, for long reacheswith large cross-sections, low slopesand/or high Manning's numbers,the routing effect can besignificant. The graph at rightshows the effects of storage-indication routing through a 5500foot long channel. Significantattenuation may also occur onshorter reaches if the inflow peak isof short duration.

Allowing for Travel Time

The storage-indication method, as illustrated above, accounts for only the storage effects of thereach. Other techniques must be used to account for the kinematic effects of long reaches, such asthe “Stor-Ind+Trans” methoddescribed on page 71. A closeexamination of the graph belowreveals that the peak discharge nolonger corresponds to a point on theinflow curve, but is translatedaccording to the travel time.

A better option for modelingkinematic e f fects i s theMuskingum-Cunge methoddescribed on page 72. This is thestandard reach routing procedurein the latest Win-TR-20 release,and is recommended whensignificant kinematic effects arepresent.


Section 17 - Pond Storage Calculations

Most pond routing procedures require basic information about the pond’s stage-storagerelationship. In addition to allowing direct entry of stage-storage data, HydroCAD canautomatically calculate storage volumes for a wide range of common shapes:

PrismatoidA volume with a square or rectangular base, with vertical or equally-slopingsides. Suitable for modeling rectangular vaults, excavations, or above-groundstorage with rectangular horizontal sections.

Upright ConeA vertical cylinder or cone with any side-slope. Suitable for modeling avertical cylinder or above-ground storage with circular horizontal sections.

Round Pipe StorageFor modeling storage in a round pipe or cylindrical tank. May be set level oron a slope.

Box Pipe StorageFor modeling storage in a rectangular pipe or trench. May be set level or ona slope.

Arch & Elliptical Pipe StorageFor modeling storage in an elliptical or arch pipe. May be set level or on aslope.

Arch ChamberFor approximate modeling of storage chambers with a parabolic arch and aflat bottom. For more accurate results, use a prefab chamber definition, asdescribed below.

Prefab ChamberFor precise modeling of prefabricated stormwater chambers, using theHydroCAD chamber library. The underground storage wizard can be usedfor automated layout, modeling, and pricing of chamber installations.

Custom StorageAllows direct entry of storage data or surface areas. Suitable for naturalponds or other shapes that cannot be readily modeled with the other storageoptions described above.


Appropriate Use

Pond storage options, including pipe storage, are intended to model water storage, not conveyance.This requires the presence of outlet controls that restricts the flow sufficiently in order to createa level-pool within the defined storage. If a level-pool does not exist, an alternate modelingprocedure (such as multiple ponds or a reach routing) may be required.

Compound Storage

Although a pond can often be described with a single storage definition, HydroCAD allows eachpond to employ multiple definitions as required to define the overall storage. For example, a singlepond might include an underground storage vault (prismatoid), a section of pipe storage, plus anabove-grade overflow area (custom storage).

When using compound storage, wateris assumed to flow freely between allvolumes in each pond, such that theymaintain essentially the same watersurface elevation throughout the pondrouting (i.e. a level pool). If this assumption is not valid, the storage volumes may need to bemodeled as separate ponds, with the appropriate outlet controls.

Embedded Storage

Storage definitions can also be embedded inside each other, such as perforated pipe or storagechambers buried in a bed of crushed stone. This procedure uses the ability to define the fractionof voids within each storage volume, in order to allow for the effects of stone, sand, or gravel fill.

When embedding a thick-walled storagechamber (such as a concrete tank or pipe), awall thickness can be specified to allowaccurate calculation of the displaced volume.When using a prefab chamber definition, thewall thickness is generally preset within thechamber definition.

You can set up imbedded storage manually, oruse the underground storage wizard for automated layout and modeling with any of the pre-definedchambers. (See page 84 for details.)

Wetted Area

For most storage definitions, HydroCAD can also determine the wetted area and surface area at anyelevation. These parameters may be used in conjunction with certain exfiltration calculations asdescribed on page 108.

The remainder of this section describes the equations used to evaluate each type of storage. Thisincludes the volume calculations, as well as the determination of surface and wetted areas.


V 'y6

( A1%4A2%A3 )

A1 ' L1W1 A2 ' L2W2 A3 ' L3W3

L2 'L1%L3

2W2 '

W1%W3

2

L3 ' L1 % 2yZ W3 ' W1 % 2yZ

Eq. 33

As ' Hs L1%L3%W1%W3

Hs ' y Z 2%1Eq. 34

Do ' D % 2T Z 2%1 Eq. 35

Prismatoid Storage

For a prismatoid with a rectangular base and four equally sloping (or vertical) sides, the volumeis given by:

V=Volumey=Water depth

A1=Bottom area (depth=0)A2=Mid area (depth=y/2)A3=Top area (depth=y)

L1,L2,L3=Bottom/Mid/Top lengthW1,W2,W3=Bottom/Mid/Top width

Z=Side Run/Rise (0=vertical)

The side area is given by the following equation:

AS=Side areaHS=Side height along slope

To obtain the total wetted area, add the bottom area to this value. (The top area is never included,even when the volume is full.)

If a wall thickness is specified, the outer volume (inclusive of the wall) is calculated by increasingthe length and width of the prismatoid by twice the horizontal extent of the wall:

DO=Outside Length or WidthD=Inside Length or WidthT=Wall ThicknessZ=Side Run/Rise

By convention, the wall thickness is not added to the top or bottom of the prismatoid.


V 'h3

A1%A2% A1A2

A1 ' π R 21

A2 ' π R 22 and R2 ' R1 % hZ

Eq. 36

As ' π (R1%R2 ) (R1&R2 )2%h 2

As ' A1% A2 A1& A22%πh 2

Eq. 37

Ro ' R % T Z 2%1 Eq. 38

Vertical Conic Storage

The volume of a horizontal section (frustum) of a vertical cone (or cylinder) is given by:

V=Volume of sectionh=Height of section

A1,A2=Area of bottom/top of sectionR1,R2=Radius of bottom/top of section

Z=Side Run/Rise (0=vertical)

The wetted side area is given by either of the following equations:

AS=Wetted (side) area of section

To obtain the total wetted area, add the bottom area to this value.

If a wall thickness is specified, the outer volume (inclusive of the wall) is calculated by increasingthe radius by the horizontal extent of the wall:

RO=Outside RadiusR=Inside RadiusT=Wall ThicknessZ=Side Run/Rise

By convention, the wall thickness is not added to the top or bottom of the cone.

25 The ability to model sloped pipe storage was added in HydroCAD 7.1.


V ' L Ae

Ae '12

R 2 [ θ & sin(θ ) ]

θ ' 2 cos&1 [1& yR

]Eq. 39

Aw ' L R θ (excluding end&areas)

Aw ' L R θ % 2A (including end&areas)e

Eq. 40

As ' 2 L R sin θ2

' 2 L y (2R&y) Eq. 41

Ro ' R % T Eq. 42

Round Pipe Storage

For a level round pipe, the storage volume at any depth is given by:

V=VolumeL=Length

Ae=Submerged end areaR=Radiusθ=Submerged central angle [radians]y=Water depth

The wetted area is given by:

For exfiltration calculations, the end-area is normally excluded.

The surface area is given by:

If the pipe lies on a slope25, each of these parameters requires a numerical integration over theportion of the pipe that is partially full. This is added to the corresponding value for any portionof the pipe that is completely full.

If a wall thickness is specified, the outer volume (inclusive of the wall) is calculated by increasingthe radius by the wall thickness:

RO=Outside RadiusR=Inside RadiusT=Wall Thickness

By convention, the wall thickness is not added to the pipe length (ends).


V ' L W y Eq. 43

Aw ' L (W%2y ) (excluding end&areas)

Aw ' L (W%2y ) % 2Wy (including end&areas) Eq. 44

As ' L W Eq. 45

Do ' D % 2T Eq. 46

Box Pipe Storage

For a level box pipe, the storage volume at any depth is given by:

V=VolumeL=Length

W=Widthy=Water depth

The wetted area is given by:

For exfiltration calculations, the end-area is normally excluded.

The surface area is given by:

If a wall thickness is specified, the outer volume (inclusive of the wall) is calculated by increasingthe width and height by twice the wall thickness:

DO=Outside DimensionD=Inside DimensionT=Wall Thickness

By convention, the wall thickness is not added to the pipe length (ends).

Elliptical and Arch Pipe Storage

Arch pipes are characterized by a top, bottom, and cornerradius. An elliptical pipe is a special case of an arch pipe, inwhich the top and bottom radii are the same.

The storage volume for an arch pipe consists of up to threecomponents, depending on how much of the bottom, corner, andtop chords of the pipe are submerged. This calculation isconsiderably more complex than other types of pipe storage, andis not detailed in this document.


V '23

Y W L Eq. 47

As ' L Ps

Ps' 4Y 2%W 2

4%

W 2

8Yln

2Y% 4Y 2%W 2

4W2

Eq. 48

Yo ' Y % TWo ' W % 2T Eq. 49

Parabolic Arch Storage

The full volume of a parabolic arch chamber is given by:

V=Storage volumeY=Parabola height

W=Parabola widthL=Length of chamber

For a partially full arch, the volume is calculated by subtracting the non-submerged volume fromthe volume of the entire arch.

The side area of a parabolic arch is given by:

As=Side areaPs=Side perimeter

For a partially full arch, the side area is calculated by subtracting the non-submerged portion ofthe arch from the entire arch.

To obtain the entire wetted area, add the bottom area to this value. For exfiltration calculations,the end-area is normally excluded.

If a wall thickness is specified, the outer volume (inclusive of the wall) is calculated by increasingthe width and height accordingly:

YO=Outside heightWO=Outside width

T=Wall Thickness

Note that the wall thickness is not added to the bottom of the arch, which is usually open.


Prefab Chamber Storage

For a prefabricated storage chamber, the submerged volume iscalculated using the width or incremental storage data from theapplicable chamber definition file.

The wetted area at any depth is the sum of the bottom area andside area, which is inferred from the supplied chamber data. Notethat this value is rarely used in exfiltration calculations (see p.108),since the exfiltration rate is usually based on the area of the outerexcavation in which the chamber is embedded, rather than the areaof the chamber itself.

Chamber Library

Click View|Chamber to open the chamber reportscreen and view the chamber definitions that arepre-installed with HydroCAD. The chamber reportallows you to select multiple chambers and comparetheir storage characteristics and cross-sections, asshown here.

Click Get Updates to download the latest chamberupdates for your HydroCAD system.

Chamber Wizard

The chamber wizard simplifies the process of modeling underground storage systems, byautomatically sizing the overall drainage field based on the amount of stone required around eachchamber. Layout parameters, such as row spacing and stone cover, can be automatically setaccording to the manufacturer’s recommendations, making it much easier to design and comparedifferent storage scenarios.

When complete, the wizardproduces a stone-filled storagevolume with the chamber systemautomatically embedded in thestone. In addition to modelingarched chambers, the wizard canbe used to model pipe storagesystems, including the pipe headerassembly.

The wizard can also be used toestimate basic system cost, basedon user-supplied pricing for thechamber, excavation, and stone,plus optional user-specified items, such as filter fabric or header assemblies.


V ' hA1%A2

2Eq. 50

V 'h3

A1%A2% A1A2 Eq. 51

Custom Storage

Custom storage may be defined in three ways:

1) Direct entry of cumulative (total) storage at various elevations, which requires no furthercalculations.

2) Entry of incremental storage, that is, the volume of horizontal sections across the pond.These sections are summed by the program to produce the cumulative storage.

3) Entry of surface areas at various elevations, from which HydroCAD determines theincremental (and cumulative) storage at each elevation.

The third option is often the most convenient, since it uses readily available data, such as thesurface area at each contour elevation. The actual storage calculation is based on the selected pondshape, as described below.

Custom Prismatic Storage

Each stage is taken as a horizontal section of a prism. This provides accurate volumes forfour-sided ponds when zero, one or two (opposing) sides of the pond are sloping, and the other sidesare vertical. This is equivalent to the traditional “average area” method.


A1=Area of bottom of sectionA2=Area of top of section

Since the aspect ratio of prismatic shapes is undefined, the wetted area cannot be determined,making this shape incompatible with exfiltration calculations. One of the other shapes should beselected when wetted area is required.

Custom Pyramidal Storage

Each stage is taken as a horizontal section (frustum) of a pyramid. This assumes four sides ofequal length and slope, and includes any shape with horizontal cross-sections that are square andconcentric, including a square box with vertical sides.




As' A1% A2 A1& A22%4h 2 Eq. 52

V 'h3

A1%A2% A1A2 Eq. 53

As' A1% A2 A1& A22%πh 2 ' π R1%R2 (R1&R2)

2%h 2 Eq. 54

R1'P1

2πand R2'

P2

2πˆ As'

P1%P2

2(P1&P2)

2

4π2%h 2 Eq. 55

The side area is given by the following equation:

As=Wetted (side) area of section


Custom Conic Storage

Each stage is taken as a horizontal section (frustum) of a cone. This includes any shape withhorizontal cross-sections that are circular and concentric, such as a cylinder or cone. Thiscalculation is appropriate for many dry wells and natural ponds. The storage volume is given by:



The side area is given by:

As=Side area of section


Custom Irregular Storage

Although the exact shape is unknown, the volume is calculated as a conic section using equation 53,as shown above. This provides reasonable accuracy for natural ponds with equally sloping (orvertical) sides, or when small section heights are used.

To calculate the wetted area, the specified perimeter is taken as the circumference of an equivalentcircle. Substituting the resulting radius into equation 54 (the wetted area of a conic section) yieldsa reasonable estimate of the wetted area:

As=Wetted (side) area of sectionh=Height of section

P1,P2=Perimeter of bottom/top of sectionR1,R2=Radius of bottom/top of section



Section 18 - Pond Hydraulics Calculations

This section details the hydraulics calculations used by HydroCAD. These equations are used todetermine the discharge of each device under specific headwater and tailwater conditions, primarilyin determining the stage-discharge relationship for a pond. All equations determine the dischargeQ, in ft³/sec or m³/sec.

The following outlet devices and flow characteristics can be modeled with HydroCAD, as detailedin the following pages:

! Sharp-Crested Rectangular Weir

! Broad-Crested Rectangular Weir

! V-Notch Weir

! Trapezoidal Weir

! Custom Weir/Orifice

! Asymmetrical Weir

! Submerged Weirs

! Dam Breach

! Orifice & Grate Flow

! Culverts

! Tubes, Siphons, and Float-Operated Valves

! Constant-Flow Devices (such as floating skimmers)

! Special Outlet (User-defined stage-discharge curve)

! Pumps

! Exfiltration

Each pond can include an unlimited number of outlet devices,discharging independently, or combined to create “compoundoutlets”, such as a standpipe or riser discussed on page 114.


Q ' C Le H 3/2 where C'23

2g Cd Eq. 56

Le' L& nH10

(but not < L2

) Eq. 58

C ' 3.27 % .4 HP Eq. 59

Q ' C Le H 3/2 & (H&M)3/2 Eq. 57

Sharp-Crested Rectangular Weir

The basic equation for a sharp-crested weir is derived in Open Channel Hydraulics p.362.

C=Weir coefficientLe=Effective crest lengthH=Head (above crest or invert elevation)g=Gravitational constant

Cd=Discharge Coefficient

If the headwater exceeds the weir rise (see page 92), orifice flow exists with:

M=Rise (vertical dimension of weir opening)

The effective crest length Le may include an adjustment for the number of end contractions.

L=Crest lengthn=Number of end contractions (0, 1, or 2)

In practice, the weir coefficient C may vary slightly based on the crest height and the resultingturbulence. If the crest height is specified, the English weir coefficient is given by the followingequation: (To obtain a metric weir coefficient, divide this value by 1.811 as described on page 43.)

C=Weir coefficient [ ]feet ªsecH=Head (above crest)P=Crest height (above approach channel)


Q ' C L H 3/2 where C'23

2g Cd Eq. 60

Broad-Crested Rectangular Weir

A broad-crested rectangular weir differs from a sharp-crested weir in that the weir coefficient mayvary as an arbitrary function of head. (See Practical Hydraulics p.274.) This allows more accuratemodeling of a wide range of real-world weirs.

C=Weir coefficientL=Crest lengthH=Head [above crest]g=Gravitational constant


C varies with H by means of a lookup-table, which supplies the appropriate weir coefficient atspecific heads. For intermediate heads, HydroCAD interpolates linearly between the given values.For heads that fall outside the given range, HydroCAD uses the first or last coefficient withoutextrapolation.

! For weirs with a square-edged crest, the coefficient lookup valuescan be supplied automatically by specifying the breadth of the crest.(Crest thickness along the direction of flow.) This causes the lookuptable to be filled with coefficients as listed on page 164.

! For other weir profiles as listed on page 165, the coefficients aresupplied automatically when the desired profile ID is specified.

! Other crest profiles can be modeled by manually entering weircoefficients at appropriate heads.

Note: Although a broad-crested weir can produce more accurate results than using a sharp-crestedweir, the effect on the overall hydrograph routing is sometimes less than expected. Unless thereare significant variations in the weir coefficient, the sharp-crested weir equation may providecomparable accuracy while requiring less data. A quick sensitivity analysis may be useful indetermining the actual effects of coefficient variations.


Q ' Cv tan θ2

H 5/2 where Cv'815

2g Cd Eq. 61

Q ' Cv tan θ2

H 5/2 &52

H (H&M)3/2 &32

(H&M)5/2 Eq. 62

Cv ' 2.46% tan(90&θ/2)25 Eq. 63

V-Notch Weir

The basic equation for a v-notch weir is derived in Handbook of Hydraulics p.5-15:

Cv=V-notch weir coefficientθ=Notch angle (between two sides)H=Head (above apex of V)g=Gravitational constant


If the headwater exceeds the weir rise (see page 92) orifice flow exists and the discharge is givenby:


This equation is equivalent to the normal v-notch weir flow (Eq.61) minus the trapezoidal weir flow(Eq.64) that would otherwise occur above the rise.

The v-notch weir coefficient Cv may be entered manually, or automatically calculated based on thenotch angle using the following equation: (To obtain a metric weir coefficient, divide this value by1.811 as described on page 43.)

Cv=English weir coefficient [ ] feet ªsec


Q ' Cv tan θ2

H 5/2 %54

L H 3/2

where Cv'815

2g Cd

Eq. 64

Q ' Cv tan θ2

H 5/2 &52

H (H&M)3/2 &32

(H&M)5/2

%54

Cv L H 3/2 & (H&M)3/2Eq. 65

Trapezoidal Weir

Trapezoidal weir flow is a combination of v-notch weir flow (Eq.61) with half of the vee on eitherside of the horizontal spillway, plus rectangular weir flow (Eq.56) over the horizontal portion of thespillway:

Cv=V-notch weir coefficientθ=Notch angle (between two sides)H=Head (above weir crest)L=Length of horizontal portion of spillwayg=Gravitational constant


The weir coefficient may be entered manually, or automatically determined by Eq. 63 above. Alsonote the factor of 5/4, which accounts for the different terms that are included in the rectangularand v-notch weirs coefficients.

Note: This is the same trapezoidal weir equation used in previous versions of HydroCAD,but is presented here in fully reduced form.

If the headwater exceeds the weir rise (see page 92), orifice flow exists and the discharge is acombination of v-notch orifice flow (Eq.62) plus rectangular orifice flow (Eq.57):



Weir Rise

The weir rise indicates the vertical height of the weir opening. This parameter is commonly usedto avoid “overlap” when multiple weir definitions are used to define a complex weir opening, suchas the superimposed rectangular and trapezoidal weirs shown here.

The weir rise may be specified for any sharp-crested rectangular,vee, or trapezoidal weir. Whenever the headwater exceeds theweir rise, the appropriate orifice-flow equation is used instead ofthe standard weir flow equation.

If the headwater exceeds the uppermost rise (M2 in this example)a warning message is issued to indicate that the entire “weir” isoperating under orifice-flow conditions, and that additional weirdata may be required to correctly model the high-head condition.

The use of the weir rise parameter is optional. If the rise is not defined (left blank), the weir isassumed to have no vertical limit, and the standard weir equation is used for all heads.

Custom Weir/Orifice

A custom weir can be used to model an arbitrary symmetrical flow area, such as a v-notch cut intoa rectangular spillway, or a non-standard orifice. It can even be used to model a device with morethan one opening, as show below.

To calculate the flow through a custom weir, the weir is divided into a number of horizontaltrapezoidal sections, starting at the weir invert and extending to the headwater elevation. Thetotal flow is determined by adding the flow through each section as given by Eq.64 (for theuppermost section) and Eq.65 (for the lower sections).

If the weir is subject to tailwater, the discharge is a combination of standard trapezoidal weir flow(for the area above the tailwater) and constant-head orifice flow (for the area below the tailwater)as given by Eq.67.

A custom weir/orifice is defined by specifying anynumber of head/width pairs as required to fullydefine the opening(s). In general, a head/width pairmust be specified at each point of inflection, markedA-H on the example at right. Note that points C andD both specify a width of zero, in order to separatethe upper and lower openings. Curves may beapproximated by using a number of closely-spacedpoints. For further details, click Help on thePond:Custom Weir screen.

A custom weir/orifice will generally yield the samedischarge relationship as using several separateweir definitions with the appropriate rise settings.


Asymmetrical Weir

An asymmetrical weir can be used to model an arbitrary weir crest, such as water spilling over aroadway. This option is similar to the custom weir/orifice (above), but it doesn’t require that theweir opening be symmetrical around a vertical centerline.

To calculate the flow through an asymmetrical weir,the weir is divided into a number of rectangular andhalf-vee sections. The total flow is determined byadding the flow through each section, using thetrapezoidal weir equation Eq.64 for partiallysubmerged sections, and Eq.65 for fully submergedsections. The flow through a half-vee section isone-half the flow for a corresponding full-vee.

If the weir is subject to tailwater, the discharge is acombination of standard trapezoidal weir flow (forthe area above the tailwater) and constant-headorifice flow (for the area below the tailwater) asgiven by Eq.67.

Note that an asymmetrical weir will give exactly the same result as using several separatetrapezoidal weirs to describe the entire weir opening. The weir crest is described by entering the crestheight or elevation at a number of horizontal offsets.Offsets (positive or negative) can be measured fromany reference point (weir center, left end, etc.), butmust be entered in ascending numerical order. Useenough values to accurately describe the shape ofthe crest. The crest height at intermediate offsets isdetermined by linear interpolation, as shown on thereal-time sketch. The first and last heights (orelevations) must be the same.


Qs ' Qf 1& H2

H1

n 0.385Eq. 66

Q ' Cd a 2gh

with Cv '815

2g Cd then Q '158

Cv a h

or with C '23

2g Cd then Q '32

C a h

Eq. 67

Submerged Weirs

The preceding equations specify the discharge for various weirs under conditions of free discharge.If the tailwater of a weir exceeds the crest elevation, the crest becomes submerged, and thedischarge must be reduced accordingly. HydroCAD provides two separate techniques for evaluatingsubmerged weirs:

If the weir rise is not specified, the final discharge is determined by the following equation fromHandbook of Hydraulics p.5-18:

Qf=Free discharge (from standard weir equation)Qs=Submerged dischargeH1=Upstream head above crestH2=Downstream head above crestn=1.5 for rectangular weirsn=2.5 for vee/trapezoidal weirs

Although this adjustment was derived specifically for sharp-crested weirs, it is also used toestimate the discharge for submerged broad-crested weirs.

If the weir rise is specified, the discharge is the sum of two components:

1) Standard weir/orifice flow for the portion of the weir that lies above the tailwater, with the headand rise measured from the tailwater elevation, rather than from the weir invert.

2) Constant-head orifice flow for the portion of the weir that lies below the tailwater:

Cd=Discharge coefficienta=Submerged areag=Gravitational constanth=Effective head (HW-TW)

Cv=V-notch weir coefficientC=Rectangular weir coefficient

This technique is based on standard weir and orifice flow calculations, and may provide moreaccurate results than the empirical solution of Eq.66.


Dam Breach

A dam breach is modeled as a weir opening whose dimensions change over time, as the breachprogresses. The breach can start at a specific time or water surface elevation, and progresses toits final dimensions over a specified time.

A dam breach is modeled as flow through atrapezoidal weir in which the weir rise (height)increases linearly over time. The top of the breachis maintained at a constant elevation as the bottomof the breach decreases in elevation until the finalbreach height is achieved.

A breach is defined by its final dimensions. Notethat the notch angle and crest length remainconstant as the breach progresses, while the heightand top width increase linearly with time.

A dam breach may be initiated at a specific time, orwhen the water surface elevation in the pondreaches a specific elevation. After the breach begins to form, the height increases linearly over thespecified time until the final height is attained.

Because the size of the breach varies over time,breach modeling requires the use of a dynamic pondrouting procedure. This also allows the breachcalculations to account for tailwater created by adownstream node, such as a reach or pond. Seepage 117 for details.

Note that a breach outlet provides for flow onlythrough the area on the breach. If flow will occurover a spillway above the breach, you must definethat spillway using a separate weir outlet.


dQ ' Cd L 2gY dY Eq. 68

Q '23

Cd L 2g H 3/2& [H&M ]3/2 Eq. 69

Q '23

Cd L 2g H 3/2 ' C L H 3/2 (English units) Eq. 70

Q ' Cd L Y 2g H&TW Eq. 71

Rectangular Orifice in a Vertical Plane

For a rectangular opening in a vertical plane, the discharge under any head is derived from thedischarge through a thin horizontal strip. (See Handbook of Hydraulics p.4-3.)

Cd=Discharge coefficient (Default is .60)L=Strip length (width of orifice)g=Gravitational constantY=Head over center of strip

dY=Height of horizontal strip

Integrating over the height of the orifice yields:

H=Head above invert elevationM=Height of orifice

When the orifice is partially submerged (H<M) the term [H-M] becomes zeroand this reduces to the rectangular weir equation:

The above equations apply to free-discharge conditions. When the tailwater exceeds the orificeinvert, the discharge is the sum of two components:

1) The discharge for the portion of the orifice (if any) that lies above the tailwater elevation is givenby the previous equations, with the head and height measured from the tailwater level rather thanfrom the invert.

2) The portion of the orifice that lies below the tailwater is subject to a constant head differential,with the discharge given by the basic orifice equation:

TW=Tailwater depth above invertY= Lesser of M or TW


Q ' Cd a 2gh

where h'H (for free discharge)or h'H&TW (for TW above invert)

Eq. 72

V ' Cd 2gh Eq. 73

h ' k V 2

2gwhere k '

1C 2

d

and Cd '1k

Eq. 74

Rectangular Orifice in a Horizontal Plane

For an orifice opening in a horizontal plane, the discharge is given by the basic orifice equation asderived in Handbook of Hydraulics p.4-3:

Cd=Discharge coefficient (default is .60)a=Orifice areah=Effective headH=Headwater depth

TW=Tailwater depth

Under low-head conditions, you may also wish to consider the possibility of weir flow as describedon page 99.

Orifice Discharge Coefficient

All orifice calculations are based on the following equation:

V=Discharge velocityCd=Discharge coefficientg=Gravitational constanth=Effective head

The discharge coefficient indicates the fraction of theoretical discharge that the orifice can actuallyhandle. The coefficient is a unit-less parameter which can vary from 0 (for no discharge) up to 1.0(for full theoretical capability). The default value of 0.60 indicates that the orifice can discharge60% of the theoretical value. Other coefficients can be used if required, although the value istypically in the range of 0.59 to 0.61.

The above expression for orifice flow can also be written as a head-loss equation:

k=Head-loss coefficient

Any device that is characterized by this equation can usually be modeled with the standard orificecalculations, with the head-loss coefficient converted to an equivalent discharge coefficient asshown above.


Q ' Cd a 2gh Eq. 75

h ' H&max( r, TW) (if fully submerged)

h ' H&max( H2, TW ) (if partially submerged) Eq. 76

h ' H (for free discharge)h ' H&TW (for TW above invert)

Eq. 77

Circular Orifice

The discharge for a circular orifice is derived in Handbook of Hydraulics p.4-3:

Cd=Discharge coefficient (default is .60)a=Submerged areag=Gravitational constanth=Effective head

Circular Vertical Orifice

For an opening in a vertical plane, the effective head is given by:

H=Headwater depth above invertTW=Tailwater depth above invert

r=Radius

When partially submerged, the head adjustment closelyapproximates the weir discharge of an orifice. It also providescontinuity between the fully and partially submergedconditions. For critical situations, the resulting discharge curve should be verified by independentmeans.

Circular Horizontal Orifice

For an orifice opening in a horizontal plane, the effective head is given by:

Under low-head conditions, you may also wish to consider thepossibility of weir flow as described on page 99.

26 The ability to define the outer grate dimensions was added in HydroCAD 9.0.


Q ' C L H 3/2 where C'23

2g Cd and Cd'0.61 Eq. 78

Orifices Under Low-Head Conditions

The previous orifice equations for openings in a vertical plane are generally valid under all headconditions. No adjustment is required under low-head (partially submerged) conditions, since theseequations reduce to the appropriate weir equation.

For orifice openings in a horizontal plane, the equations assume that the head is large in relationto the orifice size. This can lead to overestimating the discharge under low-head conditions. Toensure correct flow under all conditions, discharge can be automatically limited to that predictedby the weir equation:

Q=DischargeC=Weir CoefficientL=Crest length (orifice perimeter)H=Head (above invert elevation)g=Gravitational constant

Cd=Discharge coefficient

This will cause the weir equation to control at low heads, without affecting the high-head dischargepredicted by the orifice equation. The result is useful for a range of real-world “orifices”, such asthe top of a standpipe.

! This adjustment is performed automatically whenever “Use weir flow at low heads” isselected for a horizontal orifice.

Note: This calculation uses a preset discharge coefficient Cd=0.61, which corresponds to an Englishweir coefficient C=3.27or a metric coefficient C=1.81.

Modeling a Grate

A grate typically consists of several identical openings, eachof which can be modeled as an orifice. For a grate in ahorizontal plane, the discharge multiplier can be used tospecify the total number of openings. The overall gratedimensions can also be specified, allowing for possible weirflow control at the outer perimeter of the grate.26

For a grate in a vertical plane, the number of rowsand columns must be specified separately, alongwith the center-to-center row spacing. This allowsthe software to automatically calculate the correctinvert elevation for each row of openings. Thistechnique may also be used for any array of same-size orifice openings, such as the perforations inthe side of a vertical riser.


V 2 'H&D%SL

Ke%1

2g%

n 2 L

C R 4/3

and Q ' A VEq. 79

Culvert Flow

HydroCAD can model a wide range of culvert shapes, including circular, box, elliptical, and pipe-arch.

When evaluating a culvert, HydroCAD checks multiple flow conditions in order to determine theprevailing control at each headwater elevation. This is based on six types of culvert flow asidentified in Culverts - Hydrology & Hydraulics page E-1. (Also see Standard Handbook for CivilEngineers p.21-18,19.)

Type Inlet Outlet Slope Flow TypeTailwaterDependent? Type of Control

1a Submerged Submerged Any Pipe Yes Outlet1b Submerged Free Mild Pipe No Outlet (barrel)1c Submerged Free Any Channel No Inlet (orifice)2a Free TW>Yc Mild Channel Yes Outlet2b Free TW<Yc Mild Channel No Outlet (barrel)2c Free TW<Yc Steep Channel No Inlet (weir)

TW=Tailwater, Yc=Critical Depth

For type 1b, assuming that the culvert is full along its entire length, the velocity is given by thefollowing equation. (See Culverts - Hydrology & Hydraulics page D-11.)

V=Average velocity of flowH=Headwater depth above inlet invertD=Depth of flow (=culvert height)S=Slope [rise/run]L=Length

Ke=Entrance energy loss coefficient (See table on page 166)g=Gravitational constantn=Manning's number (See table on page 162)C=2.22 for English, 1 for metricR=Hydraulic radiusA=Cross-sectional flow area


D '34

H Eq. 80

Cd 'Cc

1%KEEq. 81

Type 2b discharge is the same as type 1b except that the depth (D) is less than the culvert height.Under these conditions, open channel flow exists and backwater calculations must be performedto precisely determine the depth. To reduce calculation time, the depth is approximated by:

Rather than directly determining whether type 1b or 2b flow exists, HydroCAD uses the lesser ofthis depth and the culvert height. This also ensures continuity between the two flow conditions,with the cross over occurring when the head is 4/3 of the culvert height.

Types 1a and 2a are similar to types 1b and 2b, except for the tailwater dependency. This isaccommodated by setting D equal to the tailwater depth whenever this value exceeds the normalflow depth.

Types 1c and 2c operate under inlet control, and the discharge is determined with the orificeequations given previously. The orifice discharge coefficient is given by:

Cc=Contraction coefficient (default is .90)

Note that for Ke=.5 this yields Cd=.6, which is the default dischargecoefficient for a sharp-edged orifice.

The final determination of culvert discharge is made by calculating the type 1a/2a, 1b/2b and 1c/2cflows as described above. The least of these values (a, b, and c) is then used as the final dischargefor a given head.

Note: The approximations used for culvert discharge have generally been found to providesufficient accuracy for most hydrograph routing purposes. However, it is strongly recommendedthat the resulting stage-discharge curve be verified using independent culvert data. If a significantdiscrepancy is found, the desired discharge data should be entered directly as a Special Outletinstead of using the built-in culvert equations.


Q ' CO a 2gHO Eq. 82

HO 'Q 2

2g C 2O a 2

'V 2

2g C 2O

Eq. 83

Tube & Siphon Flow

Tube flow is used to model a conduit flowing full, where the flow rate is determined by headwater,tailwater, and tube characteristics. This capability can also be applied to model self-primingsiphons, where the flow initiates and breaks at preset elevations. The start/break capability alsomakes this class of outlet suitable for modeling float-activated valves and other gravity-drivendevices that exhibit an on/off hysteresis with respect to head.

The operation of the tube/siphon outlet can be divided into three basic categories:

1) Regular Tube: A gravity-fed tube with a continuous downward slope to the outlet, or an“inverted siphon” with a dip in the middle of the tube. These scenarios are defined by the tube’sparameters and its inlet and outlet elevations. Flow begins when the headwater exceeds the inlet,and stops when the headwater drops below the inlet.

2) Siphon or float-activated valve: A device that requires a water surface elevation somedistance above the inlet in order to initiate flow. These devices are also characterized by a breakelevation at which flow will cease. Flow begins when the headwater exceeds the “start” elevation,and stops when the headwater drops below the “break” elevation.

3) Float valve with trickle: A float-activated valve (as above) that allows a reduced flow (trickle)in the "closed" position. The trickle flow is characterized by an equivalent orifice diameter whichcontrols the flow when the valve is “closed”.

Note that categories 2 and 3 require a dynamic routing procedure in order to allow for the multi-variable stage-discharge relationship. See page 117 for details.

When the tube length is zero, the discharge is calculated directly from the orifice equation:

CO=Orifice coefficient (default is .60)a=Tube areag=Gravitational constant

HO=Head above inlet invert or tailwater

When the tube length is non-zero, we must consider the head-loss from the orifice, plus thefrictional losses within the tube. For the orifice, we can solve the above equation for head:

V=Flow Velocity


V ' k CF R 0.63 HF

L

0.54

Eq. 84

HF ' L Vk CF

1.85

R &1.17 Eq. 85

H(V) ' HO(V)%HF(V) 'V 2

2g C 2O

%L Vk CF

1.85

R &1.17 Eq. 86

Frictional losses within the tube are derived from the Hazen-Williams equation:

k=1.318 for English units -or- 0.85 for metricCF=Hazen-Williams coefficientR=Hydraulic radius

HF=Friction head lossL=Tube length

Solving for the head loss gives:

The total head loss for any flow rate is the sum of the orifice and tube losses:

This value must equal the vertical distance from the headwater to the outlet of the tube, or thedistance to the tailwater, if the outlet is submerged. To determine the flow rate for a given head,a numerical solution for V is obtained by using Newton's Method. Multiplying by the tube areagives the final discharge Q. When using a dynamic routing procedure, the tube outflow willrespond to changes in tailwater elevation as they occur.

For all tubes, the flow calculated above is gradually phased-in over an elevation range equal tothe orifice diameter. This avoids a sudden flow increase at initiation due to the suction head whenthe tube outlet is lower than the inlet, and approximates orifice flow at low heads. This phase-inis not applied to siphons or float-activated valves, since they are subject to full-flow conditions atthe moment of initiation.

Remember that all calculations assume the tube is always flowing full. For a partially-full tube,consider a culvert outlet instead.


Q 'D1

D2QD Eq. 87

Constant-Flow Outlet Device

A constant flow outlet device may be used to model a skimmer or other outlet structure thatexhibits a relatively constant flow rate regardless of the water surface elevation. The specified flowwill occur whenever the headwater exceeds the device’s invert elevation.

For a skimmer, the invert elevation is the level at which water begins to overtop the inlet weir ororifice and starts to flow through the structure. The invert will generally be above the bottom ofthe pond, since a certain depth of water is usually required to reach the inlet and initiate operation.

After water begins to flow, an additional phase-in depth is generally required before the skimmerstarts to float and the full design flow is achieved. If the phase-in depth is set to zero, the fulldesign flow will occur as soon as the pond's water surface elevation exceeds the invert elevation.However, a more stable and accurate routing is generally achieved by using a non-zero phase-indepth.

When the water is above the invert but below the phase-in depth, the structure will discharge afraction of the total design flow calculated by:

Q=Actual dischargeD1=Depth above invertD2=Phase-In DepthQD=Design Flow

This relationship provides a linear transition from zero to full flow that approximates the behaviorof the weir/orifice arrangement at the inlet of a typical floating skimmer. Since the phase-in depthis usually small in comparison to the overall operating range of most skimmer installations, theexact shape of the transition curve will have a minimal effect on the overall pond routingcalculation. For situations where the transition curve is critical, or the skimmer discharge is notstrictly constant, a special outlet can be used for more precise modeling. However, a constant flowoutlet will provide sufficient accuracy for most applications without requiring the externalcalculation of a complete rating curve.

If the pond is subject to tailwater conditions, the discharge (and phase-in) are delayed until thepond's water surface elevation exceeds the tailwater elevation. If a phase-in depth is specified, fullflow will occur when the WSE exceeds the tailwater elevation plus the phase-in depth.


Special Outlet Device

The special outlet device is designed to handle unusual stage-discharge relationships (such as avortex valve) that can't be readily reproduced with any of the standard outlet devices.

The behavior of a special outlet device is defined by an explicit stage-discharge table. The firstdischarge value must always be zero and may occur at any desired elevation. Additional dischargevalues are specified at higher elevations as required to adequately represent the true shape of thedesired rating curve. When choosing the elevations, keep in mind that HydroCAD performs alinear interpolation to determine the discharge at any required intermediate elevations.

If the last defined elevation is exceeded and the “Extrapolate” option is selected, HydroCAD willextrapolate from the last two values. Otherwise, the final discharge value is used withoutextrapolation.

! When the Head-Loss option is not selected, special outlets have a fixed stage-dischargerelationship and cannot respond to variable tailwater conditions. (The discharge is a function ofheadwater only.)

! If the Head-Loss option is selected, the discharge is a function of the head loss (difference)between the headwater and tailwater elevations. If the tailwater is less than the device invert, thehead-loss is given by the headwater depth above the invert, and the actual tailwater is ignored.In this case, the discharge is the same as when the Head-Loss option is not selected.

Note: Rather than entering a rating table by hand, the “Load From File” button may be used to loada pre-defined rating table, such as the vortex valve data added in HydroCAD 7.1.


V ' k C R 0.63 S 0.54 where S' hL

Q'VA ˆ Q ' k C A R 0.63 hL

0.54Eq. 88

h ' L QkCA

10.54 R

&0.630.54 ' L Q

kCA

1.85R &1.17 Eq. 89

Pump Calculations

Pumps are modeled as a pond outlet device,which contributes to the pond’s stage-discharge curve. The primary input data isthe pump rating curve, which specifies theavailable pump head as a function of flow. Allowance can also be made for the frictionallosses in the supply and discharge pipes,which reduce the flow rate that can beattained for a given lift.

Frictional losses are calculated with theHazen-Williams equation, which relates pipeflow to the energy slope. (See Handbook ofHydraulics p.6.28)

V=Flow velocityk=1.318 for English units or 0.85 for metricC=Hazen-Williams coefficientR=Hydraulic radiusS=Energy slopeh=Friction head lossL=Pipe lengthQ=DischargeA=Flow area of discharge pipe

By solving for h, we can determine the head loss for any given flow rate:


Flow Determination

Based on the friction loss equation and thestatic head, a system head curve can becalculated and plotted against the pumphead curve. The intersection of the twocurves indicates the discharge that willoccur for a given static head.

During a hydrograph routing, the statichead is subject to constant change due tovariations in the headwater and/ortailwater. Rather than constantlyrecalculating the system curve, we cansimplify the process by creating an overallrating curve for the entire pump/pipesystem.

This is done by subtracting the friction lossat each flow rate from the pump ratingcurve, resulting in the static head curveshown here. Based on this single curve,system flow can be readily determined forany given static head.

Routing Considerations

Pumps should normally be modeled with atailwater-sensitive routing procedure, suchas the Dynamic Storage-Indication method.(See p.117.) This allows the stage-discharge relationship to be re-evaluated ateach time step, so that tailwater variationsand pump switching can be taken intoconsideration (in addition to headwatervariations).

Under conditions of free discharge, thestatic head is solely a function ofheadwater. But if the outlet of thedischarge pipe becomes submerged, thestatic head is determined by the liftrequired to reach the tailwater. Thisadjustment occurs automatically whenevera tailwater-sensitive routing procedure isemployed.

27 The term "Permeability" is sometimes used as a synonym for Conductivity. However, Permeability has severaldifferent meanings, and therefore is not used in this presentation.


QY ' V AY Eq. 90

V ' KS I Eq. 91

I'Y&YGW

YBot&YGWor I' M%D

M' 1% D

M Eq. 92

Exfiltration Calculations

Since exfiltration is incorporated into a pond's stage-discharge curve, it is classified as an “outletdevice.” Exfiltration is also distinct from an “inflow loss,” in that it continues to occur even whenthere is no inflow.

To separate exfiltration from other “true” outflows, it is usually directed to the discarded outflowto prevent further routing. Since there are many different approaches to modeling exfiltration,HydroCAD provides several options that can be used to implement a wide range of design methods:

Option 1 A constant exfiltration rate Q may be specified in Ft³/sec or m³/sec. This value maybe applied whenever there is water in the pond, or only when the level exceeds the specified “invertelevation.” This feature may be used to exclude exfiltration through (lower) impervious regions ofthe pond.

Option 2 An exfiltration velocity V may be specified in in/hr or mm/hr. This is multiplied bythe available exfiltration area at a given elevation to determine the final exfiltration rate.

QY=Exfiltration flow at elevation YV=Exfiltration velocity (flux velocity)

AY=Exfiltration area at elevation Y

Option 3 This is an extension of Option 2, in which the exfiltration velocity is calculated from thesaturated hydraulic conductivity. For flow through a saturated medium, Darcy’s Law states:

V=Exfiltration Velocity (flux velocity)KS=Saturated Hydraulic Conductivity27

I=Hydraulic Gradient

The Hydraulic Gradient is the head differential across the media divided by the media thickness:

Y=Pond water surface elevationYGW=Groundwater elevationYBot=Pond bottom elevation

M=Media thicknessD=Water depth


AY ' AY&Ainvert Eq. 93

Pond Water Surface

Groundwater Elevation

M1

D1

M2

D2

I = (M+D)/M = 1 + D/MFor a flat-bottom pond, the bottom elevation,gradient, and flux velocity will be constantacross the pond bottom, so the total exfiltrationcan be directly calculated by Eq.90. But for apond with a variable depth (i.e. non-flatbottom), there will be a spatial variation inthese parameters, and the total exfiltrationmust be determined by integration of Eq.90over the entire depth range.

Note that the gradient is always greater thanone as long as the depth is greater than zero.If the depth is much less than the distance togroundwater, the gradient approaches one, andthe flux velocity is equal to the conductivity.This special case is equivalent to Option 2 above.

Exfiltration Area

The exfiltration area for options 2 and 3 may bedefined in three ways: (A) if all exfiltration isassumed to be downward (none through the sides ofthe pond), you may use the pond's surface area;(B) for downward exfiltration with in-sloping sides,you may prefer to use horizontal area, whichincludes the largest surface area at or below thegiven elevation; (C) if exfiltration occurs through allexposed surfaces regardless of slope, you may use thepond's wetted area.

You can also restrict exfiltration to a certain region of the pond. Setting the invert elevation willexclude the area of the pond that lies at or below this elevation. This reduces the effectiveexfiltration area by the area at the invert:

Ainvert=Exfiltration area at invert elevation

This causes exfiltration to be calculated only over thearea of the pond that lies above the invert elevation,and is useful for excluding exfiltration throughimpervious (lower) regions of the pond.

For cases where the upper regions of the pond areimpervious, a maximum exfiltration elevation canalso be specified. Although exfiltration will continueto occur as the water level exceeds the maximum, theexfiltration area will not increase above thiselevation.


V '60P Eq. 94

Tips for Using Exfiltration

Using surface area vs. wetted area

For ponds with gentle side-slopes, the surface area and wetted area are almost identical, and thetwo methods will give similar results. In such cases, the surface area method is recommended forsimplicity. By basing exfiltration on surface area, you are stating that all flow is essentiallydownward. Only horizontal areas (above any invert) are available for exfiltration. All verticalareas are excluded.

If you wish to allow exfiltration through vertical surfaces, such as the sides of a drywell, then youmust specify wetted area (See page 78 for details on wetted area calculations.) As always, it isyour responsibility to ensure that this computation is applicable to your particular situation.

Using a percolation rate

A measured percolation rate can be converted to an equivalent exfiltration velocity by the followingequation. However, other factors must be considered to determine if this is a reasonable designvalue for a proposed exfiltration area. (For example, can a large pond be expected to perc for 24hours at the same rate as a small test pit for over a much shorter period?)

V=Exfiltration velocity [inches/hour or mm/hour]P=Perc Rate [minutes/inch or minutes/mm]

Embedded Storage

When using embedded storage volumes, (such as a chamber in a stone bed) water is assumed tomove freely between the chamber and outer storage volume, such that they maintain essentiallythe same water surface elevation throughout the routing. (Referred to as a “level pond” routing.)Any exfiltration is based on the outer storage volume only, since this is the only surface throughwhich water can actually leave the pond.

Advanced techniques

While most cases will require just a single exfiltration device, it is also possible to use severalexfiltration devices on a single pond. This could be used to model multistage exfiltration schemes,such as a drywell that overflows into a perforated pipe.

As with all pond designs, you should review and understand thestage-discharge plot to make sure the pond is exhibiting thebehavior you expect. Do not rely solely on a review of thehydrograph, in which the pond's behavior is intertwined with thecomplexities of the inflow hydrograph.


Discharge Multiplier

Each outlet device may employ an optional discharge multiplier. This factor can be used to increaseor decrease the device’s discharge under all flow conditions. The most common application is anintegral multiplier, such as “2", to double the device flow under all conditions. This is a convenientway to model several identical devices with only a single outlet definition. A fractional multiplier(such as 1.25 or 0.75) can be used to increase or decrease the normal device flow by the specifiedfactor.

Note: An integral multiplier will increase the flow cross-section anddischarge, while maintaining the same discharge velocity. Afractional multiplier will adjust the velocity and discharge, whilemaintaining the same cross-section.

Discharge Velocity

During each pond routing, HydroCAD attempts to calculate the discharge velocity for eachcontrolling outlet device. This velocity is listed on the pond outlet report, along with the maximumdischarge rate for each device.

Some devices (such as a horizontal orifice) have a uniform discharge velocity that is directlyspecified by the governing discharge equation. Other devices (such as weirs) have significantvelocity variations over the flow area, and their discharge equations do not directly yield thisinformation. In these cases, the average velocity is estimated by dividing the flow rate by theapproximate cross-sectional area of the flow.

Notes:

1) The flow cross-section is estimated using the device geometry and the adjacent headwaterelevation. In some cases (such as certain weirs) the water surface elevation may actually decreaseas it approaches the device, resulting in a somewhat lower cross-sectional area, and acorrespondingly higher velocity.

2) The reported discharge velocity does not account for any acceleration (due to gravity) ordeceleration (due to friction) after it passes through the control point.

3) When applying a fractional discharge multiplier, the discharge velocity is adjusted in directproportion to the multiplier. The discharge velocity is not adjusted for integral multipliers, sincethese are typically used to represent multiple devices, rather than an increased flow through asingle device.

4) The device velocity is reported only for informational purposes. This value does not play a rolein any other calculations, so any discrepancies in the reported velocity will not effect any othercalculations.



Section 19 - Pond Routing Calculations

Most pond routing calculations require detailed information about the pond’s stage-storage andstage-discharge relationships.

Stage-Storage Calculations

Pond storage may be defined by any combination of the techniques described in Section 17. Whenmultiple storage volumes are defined in a single pond, HydroCAD uses the total storage providedby all volumes. This requires that the volumes be interconnected so that water can flow freelybetween them, in order to obey the “level pool” assumption for pond routing.

Stage-Discharge Calculations

The stage-discharge curve is automatically compiled based on the outlet calculations described inSection 18.

The individual outlet devices arecombined into one or more stage-discharge curves based on the specifieddevice routing. In the defaultconfiguration, all outlets are routeddirectly to the primary outflow, asshown in the sample stage-dischargecurve at right. They are considered tobe independent, parallel outlets whoseflows are additive. The compositestage-discharge curve covers the sameelevation range for which pond storageis defined, with the total discharge ateach elevation determined by addingthe discharge from each individualdevice.

If any devices are routed to a secondary, tertiary or discarded outflow, additional stage-dischargecurves are compiled using the same basic procedure. Each device is included in the stage-dischargecurve to which it is routed. To perform the actual pond routing, a total discharge curve is obtainedby adding the individual curves. When routing is complete, the total outflow hydrograph is splitinto separate outflows based on the ratio of the stage-discharge curves. This provides an automaticsplit-flow, or “diversion” capability. This is most commonly used when one or more outlets requireseparate routing, such as an emergency spillway (routed through a separate channel) or anexfiltration outflow (to be removed from any further routing).

28 This procedure uses the standard hydraulic equations given previously, with consideration of the minimum tailwatercreated when a device is lower than the next device downstream. However, it does not consider more complex interactionsthat may occur between devices, except to limit the flow to the lesser of the two. Like all complex calculations, it isimportant to verify these results by independent means to ensure they are sufficiently accurate for your purposes. Insituations where the inter-device water level may constitute a significant tailwater for the upstream device, this can bemodeled by treating the lower portion of the outlet structure as a separate “pond”, and using a tailwater-sensitive routingprocedure.


Primary Discharge .))1=Culvert (standpipe outlet) /))2=Orifice/vertical (side opening(s) in riser) .))3=Orifice/horizontal (top opening of riser)

Compound Outlet Devices

More complex outlets can be modeled by placing standard devices in series.An orifice, for example, could be routed through a culvert. To calculate thedischarge at each elevation, HydroCAD evaluates the standard flow througheach device, and uses the lower (controlling) flow to build the stage-dischargecurve. By making this comparison at each elevation step, different devicesmay control the outflow at different pond stages.28

Even more complex outlets, such as a standpipe, can be modeled byutilizing simultaneous series/parallel device combinations, asshown in the schematic representation below.

Reading from the bottom up: Device 3 is a horizontal orifice representing the flow into the top ofthe riser. Device 2 is used to model one or more openings in the side of the riser. Devices 2 and 3are summed together, and routed through the final outlet culvert, device 1.

When entering a compound outlet device, start with the final device (such as theculvert shown above), and work up towards the pond, entering each device that limits flowor contributes to the discharge. Repeat for any secondary, tertiary, or discarded discharge.

This graph shows a typical stage-discharge curve for a pond with acompound outlet. A culvert ispositioned with the inlet invert at50 feet; however, no dischargeoccurs until the water level reachesan orifice at 50.5 feet. (Thisexample might represent an orificeplate used to reduce the flowthrough an existing culvert.)Above 50.5 feet, both devices areevaluated to determine which willcontrol at each elevation. Theresulting curve is labeled “pri” forprimary.

This example also includes a broad-crested weir which is directed to the secondary discharge. This might represent an emergencyspillway that is being routed separately from the culvert/orifice combination.


I & O '∆S∆ t

or I∆t & O∆t ' ∆S Eq. 95

I1% I22∆t &

O1%O2

2∆t ' S2 & S1 Eq. 96

I1% I22∆t % S1&

O1

2∆t ' S2%

O2

2∆t Eq. 97

Pond Routing Procedures

HydroCAD currently provides these basic pond routing procedures:

! Storage-Indication method! Dynamic Storage-Indication method! Simultaneous pond routing

All pond routing techniques assume that the storage volume is large in comparison to the inflow,such that the pond constitutes a zero-velocity level pool. If the velocity approaching the outletdevice(s) is significant, pond routing may underestimate the discharge and overestimate the peakelevation and storage of the pond.

Storage-Indication Method

The Storage-Indication method (SI) is based on the conservation of mass, as expressed in thefollowing relationship. (See Hydrologic Analysis and Design p.545.)

I=Inflow rate [ft³/sec] or [m³/sec]O=Outflow rate [ft³/sec] or [m³/sec]∆t=Time increment (dt) [sec]∆S=Change in storage [ft³] or [m³]

Using subscripts 1 and 2 to denote values at the beginning and end of the time interval ∆t, yieldsthe following expression:

Rearranging the equation with unknown terms on the left and known terms on the right yields:

The right hand portion of this equation is known as the storage-indication value, which can beevaluated at any stage (elevation) using the stage-storage and stage-discharge relationshipspreviously determined.

Performing the Storage-Indication Routing


1) The pond's stage-discharge relationship is calculated based on the specified outlet devices.

2) The stage-storage relationship is determined from the specified stage-area or stage-storage data.

29 If the pond has multiple outlets, the routing is performed based on the total discharge, and is then split according tothe characteristics of the individual outlets.


3) The stage-discharge and stage-storage curves are used to create the storage-indication curve.

4) Routing is performed using the specified time span and time increment. At each point in time,a storage-indication value is calculated based on the current inflow, plus the previous inflow,outflow, and volume in the pond.

5) The current storage-indication value and the storage-indication curve are used to determine thenew elevation.

6) Using the new elevation, the stage-storage and stage-discharge curves are consulted todetermine the new storage and discharge.

7) This process is repeated for all points in the inflow hydrograph, producing a complete outflowhydrograph as shown below.29

Special SI Considerations

Since the SI method is dependent on a static SI rating curve, it is unable to respond to otherfactors, such as a varying downstream tailwater. These situations may require the use of analternate routing procedure, as described below. (HydroCAD will generally issue a warningmessage if an alternate procedure is required.)

When modeling ponds with no storage capability, the SI procedure will calculate the headwaterelevation that is required to discharge the entire inflow at each time step. This eliminates anydetention effects, while still determining a water surface elevation at each time step.


Additional Routing Features

In addition to the basic routing procedure described above, HydroCAD provides the followingspecial routing capabilities.

1) The inflow hydrograph is automatically adjusted for any base flow (or inflow loss) by adding (orsubtracting) the specified value from each point on the hydrograph. If “automatic base flow” isselected, the base flow is set to the pond's discharge at the specified starting elevation. This placesthe pond in an equilibrium condition (stable water surface elevation) when routing begins.

2) If a starting elevation is specified, routing begins with the water at this level. If this is above thelowest outlet device, the pond begins discharging immediately, possibly before any inflow hasoccurred. If the starting elevation is below the lowest device, no outflow occurs until this level isattained. (The outflow volume will also be reduced by the amount of storage below this level.)

3) Routing is normally performed using the time interval (dt) of the inflow hydrograph, althougha finer interval may be specified for each pond to provide improved tracking. The normal dt isdivided by the specified finer routing value. Finer routing (usually 2) can also be used to eliminateany oscillations in the pond's outflow, which will usually be flagged by a specific warning message.

4) If the peak elevation exceeds the specified flood elevation, a warning message is issued androuting will continue. The setting of the flood elevation has no effect on the routing itself.

Dynamic Storage-Indication Method

The Storage-Indication method (SI) is one of the most widely used routing methods. Although thenature of the SI makes it exceptionally stable, it is dependent on a static rating curve. This meansthat the stage-discharge relationship must be known before routing begins, and cannot change untilthe routing is complete. This prevents the pond from responding to external conditions (such asvarying tailwater or pump switching) and enforces a sequential evaluation of the watershed.

In contrast, the Dynamic Storage-Indication method (DSI) reevaluates the storage-indication curveat each time step, allowing other variables (such as the downstream tailwater or pump switching)to be taken into consideration. And since DSI uses the same equations as SI, it can be expected togive comparable results when these variable are not present.

Special DSI Considerations

While DSI routing tends to provide greater capabilities, it is inherently more complex and requiresmore time to perform. Rather than relying on pre-established rating curves, the DSI procedurereevaluates all storage and discharge equations at every time step. The DSI procedure can alsoiterate the calculations at each time step, so that tailwater effects can propagate upstream throughthe model. (This is controlled by the “Finer Routing” parameter, whose use is automaticallysuggested by HydroCAD as required.)

DSI routing may also require that upstream nodes be recalculated in response to downstreamtailwater changes. Due to these vastly greater computational requirements, DSI routing isrecommended only when variable tailwater dependencies or pump switching are involved.HydroCAD will generally issue a warning message when these factors are present in a specificmodel.


I & O '∆S∆ t

where ∆S ' S2&S1

ˆ S2 ' S1% ( I&O )∆ tEq. 98

Simultaneous Pond Routing

The SI and DSI methods are both sequential procedures. They require that the watershed have aunique flow order. This precludes the modeling of situations with an ambiguous order, such as aflow loop or flow reversal. To model these situations, HydroCAD provides the simultaneous routingmethod, or Sim-Route.

Like the DSI procedure, Sim-Route allows for variable tailwater by re-evaluating all outlet devicesat each time step during the routing. But in order to allow for ambiguous flow order, Sim-Routeuses only the inflow at the previous time step, which is always available, regardless of the floworder.

Since the current inflow is unknown, the traditional SI equations cannot be applied. Instead, Sim-Route is based on a direct application of the basic equation for conservation of mass. (SeeHydrologic Analysis and Design p.545.)

I=Inflow rate [ft³/sec] or [m³/sec]O=Outflow rate [ft³/sec] or [m³/sec]∆t=Time increment (dt) [sec]∆S=Change in storage [ft³] or [m³]S1=Storage at start of time interval [ft³] or [m³]S2=Storage at end of time interval [ft³] or [m³]


1) Routing is performed using the specified time span and time increment. The initial storage isassumed to be zero, unless an initial elevation is specified.

2) The new pond storage (S2) is calculated using the above equation with the previous rates ofinflow and outflow.

3) Using the new storage, a new elevation is determined directly from the stage-storagerelationship.

4) New outflow(s) are calculated using the new pond elevation (the headwater) and the previouselevation of each downstream node (the tailwater). Each outlet structure is directly evaluatedunder these conditions, without use of an intermediate stage-discharge table.

5) Steps 2 through 4 are repeated at each time interval until the entire hydrograph has beendeveloped.

Note: When modeling ponds with no storage capability, the Sim-Route assumes the outflow is equalto the inflow, and calculates the headwater elevation required to produce that discharge. Thiseliminates any detention effects, while still allowing the water surface elevation (headwater) to bedetermined at each time step.


Special Sim-Route Considerations

When performing a simultaneous routing, each outflow is calculated based on the inflow, outflow,and elevations that existed at the previous time step. This eliminates the need to calculate thenodes in a fixed flow order, making it possible to model systems where the nodes may not have afixed linear sequence. However, it also introduces an inherent time delay, which can be minimizedby using the smallest feasible time increment.

Since this technique directly evaluates all outlet devices at each time increment, it eliminates theapproximations inherent in a pre-calculated stage-discharge curve, as used by the traditionalstorage-indication method. The drawback is slower calculation time, particularly with small timeincrements.

Simultaneous routing also requires a small enough time increment to permit accurate “tracking”during the routing procedure. As the amount of available storage decreases, so does the requireddt. An inadequate dt will cause the outflow to oscillate, which will usually generate a warningmessage. Very small reaches or ponds (such as catch basins) may require so small a timeincrement as to make simultaneous routing impractical.

Tailwater Capabilities

In order for tailwater effects to be automatically accommodated, two conditions must exist:

1) The upper node must use a routing procedure that is “tailwater aware.” That is, it musttake the variable downstream condition (tailwater) into account when performing therouting calculation.

2) The lower node must use a routing procedure that defines a water surface elevation,which is seen as the tailwater for inflowing nodes.

The following table summarizes these characteristics for each type of node when using the DSI orSim-Route procedure described above.

Tailwater Aware? Defines Elevation?

Subcat no no

Reach no yes

Pond yes yes

Link no optional

Applying the previous rules to all node combinations indicates that automatic tailwater calculationsoccur for the following combinations:

! A pond flowing into another pond! A pond flowing into a reach! A pond flowing into a link (when the link is used to define a fixed or tidal tailwater)

30 A reverse outlet is permitted only for a simultaneous routing, which doesn’t require a linear flow sequence. Sequentialrouting methods cannot accommodate such a “flow loop” and will report an error condition.


Reverse Flows

HydroCAD is designed to model flows that occur in the same direction as the outflow arrows on therouting diagram. If the potential exists for flow in the reverse direction, an appropriate warningmessage is issued. Since HydroCAD does not automatically model reverse flows, the user musttake appropriate action to address the situation.

1) Systems with reverse flow effects should generally be modeled with the Sim-Route procedure.This allows each pond to respond to dynamic tailwater conditions, and allows flows in bothdirections.

2) To model a reverse flow from one pond to another, create an appropriate outlet going in thereverse direction, that is, routed from the “lower” pond to the “upper” pond. (This will appear onthe diagram as a double-ended arrow.) This outlet will mirror the normal down flow from the upperpond, but is described from the standpoint of water flowing in the opposite direction.30

Note: The existence of tailwater alone does not necessarily indicate a reverse flow situation.Although tailwater can reduce discharge, reverse flow can occur only when the tailwater elevationexceeds the headwater and the flow changes direction. Reverse flow devices (and the Sim-Routeprocedure in general) are recommended only when a specific reverse flow warning occurs.Otherwise, the normal SI or DSI routing procedures are recommended.

31 Note that these calculations are not routing methods. They are a separate analysis that is performed after thehydrograph routing has been performed.


Section 20 - Detention TimeThe detention time is a measure of how long water is detained in a pond or other impoundment.This value is commonly used to meet water quality objectives, such as allowing sufficient time forsediment removal or neutralization of runoff contaminants. (For further details see AppliedHydrology and Sedimentology for Disturbed Areas p.257.) HydroCAD provides two procedures forcalculating the detention time.31

The center of mass method is one of themost basic methods of calculating detentiontime. It evaluates the difference in timebetween the center-of-mass of the inflow andoutflow hydrographs. One of the chiefadvantages of this technique is that it iseasily calculated, and can even be estimatedgraphically. However, the technique does notconsider the actual movement of waterthrough the pond, and can fail to give a goodmeasure of detention time in certainsituations.

The plug flow method provides a morephysically meaningful measure of the averagedetention time. This technique divides theinflow hydrograph into a number of “plugs” ofequal volume, and then calculates the timebetween each plug entering and leaving thepond. The average time for all plugs is thencalculated and used as an overall measure ofdetention time.

The plug flow method provides a theoreticalmaximum detention time based on theassumption that any water initially in thepond is allowed to discharge before the firstplug from the inflow is allowed to discharge.This “first-in first-out” assumption will yielda maximum detention time, and means thatthe amount of water initially in the pond willaffect the calculated result. (Since all water in the pond is displaced before any of the inflow startsto discharge, the detention time is increased by the time required to flush the initial volume.)


Detention Time Accuracy

For all detention time calculations, any water retained in the pond, or discharging after thespecified time span, is excluded from the calculation. To obtain an accurate measure of detentiontime, it is therefore important to use a time span that allows the pond to discharge fully. This canbe determined by comparing the volume of inflow and outflow, which should be roughly the same(unless the pond was surcharged or water was retained). For the plug flow method, also comparethe volume included in the plug flow calculation (this is shown to the right of the detention time).For accurate (maximum) results, this should be close to the total volume of the outflow hydrograph.

Detention Time Regulations

When evaluating detention time regulations, care must be taken to understand the exact standardthat must be met. Wording such as “detain the ten-year storm for 24 hours” is common, but issometimes incomplete or ambiguous. The plug flow method provides a more specific standard thatcan be used to address water quality issues related to detention time.

32 HydroCAD also checks for flat-topped hydrographs, where curve fitting and extrapolation are not appropriate.


Section 21 - Hydrograph Parameters

This section provides details on special hydrograph values that appear in HydroCAD reports.

The peak flow for each hydrograph is calculated using the three highest points on thehydrograph.32 A parabola is fitted to these points and the apex of the parabola specifies the truepeak. This eliminates variations in the peak that would occur if only a single point wereconsidered. This improvement in accuracy is most pronounced with a narrow peak, where the twoclosest points fall on either side of the peak and may be several percent below the actual peak.

The peak attenuation indicates the percentage reduction in peak inflow caused by a routingoperation. This is determined by comparing the peak of the inflow and outflow hydrographs ascalculated above.

The time of peak is determined by the same parabolic fit to the three highest points. The apexof the parabola establishes a time of peak with far greater resolution than the time between points.Like the peak flow, this value is not affected by the placement of the points on the “true” curve.

The time lag caused by a reach or pond is the difference between the time of peak obtained fromthe inflow and outflow hydrographs. (This is distinct from the travel time, described below.)

The hydrograph volume is determined by integrating the flow over the time span of thehydrograph. Since the volume can include flow only within the given time span, any flow beforeor after is excluded. Also note that the lag introduced by a pond or reach can cause a discrepancybetween the calculated inflow and outflow volume. If necessary, this can be remedied by increasingthe calculation time span to include the entire duration of the inflow and outflow hydrographs.

The peak elevation, peak depth, and peak storage are determined by interpolation, in thesame manner as peak flow. (This is a change from HydroCAD-5, which reported the highestdiscrete value as the peak.)

The maximum velocity is the largest value obtained by dividing each discrete flow rate by thecorresponding cross-sectional area. This result may be somewhat different than dividing the peakflow by the corresponding area.

The average velocity is determined by averaging the flow rate divided by the corresponding areaat each time interval during the routing calculation.

The travel time is calculated by dividing the length of the reach, channel, or pipe by the velocity.For a reach routing, the average and minimum travel time are calculated using the average andmaximum velocity, respectively. For some reach routing procedures, the travel time is used tofurther translate (delay) the outflow.


33 The Time Multiplier allows the use of dimensionless hydrographs, as used for runoff studies in Ohio. A link filecontaining the Ohio rural hydrograph is included in the Ohio rural hydrograph.hce file supplied with HydroCAD.


Section 22 - Link Calculations

Basic Applications

A link is a multipurpose mechanism that can be used to:

! Manually enter a hydrograph! Import a hydrograph from an ASCII file! Import a hydrograph from another project! Apply a flow threshold or flow limit! Change the duration of a hydrograph or apply a specific time lag! Rescale a hydrograph to a different peak! Sum hydrographs without performing any routing! Define a fixed or tidal water surface elevation

These capabilities are provided by the following basic types of links:

! An internal link is used to process only the inflows that are connected on the routingdiagram.

! A manual link is used to manually enter an arbitrary hydrograph.

! A file link is used to read a hydrograph from an external data file. This capability canbe used to import a hydrograph from another program, or to link several projects asdescribed below.

Note that any type of link may have inflows on the routing diagram. For a manual link or file link,the imported hydrograph is added to any inflows shown on the diagram.

Advanced Settings

A link also provides several settings that can be used to perform special hydrograph operations:

Flow Threshold If a threshold is specified, only the portion of the inflow that exceeds this value isretained. The default (blank) value causes the entire flow to be passed.

Flow Limit If a limit is specified, only the portion of the inflow below this limit is retained.

Discharge Multiplier After applying any flow threshold and/or limit, each flow is multiplied by thisvalue. The default value of one produces no net change to the hydrograph.

Time Multiplier Scales any external inflow to a different duration.33 The default value of oneproduces no net change to the hydrograph. Internal (onscreen) inflows are not affected by thissetting.

Time Lag Delays (translates) the outflow hydrograph by the specified time. This option can beused to apply a known time lag to any hydrograph.

34 When adding hydrographs with the same starting time and time increment, the ordinates at matching times areadded directly. If the starting time or time increment are different, HydroCAD will interpolate from the externalhydrograph in order to obtain values at the same time steps as the current project. The resulting hydrograph will alwayshave the same time span and increment as the current project.


Elevation Settings

A link may also be used to define an arbitrary water surface elevation. Although the elevation hasno direct effect on the link's outflow, it allows the link to define special tailwater conditions whenusing a tailwater-aware routing procedure. (See page 119 for details.)

For a fixed elevation, the desired water surface elevation is specified directly. This elevation willexist at all times during the routing calculation. This option is useful for a site that discharges toa lake or other water body whose elevation is essentially constant throughout the calculation timespan.

An elevation table may be used to specify an arbitrary water surface variation over time. Thiscan be used to simulate a river or other discharge point that lies outside the boundaries of theHydroCAD model, but which creates a known, varying tailwater effect that must be considered.

A tidal elevation may be specified by defining the high and low tide elevations, the time of hightide, and the tide cycle time. The tide cycle is measured from one high tide to the next, and defaultsto 12.42 hours (12 hours and 25 minutes). The resulting tidal elevation is defined by a sine wavewith the specified parameters, which repeats over the entire calculation time span.

Link Routing Procedure

The basic link routing procedure is as follows:

1) If a Time Multiplier is specified, it is applied to any external inflow by multiplying each timeordinate by this value. The time multiplier has no effect on any internal (onscreen) inflows.

2) The adjusted external inflow is converted to the current time span and time step, and added toany internal (onscreen) inflows.34 The result is considered to be the total inflow for the link.

3) If a Flow Threshold is specified, only the portion of the inflow hydrograph above the thresholdis retained.

4) If a Flow Limit is specified, only the remainder of the inflow that falls below the threshold isretained.

5) If a Discharge Multiplier is specified, each hydrograph ordinate (flow) is multiplied by thespecified value.

6) If the Flow Limit, Threshold, or Discharge Multiplier has caused a reduction of the inflowhydrograph, a secondary outflow will be generated containing the remainder of the inflowhydrograph. In some cases this may then be routed separately, providing a basic flow-diversionmechanism.

The results of these operations are readily apparent upon viewing the link's hydrograph, whichshows the inflow and outflow curves as described above.


Upstream section

Downstream section

Using a Link to Model a Large Watershed

If you exceed the node capacity of your program, you can break a project into two (or more) sectionsand then connect them using an automatic link. (This technique does not apply to the HydroCADSampler, which does not have the file export capability.)

1) First break the project into two or more sections. You can do this bydragging nodes from one project to another, or by cut-and-paste. One projectwill typically contain the upstream portion of the watershed, and will endwith an unrouted outflow, as shown at right.

2) Verify the calculation settings for the upstream portion of the project.Since rainfall settings must be set independently for each section of theproject, it’s very helpful to define each of the applicable rainfall events. Thiswill allow you to select between events without having to modify each of theinterconnected files.

3) Under Settings|Export, select “Export Unrouted Outflow Hydrographs” and click OK.

4) Close and save the upstream project. As the project is closed, the unrouted nodes areautomatically updated and exported.

5) Open the downstream portion of the split project and create a link. Edit the link and select FileLink. Select the File tab and press Browse to see a list of available export files. Select the desiredfile from the upstream portion of the project.

6) If the file contains multiple outflows, use the “File Hydrograph” box to select the desiredhydrograph. Click OK to save the link's description.

7) You can now route the link's outflow on the diagram as shown atthe right. The link will automatically import the hydrograph ascalculations are performed.

8) Make sure the lower portion of the projects uses exactly the sameevent definitions as the upper portion. (The easiest way to do thisis to import the events from the upper project, as described onpage 47.) This will allow you to select any event in the lower projectand the link will automatically import the correspondinghydrograph. If you don’t use events, you’ll have to re-open the upperproject each time you need to change the rainfall.

Important: If you change the upstream project while the downstream portion is open, thedownstream project will retain the earlier (outdated) inflow. To update the downstream project youmust either (1) close and reopen the downstream project, or (2) select Settings|Calculation and pressOK. In either case, you must close the upstream project in order for the exported flow(s) to becomeavailable.


External Hydrograph Files

External hydrograph files are commonly used to connect separate projects with a file link, asdescribed above. HydroCAD automatically creates and deletes these files as required, based on theexport settings for each project. External hydrograph files may also be created manually orgenerated by other software. These are ASCII text files with an .hce extension, and must follow theformat described in the sample file LinkTest.hce which is installed in the HydroCAD\Projects folder.

To edit an external hydrograph file by hand, click on the file and select “Notepad” when asked whatprogram you want to use to open it. Windows will remember this selection the next time youattempt to open an external hydrograph.

External hydrograph files created by HydroCAD use the same file name as their project, with atilde and the node number added to the end. If you create your own external files, you may use anyname that does not match this format. Any files that match this specification will be overwritteneach time the project is opened by HydroCAD!

To import an external hydrograph into any project, create a file link on the diagram and use it toimport the file.

Hydrograph Export Settings

Several options are provided under Settings|Export that control which hydrographs (if any) will beexported from the current project:

! All Inflows: All node inflows are exported.! All Outflows: All node outflows are exported.! Unrouted Outflows: Only unrouted outflows are exported.

These settings instruct HydroCAD to automatically export certain hydrographs for use with anautomatic link or with other software. These hydrographs are automatically exported when theproject is saved and closed. The exported data is not available while the project is open.

If you plan to route the current project into another project using a link, use the “UnroutedOutflows” setting. This optimizes disk space by exporting only the outflows that are not beingrouted on the diagram. This will include all outflows that are likely to be imported with a link.

The “All Inflows” and “All Outflows” settings should be used only when absolutely necessary, sincethey can consume large amounts of disk space. (A single 1000-point hydrograph requires about 15Kof storage. Multiply this by the total number of inflow or outflow hydrographs to estimate the totalstorage required.)


L ' R C A Eq. 99

R ' P PJ RV Eq. 100

Section 23 - Land-Use Analysis & Pollutant Loading

Land-Use Reporting

Land-use reporting provides a detailed breakdown of how the land within a given project is beingused. This can include industrial, residential, and other uses. Although land-use reporting canbe used on its own, it is most commonly used in conjunction with pollutant loading calculations.

To enable land-use reporting and/or pollutant loading calculations,see the Settings|Land-Use screen.

Pollutant Loading

Pollutant loading calculations are used to estimate the quantity of pollutants that are present insite runoff. This is typically used to determine the Total Phosphorus (TP), Total Nitrogen (TN), andTotal Suspended Solids (TSS) that will be discharged from a site over a given period of time.

To simplify the calculation of pollutant loading, each sub-area is assigned to a specific land-usecategory, and pollutant concentrations are defined for each category. To enable pollutant reportingand define the pollutant concentrations, see the Settings|Land-Use screen.

Pollutant loads are calculated according to the basic equation:

L=Total pollutant loadR=Runoff depthC=Average pollutant concentrationA=Runoff area

Although Eq.99 can be evaluated using the runoff depth for a single event, it is more commonlyused to calculate loading for a longer time period, such as one year. This requires an alternateprocedure for estimating the long-term runoff depth, such as the Simple Method.

The Simple Method

The Simple Method provides an alternate procedure for estimating the long-term runoff depth andthe associated pollutant loading. (See Controlling Urban Runoff: A Practical Manual for Planningand Designing Urban BMPs) The Simple Method estimates runoff depth with the equation:

R=Runoff depthP=Precipitation depth

PJ=Fraction of rainfall events that produce runoffRV=Runoff coefficient

Although this equation is most commonly used to estimate the annual runoff depth, it may be usedfor any desired time period.

35 Exceptions to this rule can be made for water surfaces, which are commonly modeled with a high CN value, but notclassified as impervious. See page 49 for details.


Rv ' 0.05 % 0.90 I Eq. 101

L ' P PJ RV C A 0.2266 Eq. 102

The runoff coefficient is calculated with the empirical relationship:

I=Fraction of runoff area which is impervious

When evaluating these equations by hand, care must be taken to use consistent units throughout.Manual calculations are commonly performed by combining equations 99, 100, and 101 andconsolidating the conversion factors:

L=Pollutant load [pounds]P=Precipitation depth [inches]

PJ=Fraction of events that produce runoff (commonly 0.9)RV= (as above)C=Pollutant concentration [mg/liter]A=Runoff area [acres]

Impervious Area Determination

When applying the Simple Method, HydroCAD performs an automatic determination of theimpervious area based on existing subcatchment data. When using the SCS runoff equation, curvenumbers of 98 or higher are generally classified as impervious.35 For the Rational method, aC value of 0.95 or higher is classified as impervious.

When applying the Simple Method to multiple subcatchments and land-use areas, HydroCADprovides several calculation options:

Option 1: Calculate imperviousness for the overall project, and use the resulting (same)runoff coefficient to calculate the pollutant loads for all land-uses.

Option 2: Calculate imperviousness and pollutant loads separately for each land-use,without regard to subcatchment delineation.

Option 3: Calculate imperviousness for each subcatchment, and use the resulting runoffcoefficient to calculate the pollutant load for all land-uses within that subcatchment.

Option 4: Calculate imperviousness for each land-use within each subcatchment, and usethe resulting runoff coefficient to calculate the loading for that land-use and subcatchment.

Option 4 is recommended for maximum accuracy. The other options are provided for compatibilitywith pre-existing calculation procedures.


Section 24 - Calculation Messages

This appendix lists samples of the various messages that may occur during runoff and routingcalculations. In actual use, the text of each message will vary to reflect the exact situation,however the message number (shown in brackets) will remain the same.

When running HydroCAD, you can click on any item in the summaryreport or message window for complete information. This willdisplay the entire text shown here, as well as links to related topics,calculations and definitions.

[01] Note: {node} Duplicate node number skipped

A node was encountered with a number that was already in use. The node is skipped, and theprogram continues to process any additional nodes.

This message usually occurs while adding a project that contains nodes that don't have uniquenumbers. Before adding a project, you should generally renumber any nodes with conflictingnumbers. To add any nodes that were skipped, renumber the applicable nodes and add the projectagain.

The message can also occur when pasting nodes from the clipboard, or opening a project thatcontains duplicate node numbers. Tip: To make a clone of an existing node use Ctrl-drag (ratherthan cut and paste).

[02] Note: Exceeded node capacity

The current project has exceeded the nominal node capacity of your program.

You can continue to work with the project, but you will not be able to save the project or producemulti-page reports unless you reduce the number of nodes.

Warning: If you're using the HydroCAD Sampler, your changes will be lost when the Sampler'stime limit expires. To save your project you must reduce the number of nodes before the time limitexpires.

[03] Note: Added x nodes and updated y nodes

This message indicates the number of nodes that have been added, updated, and/or removed by theProject|Add, Project|Merge, or Project|Import commands.


[04] Note: Repositioned x nodes

This message indicates that a number of nodes have been repositioned for better visibility. Thisoccurs automatically if the nodes don't contain the expected position information. The repositionednodes are selected and aligned in a row at the bottom of the routing diagram.

This situation occurs primarily with project files that are generated by another application whichdoesn't provide position information. To avoid automatic repositioning of nodes, be sure they havea valid position before the file is opened in HydroCAD.

[10] Note: Updating to {node} as needed

Routing calculations are being performed up to a specific node. To save time, downstream nodesare not being calculated, unless a dynamic or simultaneous routing is being performed. Any othernodes will be automatically updated when required.

[13] Note: Time span = 5-20 hrs, dt= .05 hrs, 301 Points

This message indicates the time span and time increment (dt) being used to generate and routehydrographs. The message also indicates the number of points in the resulting hydrographs.

[16] Note: Runoff by SCS TR-20 method, UH=SCS

This message indicates the method being used to produce runoff hydrographs. When using the SCSmethod, the message also indicates the Unit Hydrograph (UH) being employed.

When using the Rational method, the message indicates the hydrograph rise/fall rates. Theseparameters may be changed with Settings|Calculation.

[18] Note: Rainfall Duration=20 min, Inten=4.30 in/hr , Cf=1.20

This message indicates the rainfall duration, intensity, and frequency factor being used with theRational method. These parameters may be changed with Settings|Calculation.

[19] Note: Type II 24-hr 10-Year Rainfall= 4.30" x2, AMC=3, Ia/S=0.25

This message indicates the rainfall distribution, rainfall event name, and rainfall depth being usedto generate runoff hydrographs. The letter “x” indicates that several back-to-back storms are beinggenerated, with the specified rainfall depth occurring in each of the storms.

The Antecedent Moisture Condition (AMC) is shown only if it differs from the default value of 2(normal conditions). The ratio of Ia to S is shown only if it differs from the default value of 0.2.

These parameters may be changed with Settings|Calculation.


[22] Note: Reach routing by Stor-Ind+Trans method

This message indicates the method being used to perform hydrograph routing through each reach.The routing method may be changed using Settings|Calculation.

[25] Note: Pond routing by Stor-Ind method

This message indicates the method being used to perform hydrograph routing through each pond.The routing method may be changed using Settings|Calculation.

[28] Note: Updating {node}

This message indicates that runoff or routing calculations are being updated for a given node.

Routing calculations are automatically updated as required to view or print reports. HydroCADemploys a “minimal recalculation” feature, so that only the required nodes are updated at any giventime.

Any upstream nodes are automatically updated as required to produce a valid inflow. During adynamic or simultaneous routing, any downstream nodes that contribute to a tailwater effect areupdated concurrently. (See Tailwater Capabilities on page 119.) Otherwise, downstream nodesare just marked as “invalid” and will be updated as their results are required.

[37] Hint: Longer time span advised for full volumes

The calculation time span may not be long enough to encompass the entire hydrograph duration,causing a reduction in the reported hydrograph volume, runoff depth, and detention time. If youwish to study the entire volume, the time span may be changed using Settings|Calculation: Time Span.

[40] Hint: {node} Not described

No description has been entered for the node, or the data is incomplete. For a subcatchment, norunoff is produced. For a reach, pond, or link, any inflow is passed through unchanged.

To describe the node, select Node|Edit. An undescribed node can also be useful during initial designwork, since it allows a node to be placed on the diagram but not described until later.

[43] Hint: {node} Has no inflow (Outflow=Zero)

This node has no inflow on the routing diagram, so it will produce no outflow unless it has a baseflow and/or initial water surface elevation.


[45] Hint: Subcat Runoff=Zero

The subcatchment has produced no runoff. This can occur in any of the following situations:

1) The entire runoff may be occurring before or after the specified time span. To change the timespan use Settings|Calculation.

2) For the SCS or SBUH methods, the rainfall depth may be insufficient to generate any runoff asdetermined by the SCS runoff equation. This is normal when low rainfall depths are used incombination with low curve numbers.

[46] Hint: Subcat Tc=0 (Instant runoff peak depends on dt)

When the time-of-concentration is zero, the precipitation excess appears immediately as runoff.Since the calculated peak flow is based on the average over each time interval (dt), using a longerdt (in relation to the peak duration) will yield a lower “peak” flow.

If the instantaneous peak is required, reduce the dt. The dt may be changed usingSettings|Calculation: Time Span.

Note: The calculated runoff volume is not affected by the dt setting.

[48] Hint: Subcat Peak<CiA due to short rainfall duration

Due to the short rainfall duration, the peak runoff has not had time to attain the full valuepredicted by the Rational Method equation Q=CiA. In order to attain the full peak, the rainfallduration must be greater than or equal to the time-of-concentration multiplied by the rise rate.Shorter durations will produce a proportionately lower peak.

[49] Hint: Subcat Tc<2dt may require smaller dt

When the time-of-concentration is less than twice the time increment (dt), the instantaneous peakmay exceed the (average) hydrograph peak for a brief time (less than dt).

If the instantaneous peak is required, reduce the dt. A dt of one-half the smallest Tc will preventthis message from occurring. The dt may be changed using Settings|Calculation: Time Span.

Note: The runoff volume is not affected by the dt setting.

[51] Hint: xx Area used default ground cover / soil group

When importing watershed data, a ground cover or soil group was not specified for all areas, so thedefault value was used for those areas. The message will also indicate the default that was used,plus the area to which it was applied.

The default ground cover and soil group are specified on the Settings|Watershed screen. When usinga default value, be sure to draw all areas that should not use the default.


[52] Hint: Reach Inlet/Outlet conditions not evaluated

Reach routing calculations assume normal-flow conditions in the channel or pipe. The softwaredoes not evaluate inlet conditions or tailwater for the reach, although these may often be acontrolling factor.

If you wish to consider entrance losses, pressure flow, or other conditions for a pipe, it should bemodeled as a pond with a culvert outlet.

[55] Hint: Reach Peak inflow is 1xx% of Manning's capacity

The reach is operating above its Manning's normal-flow capacity, but has not overtopped. This maybe acceptable depending on the design criteria.

[56] Hint: Dam Breach started at x.xx hrs WSE=xxx.x’

A dam breach has commenced at the indicated time and water surface elevation.

[57] Hint: Pond Peaked at xx’ (Flood elevation advised)

The peak water surface elevation is displayed. Since a flood elevation has not been specified,HydroCAD is unable to determine if this is an acceptable elevation.

If the peak elevation is higher than expected, make sure you have defined all necessary outletdevices. For example, if the level exceeds the top of your pond or catch basin, you must define asuitable overflow device. Otherwise the elevation may attain an unrealistic level.

It is also recommended that you define a flood elevation for this node. This will allow a warningmessage to be automatically generated whenever the water surface exceeds the specified elevation.Setting a flood elevation will also improve calculation speed by allowing a better estimate of theelevation range prior to routing.

[58] Hint: {node} Peaked x.x' above defined flood level

The peak depth in a reach or pond has exceeded the defined flood elevation. Routing continues asusual. (A separate message occurs whenever a reach or pond exceeds the highest defined stage.)

The significance of this message depends on the flood level that has been specified for this node.The message does not indicate that the calculations themselves have been compromised.

If the peak elevation is higher than expected, make sure you have defined all necessary outletdevices. For example, if the level exceeds the top of your pond or catch basin, you must define asuitable overflow device. Otherwise the elevation may attain an unrealistic level.

[59] Hint: {node} Culvert Resized to xx”

The specified culvert outlet or pipe reach has been resized according to the criteria established onthe Resize tab of the Settings|Calculation form.


[61] Hint: {node} Exceeded Reach x outlet invert by x.x’ @ x.x hrs

The node's peak elevation has exceeded the outlet invert of an inflowing reach, but did not exceedthe reach’s outlet depth at any time during the routing.

This degree of tailwater is common, and does not necessarily require further action. The reachrouting calculations continue to be performed as if the reach were operating under normalManning's flow with no tailwater influence. The user is responsible for adjusting the model in anyway that is deemed necessary to accommodate this situation. In some situations, an alternaterouting method or modeling technique may be required.

[62] Hint: {node} Exceeded Reach x OUTLET depth by x.x’ @ x.x hrs

At some time during the routing, the node's water surface elevation has exceeded the flow depthat the reach outlet, but always remained below the inlet depth. The message indicates themaximum amount of exceedance, and the time at which it occurred.

This message indicates that part of the reach has been "flooded out" by the downstream node.

Important: The reach routing calculations are not automatically changed to accommodate thissituation, even though it may reduce the actual reach discharge. The routing continues to beperformed as if the reach were operating under normal Manning's flow with no tailwater influence.Since these basic routing assumptions may no longer be valid, an alternate routing method ormodeling technique may be required. The user is responsible for adjusting the model in any waythat is deemed necessary to accommodate this situation.

Reminder: In most situations, a pipe reach is best modeled as a pond with a culvert outlet, whichcan accommodate a wider range of tailwater conditions.

[63] Warning: {node} Exceeded Reach x INLET depth by x.x’ @ x.x hrs

At some time during the routing, the node's water surface elevation has exceeded the flow depthat the reach inlet, indicating a tailwater dependency, or even the potential for reverse flow. Themessage shows the maximum amount of reverse head and the time at which it occurred

Important: The reach routing calculations are not automatically changed to accommodate thissituation, even though the higher tailwater may in reality cause a reduction in flow, or even areverse flow. The routing continues to be performed without tailwater effects or reverse flow, asif a one-way valve were preventing any backflow. The user is responsible for adjusting the modelin any way that is deemed necessary to accommodate this situation.

Note: If the reach is being used to connect two ponds, you may want to remove the reach andconnect one pond directly to the next. This will provide additional capabilities for handling thetailwater effects as described on page 119.

[65] Warning: Reach Inlet elevation not specified

The reach inlet elevation has not been specified. This information is required in order to detectpotential tailwater effects. Use Node|Edit: Profile to specify the reach elevation. (This warningcommonly occurs with reaches imported from HydroCAD 5, which did not define the elevation.)


[67] Warning: {node} Inflow not allowed - Ignored

This node cannot accept an inflow hydrograph. The inflow is ignored. If you need to route ahydrograph through a subcatchment, use a separate reach routing.

[68] Warning: Input data skipped

Some of the imported data could not be fully processed. The data that was skipped is listed beforethis message. When importing watershed data, review the source data to identify and correct theproblem.

When importing a TR-20 file, the skipped data is also listed in the project notes. In order toproperly replicate the results obtained with the original data, the skipped items must be manuallyincorporated into the HydroCAD project. After incorporating the skipped items, you should openthe Project|Notes screen and update that information accordingly.

[70] Warning: Subcat Tc<8dt requires smaller dt

The rising limb of a Rational Method hydrograph contains fewer than eight ordinates. A smallerdt is recommended in order to accurately represent the hydrograph. (A dt of .01 hours is generallyappropriate for most Rational method calculations.) Inspection of the hydrograph is also advised.The dt may be changed using Settings|Calculation: Time Span.

[73] Warning: Subcat Peak may fall outside time span

The calculated runoff peak isn't within the middle 90% of the specified time span. Under theseconditions, the reported “peak” is the highest flow within the time span, and may not be the truepeak. Inspect the hydrograph and change the time span to include the peak. The time span maybe changed using Settings|Calculation: Time Span.

[74] Warning: xx Area has no ground cover / soil group

When importing watershed data, a ground cover or soil group was not specified for certain areas.The message will indicate the amount of area that had no ground cover or soil group.

Because a ground cover or soil group was not specified, the Curve Number cannot be determinedfor these areas. To resolve the problem, revise the input data or specify default values on theSettings|Watershed screen.

[75] Warning: Subcat xx area mismatch

When importing watershed data, the total area of a subcatchment did not match the sum of theindividual subareas. The message will indicate the actual difference in areas.

This message indicates a problem with the original data or the import process itself. Examine theoriginal data to identify and correct the discrepancy.


[76] Warning: Reach Detained x.xx AF (Pond w/culvert advised)

A pipe reach has filled with water, causing the flow to be limited and the excess volume to bedetained without head.

Important: Pipe reach calculations assume normal-flow conditions in the pipe. If you wish toconsider entrance losses, pressure flow, or other conditions, the pipe should be modeled as a pondwith a culvert outlet.

[77] Warning: Pond Manual tailwater submerged device # by x.x’ (SI method only)

The pond’s manual tailwater elevation exceeds an outlet device which is not tailwater-aware. Themessage occurs when the manual tailwater setting is above the invert of one of the pond’s finaloutlet devices, and that device is not able to respond to tailwater conditions.

This message can occur only for specific types of outlet devices that are unable to respond totailwater conditions, such as exfiltration and some special outlets. To resolve the situation, eitheradjust the device to accommodate the tailwater, or switch to a tailwater-capable device.

[78] Warning: {node} Submerged Pond x device # by x.x'

The node's peak elevation has submerged the specified pond outlet device. This message occurswhen the peak elevation (tailwater) rises above the invert of one of the pond's final outlet device(s),and that device is not able to respond to tailwater conditions.

Important: The pond routing calculations are not altered by this situation, even though thetailwater may reduce the pond's discharge. The routing continues to be performed based on theexisting stage-discharge relationship, as if the tailwater did not exist.

This message can occur only for specific types of outlet devices that are unable to respond totailwater conditions, such as exfiltration and some special outlets. To resolve the situation, eitheradjust the device to accommodate the tailwater, or switch to a tailwater-capable device.

[79] Warning: {node} Submerged Pond x device # by x.x' (SI method only)

The node's peak elevation has submerged the specified pond outlet device. This message occurswhen the peak elevation (tailwater) rises above the invert of one of the pond's final outlet device(s).

Important: The pond routing calculations are not altered by this situation, even though thetailwater may reduce the pond's discharge. The routing continues to be performed based on theexisting stage-discharge relationship, as if the tailwater did not exist.

Since a static stage-discharge curve cannot accommodate a variable tailwater, the tailwater mustbe specified manually using Node|Edit:Tailwater. If one of these tailwater options isn't sufficient, analternate routing method may be required, such as a dynamic routing.


[80] Warning: {node} Exceeded Pond x by x.x' @ x.x hrs (x.x cfs x.x af) (DSI & Sim-Route only)

At some point during the routing, the node's elevation has exceeded the elevation of an inflowingpond, indicating a possible reverse flow. The message shows the maximum amount of reverse head,the time at which it occurred, and an estimate of the potential reverse flow.

Important: The pond routing is not altered by this situation, even though the higher tailwater mayin reality cause a reverse flow. The routing continues to be performed as if a one-way valve werepreventing the backflow. The user is responsible for adjusting the model in any way that is deemednecessary to accommodate this situation. If the potential reverse flow volume is significant inproportion to the total outflow volume, you may be able to model the flow with a reverse outletdevice.

[81] Warning: {node} Exceeded Pond x by x.x' @ x.x hrs (SI method only)

At some point during the routing, the node's elevation has exceeded the elevation of an inflowingpond, indicating a possible tailwater dependency. The message shows the maximum amount ofreverse head and the time at which it occurred.

Important: The pond routing is not altered by this situation, even though the higher tailwater mayin reality cause a reduced discharge. To remedy the situation, a different pond routing methodshould be used that is able to accommodate tailwater effects.

[82] Warning: {node} Early inflow requires earlier time span

Some inflow may be occurring before the beginning of the specified time span, and is therefore notincluded in the routing. An earlier time span is required in order to include the early part of theinflow hydrograph.

The time span may be changed using Settings|Calculation: Time Span.

[85] Warning: {node} Oscillations may require Finer Routing>1 (SI method only)

The outflow of a pond or reach contains a greater number of peaks than the inflow. This suggeststhat oscillations were induced by the routing and that the routing results may not be valid.

If a visual inspection reveals oscillations, they can usually be eliminated by setting the finer routingvalue to 2 for that particular node. If this fails to correct the problem, the available storage maybe too small to permit an accurate routing for this node.

Note: Reducing the overall time increment (dt) may also resolve the problem, but this willunnecessarily increase the calculation time for other nodes.


[86] Warning: {node} Oscillations may require smaller dt (Sim-Route method only)


If a visual inspection reveals oscillations, reduce the time increment (dt) until the situation iscorrected. This may require a reduction to the minimum dt of 0.01 hours.

If the problem persists, the available storage may be too small to permit an accurate routing withthe simultaneous routing method. This method is intended for "coupled ponds" of reasonable size.In general, it is not intended for reaches or ponds with very small amounts of storage, such as catchbasins or manholes. The DSI method (or using a zero-storage pond) may produce better results.

[87] Warning: {node} Oscillations may require finer routing or smaller dt (DSI only)


If a visual inspection reveals oscillations, increase the Finer Routing value (on theSettings|Calculation screen) and/or reduce the time increment (dt) until the situation is corrected. If the problem persists, the available storage may be too small to permit an accurate DSI routing.

[88] Warning: {node} Qout>Qin may require Finer Routing>1 (SI method only)

The peak outflow of a pond or reach was greater than the peak inflow. This can occur if the storageis very small in relation to the inflow volume, or if there are abrupt changes in the stage-dischargecurve or inflow hydrograph.

This can usually be corrected by setting the finer routing value to 2 for that particular node. If thisfails to correct the problem, the available storage may be too small to permit an accurate routingfor this node.

Note: Reducing the overall time increment (dt) may also resolve the problem, but this willunnecessarily increase the calculation time for other nodes.

[89] Warning: {node} Qout>Qin may require smaller dt (Sim-Route method only)


This can usually be corrected by using a smaller time increment (dt) for the entire project. Thismay require a reduction to the minimum dt of 0.01 hours.

If the problem persists, the available storage may be too small to permit an accurate routing withthe simultaneous routing method. This method is intended for "coupled ponds" of reasonable size.In general, it is not intended for reaches or ponds with very small amounts of storage, such as catchbasins or manholes.


[90] Warning: {node} Qout>Qin may require Finer Routing or smaller dt (DSI only)


In some cases, this message may be triggered by normal routing conditions. If a visual inspectionof the hydrograph confirms the presence of routing problems, they can usually be eliminated byincreasing the Finer Routing value (on the Setting|Calculation screen) or by reducing the overall timeincrement (dt). If this fails to correct the problem, the available storage may be too small to permitan accurate routing for this node.

[91] Warning: Reach Storage range exceeded by xx'

The water surface elevation has exceeded the highest defined stage. Routing continues using alinear extrapolation of the storage and discharge curves.

Important: For accuracy, you must extend the stage-storage data in order to prevent extrapolation.

[92] Warning: Pond Outlet Device #1 is above defined storage (Occurs while editing)

The invert of the specified outlet device lies above the highest defined stage, and therefore does notcontribute to the pre-calculated stage-discharge curve. This can result in no flow being allowedthrough the device.

Important: In order for the device to be properly evaluated, additional stage-storage data must beprovided so that the device falls within the defined storage range. There should be at least onedefined stage above the top of the highest outlet device.

[93] Warning: Pond Storage range exceeded by xx’

The water surface elevation has exceeded the highest defined stage. All defined storage has beenfilled. Routing continues by applying additional head to the outlet(s), but without utilizing anyadditional storage. In essence, the pond has been extended upward as a pencil-thin chamber withno additional storage.

Important: For accuracy, you must define additional storage in order to accurately represent thephysical situation being modeled. This may consist of additional stage-storage data and/or storagechambers, as required to describe the actual storage. Overfilled storage can also cause otherproblems, such as oscillations.

This warning commonly results from a failure to provide storage data above the highest outletdevice, such as an emergency spillway. Although you may not intend to utilize this storage,complete storage information is required in order to perform an accurate routing. When usingcustom stage-storage data, the situation is easily resolved by entering storage at one or moreelevations above the upper-most outlet.

It is also possible that you have failed to include an overflow device for the pond, causing theelevation to rise beyond the expected level.


[95] Warning: Pond Outlet Device #1 rise exceeded

The water level has exceeded the rise of the specified outlet device (usually a weir), but no matchingdevice was found to handle the flow at higher elevations. Calculations will continue using orificeflow through the specified rise, but without using any additional flow area.

Important: To obtain accurate results, you probably need to define another outlet device (generallya weir) to handle the flow that will occur when the headwater exceeds the top of the lower weir.The invert of the upper weir must be equal to the lower weir's invert plus its rise.

[97] Warning: Reach factor X out of range

The reach routing factor “X” (see page 73) has exceeded the permissible range of 0.0 to 0.5. Thevalue is automatically limited to the respective limit. To ensure an accurate routing, you mustadjust the reach parameters to produce a value in the required range.

[98] Warning: Max. Lift of x.xx’ exceeds pump rating

At some time during the routing, the required lift was greater than the pump could deliver. Nopump flow occurs under these conditions. If you intend to operate the pump at this lift, you shouldinsert additional points at the beginning of the rating table to adequately describe the pump flowunder high head (low flow) conditions. See page 106 for further details.

[99] Warning: Min. Lift of x.xx’ is below pump rating

At some time during the routing, the pump lift was below the range specified by the rating curve.Under these conditions, the pump flow is assumed to be the highest value specified in the ratingtable. If you intend to operate the pump at this lift, you should add additional points at the end ofthe rating table to adequately describe the pump flow under low head (high flow) conditions. Seepage 106 for further details.


Section 25 - Frequently Asked Questions

Why can't I route an upstream node through a subcatchment?

A subcatchment contains only the information needed to perform a runoff calculation. Use a reachif you want to route another hydrograph through this land area. For shallow overland flow, usea wide channel with a suitable Manning's number.

Why isn't the entire outflow of my pond being routed?

Make sure there isn't an un-routed primary, secondary, or tertiary discharge, which will appearas an outflow handle under the node. If so, you must route the outflow to another node orreconfigure the outlet devices to eliminate the outflow.

Why don’t my peak inflows add up?

When hydrographs are added, HydroCAD adds the corresponding flow at each time step. The peakflows will add directly only if they occur at the exact same time. If the peaks occur at differenttimes, the peak value will be somewhat less than the sum of the individual peaks. To verify thathydrographs are being added correctly you can compare the total inflow volume, which shouldalways be the exact sum of the individual hydrograph volumes. You can also review thehydrograph summation by examining the tabular inflow hydrograph. (Right-click the table toselect the individual inflows.)

How do I compare the existing and proposed conditions for my site?

First create and save the model for the existing conditions. Then use Project|SaveAs to save a copyof the project under a new name for modeling the proposed conditions.

To compare the existing and proposed conditions, open both files at the same time. Select thenodes to be compared (one in each file), and select Comparison Report from the toolbar.

How do I compare different rainfall events?

HydroCAD can automatically calculate and print reports for multiple events. A multi-event reportis also available for each node. These capabilities are automatically enabled in any project whererainfall events have been defined as discussed on page 47. Using a separate “project” for each eventis not recommended, since each project would have to be manually updated whenever thewatershed or drainage system is revised.

What is the best way to model a pipe?

A pipe can be modeled in three ways. 1) If the pipe always operates under normal, open channelflow conditions, you can model it as a separate pipe reach. 2) Some open-channel pipes are mosteasily modeled as a flow segment within a subcatchment. 3) The most complete solution is to modelthe pipe as a culvert outlet on a pond, even if the “pond” is simply a roadway impoundment,approach channel, or drop inlet.


How do I model a catch basin?

A catch-basin is best modeled as a pond with a culvert outlet. If the catch-basin provides negligiblestorage, it can be modeled as a “zero-storage” pond. Or you can evaluate the detention effects bydefining the “pond” storage, including any above-ground storage that is used when the basinoverflows. (Always enter enough stage-storage data to prevent a “data exceeded” warning.) Thiswill allow a more accurate culvert analysis to be performed on the pipe, including the effects ofheadwater and inlet losses. In some cases the Dynamic Storage-Indication routing procedure maybe used to handle varying tailwater conditions.

What about a closed storm sewer?

A storm sewer can sometimes be modeled as a series of ponds with culvert outlets as describedabove, subject to the same tailwater considerations. Due to the computational problems of routinghydrographs through a closed storm sewer, these systems may have to be modeled with twoseparate tools, first performing a steady-state analysis of the closed drainage system, followed bya hydrograph routing to consider detention effects. However, the dynamic Storage-Indicationmethod is able to handle many of these situations.

Can HydroCAD calculate the Hydraulic Grade Line (HGL) ?

Determining the HGL traditionally involves a steady-state analysis, with the entire drainagesystem at equilibrium. Since hydrograph routing models are handling a time-varying flow, thereis no single HGL for the system. However, the peak elevation calculated at each node can be usedas the effective HGL.

When should I use the Dynamic Storage-Indication method?

The DSI procedure is intended primarily for “coupled ponds,” where one pond creates a tailwaterthat influences an upstream pond. Common applications include a set of catch basins, or aculverted road crossing with ponding occurring on both sides of the road.

If there are no tailwater effects, or the tailwater is constant, the traditional Storage-Indicationmethod is still recommended.

When should I use the Simultaneous routing method?

The Sim-Route procedure is intended specifically for ponds with reversing flows. That is, flows thatactually change direction during the course of the routing and require a reverse flow connection,as described on page 120. For normal tailwater conditions where the flow does not reverse, use theDSI procedure instead.

The Sim-Route procedure may also be precluded for very small ponds (such as catch basins) thatwill not “track” properly, even with the smallest dt. Although the Sim-Route procedure may workwith some catch basins, it is intended for larger ponds that have a significant storage volume inrelation to the inflow hydrograph.


Section 26 - References

The following publications contain additional information on the hydrology and hydraulicscalculations employed by HydroCAD. They are listed in the approximate order in which they arereferenced in this Manual.

[1] McCuen, Richard H. A Guide to Hydrologic Analysis Using SCS Methods, Prentice Hall, 1982.(Out of print. Also see [13], Chapter 8.)

[2] Soil Conservation Service Technical Release Number 20 (TR-20), National TechnicalInformation Service, 1982.

[3] Smith, P.D. Basic Hydraulics, Butterworth Scientific, 1982.

[4] Sharp, J.J. & Sawden, P. Basic Hydrology, Butterworth Scientific, 1984.

[5] King, H.W. & Brater, E.F. Handbook of Hydraulics, McGraw Hill, 1963.

[6] Simon, Andrew Practical Hydraulics, John Wiley & Sons, 1981.

[7] Chow, Ven Te Open Channel Hydraulics, McGraw Hill, 1959.

[8] Merrit, Frederick Standard Handbook for Civil Engineers

[9] Jerome M. Norman et al Culverts - Hydrology & Hydraulics, Lehigh University, 1980.

[10] NRCS National Engineering Handbook, Part 630: Hydrology (NEH, previously NEH-4)

[11] Soil Conservation Service Technical Release Number 55 (TR-55), 1986.

[12] American Concrete Pipe Association Concrete Pipe Handbook, 1981.

[13] McCuen, Richard H. Hydrologic Analysis and Design, Prentice Hall, 1989.

[14] Barfield and Warner Applied Hydrology and Sedimentology for Disturbed Areas, OklahomaTechnical Press, 1983.

[15] NRCS (Formerly SCS) Agricultural Handbook Number 590, Ponds - Planning, Design,Construction.

[16] Merkel, William H. Muskingum-Cunge Flood Routing Procedure in NRCS Hydrology Models,Second Federal Interagency Hydrologic Modeling Conference, 2002

[17] Schueler, T.R. Controlling Urban Runoff: A Practical Manual for Planning and DesigningUrban BMPs, Metropolitan Washington Council of Governments, Publication No. 87703.

For a list of on-line reference documents, visit www.hydrocad.net/library.htm



Appendices

The following appendices contain generalreference information that is commonlyused in connection with any analysis ordesign involving hydrology and hydraulics.


This appendix reprinted from S.C.S. TR-55, revised 1986.


Appendix A1: Hydrologic Soil Groups

Hydrologic soil groups

Soils are classified into hydrologic soil groups (HSG's) to indicate the minimum rate of infiltrationobtained for bare soil after prolonged wetting. The HSG's, which are A, B, C, and D, are oneelement used in determining runoff curve numbers as listed on the following pages.

The infiltration rate is the rate at which water enters the soil at the soil surface. It is controlledby surface conditions. HSG also indicates the transmission rate — the rate at which the watermoves through the soil. This rate is controlled by the soil profile. The four groups are defined bySCS soil scientists as follows:

Group A soils have low runoff potential and high infiltration rates even when thoroughly wetted.They consist chiefly of deep, well to excessively drained sands and gravels, and have a high rateof water transmission (greater than 0.30 in/hr).

Group B soils have moderate infiltration rates when thoroughly wetted, and consist chiefly ofmoderately deep to deep, moderately well to well drained soils with moderately fine to moderatelycoarse textures. These soils have a moderate rate of water transmission (0.15-0.30 in/hr).

Group C soils have low infiltration rates when thoroughly wetted, and consist chiefly of soils witha layer that impedes downward movement of water, and soils with moderately fine to fine texture.These soils have a low rate of water transmission (0.05-0.15 in/hr).

Group D soils have high runoff potential. They have very low infiltration rates when thoroughlywetted, and consist chiefly of clay soils with a high swelling potential, soils with a permanent highwater table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearlyimpervious material. These soils have a very low rate of water transmission (0-0.05 in/hr).

Note: A complete list of soil types for the United States is included in the HydroCAD Helpsystem, and on the HydroCAD support page at www.hydrocad.net.

Disturbed soil profiles

As a result of urbanization, the soil profile may be considerably altered and the listed groupclassification may no longer apply. In these circumstances, use the following to determine HSGaccording to the texture of the new surface soil, provided that significant compaction has notoccurred:

HSG Soil Textures A Sand, loamy sand, or sandy loam B Silt loam or loam C Sandy clay loam D Clay loam, silty clay loam, sandy clay, silty clay, or clay



Appendix A2: Runoff Curve Numbers



Appendix A2: Runoff Curve Numbers (continued)








Appendix A3: Curve Number Adjustment for AMC

The following table lists the automatic curve number adjustments for Antecedent MoistureCondition I and III as specified in NEH Table 10.1 and described on page 51. The first column liststhe normal (AMC II) values. The second and third columns list the adjusted values for AMC I andIII respectively. When using fractional curve numbers, the translation is performed using aninterpolated version of this table. For details see the AMC.TXT file installed with HydroCAD.

II I III

5 2 13

10 4 22

15 6 30

20 9 37

25 12 43

30 15 50 31 16 51 32 16 52 33 17 53 34 18 54 35 18 55 36 19 56 37 20 57 38 21 58 39 21 59 40 22 60 41 23 61 42 24 62 43 25 63 44 25 64 45 26 65 46 27 66 47 28 67 48 29 68 49 30 69 50 31 70 51 31 70 52 32 71 53 33 72 54 34 73 55 35 74 56 36 75 57 37 75 58 38 76 59 39 77

II I III

60 40 78 61 41 78 62 42 79 63 43 80 64 44 81 65 45 82 66 46 82 67 47 83 68 48 84 69 50 84 70 51 85 71 52 86 72 53 86 73 54 87 74 55 88 75 57 88 76 58 89 77 59 89 78 60 90 79 62 91 80 63 91 81 64 92 82 66 92 83 67 93 84 68 93 85 70 94 86 72 94 87 73 95 88 75 95 89 76 96 90 78 96 91 80 97 92 81 97 93 83 98 94 85 98 95 87 98 96 89 99 97 91 99 98 94 99 99 97 100 100 100 100


Appendix B1: HydroCAD Rainfall Library

The HydroCAD Rainfall LibraryHydroCAD includes a substantial library of rainfall distributions designed to meet therequirements of most projects. Some of the more common distributions are listed below. Additionalrainfalls may be included in your software or available at www.hydrocad.net/rainfall

Custom rainfalls may also be defined manually as described in the Rainfall.txt file, which may beviewed by selecting Start|Programs|HydroCAD|Rainfall Info. Custom synthetic rainfall distributions canalso be created from local rainfall data as discussed on page 47.

Rainfall Name(s) Comments

Type I 24-hrType IA 24-hrType II 6/12/24-hrType III 6/12/24-hr

Standard SCS/NRCS distributions used for most projects in theUnited States. (See page 157 for details.) Supplied aspolynomial curves for optimum results. Also available inconventional tabular format.

Type II FL 24-hr Modified Type II storm used by Southwest Florida WaterManagement District and Saint John’s County, Florida.

Type IIA CS 24-hr Modified Type IIA storm for Colorado Springs, Colorado.

SFWMD 24-hr & 72-hr Used by Southern Florida Water Management district.

FDOT 1/2/4/8/24-hrFDOT 3/7/10-day

Eight distributions used by the Florida Department ofTransportation. Select the table with the desired duration.

Spillway Emergency The SCS Emergency Spillway Hydrograph (ESH). Scaled to the duration specified on the Settings|Calculation screen. Also usedas a Type II 6-hr rainfall.

Spillway 1-day 10-day The SCS Principal Spillway Hydrograph (PSH). A ten-daydistribution that contains the one-day storm at its center. 40%of the ten-day rainfall occurs in the one-day period.

Constant Intensity A constant-intensity storm for modeling special situations. Maybe used for any storm duration and rainfall depth, as specifiedon the Settings|Calculation screen.

Sample A (intensity curve)Sample B (smoothed curve)Sample C (mass curve)Sample D (“Chicago” storm)

Fictional rainfall distributions that may be used as the basis forcustom storms.

Austin 3-hr n-yr Design storms for the City of Austin, Texas, provided for returnperiods (n) of 2, 5, 10, 25, 50, and 100 years. Each eventincludes a pre-set rainfall depth.

Charlotte 6-hr2-yr & 10-yr events

“Balanced Storms” for Charlotte, North Carolina. Each eventincludes a pre-set rainfall depth.

E-WA ShortE-WA Long Regions 1-4

Regional rainfall distributions for Eastern Washington State,from Washington State DOT, Highway Runoff Manual.


Fayette 5-hr 1995Fayette 6-hr 10-yrFayette 6-hr 100-yrFayette 18-hr 1992

Local rainfall distributions for Fayette County, Kentucky,Lexington-Fayette, KY Stormwater manual 1/1/01

Fayette05 6-hr 10-yrFayette05 6-hr 100-yrFayette05 24-hr 10-yrFayette05 24-hr 100yr

Updated rainfall distributions for Fayette County, Kentucky,Lexington-Fayette, KY Stormwater manual 1/1/05. Historicstorms same as above.

Fayette09 1-hr1-yr, 10-yr, and 100-yr

New 1-hour rainfall distributions for Fayette County, Kentucky. Other distributions as listed above.

Huff 0-10smHuff 10-50smHuff 50-400sm1st/2nd/3rd/4th Quartile

Illinois Huff distributions from ISWS Circular 173. Fourdistributions are provided for each watershed size, with thepeak occurring in the specified quartile. Each storm is scaled tothe duration specified on the Settings|Calculation screen.

Huff B70 0-10sm1st/2nd/3rd/4th Quartile

Updated Illinois Huff distributions from ISWS Bulletin 70.

Indy Huff 24-hr A variation of the Huff distributions from Indianapolis(Indiana) Stormwater Manual 5/10/95.

Indy Huff1st/2nd/3rd/4th Quartile

Updated Huff distributions from Indianapolis (Indiana)Stormwater Manual, SQU Appendix I, 7/20/06

LA County DPW24-hr and 96-hr

Los Angeles County DPW 2006 Hydrology Manual.

NJ DEP 2-hr 2-Hour stormwater quality event from New Jersey 2003Stormwater BMP Manual.

RSA 24-hrType 1, 2, 3, & 4

Republic of South Africa polynomial-based rainfalls. (Alsoavailable in tabular format)

San Diego 6-hrSan Diego 24-hr

San Diego County, California.

San Louis Obispo 24-hr2, 10, 25, 50, & 100 yearType U and D rainfalls

San Louis Obispo rainfall distributions for upper and lowerwatersheds. Each distribution also defines the rainfall depththe applicable event.

Seattle 24-hr Seattle, Washington, Flow Control Technical RequirementsManual, November 2000.

SEWRPC-90 South-east Wisconsin 90th percentile rainfall distribution.

Thurston 24-hr n-yrThurston 24-hr 1990Thurston 7-day 100-yr

Design storms for Thurston County, Washington, for returnperiods (n) of 0.5, 2, 5, 10, 25, 50, and 100 years. (Each stormincludes a pre-set rainfall depth.)



Appendix B2: SCS Synthetic Rainfall Distributions

SCS Synthetic Rainfall Distributions

Synthetic rainfall distributions can also be generatedfrom local rainfall data as discussed on page 47.



Appendix B2: SCS Rainfall Distributions (continued)



Appendix B4: Rainfall Depth Maps



Appendix B4: Rainfall Depth Maps (continued)



Appendix B4: Rainfall Depth Maps (continued)

This table reprinted from OPEN CHANNEL HYDRAULICS by Ven Te Chow, Copyright 1959 by McGraw-Hill, with thepermission of the publisher.


Appendix C: Manning's Number Table

This table reprinted from OPEN CHANNEL HYDRAULICS by Ven Te Chow, Copyright 1959 by McGraw-Hill, with thepermission of the publisher.


Appendix C: Manning's Number Table (continued)

This table was derived from information in HANDBOOK OF HYDRAULICS by Brater and King, 1976.


------------------Weir Breadth--(ft)------------------ Head 0.50 0.75 1.00 1.50 2.00 2.50 3.00 4.00 5.00 10.0 15.0 ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- 0.2 2.80 2.75 2.69 2.62 2.54 2.48 2.44 2.38 2.34 2.49 2.68 0.4 2.92 2.80 2.72 2.64 2.61 2.60 2.58 2.54 2.50 2.56 2.70 0.6 3.08 2.89 2.75 2.64 2.61 2.60 2.68 2.69 2.70 2.70 2.70 0.8 3.30 3.04 2.85 2.68 2.60 2.60 2.67 2.68 2.68 2.69 2.64 1.0 3.32 3.14 2.98 2.75 2.66 2.64 2.65 2.67 2.68 2.68 2.63 1.2 3.32 3.20 3.08 2.86 2.70 2.65 2.64 2.67 2.66 2.69 2.64 1.4 3.32 3.26 3.20 2.92 2.77 2.68 2.64 2.65 2.65 2.67 2.64 1.6 3.32 3.29 3.28 3.07 2.89 2.75 2.68 2.66 2.65 2.64 2.63 1.8 3.32 3.32 3.31 3.07 2.88 2.74 2.68 2.66 2.65 2.64 2.63 2.0 3.32 3.31 3.30 3.03 2.85 2.76 2.72 2.68 2.65 2.64 2.63 2.5 3.32 3.32 3.31 3.28 3.07 2.89 2.81 2.72 2.67 2.64 2.63 3.0 3.32 3.32 3.32 3.32 3.20 3.05 2.92 2.73 2.66 2.64 2.63 3.5 3.32 3.32 3.32 3.32 3.32 3.19 2.97 2.76 2.68 2.64 2.63 4.0 3.32 3.32 3.32 3.32 3.32 3.32 3.07 2.79 2.70 2.64 2.63 4.5 3.32 3.32 3.32 3.32 3.32 3.32 3.32 2.88 2.74 2.64 2.63 5.0 3.32 3.32 3.32 3.32 3.32 3.32 3.32 3.07 2.79 2.64 2.63 5.5 3.32 3.32 3.32 3.32 3.32 3.32 3.32 3.32 2.88 2.64 2.63

Appendix D1: Broad-Crested Weir Coefficients for Sharp-Edged Crests

The following table lists English weir coefficients for broad crested weirs with a sharp-edged crestof various breadths. These coefficients are automatically entered into the lookup table for a broadcrested weir whenever a crest breadth is entered as described on page 89. If breadth falls betweentwo listed values, interpolated coefficients are automatically used. Breadths outside the listedrange will use the first or last coefficient values without extrapolation. Values are automaticallyconverted to the current input units as described on page 43.

Reprinted from PRACTICAL HYDRAULICS by Andrew L. Simon, Copyright 1981 by John Wiley & Sons with thepermission of the publisher.


Appendix D2: Broad-Crested Weir Coefficients for Assorted Profiles

Coefficients for the following weirs may be entered automatically by specifying the appropriateProfile ID number on the HydroCAD weir screen.

Note: This table contains metric weir coefficients. To obtain English coefficients multiply thevalues in this table by 1.811 as described on page 43.

This table reprinted from the CONCRETE PIPE HANDBOOK, Copyright 1981 by the American Concrete PipeAssociation, with the permission of the publisher.


Appendix E: Culvert Entrance Loss Coefficients

The following table lists entrance loss coefficients for concrete, corrugated metal, and box culverts.These values are automatically provided by HydroCAD when the corresponding entrancedescription is selected for a given culvert.

Although comparable data is not available for corrugated plastic pipe, it is believed to be similarto corrugated metal, and the same entries are listed for “CPP” in the internal lookup table.


Appendix F: Sheet Flow Roughness Coefficients

HydroCAD provides the following table of roughness coefficients for use with the Sheet Flowprocedure (see page 54). This information is taken directly from NEH Table 15-1, with slightabbreviation of the descriptions. If you decide to substitute other roughness coefficients, note thatthese values are specifically for sheet flow, and are generally larger than the regular Manning'snumbers for comparable surfaces.

Surface Description nSmooth surfaces .011

Fallow .05

Cultivated: Residue<=20% .06

Cultivated: Residue>20% .17

Grass: Short .15

Grass: Dense .24

Grass: Bermuda .41

Range .13

Woods: Light underbrush .40

Woods: Dense underbrush .80

Note: These coefficients may also be appropriate when using a reach to model artificially createdsheet flow (as from a level spreader) as long as the depth of flow is limited to approximately 1/10foot.


Appendix G: Velocity Factors

The Shallow Concentrated Flow procedure (a.k.a. Upland Method) uses a velocity factor, KV, aslisted below. The first two surfaces (paved and unpaved) are the basis for TR-55 Figure 3-1, andthe factors were originally obtained from TR-55 Appendix F. The remaining surfaces were takenfrom NEH-4 Figure 15.2, with the factors derived from that chart. Subsequent revisions to NEHPart 630 provide numerical KV values which are in good agreement with the original chart, exceptfor “Grassed Waterways”, which appears to have changed from 15.0 to 16.13, making it the sameas the TR-55 “Unpaved” condition. For compatibility with previous calculations, the HydroCADlookup table continues to supply the original KV values as listed below. If different values arerequired for any reason, HydroCAD allows direct KV entry instead of using the lookup table. Seepage 55 for further details on Shallow Concentrated Flow.

Surface Description KV [ft/sec] KV [m/sec]Paved 20.33 6.2

Unpaved 16.13 4.92

Grassed Waterway 15.0 4.57

Nearly Bare & Untilled 10.0 3.05

Cultivated Straight Rows 9.0 2.74

Short Grass Pasture 7.0 2.13

Woodland 5.0 1.52

Forest w/Heavy Litter 2.5 0.76

Some descriptions have been abbreviated. Velocity factors have the same units as a velocity, andmay be converted between English and metric as described on page 43.


a'Y W%Y2

Z1%Z2

Pw'W%Y 1%Z 21 % 1%Z 2

2

Eq. 103

a '23

Y W

Pw' 4Y 2%W 2

4%

W 2

8Yln

2Y% 4Y 2%W 2

4W2

Eq. 104

Appendix H: Cross-Sectional Area & Perimeter Equations

The following equations are used to calculate the cross-sectional area and wetted perimeter ofcommon channel geometries.

Rectangular, Vee, or Trapezoidal channel

a=Cross-sectional areaPw=Wetted perimeterY=Flow depth

W=Bottom widthZ1=Left side slope Z-Value [run/rise]Z2=Right side slope Z-Value [run/rise]

Note: Side slopes are now expressed as a Z-value, which is calculated as the run divided by the rise.This is the reciprocal of the rise/run side slope used in HydroCAD-5 and earlier.

Parabolic Channel

a=Cross-sectional areaPw=Wetted perimeterY=Flow depth

W=Flow width at surface


a' θ2

R 2% (Y&R ) Y (D&Y ) '12

R 2 [θ&sin(θ) ]

Pw' R θ where θ'2cos&1 1& YR

Eq. 105

Circular Pipe (any flow depth)

a=Cross-sectional areaPw=Wetted perimeterD=DiameterR=RadiusY=Flow depthθ=Submerged central angle [radians]

(For multiple pipes, a and Pw are multiplied by the number of pipes)

Elliptical or Arch Pipe

Arch pipes are characterized by a top, bottom, and corner radius.An elliptical pipe is a special case of an arch pipe, in which the topand bottom radii are the same.

The area of an arch pipe consists of up to three components,depending on how much of the bottom, corner, and top chords ofthe pipe are submerged. This calculation is considerably morecomplex than other types of pipe storage, and is not detailed inthis document.

HydroCAD Owner’s Manual 171

Index Please see the HydroCAD help system for operating details not

contained in this Manual.

Adding Hydrographs . . . . . . . . . . . . . . . 126Antecedent Moisture Condition . . . 51, 132,

154Area,

Cross Sectional . . 55, 92, 106, 111, 123,142, 169, 170

Surface . . . . . . . 78, 81, 82, 85, 109, 110Wetted . . . . . . . . . . . . . . 78-86, 109, 110

ASCII . . . . . . . . . . . . . . . . . . . . . . . 125, 128Automatic Link . . . . . . . . . . . . . . . 127, 128Average

Land Slope . . . . . . . . . . . . . . . . . . 53, 54Velocity . . . . . . . . 55, 67, 100, 111, 123

Backwater . . . . . . . . . . . . . . . . . . . . 42, 101Base Flow . . . . . . . . . . . . . 15, 71, 117, 133Breach Calculations . . . . . . . . . . . . . . . . 95Broad-Crested Weir . . 87, 89, 94, 114, 164,

165Burst . . . . . . . . . . . . . . . . . . . . . . . 48, 59-61Calculation Time . . . . 61, 64, 101, 119, 123,

126, 133, 139, 140Carlson Hydrology . . . . . . . . . . . . . . . . . . 38Catch Basin . . . . . . . . . . . . . . . 30, 135, 144Center of Mass Detention Time . . . . . . 121Chamber,

Library . . . . . . . . . . . . . . . . . . . . . . . . 84Prefabricated . . . . . . . . . . . . . . . . . . . 84

ChannelFlow . . . . . . . . . . 15, 30, 54-56, 101, 143Slope . . . . . . . . . . . . . . . . . . . . . . . . . 55

Circular Orifice . . . . . . . . . . . . . . . . . . . . 98Compound

Outlet . . . . . . . . . . . . . . . . . . . . . 15, 114Storage . . . . . . . . . . . . . . . . . . . . . . . . 78

Conductivity . . . . . . . . . . . . . . 18, 108, 109Constant Flow . . . . . . . . . . . . . . 19, 42, 104Contraction Coefficient . . . . . . . . . . . . . 101Convolution . . . . . . . . . . . . . . . . . . . . 59, 63Coordinates . . . . . . . . . . . . . . . . . . . . 31, 68Crest

Height . . . . . . . . . . . . . . . . . . . . . 88, 93Length . . . . . . . . . . . . . . . 88, 89, 95, 99

Critical Duration . . . . . . . . . . . . . . . . 20, 66Cross Sectional Area . 55, 92, 106, 111, 123,

142, 169, 170Culvert Flow . . . . . . . . . . . . . . . . . . 15, 100Cumulative

Storage . . . . . . . . . . . . . . . . . . . . . . . . 85

Curve Number . . . 15, 18, 19, 37, 49, 50, 53,54, 56, 58, 59, 61, 62, 64, 137,

154Composite . . . . . . . . . . . . . . . 49, 50, 64

Curve Number Method . . . . . . . . 15, 53, 54Curve Numbers . . 42, 51, 63, 130, 134, 149-

154Dam Breach . . . . . . . . . . . . . . . . . . . . . . . 95Darcy’s Law . . . . . . . . . . . . . . . . . . . 18, 108Data Files . . . . . . . . . . . . . . . . . . . . . . . . 18Default

Project . . . . . . . . . . . . . . . . . . . . . 16, 35Detention . . 16, 20, 42, 46, 62, 66, 116, 118,

121, 122, 133, 144Time . . . . . . . . . . . . . . 16, 121, 122, 133

Discarded Outflow . . . . . . . 30, 32, 108, 113Discharge

Coefficient . . . . . . 88-91, 94, 96-99, 101Curve . . 87, 98, 101, 106, 108, 113, 114,

119, 138, 140, 141Multiplier . . . . . . . . . . 99, 111, 125, 126Velocity . . . . . . . . . . . . . . . . 20, 97, 111

Diversion . . . . . . . . . . . . . . . . . . . . 113, 126Duration,

Critical . . . . . . . . . . . . . . . . . . . . . 20, 66Rainfall . . . . . . . . . . . . 45, 58, 132, 134

Dynamic Storage-Indication . . . . 15, 20, 71,107, 115, 117, 144

Earlier Time Span . . . . . . . . . . . . . . . . . 139Elevation,

Peak . 115, 117, 123, 135, 136, 138, 144Starting . . . . . . . . . . . . . . . . . . . . . . 117Table . . . . . . . . . . . . . . . . . . . . . . . . 126Tidal . . . . . . . . 15, 20, 30, 119, 125, 126

EmbeddedStorage . . . . . . . . . . . . . . . . . 16, 78, 110

End Contractions . . . . . . . . . . . . . . . . . . 88English . . . 16, 17, 20, 34, 43, 55, 65, 67, 88,

90, 99, 100, 103, 106, 164,165, 168

Entrance Loss Coefficient . . . . . . . . . . . 166Exfiltration . . 15, 18, 30, 32, 78, 81-85, 87,

108-110, 113, 138Velocity . . . . . . . . . . . . . . . . . . 108, 110

Export Settings . . . . . . . . . . . . . . . . . . . 128External

Hydrograph . . . . . . . . . . . . 35, 126, 128Hydrograph Files . . . . . . . . . . . 35, 128

HydroCAD Owner’s Manual172

Extrapolation . . . . . . 89, 105, 123, 141, 164Finer Routing . . . . . . . . . . . . . 117, 139-141Float Valve . . . . . . . . . . . . . . . . . . . . . . 102Flood

Elevation . . . . . . . . . . . . . . . . . 117, 135Flow

Depth . . . 72, 73, 75, 101, 136, 169, 170Length . . . . . . . . . . . . . . . . . . . . . . 54-56Segment . . . . . . . . . . . . . 53, 67, 76, 143Threshold . . . . . . . . . . . . . . . . . 125, 126Width . . . . . . . . . . . . . . . . . . . . . 73, 169

Flow,Base . . . . . . . . . . . . . . . 15, 71, 117, 133Channel . . . . . . . 15, 30, 54-56, 101, 143Constant . . . . . . . . . . . . . . . 19, 42, 104Normal . . . . . . . . . . . . . . 67, 73, 75, 101Open Channel . . . . . . . . . . 30, 101, 143Peak . . 30, 61, 65, 66, 69, 123, 134, 143Reverse . . . . . . . . . . 120, 136, 139, 144Shallow Concentrated . . 15, 54, 55, 168Sheet . . . . . . . . . . . . . . . 15, 50, 54, 167Siphon . . . . . . . . . . . . . . . . . . . . . . . 102Tube . . . . . . . . . . . . . . . . . . . . . . . . . 102

Frequency . . . . . . . . . . . 17, 18, 45, 66, 132Frequency Factor . . . . . . . . . . . 18, 66, 132Geometry . . . . . . . . . . . . . . . 69, 70, 72, 111Grate . . . . . . . . . . . . . . . . . . . . . . . . . 87, 99Ground Cover . 17, 38, 41, 49, 58, 134, 137Hazen-Williams . . . . . . . . . . . . . . . . . . . 106Head . . . 32, 88-94, 96-99, 101-103, 105-108

136, 138, 139, 141, 142,164Loss . . . . . . . . . . . . . . 97, 103, 105, 106

Huff Distributions . . . . . . . . . . . . . . 58, 156Hydraulic

Conductivity . . . . . . . . . . . . . . . 18, 108Grade Line . . . . . . . . . . . . . . 67, 73, 144Gradient . . . . . . . . . . . . . . . . . . . . . . 108Length . . . . . . . . . . . . . . . . . . . . . . . . 53Radius . . . . . . 55, 67, 68, 100, 103, 106

Hydraulics . . . 13, 15, 28, 41, 67, 68, 87-90,94, 96-98, 100, 106, 145, 147

HydroCAD Screens . . . . . . . . . . . . . . 23, 29Hydrograph

Addition . . . . . . . . . . . . . . . . . . . . . . 126Export . . . . . . . . . . . . . . . . 35, 126, 128Routing . . . . 30, 42, 67, 74, 75, 89, 101,

107, 116, 118, 121, 133, 144IDF Curve . . . . . . . 15, 17-20, 45-47, 65, 66IDF Library . . . . . . . . . . . . . . . . . . . . . . . 46Illinois . . . . . . . . . . . . . . . . . . . . . . . 58, 156Impervious,

Area . . . . . . . . . . . . . . . . . . . 18, 50, 130Unconnected . . . . . . . . . . 15, 18, 49, 50

Import . 16-19, 28, 32, 37, 38, 47, 125, 127,128, 131, 137

Incremental Storage . . . . . . . . . . 46, 84, 85Infiltration . . . . . . . . . . . . . . . . . . . . 51, 149Inflow Loss . . . . . . . . . . . . . . . . 71, 108, 117Inlet Conditions . . . . . . . . . . . . . . . . . . . 135Installation . . . . . . 3, 18, 21, 23, 25, 26, 35

Directory . . . . . . . . . . . . . . . . . . . 26, 35Update . . . . . . . . . . . . . . . . . . . . . 25, 26

Intensity . . . . . 20, 45, 48, 65, 66, 132, 155Interpolation . . 45, 69, 70, 72, 93, 105, 123,

126Lag

Method . . . . . . . . . . . . . . . . . . . . . . . . 53Land Slope . . . . . . . . . . . . . . . . . . . . . . 53-55Land-Use . . . . . . . . . . . . . . 16, 18, 129, 130Length,

Hydraulic . . . . . . . . . . . . . . . . . . . . . . 53License . . . . . . . . . . . . . . . . . . . . . . 3, 25, 26

Pooling . . . . . . . . . . . . . . . . . . . . . 25, 26Link . . . . . 19, 20, 30, 33, 119, 125-128, 133LinkTest.hce . . . . . . . . . . . . . . . . . . . . . 128Manning's

Capacity . . . . . . . . . . . . . . . . . . . . . . 135Coefficient . . . . . . . . . . . . . . . . . . 54, 55Composite . . . . . . . . . . . . . . . . . . . . . 68Equation . . . . . . . . . . 55, 67, 69, 70, 75

Manual Link . . . . . . . . . . . . . . . . . . . . . 125Messages,

Calculation . . . . . . . . . . . . . . . . 33, 131Warning . . . . . . . . . . . . . . . . . . . 33, 136

Metric . 16, 17, 20, 34, 43, 55, 65, 67, 88, 90,99, 100, 103, 106, 165, 168

Minimal Recalculation . . . . . . . . . . 27, 133Modified Rational Method . . . . . . . . 15, 65Mouse . . . . . . . . . . . . . . . . . . . . 20, 23, 28-30Multiplier . . . . . . . . . . . . . 99, 111, 125, 126Muskingum-Cunge . . 15, 19, 71, 72, 74, 76,

145No Inflow . . . . . . . . . . . . . . . . . . . . 108, 133Node

Numbering . . . . . . . . . . . . . 31, 128, 131Undescribed . . . . . . . . . . . . . . . . . . . 133

Normal Flow . . . . . . . . . . . . 67, 73, 75, 101Not Described . . . . . . . . . . . . . . . . . . . . 133Notch Angle . . . . . . . . . . . . . . . . . 90, 91, 95Ohio . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125Open Channel Flow . . . . . . . . . 30, 101, 143Orifice . 19, 87, 88, 90-94, 96-104, 111, 114,

142Orifice,

Circular . . . . . . . . . . . . . . . . . . . . . . . 98Low-head . . . . . . . . . . . . . . . . . . . . 97-99Rectangular . . . . . . . . . . . . . . 91, 96, 97


Oscillations . . . . . . . . . . . . . . . 117, 139-141Outflow,

Discarded . . . . . . . . . . . 30, 32, 108, 113Primary . . . . . . . . . . . . . . . . . . . 32, 113Secondary . . . . . . . . . . . . . . . . . 30, 126Tertiary . . . . . . . . . . . . . . . . . . . . . . . 32

Outlet Device . . 30, 104-106, 108, 111, 114,115, 117, 138, 139, 141, 142

Outside Time Span . . . . . . . . . . . . . . . . 137Override . . . . . . . . . . . . . . . . . . . . . . . . . . 72Pan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20Parabolic . . . . . . . . 67, 70, 77, 83, 123, 169Peak

Attenuation . . . . . . . . . . . . . . . . . . . 123Depth . . . . . . . . . . . . . . . . . . . . 123, 135Elevation . 115, 117, 123, 135, 136, 138,

144Factor . . . . . . . . . . . . . . . . . . . 48, 58, 59Flow . . 30, 61, 65, 66, 69, 123, 134, 143Storage . . . . . . . . . . . . . . . . . . . . . . . 123Time . . . . . . . . . . . . . . . . . . . . . . . . . 123

Perc Rate . . . . . . . . . . . . . . . . . . . . . . . . 110Permeability . . . . . . . . . . . . . . . . . . . . . 108Pipe Networks . . . . . . . . . . . . . . . . . . . . . 42Plug Flow Detention Time . . . . . . 121, 122Pollutants . . . . . . . . . . . . . . . . . . . . . . . 129Pond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Routing . . . . 15, 56, 66, 75, 77, 78, 104,111, 113, 115, 118, 133, 138,

139Shape . . . . . . . . . . . . . . . . . . . . . . . . . 85

Potential Maximum Retention . . . . . 53, 59Precipitation Excess . . . . 48, 58, 59, 61, 63,

134Primary Outflow . . . . . . . . . . . . . . . 32, 113Printer . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Probability . . . . . . . . . . . . . . . . . . . . . . . . 45Project

Default . . . . . . . . . . . . . . . . . . . . . 16, 35Read-Only . . . . . . . . . . . . . . . . . . 35, 36Selector . . . . . . . . . . . . . . . . . 29, 35, 36Storage . . . . . . . . . . . . . . . . . . . . . 28, 35

Pumps . . . . . 15, 18, 87, 106, 107, 117, 142Rainfall

Burst . . . . . . . . . . . . . . . . . . . . 48, 59-61Data . . . . 16, 27, 37, 45-47, 57, 155, 157Distribution . . 15, 17-19, 46, 47, 57, 63,

155-158Duration . . . . . . . . . . . 45, 58, 132, 134Event . . . . . . . . . . . . . . . . 45-47, 58, 132Intensity . . . . . . . . . . . . . . . . . . . 45, 65Library . . . . . . . . . . . . . . . . . . . . . . . . 47Synthetic . . . . . . . . . . . . . . . . . . . . . . 46Type . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Rainfall.txt . . . . . . . . . . . . . . . . . . . . . . . 155Rational Method . 15, 18, 20, 42, 46, 65, 66,

130, 132, 134, 137Reach . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Discharge . . . . . . . . . . . . . . . . . . . . . 136Routing . . . 15, 19, 67, 69-71, 74-76, 78,

123, 133, 135-137, 142Read-Only . . . . . . . . . . . . . . . . . . . . . 35, 36Rectangular Orifice . . . . . . . . . . . 91, 96, 97Reinstall . . . . . . . . . . . . . . . . . . . . . . . . . . 26Retention . . . . . . . . . . . . . . . . 49, 51, 53, 59Return Period . . . . . . . . . . . . . . . 45, 58, 66Reverse Flow . . . . . . . . . 120, 136, 139, 144Routing

Diagram . . . 16, 20, 27-35, 38, 120, 125,132, 133

Routing,Dynamic Storage-Indication . 15, 20, 71,

107, 115, 117, 144Sequential . . . . . . . . . . . . . . . . . 72, 120Simultaneous . . . . 71, 75, 118-120, 132,

133, 140, 144Storage-Indication . . 15, 20, 71, 76, 107,

115-117, 119, 144Runoff

Curve Numbers . . . . . . . . . . . . . 149-153Interval . . . . . . . . . . . . . . . . . . . . . . . 61Method . . . . . . . . . . . . . . . . . . . . . 50, 66

Santa Barbara method . . 15, 50, 63, 64, 66,134

Scroll . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30SCS . . 15, 43, 46-50, 57-59, 61-63, 66, 130,

132, 134, 145, 149, 155, 157,158

Runoff Equation . 43, 49, 50, 59, 61, 63,130, 134

Unit Hydrograph method . . . 57, 63, 66Secondary Outflow . . . . . . . . . . . . . 30, 126Sequential Routing . . . . . . . . . . . . . 72, 120Serial Number . . . . . . . . . . . . . . . . . . . . . 26Shallow Concentrated Flow . . . . 15, 54, 55,

168Sharp-Crested Weir . . . . . . . . 87-89, 92, 94Sheet Flow . . . . . . . . . . . . . . 15, 50, 54, 167

Roughness Coefficients . . . . . . . . . . 167Side Slope . . . . . . . . . . . . . . . . . . . . . . . 169Simple Method . . . . . . . . . . . . . . . 129, 130Simultaneous Routing . . . . 71, 75, 118-120,

132, 133, 140, 144Siphon Flow . . . . . . . . . . . . . . . . . . . . . . 102Skimmer . . . . . . . . . . . . . . . . . . . . . . . . 104


Slope,Channel . . . . . . . . . . . . . . . . . . . . . . . 55Land . . . . . . . . . . . . . . . . . . . . . . . . 53-55Side . . . . . . . . . . . . . . . . . . . . . . . . . 169

SoilGroup . . . . . . . . . . . . . . . . . 49, 134, 137Type . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Special Outlet . . . . . . . . . 87, 101, 104, 105Stage-Discharge . . . . 42, 67, 69, 71, 75, 87,

101, 102, 105-108, 110, 113-119, 138, 140, 141

Stage-Storage . 16, 20, 67, 71, 77, 113, 115,116, 118, 141, 144

Standpipe . . . . . . . . . . . . . . . . . . 87, 99, 114Starting Elevation . . . . . . . . . . . . . . . . . 117Storage Range Exceeded . . . . . . . . . . . . 141Storage,

Arch Pipe . . . . . . . . . . . . . . . . . . . . . . 82Box Pipe . . . . . . . . . . . . . . . . . . . . . . . 82Compound . . . . . . . . . . . . . . . . . . . . . 78Cumulative . . . . . . . . . . . . . . . . . . . . 85Custom . . . . . . . . . . . . . . . . . . . . . . . . 85Elliptical Pipe . . . . . . . . . . . . . . . . . . 82Embedded . . . . . . . . . . . . . . 16, 78, 110Incremental . . . . . . . . . . . . . . 46, 84, 85Parabolic Arch . . . . . . . . . . . . . . . . . . 83Peak . . . . . . . . . . . . . . . . . . . . . . . . . 123Pond . . . . . . . . . . . . . . . . . . . . . . . . . . 77Prefab Chamber . . . . . . . . . . . . . . . . 84Prismatoid . . . . . . . . . . . . . . . . . . . . . 79Round Pipe . . . . . . . . . . . . . . . . . . . . 81Vertical Cone . . . . . . . . . . . . . . . . . . . 80

Storage-Indication . 15, 20, 71, 76, 107, 115-117, 119, 144

Storm Sewer . . . . . . . . . . . . . . . . . . 42, 144Subcatchment . . . . . . . . . . . . . . . . . . . . . 30Summary . . . . . . . . . . . . 13, 15, 19, 74, 131Surface

Area . . . . . . . . . 78, 81, 82, 85, 109, 110Swamp . . . . . . . . . . . . . . . . . . . . . . . . . . . 30Table . 17, 19, 43, 47, 49, 51, 55, 58, 67, 68,

70, 72, 89, 100, 105, 118, 119,126, 142, 143, 149, 154, 155,

162-168Tabular Method . . . . . . . . . . . . . . . . . . . . 62Tailwater . 15, 18-20, 30, 69, 71, 74, 75, 87,

92-98, 100-105, 107, 114, 116-120, 126, 133, 135, 136, 138,

139, 144Tailwater,

Tidal . . . . . . . . 15, 20, 30, 119, 125, 126Tertiary Outflow . . . . . . . . . . . . . . . . . . . 32Text/Image . . . . . . . . . . . . . . . . . . . . . . . . 31

TidalElevation . . . . 15, 20, 30, 119, 125, 126

TimeIncrement . . . . 46, 64, 71, 75, 115, 116,

118, 119, 126, 132, 134, 139-141

Interval . . . 75, 115, 117, 118, 123, 134Lag . . . . . . . . . . . . . 19, 53, 71, 123, 125of Peak . . . . . . . . . . . . . . . . . . . . . . . 123Span . 19, 61, 71, 75, 116, 118, 122, 123,

126, 132-134, 137, 139Travel . . . . 15, 53-56, 67, 71, 73, 76, 123

Time of Concentration . . 38, 42, 53, 56, 58,59, 61, 63, 64, 134

TR-20 . . . . 18, 57, 62, 72, 76, 132, 137, 145TR-55 . . . . . 49, 50, 55, 59, 61, 62, 145, 168Translation . . . . . . . . . . . . . . . . 15, 71, 154Trapezoidal . . . . . 15, 67, 72, 87, 90-95, 169Travel Time . . 15, 53-56, 67, 71, 73, 76, 123Tube Flow . . . . . . . . . . . . . . . . . . . . . . . 102Tutorial . . . . . . . . . . . . . . . . . . . . . . . 23, 32Underlining . . . . . . . . . . . . . . . . . . . . . . . 23Undescribed Node . . . . . . . . . . . . . . . . . 133Uninstall . . . . . . . . . . . . . . . . . . . . . . 25, 26Unit Hydrograph . . 15, 17, 48, 57-59, 61-63,

132Procedure . . . . . . . . . . . . . . 15, 57, 61-63

Units . 16, 17, 20, 34, 43, 55, 65, 67, 88, 90,99, 100, 103, 106, 164, 165,

168Custom . . . . . . . . . . . . . . . . . . . . . . . . 34

Upland Method . . . . . . . . . . . . . 15, 55, 168Velocity Factor . . . . . . . . . . . . . . . . 55, 168Velocity,

Average . . . . . . . . 55, 67, 100, 111, 123Exfiltration . . . . . . . . . . . . . . . 108, 110

Volume . . . 18, 41-43, 46, 48, 50, 58, 59, 61,65, 66, 71, 75, 77-86, 110, 115-

117, 121-123, 133, 134, 138-141, 143, 144

V-Notch Weir . . . . . . . . . . . . . 87, 90, 91, 94Wall Thickness . . . . . . . . . . . . . . . 18, 78-83Warning Messages . . . . . . . . . . . . . . . . . 33Weir,

Broad-crested . . . . 87, 89, 94, 114, 164,165

Coefficient . . 43, 88-91, 94, 99, 164, 165Sharp-Crested . . . . . . . . . . 87-89, 92, 94V-Notch . . . . . . . . . . . . . . 87, 90, 91, 94

WettedArea . . . . . . . . . . . . . . . . 78-86, 109, 110Perimeter . . . . . . . . 55, 67, 68, 169, 170

Zoom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20


Notes


Notes