Section 6:Pond Routing

Transforming an inflow hydrographinto an outflow hydrograph

35 Minutes

Copyright 2004 HydroCAD Software Solutions LLCAll Rights Reserved - Duplication Prohibited

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Routing IntroductionWhat is a pond routing?

PWhat role does a pond play in the model?PHow is the inflow hydrograph altered by the

pond routing?

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Pond Routing Effects #1How is the inflow hydrographaltered by the routing?

T i m e

Peak occurs later (time lag)

Peak flow is reduced (attenuation)

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Pond Routing Effects #2How much water is stored in the pond?

T i m e

PMaximum storageoccurs at peak outflow< Volume is equal to area

between curvesPPond is at equilibrium< QOUT = QIN

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Estimated Pond SizingHow much storage do I need to attenuatethe peak to the target level?

PAfter you perform a pond routing, you knowthe actual peak outflow and storage used.< But how do you estimate the required storage in

advance?PThe pond sizing report will tell you the

approximate storage required to attenuateyour actual inflow to any desired level< First, create a pond with the actual inflow

– Pond data is optional< Then open the sizing report...

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Using the PondSizing ReportHow much storage do I need?

Required Storage (AF)

PFind your target outflow onthe left axis

PMove right to the curve anddown to the bottom axis

PThis is the approximaterequired storage!

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Pond Sizing NotesHow much storage do I need?

PThe pond sizing report provides an initialestimate only< Use this to design a trial pond and then refine

the designPThe sizing report is based on the actual

inflow and an assumed discharge curve< Your sizing may vary depending on your exact

outlet configuration

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Other Detention Effects #1What happens to the larger watershed?

7 Existing Site

7 AdjacentWatershed

T i m e

7 Existing Combined Flow

PExisting peaks may occur at different timesPThis avoids direct peak addition and

minimizes downstream flooding

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Secondary Detention Effects #2What happens to the larger watershed?

Developed Site 7 (same peak)

T i m e

Developed Combined Flow7 (peak has increased)

PDeveloped discharge may have same peakPBut “peak widening” increases total flows

and contributes to downstream flooding

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Trends in StormwaterManagementSometimes smaller is better

PManage stormwater near the source< Distribute mitigation measures throughout the site< Minimize use of large-scale ponds< Utilize all detention opportunites< Maximize inflitration

P Integrate site design and stormwatermanagement

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Starting ElevationAllows for a pre-storm waterlevel in a pond

PBy default, all ponds start emptyPStarting Elevation sets pre-storm water level< If above outlet, will cause immediate outflow< Can be used without inflow hydrograph to observe

draw-down of pre-charged pond

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Base Flow CalculationsBase flow can be used to modelconstant pre-runoff flows

PBase flow is added to inflow hydrograph< Base-flow is included in all downstream flows

PAutomatic base flow option calculates flowrequired to maintain starting elevation< Keeps the pond at equilibrium< Prevents draw-down from starting elevation< Useful for ponds with pre-storm flows

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Detention TimeHow long is the water stored in the pond?

PDetention time is the average length of timethat a molecule of water stays in the pond< This is not a routing procedure - It is an analysis

that is performed after the routing is completePThere are several ways to calculate TDET

PHydroCAD provides two procedures:< Center-of-Mass method< Plug-Flow method

PLets see how these are calculated...6-110

Center-of-Mass Detention Time #1A basic procedure for calculating detention time

T i m e

PFirst, calculate thecenter-of-mass time forthe inflow hydrograph

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Center-of-Mass Detention Time #2A basic procedure for calculating detention time

T i m e

PThen calculate thecenter-of-mass time forthe outflow hydrograph

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Center-of-Mass Detention Time #3A basic procedure for calculating detention time

T i m e

PThe detention time isthe difference betweenthe inflow and outflowcenter-of-mass times

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Center-of-Mass Detention Time #4

PAdvantages:< Simple, graphical solution< Fairly easy to verify results by hand

PLimitations:< Reduces each hydrograph to a single time value

– Doesn’t consider overall hydrograph shape< Can be “fooled” by certain hydrographs< Not a physical evaluation of the flow

PA better alternative:< The Plug-Flow method...

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Plug-Flow Detention Time #1A better detention time procedure

PFirst, divide theoutflow into plugs ofequal volume

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Plug-Flow Detention Time #2

PNext, identify when the firstplug enters the pond< Inflow and outflow plugs have

equal volume, but differentduration.

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Plug-Flow Detention Time #3

PNow calculate the timebetween the plugs< This is the detention time for

a single plug

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Plug-Flow Detention Time #4

PRepeat the process for all theplugs, calculating the averageof all the detention times< The average gives the overall

detention time for the entire event

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Plug-Flow CommentsPAdvantages:< Calculation is based by physical situation< Sensitive to exact hydrograph shape< Works with complex situations such as wet ponds

PThings to understand:< Produces a “best case” (maximum detention time)

based on a first-in first-out scenario.< Any pre-storm water is displaced before any of the

new runoff leaves the pond.– Wet ponds have a greater plug-flow detention time!

– Center-of-mass method ignores any pre-filled storage

PHydroCAD provides BOTH techniques6-200

Other Water QualityRequirementsUsing HydroCAD to meet yourlocal regulations

P “Detain the first inch for 24-hours”< Exactly what does this mean?< Read your regulations very carefully!

PTo verify compliance you can use:< Tabular inflow-outflow hydrograph

– Shows the pond volume at each time step

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Software ExercisesExploring these topics in HydroCAD

PBefore we begin the software demonstration,are there any other questions about pondrouting calculations?

P If you wish, you may perform the followingexercises yourself, or just watch the demo.

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Lets examine a pond routingTo save time, we’ll work with an existing project

PGetting ready< Start HydroCAD< Open the “Mountain View Housing Complex”

PExamine the routing< Double-click pond “3P”< Examine the summary report

– Note the detention time calculations– In this case PF and CM results are very close

– Click any line for details (HydroCAD 7.1)< Examine the hydrograph

– Note the storage volume

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Using the pondsizing reportEstimating the required storage

PExamine the sizing report< Make sure the report is open for pond 3P< Click the “Sizing” tab

– Try the 2D view– Select Table format– To see more values, remove the “Shrink” option– Click Help for detailed instructions

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Modeling a “wet pond”How do we handle the initial waterlevel in the pond?

PUse the options on the “Advanced” tab:< Set the Starting Elevation:

– Set a Starting Elevation of 20.5' and click OK– Note immediate outflow at start of hydrograph

– But the outflow (and elevation) are dropping!

< Continue to next slide...

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Pre-Storm FlowsWhat if the pond is dischargingbefore the storm begins?

PUse the options on the “Advanced” tab:< Add a base flow:

– Set a Base Flow of 1 CFS and click OK– Note initial decline is reduced but not eliminated

< Try Automatic Base Flow:– Select Automatic Base Flow and click OK– Pond is now at equilibrium!– Exact base flow is shown on Summary

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*** End of Section ***

PAre there any other questions about pondrouting calculations?

PFor specific routing equations, please see theHydroCAD Owner’s manual.

P If you performed the exercises, you mayclose HydroCAD at this time.< You do NOT need to save your changes.

P ** Take a 10-minute break **< Please complete your evaluations!

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