The term long-throated flume describes a broad class of critical-flow flumes and broad-crested weir devices used to measure flow in open channels.

Introduction to Long-Throated Flumes and Broad-Crested Weirs

Critical-Flow Measurement Devices

• Flumes, sharp-crested weirs, broad-crested weirs– Produce critical-depth flow in a control section

• Froude number = V/(gD)1/2=1

– Critical depth occurs at locations where the downstream depth does not “hold the flow back”

• Minimum specific energy for a given flow

• Maximum flow for a given specific energy

• Shallow-water waves cannot travel upstream

• Tailwater does not affect headwater elevation

– Flow rate through the critical section is a function of the upstream head, acceleration of gravity, and the control section size

Traditional Critical-Flow Devices

• Most critical-flow devices have curvilinear,three-dimensional flow fields in the control section

• All such devices require laboratory calibration• Flumes

– Parshall flumes, cutthroat flumes, H-flumes, etc.

• Weirs– V-notch weirs, Cipoletti weirs, contracted and

suppressed rectangular weirs, etc.

Long-Throated Flumesand Broad-Crested Weirs

• Long-throated flumes have one-dimensional flow in the control section -- Long-throated means long enough to eliminate lateral and vertical contraction of the flow at the control section…streamlines are essentially parallel– Can be calibrated using well-established hydraulic theory

• No laboratory testing needed

– Calculations are iterative, so computer models that do the calculations have made long-throated flumes reasonable to implement in recent years

Submergence of Flumes and Weirs

• h2/h1 or hb/ha is thesubmergence ratio

• Sharp-crested weirs– NO SUBMERGENCE

ALLOWED

• Parshall flume– Some submergence allowed

• Long-throated flume– Most submergence allowed

Submergence of Parshall and Other Short-Throated Flumes

– Some submergence allowed

– Modular limit varies with flume size (50-80 percent)

– Submergence correction needed when above modular limit (no longer a true critical-flow device)…even with correction, accuracy suffers, especially above 90 percent submergence

Submergence of Long-Throated Flumes and Broad-Crested Weirs

• Highest modular limit (lowest head loss) of any critical-flow device– Modular limit varies, but can be predicted using WinFlume

– Typically 80-90+ percent

• No submergence correction needed as long as submergence is less than modular limit– Flow rate is always a function of ONLY the upstream water level

• Cannot correct for submergence if modular limit is exceeded

Long-throated flumes are the measurement device of choice for many applications. Advantages include:

• Rating tables with error of less than 2% in the computed discharge can be computed for any combination of prismatic control section and an arbitrarily shaped approach channel

• If the throat is horizontal in the direction parallel to the flow, accurate rating tables can be computed using as-built dimensions

• Throat can be any shape in the direction perpendicular to the flow, allowing the complete range of discharges to be measured with good precision

• Required head loss across the flumes is minimal (modular limit is high)

• Long-throated flumes can be operated in free-flow with greater submergence than all other critical-flow devices, and the submergence limit (modular limit) and the associated head loss requirement can be determined using the WinFlume program

• With properly constructed gradual converging transition, there is virtually no problem passing floating debris

• Can be designed to pass sediment transported by channels having subcritical flow

• Economical to construct

• Very adaptable to installation in existing canals

Principal Design Issues• Upstream channel - 3 issues

– Uniform, fully-developed flow conditions approaching structure so that hydraulic theory is applicable

– Stable water surface to allow accurate measurement of sill-referenced head

– Effect that the flume may have on upstream structures and freeboard in upstream channel

• Flume contraction - 2 issues– Ensure that flume is not submerged by tailwater (contraction

must be enough to force critical depth)

– Ensure that contraction produces enough head to make an accurate flow measurement

• Component lengths - must meet “long-throated” criteria

Lengths of Flume Components• Throat section length, L

0.07<H1/L<0.7 for ±2% uncertainty

0.05<H1/L<1.0 for ±4% uncertainty

• Floor and sidewalls of converging transition2.5 to 4.5:1 transition slope

• Diverging transitionNo flatter than 10:1

• Gaging station location (approach channel length)> H1 upstream of start of converging transition

(2 to 3) * H1max from start of throat

Throat Section Shape Selection

• Constructability• Range of Flows to be Measured

ShapeQmax/Qmin

±2% uncertaintyQmax/Qmin

±4% uncertaintyRectangular 35 100

Triangular 350 1970

Trapezoidal – wide at top 55 180

Trapezoidal – narrow at top 210 1080

Parabolic 105 440

Complex – wide at top > 100 > 200

Complex – narrow at top > 250 > 2000

Typical Flume/Weir Configurations

• Sill in a concrete-lined canal• Rectangular-throated flumes for earthen canals• Triangular-throated flumes for natural channels• Flumes in circular pipes• Portable and temporary flumes

Flume Design & Selection• Pre-computed flume designs can be chosen using

tables in the Water Measurement Manual• Designs can be developed using the WinFlume

computer program– Allows for customization

– Provides best rating table accuracy

– Simplifies checking of design

WinFlumeSOFTWARE FOR THE DESIGN AND CALIBRATION OF

LONG-THROATED FLUMES AND BROAD-CRESTED WEIRS

U.S. Water Conservation LaboratoryPhoenix, Arizona

Water Resources Research LaboratoryDenver, Colorado

International Institute for Land Reclamation & Improvement

Wageningen, The Netherlands

Evolution of Long-Throated Flume Design Software

• Batch oriented FORTRAN codes developed by ARS at U.S. Water Conservation Laboratory, Phoenix.

• Interactive program written in Clipper for DOS-based computers, available since early 1990’s (FLUME 3.0). Joint effort of ARS and ILRI.

• WinFlume program written in Visual Basic for Windows 95/NT environment. This is an upgrade of FLUME 3.0, and is a joint effort of Reclamation and ARS, with programming work being performed by Reclamation.

WinFlume Usability Improvements

• Designed for Windows 95 / Windows NT– Graphical user interface

• Freedom from DOS memory limitations of FLUME 3.0

• Supports all Windows-compatible printers and plotters

• New *.FLM file format simplifies transfer of flume designs among users

– Backward compatible (loads FLUME 3.0 designs)

• Improved plotting of discharge curves

• Output to printer, file-on-disk, or system clipboard

• Online help system

New Hydraulic Analysis & Design Features

• New design module simplifies algorithms used to find an acceptable flume design…solves problem of the software not finding acceptable design in some complex cases

• Additional parameters in rating tables– (e.g., actual tailwater and submergence ratio at all points in rating

table)

• Direct interrogation of rating tables to determine applicable range of hydraulic errors or warnings

UKhKQ 211 Q K h U 1 1

New Output Features• Full-scale plotted wall gauges

– Can be provided to fabricator for construction of durable wall gauges graduated in head or discharge units, adjusted for installation on sloped canal bank

– On-screen preview of finished wall gauge– Plotted continuously on roll-feed plotters, or in sections with match marks on

page-type printers (e.g. laser printers)

• Ditchrider’s rating table

• Alternative equation forms in curve-fitting module:

Calibrating an Existing Structure

• Define geometry of canal and flume• Provide hydraulic data and other properties

– Construction material

– Tailwater conditions

• Generate output– Rating tables and curves

– Curve-fit equation for data logger

– Wall gage data and/or plot

Designing a New Structure

• Define canal geometry and initial flume control section shape

• Provide hydraulic data, canal/flume properties, design requirements– Construction material

– Tailwater conditions

– Water level measurement method and required flow measurement accuracy

– Required freeboard in upstream channel

• Evaluate designs and choose final design

• Generate output

Design Criteria, Methods, Objectives

• Control Section Adjustment Methods– Raise sill height– Raise entire throat section– Raise inner section of throat (used with complex cross-

sections)– Adjust lateral contraction by changing bottom width of

throat section or changing diameter or focal distance of circular or parabolic sections (only option for movable-crest flumes)

• Head Loss Objectives– Minimum head loss– Maximum head loss– Intermediate head loss– Match head loss to drop in

canal invert at site

• Primary Design Criteria (4)– Maintain necessary freeboard at maximum flow– Maintain modular (critical) flow at minimum and maximum flow (ensure free flow)– Froude number at maximum flow must be less than 0.5

• Secondary Design Criteria (2)– Control section must produce sufficient head to provide a user-specified level of

discharge-measurement precision at both minimum and maximum flow, based on the precision of the device used to measure upstream water level.

Control Section Adjustment Methods

WinFlume’s Design Procedure

• User chooses one of the four methods of contraction change, and an increment at which to evaluate designs (e.g. evaluate designs at sill height increments of 0.1 ft).

• WinFlume brackets the range of possible designs by evaluating flume performance at the maximum design flow:

– The maximum possible throat-section contraction is that needed to produce a maximum upstream water level equal to channel depth.

– The minimum contraction is that which produces an upstream Froude number of 0.5 or less at maximum discharge, and an upstream water level that is at least as high as the downstream tailwater at maximum discharge.

• WinFlume builds and evaluates designs of “virtual” flumes between the lower and upper contraction limits at the interval specified by the user.

Design Procedure (continued)• Results are presented to the user, who may choose to accept one of the designs

or discard the results of the analysis.– Only designs meeting the four primary design criteria (freeboard, Froude

number, no submergence at minimum and maximum flow) are presented to the user, unless there are no acceptable designs.

– Designs that meet the four primary criteria, but do not meet measurement precision requirements may be improved by the user by specifying a better water level measurement method.

• Acceptable designs that have minimum head loss, maximum head loss, intermediate head loss, or head loss matching the bed drop at the site are listed in the output, along with any other acceptable designs. The user can evaluate the different head loss characteristics of the possible designs and then choose the design that best meets their needs.

If An Acceptable Design Is Not Found On The First Trial

• If contraction increment is too large, or if design criteria are too limiting, no acceptable design may be found.

• WinFlume searches for two adjacent designs for which the unsatisfied criteria in each design are satisfied in the adjacent design.– An acceptable design may exist between those two designs

– Analysis is repeated using a smaller increment of contraction change within that range.

• If no region of acceptable designs is found, then the results of the analysis are presented to the user, with suggestions for how to relax the design criteria or change the initial design so that an acceptable design can be found.

HOW TO OBTAIN WINFLUME• The WinFlume program is available on the World Wide Web at:

http://www.usbr.gov/wrrl/winflume

• There are 16-bit and 32-bit versions available that are appropriate for Windows 3.1x, Windows 95, Windows 98, and Windows NT systems.

This work has been funded by the U.S. Bureau of Reclamation’s Water Conservation Field Services Program.

Off-season drainage• For most irrigation-system flumes, flow through a

2-inch or smaller pipe is negligible compared to flow over structure

• Larger drain pipes can be plugged with rags or closed with valves on the downstream side while operating

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Head loss across structure, ft

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4 inches

3 inches

2.5 inches

2 inches

1.5 inches

1.25 inches

1 inches

0.75 inches

Approximate flow through flume drain pipeAssumes drain pipe length is 50 times diameter