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Section 6: Pond Routing Routing.pdfRouting Introduction What is a pond routing? PWhat role does a...
Transcript of Section 6: Pond Routing Routing.pdfRouting Introduction What is a pond routing? PWhat role does a...
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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