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DES 606 : Watershed Modeling with HEC-HMS Module 12 Theodore G. Cleveland, Ph.D., P.E 29 July 2011

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Detention Modeling Detention (ponds) are probably the most common management practice for stormwater quality enhancement and for peak discharge reduction. In the context of HEC-HMS they (detention) are usually appropriately modeled as small reservoirs –External stage-discharge relations –Internal (to HMS) regulation features

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Detention Modeling With some creativity, a roadway itself could be modeled as a reservoir (assuming there is some kind of curb structure to actually store water) –Advanced concept, beyond scope of this training course. –Point is to consider the processes rather than actual physical features when presented a challenging modeling problem.

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Sizing Detention Detention facility sizing –Dictated by desired water outflow rate (peak discharge reduction) –Dictated by desired particle size control (water quality enhancement) Water quality is beyond scope of this training course. Point is that quality and quantity are interdependent, analysis will sometimes use common tools

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Detention Routing Detention “hydraulics” –HEC-HMS is a hydrologic model, so we really are approximating hydraulics with hydrologic concepts –In particular, reservoir routing (also called level-pool routing) is common in detention analysis

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Data Requirements Detention “hydraulics” data requirements include one of: –Stage-discharge table –Stage-area table Detention reservoirs also need some kind of initial condition –Dry –Starting stage

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External vs. Internal Methods Externally developed tables –Consider detention facility geometry to develop stage- storage tables Use cut-and-fill type computations from topographic data to develop stage-storage tables. Straightforward extension of watershed area calculation –Consider outlet works for stage-discharge tables Decide if the detention facility completely drains (probably through some kind of low-flow orifice) or stores some “permanent” water.

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Outlet Works Typical outlet works (unregulated) –Rectangular weirs –V-notch weirs –Orifice (short pipe) –Combinations Combination – Rectangular weir for large overflow Orifice to drain facility

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Typical Rating Curves Rectangular weir –Discharge vs. Stage Read points from the chart to build a table for import into HMS Use the “equation” on the curve to build a table. Can have multiple weirs that activate at different pond depths – assume superposition applies for total discharge

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Typical Rating Curves V-notch weir –Discharge vs. Stage Read points from the chart to build a table for import into HMS Use the “equation” on the curve to build a table. Can have multiple weirs that activate at different pond depths – assume superposition applies for total discharge

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Typical Rating Curves Orifice –Discharge vs. Stage ASSUMES orifice is completely “full” Note sizes tend to be small – these are generally to drain a pond Drawdown times determine size and count (typical 24 hour)

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Stage-Storage-Discharge Once the stage-storage and stage- discharge are known for a detention facility –Treated as a reservoir in HMS –Enter the tables as “paired data”

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External Tables Consider some hypothetical detention facility with the Stage-Area-Storage- Discharge –Route a design storm through this detention facility to examine the effect on peak discharge attenuation

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External Tables 2-ways –Externally build the paried tables –Internally using the regulation devices (in HMS) in reservior elements Anticipate close if not identical results

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In HEC HMS Build a generic model –Sub-basin and a reservoir Sub-basin AREA = 10 ac. Sub-basin NO LOSS Sub-basin NO TRANSFORM –Sub-basin is to generate runoff –Reservoir is our detention facility

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Paired-Data Enter by paired-data manager the various tables –Stage-Discharge –Stage-Area –Stage-Storage

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Design Storm Enter some kind of design storm (or use a hyetograph) –Just made up a storm for this lecture –Total depth is 2-inches in 3 hours –Anticipate 20 cfs peak discharge at most.

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Peak Reduction Enter some kind of design storm (or use a hyetograph) –Estimate a pretty decent peak discharge reduction –However the detention facility has nearly 2 acre- feet of storage for 10 acres of contribution watershed.

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Modify Outlet Modify the outlet from a V-notch to a rectangular weir –HMS does not have V- notch weirs built-in –OK, now repeat using reservoir regulation devices

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Internal Method Method: Outflow structures –Note we still need a stage-area curve!

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Internal Method -- Spillway Method: Outflow structures –Set initial elevation –Set spillway count to one (1)

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Spillway Set spillway properties –Type –Length (width) –Weir coefficient Run model Results appear identical

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What if? Simulations Now we have a model (of the detention) facility –How many acres of contribution watershed could it really support for such a design storm? –Here is the power of the simulation, systematically adjust the area (of the sub-basin) until we break the detention facility 20 acres OK!

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What if? Simulations Now we have a model (of the detention) facility –How many acres of contribution watershed could it really support for such a design storm? –Here is the power of the simulation, systematically adjust the area (of the sub-basin) until we break the detention facility 40 acres OK!

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What if? Simulations Now we have a model (of the detention) facility –How many acres of contribution watershed could it really support for such a design storm? –Here is the power of the simulation, systematically adjust the area (of the sub-basin) until we break the detention facility 80 acres ??? Close to max fill depth

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What if? Simulations Now we have a model (of the detention) facility –How many acres of contribution watershed could it really support for such a design storm? –Here is the power of the simulation, systematically adjust the area (of the sub-basin) until we break the detention facility 100 acres ??? max fill depth

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What if? Simulations Now we have a model (of the detention) facility –How many acres of contribution watershed could it really support for such a design storm? –Here is the power of the simulation, systematically adjust the area (of the sub-basin) until we break the detention facility 110 acres ??? Note the error – broke the “model”

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What if? Simulations Now we have a model (of the detention) facility –How many acres of contribution watershed could it really support for such a design storm? –Based on our analysis about 100 acres for the particular design storm! –But also notice the discharges – our downstream “channel” would need to be able to handle the discharge 110 acres ??? Note the error – broke the “model”

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Summary Discussed use of detention to reduce peak discharge magnitude Illustrated how to model using externally developed tables Illustrated how to model using reservoir “regulation” devices in HMS –Note that even built-in components need some external tables

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