Environmental Engineering Reference
In-Depth Information
Guidelines: Volume Credits for Detention Routing
(For projects required to demonstrate peak rate control / extended detention for
1-year through 100-year storm events)
The utilization of volume reduction BMPs as part of either CG-1 or CG-2 will obviously reduce
the amount of storage required for peak rate mitigation because less runoff is ultimately
discharged. As quantifying the peak rate mitigation benefits of volume control BMPs can be
difficult and cumbersome with common stormwater models, applicants are strongly
encouraged to use these or other approaches to give credit for volume control when
demonstrating peak rate control:
1) Volume Diversion. Many computers models have components that allow a
"diversion" or "abstraction". The total volume reduction provided by the applicable
structural and non-structural BMPs can be diverted or abstracted from the modeled
runoff before it is routed to the detention system(s). This approach is very conservative
because it does not give any credut to the increased time of travel, ongoing infiltration,
etc. associated with the BMPs. Incorporating the CG-2 capture volume in the model
reduces the required detention storage by about 22% for this example.
2) Composite BMPs. For optimal stormwater management, this manual suggests
widely distributed BMPs for volume, rate, and quality control. This approach, however,
can be very cumbersome to evaluate in detail with common computer models. To
facilitate modeling, similar types of BMPs can be combined within the model. For
modeling purposes, the storage of the combined BMP is simply the sum of the BMP
capacities that it represents. A stage-storage-discharge relationship can be developed
for the combined BMP based on the configuration of the individual systems. The
combined BMPs can then be routed normally and the results submitted.
3) Travel Time/ Time of Concentration Adjustment. The use of widely-distributed,
volume-reducing BMPs can significantly increase the post-development runoff travel
time and therefore decrease the peak rate of discharge. The Delaware Urban Runoff
Management Model (DURMM) calculates the extended travel time through storage
elements, even at flooded depths, to adjust peak flow rates (Lucas, 2001). The extended
travel time is essentially the residence time of the storage elements, found by dividing
the total storage by the 2-year peak flow rate. This increased travel timecan be added to
the time of concentration of the area to account for the slowing effect of the
volume-reducing BMPs. This can reduce the amount of detention storage required for
peak rate control. The detention storage requirements are reduced by almost 40% by
the CG-2 capture volume.
4) Other Methods. Other methods, such as adjusting runoff curve numbers based on
the runoff volume left after BMP application, or reducing net precipitation based on the
volume captured, can be applied as appropriate.
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