Stormwater Runoff Detention Basin Storage Volume
Compute storage volume for detention basin to attenuate peak discharge from a design storm. TR-55 method.
Stormwater storage volume calculation is mobile-device-friendly as of August 1, 2015
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Units: cm=centimeter, ft=foot, gal=U.S. gallon, gpm = U.S. gallon per minute, km=kilometer, m=meter, min=minute, s=second.
Detention basin storage can be incorporated into developments to attenuate (reduce) the peak stormwater discharge. For example, say a city requires the 25-yr, 24-hr storm to be the basis for design. Prior to development, the peak discharge from this storm is, say, 150 cfs (ft3/s) at a specified location, and the peak discharge due to development is predicted to be, say, 300 cfs at the same location. The city won't approve the project unless the developer incorporates enough detention storage to reduce the predicted stormwater peak discharge to the pre-development flow of 150 cfs at the specified location. The engineer can use our calculation to determine the detention basin storage volume required to attenuate the peak discharge from 300 to 150 cfs. The storage volume can then be implemented as a single pond with that volume or several ponds, basins, or depressions that add up to the required volume. The ponds/basins/depressions must go dry between storm events and should be located just upstream of the specified location. (To compute stormwater runoff depth and pre- and post-development peak discharge, use our SCS TR-55 Peak Discharge Calculator or Rational Method Peak Discharge Calculator.)
Our stormwater detention storage basin volume calculation is based on methodology presented in Technical Release 55, Chapter 6 (SCS, 1986), of the USA Soil Conservation Service (now called the Natural Resources Conservation Service, NRCS), division of the USDA (USA Department of Agriculture). The NRCS has worked for decades developing equations and conducting experiments to determine reliable models for predicting stormwater runoff storage volume for detention basins to reduce peak discharge from storm events.
We have made the calculation useful for the international community by
allowing a variety of units. Unfortunately, TR-55 only presents rainfall distribution maps for the USA.
Therefore, non-USA users need to determine whether a typical 24-hr rainfall resembles a Type I or IA or Type II or III distribution.
Equations (SCS, 1986)
Vr = QA Vs = Vr (Vs / Vr) qo = qi (qo / qi)
where (L=Length units, T=time units): A = watershed area [L2]. Q = Runoff Depth [L]. q i = Inflow, or post-development peak discharge upstream of detention ponds [L3/T]. q o = Outflow, or post-development peak discharge downstream of detention ponds; often the pre-development peak discharge is used as this value [L3/T]. Vr = Runoff Volume [L3]. Vs = Detention Storage Volume [L3].
SCS (1986) provides equations and a graph for Vs / Vr vs. qo / qi . Our calculation uses the equations, but solves them backwards when solving for qo.
Vs / Vr = C0 + C1
(qo / qi) + C2 (qo / qi)2
+ C3 (qo / qi)3
Error Messages given by calculation
"Need 0.1 < qo / qi < 0.8." Vs not computed. The calculation is only valid for qo / qi in the proper range.
"Vs / Vr out of range."
qo not computed. The calculation is only valid for Vs / Vr
in the proper range as indicated in the above figure, depending on the rainfall
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