Stormwater Runoff Detention Basin Storage Volume 
Compute storage volume for detention basin to attenuate peak discharge from a design storm. TR55 method. 

Stormwater storage volume calculation is mobiledevicefriendly as of August 1, 2015 Register to enable "Calculate" button. Units: cm=centimeter, ft=foot, gal=U.S. gallon, gpm = U.S. gallon per minute, km=kilometer, m=meter, min=minute, s=second. Introduction Detention basin storage can be incorporated into developments to attenuate (reduce) the peak stormwater discharge. For example, say a city requires the 25yr, 24hr storm to be the basis for design. Prior to development, the peak discharge from this storm is, say, 150 cfs (ft^{3}/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 predevelopment 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 postdevelopment peak discharge, use our SCS TR55 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, TR55 only presents rainfall distribution maps for the USA.
Therefore, nonUSA users need to determine whether a typical 24hr rainfall resembles a Type I or IA or Type II or III distribution. Equations (SCS, 1986) V_{r} = QA V_{s} = V_{r} (V_{s} / V_{r}) q_{o} = q_{i} (q_{o} / q_{i}) where (L=Length units, T=time units): A = watershed area [L^{2}]. Q = Runoff Depth [L]. q_{ i} = Inflow, or postdevelopment peak discharge upstream of detention ponds [L^{3}/T]. q_{ o} = Outflow, or postdevelopment peak discharge downstream of detention ponds; often the predevelopment peak discharge is used as this value [L^{3}/T]. V_{r} = Runoff Volume [L^{3}]. V_{s} = Detention Storage Volume [L^{3}]. SCS (1986) provides equations and a graph for V_{s} / V_{r} vs. q_{o} / q_{i} . Our calculation uses the equations, but solves them backwards when solving for q_{o}. V_{s} / V_{r} = C_{0} + C_{1}
(q_{o} / q_{i}) + C_{2} (q_{o} / q_{i})^{2}
+ C_{3} (q_{o} / q_{i})^{3} Error Messages given by calculation "Need 0.1 < q_{o} / q_{i} < 0.8." V_{s} not computed. The calculation is only valid for q_{o} / q_{i} in the proper range. "V_{s} / V_{r} out of range."
q_{o} not computed. The calculation is only valid for V_{s} / V_{r}
in the proper range as indicated in the above figure, depending on the rainfall
distribution type.
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