Preliminary Sizing of Detention Reservoirs to Reduce Peak Discharges

Some simple relationships derived from a generalized reservoir‐routing analysis provide a sound basis for the preliminary sizing of stormwater detention reservoirs. The analysis indicates that the ratio of required storage to flood volume (the flood‐storage factor) is determined primarily by the ratio of peak outflow to peak inflow (the peak‐discharge factor), the type of outlet, and the shape of the inflow hydrograph. A submerged outlet is more efficient than an overflow outlet in that considerably less detention storage is needed to achieve the same reduction in peak discharge for the same inflow hydrograph. Two algebraic formulas approximate the relationships between the flood‐storage and peak‐discharge factors for detention reservoirs with submerged outlets and overflow outlets. Use of the graphical relationships of U.S. Soil Conservation Service leads to underestimation of detention‐storage requirements.