Graphical Sizing of Small Single-Outlet Detention Basins in the Semiarid Southwest

Abstract

A graphical procedure is presented for sizing single-outlet detention basins to control stormwater runoff from small catchments in the semiarid southwestern United States. The approach relies on the modified rational method to provide the catchment runoff hydrograph, a linearized relation between basin water depth and storage volume, and a rainfall-intensity-duration equation for which coefficients can be found easily for any location covered by NOAA Atlas 14, Volume 1 (Arizona, Southeast California, Nevada, New Mexico, and Utah). Numerical solutions of a dimensionless form of the differential detention storage continuity equation are carried out, and solution values are related to a dimensionless discharge coefficient that provides a comprehensive characterization of catchment hydrology, outlet structure type and size, and regional precipitation. Graphs that supply dimensionless values of the maximum peak outflow rate from a basin, the maximum storage volume, and the critical storm duration (that is, the storm duration that produces the peak outflow and maximum storage volume) are prepared from the numerical solutions. The method avoids repetitive calculations and provides accurate solutions rapidly in an uncomplicated way for small catchments of about 12 ha or less with times of concentration less than about 30 min.