The simplified methods of evaluating detention storage volume for small catchment

Massive land development reduces the plant-cover area and increases the impervious area of watershed, which induces downstream flooding. A detention pond stores runoff from watershed and releases the stored runoff slowly to reduce the flood threat in the downstream area. While the runoff hydrograph is irregular, several researches verify that the runoff hydrograph can be represented by simple geometrical-shaped (such as triangular or trapezoidal) hydrographs. This study collects and develops simplified detention volume design models. The study also develops a suitable hydrological condition and the calculation method of detention volume for various models. A real world case is used to demonstrate the calculation procedure used in the detention pond design. The case shows that the combination of triangular inflow and triangular outflow produces a maximum detention volume, which implies a large reduction of peak flow needs greater detention volume. The combination of trapezoidal inflow and triangular outflow produces a second large detention volume. It implies a long duration of inflow hydrograph (IH) needs greater detention volume. A combination of triangular inflow and trapezoidal outflow results in a minimum detention volume, which indicates that either the storm water drains completely before the permissible outflow occurs, or storm water is kept from entering the detention pond before the inflow rate reaches the permissible outflow. In either case, the detention pond has more space to accommodate the flood inflow so as to reduce the rate of peak outflow.