Graphic Estimation of Peak Flow Reduction in Reservoirs

A common problem in engineering hydrology is estimating the attenuation effects of storage reservoirs on peak inflow rates. While this may always be accomplished by a detailed hydrologic or hydraulic routing of the inflow hydrograph, approximate solutions are often desirable. A simple graphic procedure is developed to permit a direct determination of peak outflow as a function of parameters describing the inflow hydrograph, storage, and outlet characteristics. This procedure is based on the use of a dimensionless two-parameter inflow hydrograph and the solution of a nonlinear differential routing equation with reservoir storage expressed as an exponential function of outflow. Curves obtained from the results of the complete hydrograph routing indicate that the attenuation can be related to a dimensionless routing number, which in turn is defined by the reservoir and inflow parameters. The approach is judged to provide an accurate solution for routing problems that satisfy the methodology assumptions and permits a rapid evaluation of the effects of parameter changes on attenuation.