ABSTRACT
Storm sewers, street gutters, and roadside ditches are major facilities for collecting and delivering stormwater out of urban areas. Since these stormwater conveyance systems are designed to pass peak runoff rates, the Rational method has been the most widely used approach. In this chapter, the tributary watershed is analyzed as a system that is subject to the design rainfall as the inputs and produces a runoff hydrograph as the outputs. The design rainfall distribution can be a uniform or non-uniform distribution. Using the moving average procedure, the Rational method is expanded from a peak runoff prediction method under a uniform rainfall distribution to a hydrograph prediction method under a non-uniform rainfall distribution. A moving average procedure can generate a complete hydrograph with a peak flow consistent with the Rational method. With this modification, the conventional Rational method is employed as a peak-flow prediction method for stormwater conveyance designs, and it can be expanded into a hydrograph prediction method wherever a stormwater storage facility needs to be sized in the drainage system.
