ABSTRACT
The rainfall and runoff process through a watershed depends on the nature of the drainage network. Urban catchments are often classified as "small watershed" because their tributary areas are small and highly impervious. This chapter applies the concept of system to a small watershed. It leads to the development of the rational hydrograph method (RHM) using the time of concentration as the system memory and the runoff coefficient as the system parameter. The rational method is a simplified kinematic wave approach for peak flow estimation. The major variables in the rational method are rainfall intensity, watershed tributary area, and runoff coefficient. For the purpose of rainfall–runoff designs, the rational method requires intensity–duration–frequency (IDF) information. A lumped system represents a cascading flow process that drains stormwater generated from the upstream impervious areas onto the downstream pervious areas. Hydrographs predicted by the RHM are comparable with sophisticated models such as CUHP, SWMM, and HEC-1 models.
