ABSTRACT
Stormwater is the second major urban flow of concern to the drainage engineer. Stormwater is generated by precipitation, typically rainfall, and consists of that proportion that runs off from urban surfaces. This chapter focuses on the generation and characteristics of stormwater. It explains the mechanisms of runoff generation, and discusses the overland flow processes. The transformation of a rainfall hyetograph into a surface runoff hydrograph involves two principal parts. First, losses due to interception, depression storage, infiltration, and evapo-transpiration are deducted from the rainfall. Second, the resulting effective rainfall is transformed by surface routing into an overland flow hydrograph. The urban catchment wetness index (UCWI) represents the degree of wetness of the catchment at the start of a storm event. Once the losses from the catchment have been accounted for, the effective rainfall hyetograph can be transformed into a surface runoff hydrograph—a process known as overland flow or surface routing.
