Selecting appropriate synthetic hyetographs for use in SWMM to model runoff from discretised catchments in South Africa

A perceived challenge when using SWMM is the discretisation of a large catchment into numerous sub catchments and the implication this has on the selection of an appropriate synthetic design hyetograph. The design hyetograph represents the temporal distribution of rainfall intensity associated with a return period. The choice of the design hyetograph will have a significant influence on the shape and peak value of the hydrograph. In a SWMM model with numerous discretised sub-catchments, there is no single critical storm but a range of these, each associated with any one of the total number of discretised sub catchments. Also, SWMM does not prescribe the input hy-etograph which could be a synthesized rectangular or triangular hyetograph, the SCS type design storm hyetographs, actual recorded precipitation events or any other custom hyetograph. It is left to the user to ascertain the appropriateness of the input storm event for the design purpose. In this paper the literature on the impact that catchment and model discretisation have on the predicted flood peaks is reviewed. Procedures for selecting and synthesizing appropriate design hyetographs are then discussed, providing a framework for selecting storm durations which represent the spectrum of possible critical storms. These are dependent on the levels of model and catchment discretisation for the study area de-fined. The framework is then illustrated by means of an example, where the peak discharges and volumes for a rural discretised catchment in Mpumalanga, South Africa, are calculated.