Complex versus Simple Stormwater Network Models - Modeling of the Keysers River Catchment
In accordance with their objective of ensuring adequate stormwater drainage and the reduction of flood risk within the Cape Metropolitan Area, the Catchment, Stormwater and River Management Branch of the City of Cape Town commissioned a study to determine floodlines adjacent to the lower reaches of the Keysers River in the South Peninsula.
Information relating to the hydrological characteristics of the catchment, as well as the physical stormwater network, was collected and incorporated into a complex model of the catchment. When runoff from a recorded rainfall event was routed through the model, the simulation output was found to be significantly different from the observed catchment response.
A simplified model, which incorporated the overall physical characteristics and average hydrological parameters of the catchment, was run using the same rainfall event. In contrast to the results from the complex model, the output from the simple model showed a close match with the observed data. By reducing the time of concentration in the simple model, it was possible to produce a similar output to that of the complex model.
The various physical and hydrological parameters in the complex model were re-examined and it was found that the roughness coefficient used in the natural watercourse elements was the parameter with the most significant uncertainty. By increasing this parameter, the complex model response could be adjusted to reflect the observed catchment response.
We conclude from the modeling exercise that a complex system would not necessarily require complex modeling. Also, since the roughness coefficients used in natural channels and floodplains could significantly affect the response of a complex model, these need to be estimated as accurately as possible.
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