Development of appropriate synthetic design storms for small catchments in Gauteng, South Africa

Abstract

Urban stormwater drainage networks are frequently analysed in dynamic rainfall-runoff simulation models. These models use hypothetical rainstorm events (synthetic design storms) as input in the case of single event-based modelling. A significant number of methods to generate synthetic design storms are described in the literature. However, due to the abundance of methods some engineers are likely to base their method choice on familiarity with a method and preference. This could lead to the selection of an inappropriate synthetic design storm that will generate unrealistic peak discharge results. Therefore, the need to develop appropriate synthetic design storms applicable to single event-based modelling of small urban catchments in South Africa was identified. The aim of this study was to test the performance of the existing methods, and to identify the method, or methods, best suited for single event-based modelling of small urban catchments in the selected pilot study area. The completeness of the data was assessed, at-site design rainfall was determined, storm events were identified and analysed to obtain the general storm parameters, and synthetic design storms were generated and compared with the observed rainfall mass curves. The performance was evaluated based on the shape of the storm and the intensity, whereas the peak discharge and the runoff volume was evaluated using dynamic rainfall-runoff simulation models. The Mean Absolute Relative Error (MARE) was used as a measure to determine the Goodness-of-Fit (GOF) of the data. It was concluded that the Chicago Design Storm and SCS-SA curves are most suited for single event-based models. Improvements to the Chicago Design Storm and SCS-SA curves are proposed to better simulate design rainfall events and guidance is provided for further refinement.