Direct rainfall flood modelling: The good, the bad and the ugly

Abstract

The direct rainfall technique for flood modelling is relatively new to the industry, and is treated with some suspicion. The approach involves the application of direct rainfall to all cells in a 2D model domain, and runoff is then routed within the hydraulic model. Advantages of this approach include the facilitation of cross catchment flows, a high definition of flow behaviour in catchments, and the approach can replace the need for hydrological models within the 2D model domain. Complications can occur when applying the direct rainfall technique, leading to problems including mathematical instabilities, unrealistic flow responses and large model errors. Issues are generally associated with model losses, run-times, grid issues and very shallow flow. The direct rainfall technique was used to develop a flood model in the Serpentine area on the Swan Coastal Plain in south-west Western Australia (approximately 20km south of Perth). The approach was suited to the project, as the study area has an extremely flat terrain traversed by drains of various sizes where cross catchment flows are common; there is little information or literature on hydrological model parameterisation in the region; and the project required floodplain mapping and flood flows at many locations. The project successfully produced a fit-for-purpose flood model which was used for design flood simulations, floodplain mapping and management scenario assessment, demonstrating the suitability of the direct rainfall technique for this project. Due to the problems associated with the approach, a series of checks and quality assurance procedures need to be accounted for in project planning if direct rainfall modelling is undertaken. This paper explores the pros and cons of the direct rainfall approach through the processes of model construction, a comprehensive model calibration, predictive scenario simulations, and a series of checks including mass balances, sensitivity analysis and comparison to other modelling techniques.