Direct Rainfall Flood Modelling: The Good, the Bad and the Ugly

Abstract

The direct rainfall method (DRM) for flood modelling involves the application of rainfall to all cells in a 2D model, and runoff is 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 requirement for hydrological models within the 2D model domain. Complications can occur when applying the DRM which can lead to unrealistic flow responses and large model errors. Issues are generally associated with model losses, run-times, grid-scale effects and very shallow flow. The DRM was used to develop a flood model in the Serpentine area on the sandy Swan Coastal Plain in southwest Western Australia (approximately 20 km south of Perth). The project demonstrates the suitability of the DRM for design flood simulations and floodplain mapping. Due to the problems associated with the approach, a series of checks and quality assurance procedures are recommended if DRM flood modelling is undertaken. This paper explores the pros and cons of the DRM through the processes of model construction, a comprehensive model calibration, validation and sensitivity analysis, and a series of checks including mass balances, and comparison to traditional modelling techniques.

Key Words:

• Direct rainfall method
• flood modeling
• hydraulic modeling
• sensitivity
• calibration

Additional information

Notes on contributors

J Hall

Joel Hall is a senior engineer at the Department of Water, Western Australia. He has over 10 years’ experience modelling floods, groundwater systems, surface water systems and contaminant transport. His major interest is in developing robust models to guide management and policy decisions by applying the latest software and technology available. This has included the application of MIKE SHE to model surface-groundwater interactions to guide drainage management plans in southwest Western Australia, direct rainfall methods using MIKE FLOOD to develop floodplain management strategies on the sandy Swan Coastal Plain and the application of eWater Source to develop custom tools for licensing officers to assess farm dam reliability in Western Australia’s southwest. He has a particular interest in model governance, and has developed a series of model guidelines and policy for the Department of Water, as well as being on technical advisory groups for national modelling guidelines.