A case study on urban flood modelling in Sydney using DRAINS and HEC-RAS

Flood mapping is a crucial element of flood risk management in both rural and urban development projects. In small and ungauged basins, empirical and regionalisation approaches are often adopted to estimate design flood hydrographs that represent input data into a hydraulic model. The limitations of observed runoff data in urban catchments in Australia present a major challenge with respect to direct model calibration and verification. This paper presents a case study on flood risk assessment in a small urban development project in Sydney, Australia. For this purpose, DRAINS and HEC-RAS models are adopted. It is noted that in Australia a more holistic approach of flood modelling (e.g. Monte Carlo simulation) is advocated in the recent edition of Australian Rainfall and Runoff (ARR), the national guide. However, there is limited data availability in applying such holistic approaches, Outcomes provided from the HEC-RAS modelling of this case study will provide the estimated level of flood or flood height for that particular site according to the proposed plans. Flood level has been based on the 1:100-year ARJ storm events. A minimum Finished Floor Level for the proposed development is reached with the addition of the minimum free board provided from this local council.