Overcoming the joint probability problem associated with initial loss estimation in desgn flood estimation

Abstract

Design flood estimation techniques currently used in Australia only partially account for the joint probability interactions between flood producing variables. The joint probability problem associated with initial loss estimation is particularly important. Failure to deal with it can introduce considerable bias into estimated flood frequency distributions for ungauged catchments. One solution to this problem is to use continuous simulation whereby a rainfall generation model is linked to a catchment water balance and flood routing model. This technique has been used to explore less complicated ways of overcoming the joint probability problem by allowing design rainfall obtained from Australian Rainfall and Runoff (ARR) to be directly converted into rainfall excess. This eliminates the need for assumptions regarding initial loss. A continuous simulation approach has been utilised here by means of a stochastic point rainfall model, coupled with a stochastic evaporation model and modified AWBM model. This enables the determination of the scaling required for the conversion of rainfall into rainfall excess. The technique is validated using a kinematic wave catchment runoff model and comparison with observed flood frequency curves. Through this method, the simplicity of the design flood estimation approach is retained by the use of rainfall excess frequency duration (REFD) curves instead of rainfall IFD curves. The REFD method has been calibrated and tested on a number of catchments in Australia. Presented here are the results from the Boggy Creek catchment in Victoria. The scaling of the ARR design rainfall required has been very similar across all average recurrence intervals and event durations. This should allow the approach to be extended to other catchments.