Launceston, TAS The Impact of Embedded Design Storms on Flows Within a

Abstract: Previous investigations into design flood estimation, found that the AR&R burst based procedure can under-predict discharges at a catchment’s outlet, relative to those obtained from an embedded design storm (EDS) approach. This under-prediction was quite significant in some catchments (up to 40%), causing concern as to the ability of the present AR&R burst based procedure to adequately simulate design discharges from ‘storage sensitive ’ catchments. Application of the EDS procedure in recent studies further suggested that the AR&R burst based procedure could result in even greater levels of under-prediction at internal locations within a catchment. An investigation into these internal flows was therefore undertaken to quantify and better understand the differences between AR&R burst based discharges and EDS discharges within a catchment. This paper reports on the findings of this investigation. A series of catchments located in Wollongong, ranging in size from 0.5 km2 to 105 km2, were used to explore these impacts. All catchments were relatively complex partly urbanized catchments, with a number of storages and diversions present in each. WBNM was used to simulate the hydrologic response of each catchment. All catchments modelled with a burst responded with decreased internal discharges (relative to an EDS approach), at times less than half that of the EDS based discharge. In each catchment investigated, the difference between the EDS and AR&R discharges were much higher internally than at the outlet. This work reinforces, in the authors view, the inability of the AR&R burst based procedure to properly represent design flows in storage sensitive catchments, and their earlier conclusion that it is time to replace the present burst based procedure for design flood estimation with a storm based procedure, such as that provided by an embedded design storm