Scaling Issues in Design Flood Estimation for Ungauged Catchments: A Case Study for Eastern Australia

Design flood estimation in smaller ungauged catchments is often required in hydrologic design of water infrastructures such as bridge, culverts and water retention weirs. Regional flood estimation methods, which are generally developed using data from medium to large catchments, are often applied to very small catchments. The question often remains unresolved whether these regional flood estimation methods are applicable to very small catchments since independent testing cannot be undertaken in most situations due to absence of recorded streamflow data for these very small sized catchments. This paper uses data from 429 catchments in eastern states of Australia, which are grouped into a number of class intervals based on catchment size. It has been found that smaller catchments generally produce a larger unit area runoff. A method is developed to account for this incremental unit runoff in the developed regional flood prediction equations.