The design flood under two approaches: synthetic storm hyetograph and observed storm hyetograph

This paper evaluates the design peak flow for several return periods using two approaches: 1) a flood frequency analysis; 2) rainfall-runoff model applied to a design storm. The design storm originates from the design rainfall depth, which is distributed throughout the rainfall duration via synthetic hyetographs. In the flood frequency analysis, the design peak flow was obtained from a 30-year series of flows estimated from rainfall observations at a five-minute time step. The design storm was routed with the same rainfall-runoff model and parameters to yield the design peak flow. The hyetographs tested were categorized as having advanced, delayed, alternating, uniform and empirical blocks that were built from rainfall observations. All design storms analysed undersized the hydraulic structures. The results, which were valid for the study region, indicated the need to rethink the concept of design storms, which associate the design storm depth with synthetic hyetographs, for a conservative standard.