Design flood estimation for South East Queensland

Abstract

This report details the derivation of a Rational Method procedure for the estimation of design peak flows from ungauged, small to medium sized rural catchments within the Brisbane River, Logan River, Pine River and South Coast catchments of South East Queensland. In doing this, the objective is to improve upon the current recommended procedure for Queensland outlined in lAustralian Rainfall and Runoff: A guide to Flood Estimationr (ARaR), which is not based on observed flood data and as such does not meet the requirements generally expected. Flood frequency analysis of 21 gauged catchments with areas of less than 100km2 and more than 20 years of stream flow record, along with rainfall intensities derived using the standard method given in ARaR, were used to evaluate the runoff coefficients for the average recurrence intervals of 1 in 10, 20, 50 and 100 years. A function for determining design runoff coefficients was then derived using easily obtainable catchment characteristics. The procedure developed has been shown to satisfactorily reproduce the observed peak discharges for the 1 in 10, 20 and 50 year average recurrence intervals while the 1 in 100 year peak discharge is of questionable reliability. Discussion of the background research and methodology used, as well as presentation and evaluation of the design procedure are given in the following report. n