Extreme flood estimation for the Onkaparinga River Catchment

Abstract

The methods used for determining extreme floods were critically examined in an attempt to reduce the large uncertainties associated with their estimation. The Onkaparinga River Catchment and the Mt Bold Dam in South Australia were used in the analysis. The sensitivity of the probable maximum flood (PMF) estimate to the choice of various parameters was tested. It was found that the PMF estimate was particularly sensitive to the model non-linearity and the choice of uniform or spatially varying rainfall. The RORB model was used to determine the PMF from an estimate of the probable maximum precipitation (PMP). The model was calibrated using recorded events by the method of sensitivity fining. This method considers the errors associated with a choice of model parameters for a particular size flood. The optimal choice of model parameters was used in the calculation of the PMF. The inflow frequency curve for Mt Bold Reservoir was determined from an extended data set derived from modified downstream records prior to the construction of the dam. The outflow frequency curve was calculated considering the joint probability of inflow and initial reservoir level. In order to rout the floods through the reservoir, a new spillway rating curve for Mt Bold was developed. The best estimate of the outflow PMF for the Mt Bold Reservoir was 9,300 m3/sec from a 4 hour duration PMP. The inflow PMF was determined to be 10,200 m3/sec from a 3 hour duration PMP. Both high and low bounds of a reasonable estimate of the PMF were calculated. The high and low bounds of the inflow PMF were l2,4AO m3/sec and 5,600 m3/sec respectively. The high and low bounds of the outflow PMF were 10,800 m3/sec and 5,200 m3/sec respectively.