2019 – Estimating Extreme Rainfall Probabilities for Large Catchments in Northern Australia

S. Lang, R. Nathan, D. O’Shea1, M. Scorah, J. Zhang, G. Kuczera, M. Schaefer

If a risk-based approach is used to assess the spillway adequacy for large dams, then an estimate of the annual exceedance probability (AEP) of extreme rainfalls is required up to and beyond the Probable Maximum Precipitation (PMP). This paper describes how two site-specific approaches described by Nathan et al. (2015; 2016) were used to estimate the AEP of extreme rainfalls for seven catchments, ranging from 1300 km2 to 114,000 km2, in the northern-coastal region of Australia. The results are then compared with the regionally-based Laurenson and Kuczera (1999) relationship for estimating the AEP of the PMP, which is recommended by the Australian Rainfall and Runoff 2019 guide to flood estimation (Nathan and Weinmann, 2019). This shows that the site-specific assessments have produced a rarer estimate of the AEP of the PMP compared with Laurenson and Kuczera (1999), particularly for the catchments >10,000 km2. For some of these locations, this has allowed the dam owners to plan risk-based upgrades with more confidence.

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