Holocene palaeohydrology of the East Alligator River, for application to mine site rehabilitation, Northern Australia

Abstract As a contribution to the estimation of extreme floods and rainfall, palaeofloods in the East Alligator River in tropical Australia were examined to derive estimates of palaeodischarges and their frequency. Nine extreme floods have occurred over 8400 years in a non-stationary series, the youngest five of which are stationary, and the youngest three of which occurred during wet periods produced by variations of the Southern Oscillation Index (SOI) modulated by the Pacific Decadal Oscillation (PDO). The magnitudes of these floods are similar, with one possible exception (in 2007 CE). The extreme floods in the region lie close to, or above, the Australian flood envelope curve showing that floods larger than the expected are possible. Their magnitudes also approximate the Probable Maximum Flood (PMF) but with a much higher frequency than expected. The annual exceedance probability (AEP) of the palaeofloods is 0.3%, which may also be the AEP for the rainfall. These results will be used as input to the design of an artificial landform at the Ranger uranium mine, within the East Alligator catchment, that has to last for 10,000 years. The 2007 CE flood was probably the most extreme flood (and associated rainfall) likely to occur in the vicinity of Ranger under the current climate and will occur in the future along with climate change. Because of the uncertainties associated with projections of future extremes the best design rainfall AEP for the Ranger landform is 0.3%.