Flood risk in the coastal zone - the importance of understanding drivers of hydroclimatic variability for robust urban and coastal planning

Previous work has established that the risk of climate related emergencies (e.g. floods, droughts, bushfires etc.) in Australia, and many other parts of the world, is non-stationary. That is, the chance of an extreme climatic event occurring is not the same from one year to the next and is in fact dependent on the state of the various climate phenomena that are responsible for Australia's hydroclimatic variability. This previous work demonstrated how, on average for New South Wales, the probability of a flood occurring that is equal in magnitude to the '1 in 100 year flood' is about five times greater during La Nina events compared to all other years and twelve times greater during a La Nina event that occurs during the negative phase of the Inter-decadal Pacific Oscillation compared to all other years. This work has recently been extended to focus specifically on urban coastal areas where it has been found that the non-stationarity of flood risk is even further enhanced when compared to the noncoastal catchments. Also investigated is whether this non-stationarity of flood risk is due to non-stationarity of antecedent conditions or non-stationarity of extreme daily and sub-daily rainfall events, with the finding being that both are important. This is contrary to recent studies that claim there is no evidence of non-stationarity in extreme daily and sub-daily rainfall across Australia. The implications of these results are significant given the large populations and infrastructure investment along the eastern seaboard and also timely given current updates to Engineers Australia's "Australian Rainfall and Runoff: A Guide to Flood Estimation", the standard for flood estimation in Australia.