Dependence between extreme storm surge and rainfall can have significant implications for flood risk in coastal and estuarine regions. To supplement limited observational records, we use reanalysis surge data from a hydrodynamic model as the basis for dependence mapping, providing information at a resolution of approximately 30 km along the Australian coastline. We evaluated this approach by comparing the dependence estimates from modeled surge to that calculated using historical surge records from 79 tide gauges around Australia. The results show reasonable agreement between the two sets of dependence values, with the exception of lower seasonal variation in the modeled dependence values compared to the observed data, especially at locations where there are multiple processes driving extreme storm surge. This is due to the combined impact of local bathymetry as well as the resolution of the hydrodynamic model and its meteorological inputs. Meteorological drivers were also investigated for different combinations of extreme rainfall and surge—namely rain‐only, surge‐only, and coincident extremes—finding that different synoptic patterns are responsible for each combination. The ability to supplement observational records with high‐resolution modeled surge data enables a much more precise quantification of dependence along the coastline, strengthening the physical basis for assessments of flood risk in coastal regions.

Current climate 1‐in‐100‐year storm tide heights along the coast of Victoria, southeast Australia were estimated by combining probabilities of storm surge and tide heights determined from hydrodynamic modelling. For this return period, levels lie between 1 and 2 m above mean sea level along much of the coastline. Future climate 1‐in‐100‐year storm tide heights were estimated by adding high‐end estimates of future sea‐level rise from recent literature. The effect of climate change through consistent wind‐speed increases was also examined and it was found that, for the late 21st Century, the contribution of wind‐speed increase to the increases in extreme storm surge heights is considerably smaller, by a factor of more than 2, than the contribution of sea‐level rise.A computationally inexpensive approach to assessing current and future vulnerability to coastal inundation due to sea‐level extremes is then demonstrated for the Victorian coast. A simple inundation algorithm was used with high‐resolution terrestrial elevation data from a Light Detection and Ranging (LiDAR) survey of the Victorian coast to evaluate the potential vulnerability of nine coastal regions to inundation by current and future climate 1‐in‐100‐year storm tides. The response of different regions varied from exhibiting proportional increases in inundation to sea‐level rise to nonlinear responses, where the exceedance of critical sea‐level thresholds led to large stepwise increases in land area or number of land parcels affected by inundation. These responses were a function of both coastal topography and the spatial density of land parcels. The low computational cost of the methodology permits different time horizons and uncertainties in future climate change to be considered using a scenario‐based approach and is therefore useful in assessing options for adaptation to climate change. Copyright © 2011 Royal Meteorological Society

Sea levels along Australia’s coastline are influenced by natural climate variability and anthropogenic climate change. Projections of sea-level rise (SLR) for 2090 for the Australian coastline are similar to the global mean sea-level projections. The global and regional projections are almost independent of the Representative Concentration Pathways (RCPs) for greenhouse gas emissions chosen for the first decades of the 21st century, but they begin to diverge significantly from about 2050. For the business-as-usual scenario (RCP8.5), the rates increase steadily through the 21st century, reaching almost 12 mm yr-1 by 2100 at all locations. For the intermediate scenarios of RCP 6.0 and RCP 4.5, the rates stabilise in about 2090 and 2060 at about 7-8 and 6 mm yr -1 , respectively. For the strong mitigation scenario (RCP 2.6, requiring significant and urgent mitigation of greenhouse gas emissions), the rate of rise stabilises much earlier than the other scenarios and then reduces slightly to about 4 mm yr-1 . On the north and west coasts of Australia, the observed sea-level trends from the start of the projections in 1996 to 2010 are larger than the model projections as a result of the combined effect of SLR and internal modes of climate variability. The impacts of SLR will be felt most profoundly during extreme sea-level events. The meteorological and climate processes that contribute to extreme sea levels around the coast of Australia are highly regionally heterogeneous. Allowances were calculated that provide estimates of the height that present assets or their protective measures would need to be raised to ensure that the likelihood of exceedance of those levels in the future does not change from the present climate. These allowances vary according to SLR projections, their uncertainties and the variability of extreme sea levels. For 2030, when the uncertainty surrounding future SLR scenarios is small, the allowances are approximately the median projected SLR. However by 2090, the larger uncertainties associated with the projections lead to allowances that typically lie towards the upper end of the range of projected SLR.