Assessing the Impact of Climate Change on Storm Surges in Southern Australia

Global warming induced increases in mean sea level and possible changes to weather patterns that drive extremes of sea level such as storm surges are likely to increase the frequency and severity of coastal flooding and erosion in the future. Information about the present threat of storm surges and how this threat will change in the future is essential to assess the impact of climate change on the coast and to subsequently formulate adaptation responses to changing climate conditions. Tide gauge data provides an opportunity to evaluate the average recurrence intervals of extreme sea levels. However the often short duration of records collected at many gauges prevents a reliable analysis of extreme event probabilities. Furthermore, the limited number of tide gauges means that large stretches of coastline are without direct data coverage to enable an assessment of this hazard even under present climate conditions let alone future conditions due to greenhouse warming. Along Australia's south coast, storm surge events are driven by severe weather events such as cold fronts and mid-latitude storms. The strong winds and falling pressure elevate sea levels in the vicinity of the coast. Many of the events affect large stretches of coastline leading to a high coherence of the storm surge signal in the available tide gauge records. This aspect has been exploited in the design of an approach to quantify the storm surge hazard over a large stretch of coastline. The severity and frequency of storm tide events in the future will increase with rising sea levels and additionally, climate change may also change the frequency and intensity of the meteorological drivers of storm surge. The modelling framework for assessing the current climate storm surge hazard must be flexible enough to allow exploration of future climate change scenarios and their associated uncertainty, bearing in mind that estimates of future climate change are undergoing constant refinement and revision. The modelling approach described here has been developed with these issues in mind. In this paper it has been used to estimate the impact of climate change on storm surge return periods over the southeastern Australian coastline. The approach requires the identification of a large population of extreme sea level events from tide gauge records along the stretch of coastline of interest, and modelling each event with a hydrodynamic model. Extreme value analysis is then applied to the modelled events to enable the generation of event probabilities and return periods for storm surge. Joint probability analysis is then used to combine the tide with the storm surge to produce storm tides. The impact of climate change on storm surge return periods and inundation extent is investigated by considering the wind speed changes simulated by a range of climate models, as well as projected increases in mean sea level. The results indicate that under current climate conditions, the modelling approach yields results at the locations of tide gauges that are consistent with results obtained by directly analysing the tide gauge data. Under future climate conditions it is found that a 1 in 100 year storm tide level estimated for the late 20th century would occur every 10 years or less under a 2030 high sea level rise scenario once every 4 years or less for a 2070 high sea level rise scenario. Wind speed changes over the region of interest may increase or decrease but will have a considerably smaller impact than sea level rise. In ongoing work the approach will be extended to broader coastal regions by including additional tide gauge data for selection of extreme sea level events and using the model to develop tidal information over the same coastal region.