Climate Futures for Tasmania: extreme tide and sea-level events technical report

The purpose of this study has been to investigate the causes of extreme sea levels and evaluate their return periods for late 20th century conditions along the Tasmanian coast. These data provide a basis for estimating the impact of rising sea levels on the extreme sea-level return periods and of estimating land at risk from inundation due to projected future sea levels. Tide-gauge records show that storm surges on the southeast coast of Tasmania attain maximum heights of typically less than a metre and the weather systems most commonly responsible for generating elevated sea levels at these locations are the west to east moving cold frontal systems associated with low pressure centres situated to the south of Tasmania. While such systems were also the cause of elevated sea levels on the Tasmanian north coast, the largest storm-surge events there occurred when a low pressure centre was situated to the west of Tasmania or Bass Strait and brought strong northerly winds to this coastline. The high correlation between storm surges on the Tasmanian and Victorian coasts meant that a more efficient modelling approach, previously applied to the Victorian coast, could be used to evaluate storm-surge return levels for the Tasmanian coast. In this, a population of extreme sea-level generating weather events was identified from a limited number of long tide-gauge records and each of the events modelled with a hydrodynamic model forced with archived wind and pressure patterns. The peak residual sea levels from each event were then statistically analysed to obtain event probabilities. This modelling indicated that the highest storm-surges occurred on the southeastern coastline of Tasmania and storm surges were lowest on the northern Tasmanian coast. Tide prediction models are used to generate probability distributions of tide height. The tidal range was largest on the Tasmanian north coast and smallest on the southwest coast. Tide height probability distributions were developed and combined with storm surges to yield return periods of storm tide (the total sea level arising from the combination of a storm surge and tide). The one-in-100-year sea levels for Hobart, Georgetown, Burnie and Spring Bay were estimated to be 1.29, 2.00, 1.92 and 0.99 m relative to Australian Height Datum (AHD) respectively. Changes in wind speed are not projected to significantly affect storm surge. The effect on storm surges of wind speed changes was investigated by adjusting the winds in each event by the wind changes in the 2080 to 2099 climate relative to the 1980 to 1999 climate. The changes were found to vary from no change to negligible reduction in storm tide height around the Tasmanian coast. Further investigation is needed to understand the role of wind changes on the Tasmanian coast results presented here, but are beyond the scope of the current project. By 2030 a 100-year event based on late 20th century conditions will occur around twice to 10 times more often (i.e. about once every 10 to 50 years) if sea-level rise follows the upper end of the IPCC (2007) projected range. By 2090, a 100-year event based on late 20th century conditions will occur as frequently as once every five years and up to several times a year if the high-end projections for sea-level rise eventuate.