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Stage 2 Report, July 2004

Report cover

Report cover

Tropical Cyclone-Induced Water Levels and Waves: Hervey Bay and Sunshine Coast

Executive summary

The frequencies of storm tide and waves during tropical cyclones were determined for the Hervey Bay and Sunshine Coast regions of southeast Queensland, Australia. The goal was to produce return period curves for storm tide (storm surge plus wave setup plus astronomical tide) and for significant wave height for return periods between 10 and 1000 years.

A series of sophisticated models was employed. First a tropical cyclone track and pressure model produced a synthetic dataset consisting of the time series of position and pressure for almost 10,000 storms. This represents 3000 years of data for the western Coral Sea. Numerical models with three nested grids with increasing spatial resolution for both storm surge and wave generation were established for the study areas. The storm surge model had been validated in Phase 1 of this overall study. The wave model was validated by comparing measurements and modelled significant wave heights during T. C. Simon (1980). It was computationally impossible to model all 10,000 storms to the needed resolution; therefore, a system was developed to determine which storms would contribute to returns periods above 10 years. This reduced the number of storms to be modelled from 10,000 to about 500 for each of the two study areas.

Wave setup was calculated as a function of wave height and peak wave period using an empirical formula. Wave setup is the most uncertain of all the components since the surf zone width during a severe tropical cyclone would be very much wider than those used to derive the empirical wave setup formula. Although all other components of this study received state-of-the-art treatment it was infeasible to do this for the wave setup. At locations such as the Sunshine Coast where wave setup is more important that storm surge, developing state-of-the-art wave setup calculations has much more importance than at Hervey Bay where storm surge is the dominant component.

It is important to note that water levels created by wave setup will not translate far inland after overtopping frontal dunes, flowing overland over low lying areas, or proceeding through inlets. Once flow starts wave setup reduces markedly; therefore, the storm tide curves that include wave setup should apply only to areas with direct wave attack. For areas more than a couple hundred metres landward of the shoreline during the storm, a better estimate of potential flood levels is obtained from the storm surge plus tide return period curves. An overland flooding study might be necessary to provide definitive results for inland locations, especially if the inland floodable area is large.

An astronomical tidal signal was created using tidal analyses. Each coupled set of storm surge and wave setup time series from a single storm were linearly added to 500 separate tidal time series. The tide series were randomly chosen (with a weighting to reflect the monthly change in cyclone frequency) from a long tidal record. The maximum water level and significant wave height during each storm-tide event were determined and these values were ranked by magnitude and return period curves were created.

Establishing a datum and a tidal range at the project output points caused considerable difficulty. Most of the output points were not at established tidal measurement stations. The elevations of the modelled storm tide time series were assigned relative to the official MSL and then the official MSL to AHD correction was used to transfer the results to AHD.

In general storm surge is much more important in Hervey Bay with its broader continental shelf and protection by Fraser Island from severe wave directions. In Hervey Bay storm surge is up to twice the magnitude of wave setup at the 100 year level.

For the Sunshine Coast with its narrower and more open shelf, wave setup is more important than storm surge. Wave setup is more than twice the magnitude of storm surge at the 100 year level.

The effect of greenhouse-induced climate change was investigated. Three separate scenarios were tested. These were (A) combined effect of an increase in maximum intensity (MPI) of 10% and a poleward shift in tracks of 1.3°. (B) increase in frequency of tropical cyclones by 10%. (C) mean sea level rise of 300 mm. In general the mean sea level rise is the most important effect especially at lower return periods. The 10% increase in tropical cyclone frequency is insignificant. The combined increase in intensity and poleward shift in tracks becomes increasingly significant with larger return periods. This is more evident in Hervey Bay where storm surge is more important than at Sunshine Coast where depth limitation reduces the effect of increased wave setup. Both the magnitude and probability of greenhouse-induced mean sea level rise are more certain than greenhouse-induced changes in tropical cyclone frequency, central pressure, or track.

Note that the results in this study are for tropical cyclone-induced water levels. For return periods below about 100 years, extra tropical events will be increasingly important; therefore, the combined curve of tropical and extra-tropical storm tides will be higher than the cyclone-induced storm tide curves shown in this report.

There are several different non-cyclonic influences that affect water levels that are not considered in this study. These include not only non-cyclonic winds, low pressures and waves during storms, but non-storm events such as changes in oceanic currents and continental shelf waves which can also alter water levels.

For all project reporting locations, the occurrence of a tropical cyclone was defined as any that occurred in the western Coral Sea regardless of its distance from the location. This has the property of merging the return period curve for tropical cyclone-induced storm tide into the return period curve for astronomical tide at the lower end of the curves. This definition was used to avoid any misinterpretation of the frequency of water levels at return periods that may be dominated by non-cyclonic events.

A caution is necessary on the possibility of water levels much higher than the 1000 year levels that are presented in this report. The occurrence of the probable maximum water level could have devastating consequences for a nearby community. Although the probability of occurrence is very rare, a calculation of the risk (probability times consequences) is an important component of both disaster and longer term land use planning.

The probable maximum water level at a given location would be caused by a tropical cyclone and tide with the following characteristics. (1) landfall point at a distance equal to the radius of maximum winds to the north, (2) very severe central pressure, (3) large radius to maximum winds, (4) forward speed of the eye equal to the short wave speed offshore and the long wave speed over the shelf, and very importantly, (5) an astronomic tide level that is close to HAT at the time of maximum surge plus wave setup. The combination of these characteristics would be very rare, but not impossible.

An estimate of the probable maximum level could be calculated by adding the largest storm surge and wave setup from the 3000 years of simulations to the HAT level. For example, this would result in a storm tide of about 4.1 m (AHD) at Coolum, Sunshine Coast (see Figure 17) and 6.7 m (AHD) at Torquay, Hervey Bay (see Figure 18). These are approximately 1 and 2 m, respectively above the 1000 year water levels. It must be emphasised that these probable maximum figures are only a rough estimate. The 3000 year storm surges and wave setups are not necessarily the largest possible (i.e. they don’t meet the first four of the above criteria).

Wave information at near coastal and offshore sites was generated in both study areas. Sheltering from southeasterly wave directions by large Islands to the south of Hervey Bay and Sunshine Coast (Fraser Island and Moreton Island, respectively) was evident at all near coastal sites. The offshore Hervey Bay location, had the widest spread of directions and the largest significant wave heights at the 500 year level (16.5 m) as compared with the Sunshine Coast (14 to 15 m). The more northerly position and the more northerly aspect of Hervey Bay is thought to allow larger waves from more frequent and more severe tropical cyclones.

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Last updated 22 June 2009

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