Matthew S. Mason

Estimating present day extreme water level exceedance probabilities around the coastline of Australia: tides, extra-tropical storm surges and mean sea level

The occurrence of extreme water levels along low-lying, highly populated and/or developed coastlines can lead to considerable loss of life and billions of dollars of damage to coastal infrastructure. Therefore it is vitally important that the exceedance probabilities of extreme water levels are accurately evaluated to inform risk-based flood management, engineering and future land-use planning. This ensures the risk of catastrophic structural failures due to under-design or expensive wastes due to over-design are minimised. This paper estimates for the first time present day extreme water level exceedence probabilities around the whole coastline of Australia. A high-resolution depth averaged hydrodynamic model has been configured for the Australian continental shelf region and has been forced with tidal levels from a global tidal model and meteorological fields from a global reanalysis to generate a 61-year hindcast of water levels. Output from this model has been successfully validated against measurements from 30 tide gauge sites. At each numeric coastal grid point, extreme value distributions have been fitted to the derived time series of annual maxima and the several largest water levels each year to estimate exceedence probabilities. This provides a reliable estimate of water level probabilities around southern Australia; a region mainly impacted by extra-tropical cyclones. However, as the meteorological forcing used only weakly includes the effects of tropical cyclones, extreme water level probabilities are underestimated around the western, northern and north-eastern Australian coastline. In a companion paper we build on the work presented here and more accurately include tropical cyclone-induced surges in the estimation of extreme water level. The multi-decadal hindcast generated here has been used primarily to estimate extreme water level exceedance probabilities but could be used more widely in the future for a variety of other research and practical applications.

Application of insurance modelling tools to climate change adaptation decision-making relating to the built environment

Decision-making concerned with managing the possible increased risk of disasters arising from climate change requires tools to forecast changes in disaster risk with time. These changes will be a function of the projected changes not only in weather-related hazard activity due to climate change but also in the vulnerability of the built environment and the aggregate value of assets exposed due to the growth of communities and associated increased concentrations of wealth. Tools developed for the insurance industry over the past three decades to assist decision-makers in estimating and managing catastrophe insurance risk can be adapted to assess the impact of these changes. This paper presents a probabilistic method for undertaking cost–benefit analyses of proposed building adaptation measures using these insurance-based models. The approach accounts for the direct and indirect cost of disasters on a community, including the transfer of risk through insurance and the associated aleatory and epistemic risks. A simplified hypothetical case study focussed on the impact of potential changes to structural design standards for tropical cyclone winds is presented to demonstrate the application of the proposed approach.

Cyclone Tracy and the road to improving wind-resistant design

Early on Christmas morning 1974, tropical cyclone Tracy devastated the city of Darwin leaving only 6 per cent of the citys housing habitable and instigating the evacuation of 75 per cent of its population. The systematic failure of so much of Darwins building stock led to a humanitarian disaster that proved the impetus for an upheaval of building regulatory and construction practices throughout Australia. Indeed, some of the most enduring legacies of Tracy have been the engineering and regulatory steps taken to ensure the extent of damage would not be repeated. This chapter explores these steps and highlights lessons that have led to a national building framework and practice at the fore of wind-resistant design internationally. Tropical cyclone Tracy was a small but intense cyclone, with a landfall radius to maximum winds of 7 km, a forward speed of 7 km/h and central pressure of 950 hPa (Bureau of Meteorology, 1977) (Figure 9.1). Tracy was an Australian Category 4 cyclone with estimated maximum-gust wind speeds on the order of 250 km/h (70 m/s) (Walker, 1975). The recorded gust of 217 km/h (60 m/s) at Darwin Airport before the anemometer failed was, to that time, the highest wind speed measured anywhere on mainland Australia. Tracys small size minimised the spatial extent of damage, but its slow translational speed meant areas impacted suffered more damage than might otherwise have been the case. Of cyclones that form in Australian waters, one passes within 200 km of Darwin every one to two years. The expected recurrence interval of an event similar to or stronger than Tracy impacting Darwin is greater than 100 years based on historical records.

Comparison of Responses of Guyed and Freestanding Transmission Line Towers Under Conductor Breakage Loading

Cascades have been a prominent feature of many transmission line (TL) accidents around the world. During these events, a localized member failure changes the boundary conditions of the structural system, magnifying member loads over a considerable extension of the line. This results in the progressive failure of successive support structures. Phase conductor breakage is regarded as the most severe cascade triggering event, generating a shock wave that propagates through the conductor and induces large unbalanced longitudinal loads on the transmission line components. Under such an extreme loading event, nonlinear time-history dynamic analysis has to be employed to predict the response of the TL system. In the present work, an explicit dynamic analysis scheme is developed to predict the response of a multi-span TL section. The analysis includes all the main structural components of a TL section. This procedure was used to investigate the response of two types of suspension steel latticed towers, guyed and freestanding, subjected to conductor breakage loading. Results indicate the occurrence of peak dynamic loads (PDLs), that significantly increase the internal forces in system components, exceeding their design capacities, particularly in towers members. The results also show that the guyed tower accommodates such a loading event more adequately than the freestanding tower.