11.4.5 Coasts

Over 80% of the Australian population lives in the coastal zone, with significant recent non-metropolitan population growth (Harvey and Caton, 2003). About 711,000 addresses (from the National Geo-coded Address File) are within 3 km of the coast and less than 6 m above sea level, with more than 60% located in Queensland and NSW (Chen and McAneney, 2006). These are potentially at risk from long-term sea-level rise and large storm surges.

Rises in sea level, together with changes to weather patterns, ocean currents, ocean temperature and storm surges are very likely to create differences in regional exposure (Walsh, 2002; MfE, 2004a; Voice et al., 2006). In New Zealand, there are likely to be more vigorous and regular swells on western coasts (MfE, 2004a). In northern Australia, tropical cyclones are likely to become more intense (see Section 11.3). The area of Cairns at risk of inundation by a 1-in-100 year storm surge is likely to more than double by 2050 (McInnes et al., 2003). Major impacts are very likely for coral reefs, particularly the Great Barrier Reef (see Section 11.6).

Future effects on coastal erosion include climate-induced changes in coastal sediment supply and storminess. In Pegasus Bay (New Zealand), shoreline erosion of up to 50 m is likely between 1980 and 2030 near the Waipara River if southerly waves are reduced by 50%, and up to 80 m near the Waimakariri River if river sand is reduced by 50% (Bell et al., 2001). In New Zealand, emphasis has been placed on providing information, guidelines and tools such as zoning and setbacks to local authorities for risk-based planning and management of coastal hazards affected by climate change and variability (Bell et al., 2001; MfE, 2004a) (see Section 11.6). In Australia, linkages between the IPO, ENSO and changes in coastal geomorphology have been demonstrated for the northern NSW coast (Goodwin, 2005; Goodwin et al., 2006) and between historic beach erosion and ENSO for Narabeen Beach (NSW) (Ranasinghe et al., 2004).

Sea-level rise is virtually certain to cause greater coastal inundation, erosion, loss of wetlands and salt-water intrusion into freshwater sources (MfE, 2004a), with impacts on infrastructure, coastal resources and existing coastal management programmes. Model simulations indicate that the loss of wetlands and mangroves in Spencer Gulf due to sea-level rise is influenced largely by elevation and exposure (Bryan et al., 2001). At Collaroy/Narrabeen beach (NSW), a sea-level rise of 0.2 m by 2050 combined with a 50-year storm event leads to coastal recession exceeding 110 m and causing losses of US$184 million (Hennecke et al., 2004). Investigations for metropolitan coasts reveal increased costs of protection for existing management systems (Bell et al., 2001). Mid-range sea-level rise projections for 2005 to 2025 are likely to increase the cost of sand replenishment on the Adelaide metropolitan coast by at least US$0.94 million/yr (DEH, 2005). Uncertainties in projected impacts can be managed through a risk-based approach involving stochastic simulation (Cowell et al., 2006). Coasts are also likely to be affected by changes in pollution and sediment loads from changes in the intensity and seasonality of river flows, and future impacts of river regulation (Kennish, 2002). In the next 50 to 100 years, 21% of the Tasmanian coast is at risk of erosion and significant recession from predicted sea-level rise (Sharples, 2004).