11.4.1.2 Flood and waste water management

Little quantitative information is available about potential changes in flood risk in Australia. Sufficient capacity exists within the Melbourne sewerage and drainage systems to accommodate moderate increases (up to 20%) in storm rainfall totals with minimal surcharging (Howe et al., 2005). For the Albert-Logan Rivers system near the Gold Coast in Queensland, each 1% increase in rainfall intensity is likely to produce a 1.4% increase in peak runoff (Abbs et al., 2000). However, increases in runoff and flooding are partially offset by a reduction in average rainfall, which reduces soil wetness prior to storms. A high-resolution atmospheric model of storm events coupled with a non-linear flood event model has been applied to flooding around the Gold Coast caused by tropical cyclone Wanda in 1974. If the same event occurred in 2050 with a 10 to 40 cm rise in mean sea level, the number of dwellings and people affected is likely to increase by 3 to 18% (Abbs et al., 2000).

In New Zealand, rain events are likely to become more intense, leading to greater storm runoff, but with lower river levels between events. This is likely to cause greater erosion of land surfaces, more landslides (Glade, 1998; Dymond et al., 2006), redistribution of river sediments (Griffiths, 1990) and a decrease in the protection afforded by levees. Increased demands for enhancement of flood protection works are likely, as evidenced by the response to large floods in 2004 (MCDEM, 2004; CAE, 2005). Flood risk to Westport has been assessed using a regional atmospheric model, a rainfall-runoff model for the Buller River, projected sea-level rise and a detailed inundation model. Assuming the current levee configuration, the proportion of the town inundated by a 1-in-50 year event is currently 4.3%, but rises to 13 to 30% by 2030, and 30 to 80% by 2080 (Gray et al., 2005). Peak flow increases 4% by 2030 and 40% by 2080. In contrast, a flood risk study for Auckland using 2050 climate scenarios with 1 to 2°C global warming indicated only minor increases in flood levels (Dayananda et al., 2005). Higher flows and flood risk are likely in the Wairau catchment in North Shore City (URS, 2004).