Climate Change Impacts on Urban Flooding

This paper estimates changes in the potential damage of flood events caused by increases of CO2 concentration in the atmosphere. It is presented in two parts: 1. the modelling of flood frequency and magnitude under global warming and associated rainfall intensities and 2. the use of greenhouse flood data to assess changes in the vulnerability of flood prone urban areas, expressing these in terms of direct losses. Three case studies were selected: the Hawkesbury-Nepean corridor, the Queanbeyan and Upper Parramatta Rivers. All three catchments are located in southeastern Australia, near Sydney and Canberra. These were chosen because each had detailed building data bases available and the localities are situated on rivers that vary in catchment size and characteristics. All fall within a region that will experience similar climate change under the available greenhouse scenarios. The GCMs' slab model scenarios of climate change in 2030 and 2070 will cause only minor changes to urban flood damage but the double CO2 scenarios estimated using the Stochastic Weather Generator technique will lead to significant increases in building damage. For all the case studies, the hydrological modelling indicates that there will be increases in the magnitude and frequency of flood events under the double CO2 conditions although these vary from place to place. However, the overall pattern of change is that for the Upper Parramatta River the I in 100-year flood under current conditions becomes the 1 in 44-year event, the 1 in 35-year flood for the Hawkesbury-Nepean and the 1 in 10 for Queanbeyan and Canberra. This indicates the importance of using rainfall-runoff modelling in order to estimate changes in flood frequencies in catchments with different physical characteristics.