The effects of urban growth and climate change on flood runoff generation in deltaic regions

Climate change generates a larger variability as well as a change in trends in the distribution of flood events (Kabat, 2005). According to the predictions made by Milly et al. (2002) more extreme rainfall and greater flooding particularly in regions that already receive the most precipitation will occur in this century. Cities may adapt to gradual climatic trends but the occurrence of extreme rain events poses a grave danger to our urban environments, possibly resulting in substantial economic losses and casualties. From a risk perspective, the vulnerability of cities is increasing because of the increase in uncertainty about the occurrence of extreme events (Rose et al., 2000). Yet, changes in our climate might not be the only cause for increased vulnerability. In the past century many cities have witnessed a process of large scale urbanization (The World Bank, 2000). This process is still continuing (often at a very high rate) for metropolitan areas, most notably in delta areas within South-East Asia, Latin America and Africa. Apart from the fact that concentration in assets and population increases potential losses in case of flood disasters, the susceptibility towards floods might also be increased by a loss of infiltration capacity as a result of urban densification (Tucci and Villanueva, 2005). Furthermore, the runoff of both pluvial floods as well as river floods is affected by the altered morphology; a decreased urban drainage capacity can dramatically increase flow resulting in flash floods. While in some cities ‘greenbelt’-policies (e.g. parks, recreation areas) provide some relief by providing urban water retention areas, the growth pressures of many cities result in dense urban agglomerates (note). Although climate change is undoubtedly an important factor in the increased vulnerability of metropolitan areas, the process of urban growth itself might prove to have a much more dramatic impact. In this paper a study is performed on the influence of urban growth for a set of metropolitan areas, located in delta regions, on their runoff capacity. This analysis is performed by taking time-series data on actual geographic coverage while topologically and quantitatively interpreting the distribution of built-up areas within the urban regions. This data is combined with precipitation data thus building up a measure for cities’ infiltration capacities which could possibly lead to a quantification of additional retention areas (including their distribution over the actual urban topology). Furthermore, several growth scenarios are modeled (e.g. Batty, 2005) to get an indication on if specific growth behaviors (e.g. a classification of urban growth morphologies) lead to an increased vulnerability towards flooding (and at which rate this increase progresses). Along similar lines as Milly, the latter will be compared with projected impacts due to climate change by examining different scenarios of precipitation patterns and trends.