Introduction

1.1 NEED FOR ASSESSING CLIMATE CHANGE IMPACTS ON URBAN DRAINAGE

For more than a century, large-scale separate and combined sewer systems have been constructed across many cities worldwide. As the name suggests, combined sewer systems convey both urban runoff and sewage in the same (combined) pipe drainage system. This is the most common type of urban drainage system in Western Europe and North American regions. The alternative solution is a separate system, which consists of parallel sewers for storm and waste water (e.g. Burian et al. 1999; Butler & Davies, 2010). Separate systems are widely used in many countries in Asia, Australia, Europe and North America for newly developed urban areas. In separate sytems, sewage is conveyed in smaller diameter pipe systems while urban runoff is conveyed separately, usually in either open channels or street pipe drainage systems. They are built to reduce the pollution effect of urban drainage on receiving waters, and to enhance the efficiency of the wastewater treatment plant (less diluted wastewater). For instance, in Japan separate systems are only constructed since the 1980s and currently about 20% of the sewer systems are of the combined type. This percentage of combined systems is much higher in Europe, for example about 70% in the UK (Butler & Davies, 2010). For clarity in this book, both combined and separate systems will henceforth be referred to as urban drainage systems.

In general, these urban drainage systems have reduced the vulnerability of the cities to the health risks since they are often built as part of municipal sanitation programs. However, the installation of these systems could make them more vulnerable to rainfall extremes, partly due to the lack of consideration to what occurs when the design criteria are exceeded. In particular, urban land use is constantly changing in response to the continuous changes in demographic and socio-economic conditions of the population (O'Loughlin et al. 1995). As a consequence of these environmental changes, designers and managers must now cope with the increase in surface imperviousness and the shorter response time of urban catchments, which boost stormwater runoff volumes and velocities beyond the capacity of existing drainage systems.

For most cities, it is expected that these trends will continue over the coming decades. At the same time, many highly developed regions already realise that their urban design and planning processes urgently need to incorporate more sustainable approaches. Many urban water systems are particularly vulnerable to rapid population growth and climate change (Semadeni-Davies et al. 2008). In the presence of climate change induced uncertainty, urban water systems need to be more resilient and multi-sourced. This is partly because of decreasing volumetric rainfall trends in many parts of the world, which might have severe effects on reservoir yields and operational practices. In addition, severe intensity rainfall events can cause failure of drainage system capacity and subsequent urban flood inundation problems (Beecham & Chowdhury, 2012).

Besides this increased vulnerability, there is also strong evidence that the probabilities and risks of urban flooding and sewer surcharge are changing due to the increasing trends of some climatic parameters such as precipitation and temperature extremes (Stone et al. 2000; Alexander et al. 2006; Allan & Soden, 2008). In particular, in their Fourth Assessment Report (AR4) the Intergovernmental Panel on Climate Change (IPCC) of the World Meteorological Organization and the United Nations Environment Program reports for the late 20th century a worldwide increase in the frequency of extreme rain storms as most likely a result of global warming (IPCC, 2007a; WMO, 2009a; Giorgi et al. 2011). Extremes were by the IPCC (2007a) defined as events that are relevant from a disaster risk management perspective, for example urban flood disasters. The increase in rainfall extremes is most pronounced in the period of anthropogenic greenhouse gas (GHG) induced twentieth-century warming (approximately 0.5 deg. C worldwide in the period 1976–2000) after the so-called climate shift (IPCC, 2007a). The study by Min et al. (2011) revealed that human-induced increases in GHG have contributed to the observed intensification of heavy rainfall events over approximately two-thirds of the data-covered parts of the Northern Hemisphere land areas. Based on climate model simulations with different future GHG emission scenarios, IPCC (2007a) furthermore concluded that it is very likely that this trend will continue in the 21st century. The consequences of these changes have to be assessed in a perspective of sustainable development. Water managers have to anticipate these changes in order to limit flood risks for communities. Also the insurance industry, as well as the various water users and policy makers, need quantification of these risks so as to develop and adapt policies.

Consequently, the number of hydrological impact studies of climate change has increased greatly in recent years. These studies, however, most often focus on river discharge extremes and low flow risks. The number of climate change studies dealing with urban drainage impacts is still rather limited, partly because they require a specific focus on small urban catchment scales (normally on a scale of 1–10 km2) and short duration precipitation extremes (normally less than 1 hour). This is because of the small characteristic time scales of the processes involved in the hydrological cycle within urban areas. These processes react very quickly to rainfall.

Despite a significant increase in computational power in recent years, the spatial resolution of climate models still remains relatively coarse and they are therefore unable to resolve significant climate features relevant at the fine scales of urban drainage systems. They also have limitations in the accuracy with which they describe precipitation extremes (e.g. high-intensity convective storms leading to urban flooding). This is due to an incomplete...