Impacts of Climate Change on Rainfall Extremes and Urban Drainage Systems

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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.