Flooding in urban areas can be caused by flash floods, or coastal floods, or river floods, but there is also a specific flood type that is called urban flooding. Urban flooding is specific in the fact that the cause is a lack of drainange in urban area. As there is little open soil that can be used for water storage nearly all the precipitation needs to be transport to surface water or the sewage system. High intensity rainfall can cause flooding when the city-sewage system and draining canals do not have the necessary capacity to drain away the amounts of rain that are falling. Water may even enter the sewage system in one place and then get deposited somewhere else in the city on the streets.
Climate change acts as a trend-breaker as well as creating a larger variability in the occurrence of extreme events. This result in increasing degrees of uncertainty towards which traditional probability based flood management policies might not provide adequate responses. Furthermore, ongoing processes of urbanization (both expansion and densification) increase susceptibilities of asset concentrations to floods, thus increasing overall vulnerability of urban areas to an inceasing degree.
Although we can’t say whether climate change caused the heavy rainfall, scientist predict we will see more heavy raianfall days in the future than we curently get. The Environment Agency Sustainable Development Unit said in June 2001:
‘Major floods that have only happened before say, every 100years on average, may now start to happen every 10 or 20years. The flood season may become longer and there will be flooding in places where there has never been any before’.
Thus, the risk of flooding looks greater than ever and not just in one country or the other, but throughout the whole world.
In discussion on urban flood risk, the focus is mainly on the financial consequences of flood events (Ashley et al, 2005). Consequences related to public health are easily overlooked. Two important types of health consequences are: ingestition of contaminated water and physical injuries as a result of stumbling over objects under the water surface or even falling into manholes.
When combines sewer systems flood as a result of heavy rainfall, humans can come into contact with the mixture of water – and stormwater that flows onto the surface and may contain various types of pathogens. Different contamination routes are possible; the most important route is ingestion of the flood water, as a result of playing in it or being splashed. Possible healthrick include, mental health impacts, physical effects (such as falls, cuts, heart attacks), infection (due to exposure to pathogenic microorganisms) and exposure to chemical.
Persistent organic and inorganic chemical contaminants can enter urban riverine systems from a range of different sources and in a range of forms. Atmospheric disposition, urban runoff, agricultural runoff, remobilization from floodplains and within – channel deposits, industrial discharges, and sewage outfalls all contribute to the chemical contaminant load of the fluvial system. Table 1, outlines the chemical contaminants that may be present in run-off from road surfaces and their primary sources. Chemical contaminants may be dissolved in the water or be associated with sediments (suspended or deposited).