Heavy rain and snow induced by climate change could cause significant damage in urban areas.

A recent model study estimates the impact on a Swedish city and suggests there could be an increase in the number of surface floods by 25-45 per cent during this century. Since 1998, floods in Europe have caused some 700 deaths, the displacement of about half a million people and at least EUR 25 billion in insured economic losses. Floods in urban areas are particularly detrimental.

The European Floods Directive aims to reduce and manage the risks of floods1 and the prediction of future flooding could help its implementation, particularly in urban areas where risks are higher. The researchers compared precipitation (rain and snow) data from a climate change model with current observations and provided some preliminary results. The city of Kalmar in south-eastern Sweden was studied.

The research compared precipitation observations from a rain gauge to data produced by a regional atmospheric climate model (Rossby Centre Regional Atmosphere Model, RCA3) for the period 1991-2004. It used two of the IPCC scenarios that represented a high impact on climate and a more moderate one. It found the model overestimated the frequency of low rainfall intensities and therefore the total volume. However, it reproduced the number of high intensity rainfalls reasonably accurately.

From this, the researchers suggested a way of adapting the climate model to urban systems. Currently the most popular method is the Delta Change (DC) method that transfers the output of a model into a local rainfall time series using a number of factors called the Delta Change factors (DCFs). Using the results of the comparison, the study produced DCFs that were related to rainfall intensity level.

Applying this method to Kalmar indicated that in summer and autumn high intensities will increase by 20-60 per cent by the year 2100, whereas low intensities will remain stable or decrease. In winter and spring most intensity levels increase similarly. The research suggested some possible impacts of these results. For example, low intensity rainfall events would cause no direct harm to the urban drainage system.

However, they may worsen the effect of events that follow due to saturation of the area. Very high intensity and extreme rainfall events would be likely to cause increased basement floods and surface floods, as well as sewer overflow. Preliminary results indicate the number of surface floods could increase by 20 to 45 per cent. If this rainfall is combined with thunderstorms, additional problems such as electrical failures could worsen the consequences because pumping facilities may stop. The study stresses the importance of knowing where urban water systems have a low capacity and identifying their most vulnerable locations.