During the last decades an increase in extreme rainfall has led to more urban flooding. This study is based on insurance claims of damages caused by heavy rain during 2006-2012 in Fredrikstad, Norway. Data are analysed using Principal Component Analysis. The purpose has been to find characteristics of extreme rainfall and its influence on the extent of urban flooding. The number of claims seems to be peaked in the late summer period. Furthermore, the precipitation depth the week before an extreme rainfall seems to have significantly influence for the pay out from insurers, and thus the changing in runoff factor due to soil wetness is of importance. Compared to 25-year frequency rainfall with 30 min duration, relatively less intensive, but more stable and long-lasting rain seems to lead to more claims. Experiences from previous events may help to determine the level of flood risk when extreme rainfall is forecasted.

Pluvial flooding already challenges the capacity of drainage and sewerage system in urban areas in Scandinavia. For system owners this requires a stricter prioritization when improving the systems. Experts seem to agree that a regime shift from improving old combined sewers by piped solutions to more sustainable drainage systems (SuDS), must take place. In this paper results from an investigation amongst the largest cities in Norway, Denmark and Sweden concerning drivers and preferred methods for improving the old system are presented. The results indicate that Norway ranks flood prevention lower than the other Scandinavian countries. During the last decades, Norwegian authorities have had a strong focus on pollution from wastewater treatment plants (WWTP). The attention to drainage and sewerage system regarding flooding, water leaks, infiltration or pollution has been neglected. Renewal or rate of investment in relation to existing drainage and sewerage system is easy to register, and provides a measure of the activity. In order to optimize flood prevention, and may be promoting the use of SuDS, the cities should be required to measure the efficiency, either by monitoring or modeling the impact of stormwater to the system. Lack of such requirements from Norwegian authorities seem to be a plausible explanation to why Norwegian cities are less focused on flood prevention compared to Swedish and Danish cities. OPEN ACCESSWater 2014, 6 840

Abstract:During the last decades an increase in extreme rainfall has led to more urban flooding. This study is based on insurance claims of damages caused by heavy rain during 2006−2012 in Fredrikstad, Norway. Data are analysed using Principal Component Analysis. The purpose has been to find characteristics of extreme rainfall and its influence on the extent of urban flooding. The number of claims seems to be peaked in the late summer period. Furthermore, the precipitation depth the week before an extreme rainfall seems to have significantly influence for the pay out from insurers, and thus the changing in runoff factor due to soil wetness is of importance. Compared to 25-year frequency rainfall with 30 min duration, relatively less intensive, but more stable and long-lasting rain seems to lead to more claims. Experiences from previous events may help to determine the level of flood risk when extreme rainfall is forecasted.

Urban flooding caused by heavy rainfall is expected to increase in the future. The main purpose of this study was to investigate the variables characterizing the placement of a house, which seem to have an impact when it comes to the exposure to floods. From the same region in Norway, data from 347 addresses were derived. All addresses were either associated with insurance claims caused by flooding or were randomly selected. A multivariate statistical model, Partial Least Square Regression (PLS), was used. Among others, the analysis has shown that the upstream, sealed area is the most significant variable for characterizing properties' exposure to urban flooding. The model confirms that flooding tends to occur near old combined sewer mains and in concave curvature, and houses located in steep slopes seem to be less exposed. Using this method, it is possible to rank and quantify significant exposure variables contributing to urban floods within a region. Results from the PLS-analysis might provide important input to professionals, when planning and prioritizing measures. It can also predict flood-prone areas and make residents aware of the risks, which may induce them to implement preventive measures.