Quantitative risk analysis of urban flooding in lowland areas

Urban flood risk analyses suffer from a lack of quantitative historical data on flooding incidents. Data collection takes place on an ad hoc basis and is usually restricted to severe events. The resulting data deficiency renders quantitative assessment of urban flood risks uncertain. The study reported in this thesis reviews existing approaches to quantitative flood risk analysis and evaluation of urban flooding guidelines. It proceeds to explore historical data on flooding incidents from municipal call centres in two cities in the Netherlands with the final aim to quantitatively assess urban flood risk. The data from municipal call centres consist of texts describing citizens’ observations of urban drainage problems. The texts provide information about causes, locations and consequences of flooding incidents. Call information on flooding causes is used to identify causes of urban flooding through application of probabilistic fault tree analysis. Urban flooding probabilities are quantified as well as contributions of a range of causes to the overall flood probability. Call information on flooding consequences is used to draw risk curves for a range of consequence classes: separate risk curves are drawn for consequences associated with human health, damage to private property and damage related to traffic disturbance. The curves depict a combination of flood consequences of increasing severity and associated probabilities of occurrence. Health risk associated with urban flooding is evaluated additionally in a screening-level quantitative microbial risk assessment. The assessment is based on analyses of samples from flooding incidents and from combined sewers. Risk values from call data analysis are translated into monetary values and into numbers of people affected by flooding in order to obtain risk outcomes that can be weighed against investments to reduce flood risk. It is discussed how outcomes in monetary terms differ from those based on numbers of affected people affected. The effectiveness of urban flood reduction strategies is assessed based on a comparison of flood risk values associated with three main failure mechanisms causing urban flooding. The effectiveness of existing strategies for flood risk control is discussed and potential improvements are indicated. Finally the acceptability of flood risk is discussed in view of the quantitative flood risk outcomes of this thesis. It is shown how quantitative risk values based on call data provide a starting point for the development of risk-based standards for urban flooding.WatermanagementCivil Engineering and Geoscience