Neglecting property-level flood risk adaptation measures lead to overestimation in flood risk analysis – An empirical study

Combining structural and non-structural mitigation measures is a strategy for managing flood risk. Besides structural flood alleviation schemes and land-use planning, property-level flood risk adaptation (PLFRA) measures are complementary measures for effective flood risk management. However, quantitative knowledge about the implementation and damage-reducing effects on building structure of PLFRA measures is scarce. Accordingly, the mitigation of vulnerability is rarely considered in flood risk assessment. Here, we collect data on PLFRA measures through a field survey, present a method for incorporating PLFRA into flood risk analysis, and conduct an analysis of their damage-reducing effects. With this approach, flood risk analysis is based on known object-specific vulnerability, rather than on assumptions on overall risk reduction by PLFRA measures. The results show that 16 % of the buildings are protected through PLFRA measures, and the expected annual damage (EAD) is reduced by around 18 %. On average, the PLFRA measures protect the respective houses against flood damage up to a flow depth of 0.6m. Further, 17 % of the buildings had a level of protection that could not be attributed to explicit PLFRA measures but was still considered effective. The average protection level of all buildings is up to 0.3m, and the EAD is reduced by around 23 %. If all buildings in the hazard zones were protected by PLFRA measures with a protection level of 0.5m, the EAD could be reduced by 50 %. The results presented provide robust evidence that neglecting PLFRA measures in flood risk analysis leads to an overestimation of flood risk.