In the summer of 2021 severe flooding occurred in Belgium, Germany and The Netherlands. High damages have been estimated in the Geul catchment, leading up to 250 million euro. Flood risk management in The Netherlands is mostly focussed on reducing the probability of flooding and in lesser extent on damage mitigation. Especially in areas such as the Geul catchment, reducing the probability of flooding through flood defences is difficult to achieve. Also taking climate change and urban development into consideration, it is challenging to meet flood safety standards in the (near) future, as the frequency and intensity of flooding is expected to increase. Under these circumstances a shift is observed in scientific and policy discourses towards a flood risk reduction strategy that includes resilience, in which the capacity to resist, mitigate and recover has a larger role. As a consequence, there is an increasing interest to include private actors in the strategy by promoting private flood mitigation measures. However, it is difficult to quantify the effects of these private measures and little research in The Netherlands has been performed. This thesis investigates which improvements can be made to the flood risk reduction strategy in the Geul catchment, thereby increasing resilience through private flood mitigation measures. Thereto a method to include and asses the effect of the measures in a risk based approach is derived.

An international literature study about flood resilience and society provided two applicable private mitigation measures and their damage reductive effect. Dry-proofing types of measures are defined as measures that prevent water from entering an object and can reduce damages between 60%-100% up to an inundation depth of 1 meter. Wet-proofing measures reduce damages and recovery time, while allowing water inside the object, by flood adapted use or structural measures. The maximum damage reduction is 53% up to an inundation depth of 2 meter. Investigation of the Protection Motivation Theory provided important factors that improve the uptake of these measures, by identifying connections between awareness, preparedness and self-reliance. The two main findings are that the communication strategy should focus on the relative risk reduction effect of private measures and should provide guidance on implementation to motivate private actors to initiate action. Five interviews were used as a reference to these findings. To describe the relative effect of measures, they are modelled in a publicly available risk assessment method (SSM-2017), based on hazard, exposure and vulnerability. The effect of the measures is calculated for two scenarios, that describe at which locations the measures are implemented. In the first scenario, top risk locations are identified and measures are applied. This is representative for high awareness and object specific guidance. The second scenario randomly distributes measures over the flood prone area and represents the current approach without specific guidance. For the modelling of measures and scenarios, an automatic procedure using a Python script is developed, which is applicable in The Netherlands in combination with SSM-2017.

The results showthat both dry-proofing andwet-proofing houses and shops are applicable in the Geul catchment. At many locations measures have a positive benefit-cost ratio, indicating it is worthwhile to implement them compared to a situation without measures. More importantly, it is found that high awareness of flood risk or object specific guidance increases the effectiveness and cost-efficiency significantly. For residential objects, an optimum was found to implement measures at the top 40% locations most at risk. The achieved risk reduction was 47% and the benefit-cost ratio 1.3-2.2 for dryproofing. At this coverage the benefit-cost ratio ofmeasures is a factor 2 higher compared to a situation without specific guidance. Location specific investigation revealed that the inundation depth is the main driver for determining the most effective measure. Wet-proofing is cost-efficient in a smaller number of areas at risk for inundation depths higher than 1 meter. For most areas dry-proofing is more cost-efficient as the expected inundation depths in the catchment are mostly below 1 meter.

The research demonstrates the potential of privatemeasures with a significant reduction of flood risk. Given the limitations to reduce the probability of flooding in the Geul catchment, the measures are highly interesting. The method itself was able to determine the type of measure thatwas most effective with confidence. However, based on uncertainties in the model, the benefit-cost ratios have to be interpreted carefully. An extension of this research with a higher resolution of input data can address this uncertainty. Current approaches and communication to motivate private measures were found to be ineffective, as they mainly focus on the type of measures and costs instead of the effect and implementation. It is suggested to improve this by explicitly including the before mentioned factors and bymaking use of social network on the spatial scale of a neighbourhood.