Investigating the necessity of updated high-resolution flood models for decreasing flood risk by interventions in consequence limitation

Abstract

In the Netherlands, flood risk is managed by the concept of multilayer safety. Herein, risk is impacted by interventions in three layers: prevention, spatial planning and crisis management. Currently, the focus of decision makers is mainly on layer one: reducing the chance of flooding by reinforcing flood defences. In fact, this has been the status quo for the past decades. Local decision makers often do not see the potential of spatial developments in reducing flood risk and the responsibilities for such measures are scattered. Furthermore, several researches have demonstrated that measures in spatial planning are currently not cost-effective as an alternative measure for dike reinforcements in most flood prone areas of the Netherlands. However, due to a continuously rising sea level and a further expansion of urban area in the flood prone West of the Netherlands, solely investing in strengthening flood defences might not be sufficient or cost-effective in the future. Therefore, the possibilities of measures in layer two and three require additional awareness. The decision-making is currently based on maximum water depth maps, resulting from breaches in primary flood defences. These originate from the VNK2 project that started in 2006. It is debatable whether these maps, with resolutions of 100×100m, provide sufficient information for well-considered decision making. Therefore, the goal of this research was to get more insight to what extent high-resolution flood maps could assist local decision makers in the process of decreasing flood risk by interventions in consequence limitation. This was done by creating a high-resolution flood model and running a set of flooding scenarios in 3Di. The resulting flood maps (max water depth, max flow velocity, arrival time) were compared to VNK2 maps, which were used for determining the dike safety standards in the Netherlands. Furthermore, the resulting damage and casualties from different scenarios were compared to those resulting from the VNK2 scenarios. It was shown that the resolution of flooding scenarios significantly impacts the resulting flooding image. Opposed to the VNK2-scenarios, the high-resolution scenarios clearly showed small scale variations in water depth, resulting from topographical variations, such as buildings, roads and channels. Furthermore, notable differences in damage and casualty calculations were observed. Thereafter, interviews were conducted with a set of stakeholders to get more insight in their point of view on impact limitation. These interviews provided information on responsibilities, potential and requirements of measures in impact limitation. From the interviews it was concluded that it is difficult to explicitly define the responsibilities in layer 2 of multilayer safety, because many different parties are involved in spatial planning. Furthermore, opportunities to link measures against rainfall flooding to flood mitigation are stated to be not very promising, because there is a great difference in water depth between such events. However, all interviewees indicated that it is highly important to get more insight in the location and availability of infrastructure during a flood. For instance, the position (horizontally and vertically) of roads, electricity- and internet infrastructure can have major influence on the accessibility for emergency services, evacuation patterns and compartmentalisation of areas. It was concluded that updated and more detailed flooding scenarios can improve the decision-making on consequence limitation as a flood safety measure. Especially measures improving the infrastructure and accessibility of flooded areas are stated to be promising. Presenting images showing the availability of infrastructure or the effects of a compartmentalisation levee, might encourage decision makers to make improvements. Herein, not only final images are important, but also the development of a flood. In order to get detailed temporary information on the functionality of local infrastructure during flood events, high resolution scenarios are essential. Opposed to the new high-resolution scenarios, VNK2 scenarios with resolutions of 100×100m, are not able to show water levels at individual objects. Furthermore, the new scenarios and damage and casualty calculations might be a reason to critically analyse the dike safety standards, resulting from the VNK2 scenarios.