Impact assessment for the area of dikering 14 under the influence of sea level rise

Abstract

Climate change is a major problem for today's society, which has a huge impact on water safety issues. Recent IPCC scenarios show that sea level rises of 1m by 2100 and 2m beyond 2200 should be seriously considered. Several scenarios show an increase in river discharge of between 10 and 20 percent of the Rhine generated by extreme precipitation by the year 2050. The combination of the sea level rise and the increase in river discharge has consequences for the flood risk in the Netherlands. The area where most people live, about 3,5 million, and where most of the gross national product, around 65 percent, is earned is the area of dikering 14. This thesis determines how the flood risk changes for future climate change scenarios, considering a sea level rise up to 2m and an associated increase in river discharge. One base case scenario with the current situation and four future scenarios with a sea level rise up to 2m with increments of 0,5m were evaluated. The river dikes along the trajectories of the Hollandse Ijssel, Nieuwe Maas and Nieuwe Waterweg and the dunes between Hoek van Holland and Ijmuiden were assessed and the consequences of flooding due to a dune or dike breach were investigated for all scenarios.

Flood risk is determined by the probability of failure of a flood defence and the consequences in case of a flood, which are expressed in economic damage, casualties and affected persons. The influence of climate change on the probability of flooding of 8 river dike profiles was investigated by assessing the failure mechanism overtopping and overflow, using a relationship between height shortage and the probability of failure. The safety of the dunes was evaluated using the Duros-plus model, with which dune erosion calculations were made. The results show that climate change has a big impact on the probability of failure, which is highest for the trajectory along the Hollandse Ijssel with a probability of failure of 1/370.000 per year in the current situation and 1/170 per year in the scenario involving a 2 m sea level rise. Although the failure probabilities of the dunes are very low in the current situation, the influence of sea level rise is shown for the dunes with failure probabilities that are a factor of 2000-3000 higher in a scenario with 2m sea level rise compared to the base case scenario.

For determining the consequences, both existing flood scenarios for the river side and new flood scenarios for the seaside were used. The new flood scenarios show the effect of sea level rise on the increased flood extent caused by a dune breach for each scenario. The potential economic damage, which is determined by the flood depth, damage curves and the land use map, is highest for a dike failure along the Hollandse Ijssel, as the highest flood depths are reached in these deep polders. The highest number of casualties, 7900, determined by the mortality rates based on flood depth, flow velocity, rise rate and evacuation factor, are expected in case of a dike failure along the Nieuwe Maas, as the densely populated cities of Rotterdam and Schiedam are flooded. By assigning monetary values for casualties and affected persons, the total damage is determined, to which the damages resulting from casualties and affected persons contribute most.

After determining the costs of several reinforcement projects, these costs and the potential total damage were used as input in a cost-benefit analysis, where economic optimums were determined expressed in a probability of failure and associated investment costs. The conclusion is that it is economically efficient to reinforce all flood defences except for the trajectory along the Nieuwe Waterweg in case of a scenario corresponding to 2m sea level rise. It was also examined whether the economic optimums met the requirement that everyone should have a maximum risk of dying due to a flood of 10^-5 per year. A total length of 40,5 km river dikes and 63,5 km of dunes will need to be reinforced, for which the costs of reinforcing the river dikes are significantly higher per km than for the dunes, approximately 20 and 3,8 million euros/km respectively. The total estimated costs determined in this study are around 1 billion euros for a 2m sea level rise scenario, but these costs for keeping the area safe will be a factor of 2-3 higher, as the method used for determining the costs of river dikes leads to an underestimation compared to the costs in reality. In addition, hydraulic structures and future subsidence are not included, which will also lead to higher total costs. With these investments the flood risk will remain acceptable and the river dikes and dunes will continue to offer sufficient protection against floods with a total potential damage of 230 billion euros, consisting of 70 billion euros in economic damage, 20.000 casualties and 2,5 million affected persons.