Assessment of the consequences of higher safety standards for flood defences along rivers in The Netherlands

Abstract

A large part of the flood defences in The Netherlands does not meet the present safety standards. In 2014 the delta programme has proposed new safety standards, which are based on the flood risk approach. These new safety standards will for a lot of river dikes be stricter than the currently prevailing safety standards. A lot of work has to be done to make sure all the river dikes will meet the new safety standards. The safety level of river dikes can be improved by means of spatial measures, which decrease the hydraulic load on the dikes by increasing the flood conveyance capacity, or by dike improvements, which increase the strength of the dike. The effectiveness of spatial measures has always been assessed in terms of water level reduction and not in terms of reduction of the flood probability. In this research a rapid assessment tool is developed with which the reduction of the flood probability by spatial measures can be assessed. This reduction has been calculated for several types of measures and it is compared with the effectiveness of more traditional dike improvements. The delta programme prescribes a strategy in which the increase of the flood probability because of climate change is counteracted with spatial measures. The difference between the current state of the dikes and the new safety standards should be resolved with dike improvements. This research shows that the flood probability reduction by spatial measures is overestimated. Due to this overestimation more spatial measures will be necessary to counteract climate change than was expected. A new strategy is proposed in which the flood probability for failure mechanism overflow/overtopping is reduced with a combination of dike improvements and spatial measures. The probability of failure for other failure mechanisms (piping, macro stability) is reduced with only dike improvements.