Flood risk reduction in the Rhine-Meuse estuary

Abstract

Large parts of the Netherlands are exposed to the threats of flooding due to the influence of high water coming from the North Sea region, due to high discharge of rivers like the Meuse and Rhine or due to a combination of North Sea water levels and river discharges. In order to cope these threats, strategies are developed in the Delta Program in which measures with respect to flood risk reduction are elaborated. The current preferred strategy by the Dutch government, 'DP2015', accounts for flood safety of the Rhine-Meuse estuary by means of functioning of the Europoort barrier in combination with an extensive dike reinforcement program and room for the river measures. In 2017 new safety standards for flood risk become effective and it is expected that many dike trajectories within the Rhine-Meuse estuary need to be reinforced as a consequence of climate change and the new standards. Opposed to strategy `DP2015', an alternative strategy in order to meet the required level of flood safety is developed by a group of six engineers under guidance of ir. F. Spaargaren. An alternative strategy, `Sluices', is developed in which the Eastern Scheldt will function as a retention basin and the Maeslant barrier is replaced by a closed dam with navigation locks, sluices and pumping stations, reducing the hydraulic loads within the system significantly. It is investigated how the reduction in hydraulic load levels relate to dike reinforcement tasks for dike ring 16 ‘Alblasserwaard & Vijfheerenlanden'. This dike ring is located in the so-called `transition zone' where hydraulic loads acting on the dikes are determined by both influence of high water levels from the North Sea (being tide and storm surge amongst others) as determined by discharge of the rivers Rhine and Meuse. The failure probabilities for these trajectories are largely determined by failure due to piping and failure due to overtopping/overflow. The failure probabilities for these mechanisms are calculated for the situation in 2015 and in 2100 where influence of climate change is taken into account. The largest risk reduction of strategy `Sluices' compared to strategy `DP2015' is found in trajectory 16-2, where the failure probabilities due to overtopping/overflow and piping were reduced significantly. Based on the results of risk reduction, the shortage on dike height and width is determined and costs are determined. With the calculated shortage on dike height and berm width, a cost calculation is made in nominal and net present terms for the trajectories. It is found that in nominal terms a slight cost reduction can be realized and that most dikes need to reinforced within the time horizon of 2100. However, reinforcements for trajectory 16-2 may be postponed 37 years, leading to a significant cost reduction in terms of net present values.