Influence of Sandy Foreshores on Overtopping in Non-Tidal Low-Energy Shallow Lake Environments

Abstract

Sea level rise will increase the risk of flooding in coastal areas. This poses a risk to the coastal protection as well as rivers and lakes close to the coast. Solutions are needed to cope with this threat. The past decade, nature based solutions have gained significant interest. One of these solutions could be sandy foreshores. Due to the use of natural materials, sandy foreshores are a ‘nature-based solution’ opposed to a more traditional approach of dike reinforcement. For sandy foreshores to be a viable alternative to regular dike reinforcements, the order of magnitude of construction and maintenance costs need to be known. For this reason, it is necessary to be able to calculate a failure probability of a dike with sandy foreshore, to predict the required maintenance and to optimize the design based on life-cycle costs.

To improve the calculation method, a step-wise calculation of the failure probability for wave overtopping of a hybrid structure was developed. The calculation included an iterative process. The calculation methods consists out of calculating the failure probability for wave overtopping in Riskeer and a dune erosion calculation in Xbeach. In this research, overtopping was considered as the dominant failuremechanism. For the assessment, the dike was seen as an impermeable hard layer and the foreshore as a beach. Therefore, dune erosion could not erode the main structure and is considered a sub-mechanism of overtopping.

Next, a calculation was performed to predict the longshore transport along the beach in Almere Duin. The occurrence of different wind directions, together with a Delft3D model of the Markermeer, was combined to find the longshore transport. The LST was calculated with transport formula calibrated for coastal areas. The life cycle costs (LCC), including design and maintenance costs, were calculated for different strategies. The calculated alongshore transport together with cost estimates were used to calculate the LCC. Subsequently, the net present value of the maintenance costs was calculated to determine the LCC for each maintenance strategy. At the end the uncertainty and sensitivity of each strategy were analysed.

The time-varying protection level can be optimized by calculating the failure probability due to wave overtopping, with Riskeer and calculating the erosion of the foreshore, with Xbeach. To use this method, Riskeer and Xbeach should have the same design point and if the design point shifts, an extra iteration is necessary. For a 1/10 profile, only a 0.25 m lowering of the foreshore height was found and one extra iterative step was required to carry out the calculation. A maintenance strategy with a 100 year maintenance interval period was found to be optimal in this thesis and a Monte Carlo simulation, which included uncertainties, led to similar results. However, the total LCC were 21% to 34% higher, when uncertainties were included in the calculation. These findings suggest that it is not necessary, to carry out a probabilistic calculation, to find the optimal maintenance strategy. However, the models used, aswell as the local boundary conditions, such as the longshore transport and the cost of sand, were found to influence the life cycle costs significantly. For this reason, it is concluded that a location specific analysis is important to optimize the maintenance strategy.