Dune erosion along curved coastlines

Abstract

The most densely populated and economical most valuable areas in The Netherlands lie below mean sea level. These areas are protected against the sea by a joint coastal dune system. The vital importance of this dune system is reflected in the extensive collection of Dutch legal regulations that ensure the safety level of the dunes. The assessment methods currently prescribed by these regulations are developed in the early eighties. These methods are strictly speaking only valid for infinitely long, uniform and straight coasts. The Dutch coast with its curvatures, islands and estuaries does not meet these prerequisites. The wide application of these assessment methods is mainly justified due to intentional overestimation of the expected dune erosion. In virtue of the expected climate change and corresponding sea level rise, an effective and reliable coastal management becomes increasingly important. Dutch water management authorities therefore commissioned the development of a renewed probabilistic assessment method for dunes. Such method will provide a more accurate and elegant way of dealing with the large number of uncertainties involved in the dune erosion process. The original assessment methods for dunes along the Dutch coast are extended with a computational method for additional erosion along convex coastlines. This method is based on a limited data analysis, which resulted in a cautious guideline. It is therefore expected that current regulations underestimate the influence of coastal curvatures on the amount of dune erosion. In context of the development of a probabilistic assessment method for dunes, little is known about the relevance of coastal curvatures. It is questioned if longshore phenomena in general and coastal curvatures in particular should be incorporated in the newly developed assessment methods. A probabilistic model set-up is created using the numerical semi-two-dimensional dune erosion model DurosTA and a Monte Carlo routine. The dune erosion model is developed especially to investigate the influence of coastal curvatures. Using this probabilistic model set-up, series of computations with varying coastal curvatures and incident wave angles are made for different types of relevant probabilistic hydraulic conditions. From the results the influence of coastal curvatures on the erosion volumes, and thus the retreat distances and probability of failure, is obtained. The results are compared to current regulations. Furthermore the aspects of the dune erosion process that are particulary interesting for curved coastlines are determined. The physical and location independent importance of different model parameters is investigated by conducting a traditional sensitivity analysis. The probabilistic and location dependent importance of the model parameters is investigated by a probabilistic analysis using, amongst others, the design point. Results show that curvatures found along the Dutch coast can increase the 1/4,000 year retreat distance over 100%. Also coastal stretches with curvatures that are neglected in the current regulations still appear to suffer from a larger retreat distance up to 15%. In comparison, the maximum additional retreat distance to account for coastal curvatures is according to current regulations less than 20% in the observed situations. Furthermore the physical importance with respect to the amount of dune erosion of the sediment diameter increases along curved coastlines due to the more efficient transport of suspended sediments. From a probabilistic perspective the storm surge level gains importance due to the common formulation of the correlation between the storm surge level, the significant wave height and peak wave period.