Modelling the 1775 storm surge deposits at the Heemskerk dunes

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Published Date

03-09-2009

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Faculty

Civil Engineering and Geosciences

Department

Hydraulic Engineering

Abstract

After a storm surge in November 2007, older storm surge deposits were discovered in the eroded dunes near Heemskerk, the Netherlands. These deposits undulate in height with a maximum elevation of over 6 m above mean sea level. Luminescence dating suggests that the layers were deposited by either the 1775 or the 1776 storm surge. The aim of this thesis is to model the 1775 storm surge and its capability to reach the height of NAP + 6.5 m at which the deposits have been discovered. Secondary objectives are (a) to give an estimation of the probability of exceedance of the 1775 storm surge and (b) to compare the effects of this storm surge on a open dune front (historical situation) and a closed dune front (present situation). A modelling framework has been set up to transform the available historical data into boundary conditions for the process-based model XBeach. A 1D probabilistic approach resulted in distributions for the 2% exceedance height for the storm surge level including set-up and wave run-up for six characteristic profiles. The distributions do not give reason to reject the hypothesis that, during the 1775 storm surge, the water level has reached the level of NAP + 6.5 m. Based on the existing exceedance line for IJmuiden, the probability of exceedance of the 1775 storm surge is estimated to be 3/10000. This is close to the existing Dutch design criteria for primary sea defences. A comparison of several 2DH simulations with a historical dune topography containing an open dune front and the same boundary conditions with a present dune topography containing a closed dune front, give the following results. With the open dune front, storm surges can easily enter the dune valleys behind the first dune row, but the water is almost always stopped by the second dune row. Wave energy dissipates quickly in the dune area. The present situation with a closed dune front gives high erosion rates along the entire dune front, while the historical situation with an open dune front gives more variation in erosion and deposition rates. Erosion rates are generally higher in the present situation with the closed dune front.