The morphological impact of the deepening of the deep foreshore on the Dutch coast

The morphological impact of the deepening of the deep foreshore on the Dutch coast

Van Walsem, T.D.B.

Stive, M.J.F. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

Coastal Engineering

2011-09-27

The deep foreshore is the part of the cross-shore profile that is between 8 m and 20 m deep. Indications exist that this area might be deepening. In this report the long term morphological consequences of this possible trend on the sandy coast of Holland (the stretch between Hook of Holland and Den Helder, excluding artificial defences) are discussed. It is assumed that the deep foreshore is deepening at a rate of 2 cm a year. Two scenarios, representing the hypothetical situation after 50 and 100 years, namely, scenarios where the deep foreshore has deepened 1 metre respectively 2 metres, are compared to the scenario where the deep foreshore does not deepen – more or less the actual situation. Six locations, spread along the sandy coast of Holland, are subjected to calculations to determine the effect that deepening of the deep foreshore has on the sandy coast of Holland. The focus is on three morphological aspects: 1. Dune erosion and extent of area of offshore deposition of sand storm surge. The process-based DurosTA model is used for calculations. The sandy coast is subjected to a storm surge with wave heights and water levels reaching maximum design values. The calculations show that dune erosion during storm surge does not significantly increase as a result of the deepening of the deep foreshore. The area of deposition – the extent to which sand is deposited offshore during a storm surge – increases slightly due to deepening of the deep foreshore. 2. Longshore transport The impact of deepening of the deep foreshore on the Dutch coast is determined by calculating the longshore transport created by two wavetypes, that together form a couple that is representative for the morphologically relevant wave types along the sandy coast. The process-based Unibest LT model is used for calculations. The Van Rijn formulation of sediment transport is applied. Increases of longshore transport vary between 3 to 8%, depending on the waveheight and angle of incidence. The increase of longshore transport is assumed to be proportional to the increase of longshore transport gradients along the sandy coast of Holland. 3. Cross-shore transport The process-based Unibest TC model is used for the calculations. Unibest TC is capable of simulating the various processes that contribute to sediment transport such as streaming, wave asymmetry, undertow and long bound waves. Deepening of the deep foreshore leads to an overall clear seaward shift of calculated transport rates in the order of several cubic metres per year per running meter (m’) of coastline. General conclusion: altogether, these effects are limited when put into the perspective of the yearly volume of suppletions that are required now and in the future. However, they would be lasting and would endure many decades. Student: Thomas van Walsem; Graduation Committee: Prof. dr. ir. M.J.F. Stive (Delft University of Technology), ir. C. den Heijer (Delft University of Technology), dr. ir. P.J. Visser (Delft University of Technology), and drs. N. Geleijnse (Delft University of Technology)

coastal morphology

http://resolver.tudelft.nl/uuid:b5235d0f-fa37-45ca-a59a-2ad60e88574c

Student theses

master thesis

(c) 2011 Van Walsem, T.D.B.