Three-line modelling of the Terschelling supply

Abstract

In the framework of the NOURTEC project experimental nourishments have been executed. The NOURTEC project is an EU-research project in the scope of the program for Marine Science and Technology (Mast II). NOURTEC stands for Innovative Nourishment Techniques Evaluation. Full-scale experiments have been carried out in three different countries (Denmark, Germany and the Netherlands). Different nourishment techniques have been applied (i.e. shoreface nourishments and beach nourishments). In the period May to November 1993 a shoreface nourishment was carried out at the coast of Terschelling (the Netherlands). An extensive monitoring program has accompanied this exercise. This study is about the simulation of the evolution of the nourishment with a three-line model. The basic idea of line modelling is that the magnitude of cross-shore transport is proportional to the deviation from the equilibrium shape of the cross-shore proflle. For the modelling the cross-shore profile is schematised in zones. Each zone is characterised by "a line which represents the sediment volume of the zone. Also longshore dispersion is taken into account. Coastal constants which quantify the magnitude of transport in cross and longshore direction have been determined. For that purpose use was made of the available wave and bathymetric data. A study was made of the behaviour of the coast prior to the nourishment. The autonomous behaviour was taken into account for determining the shoreface nourishment behaviour. A comparison between measured development and calculations with the three linemodel shows that there are similarities but also considerable differences. One of the difficulties is how to distinguish between autonomous proflle development and profile development caused by the nourishment. The behaviour of the nourishment is partly understood by the model runs. The calculated development of the upper zone is very similar to the measured development. The results for the middle zone are less good. The three-line model will in principle predict symmetrical proflle development in longshore direction in case of an alongshore symmetrical supply. From observations it is clear that the nourishment is moving eastward. This can not be modelled with the present three-line model. The differences between measured and calculated development in the lower zone are also considerable. The research has yielded an increased insight in the complex behaviour of the shoreface nourishment along the coast of Terschelling.