Modelling coastline maintenance : a review of three coastline models

One-line and multi-line process-based coastal evolution models are commonly used in coastal engineering projects to predict the effect of natural or anthropogenic changes on coastlines over periods of many decades. As such, several coastal evolution models were used to assess the nourishment volumes required to maintain the Dutch coastline during and after the reinforcement of the so-called Zwakke Schakels (Weak Links) coastal zones. Although applied in apparently similar manners, the various coastal evolution models predicted substantially different nourishment volumes. In order to be able to effectively develop and manage long-term coastal management plans, it is essential to understand how coastal evolution models work, and why they can lead to different model predictions. This information can help to better inform decision-makers and can be used to more accurately assess predictions of future nourishments. The reasons that the various coastal evolution models predicted different nourishment volumes for the Zwakke Schakels coastal zones are unknown. However, possible sources for these inconsistencies can be identified: differences in the data of the individual sites available to the modellers; differences in the interpretation of these data and the translation of these data into a model; differences in the numerical calculation within the coastal evolution models themselves; and differences in the interpretation of the model results by the modellers and by the clients. This report will focus solely on identifying differences in coastal evolution model results due to differences in the numerical processing in the models themselves. The effect of differences in the way data, model input and model output are handled are reduced as much as possible by imposing identical boundary conditions on all models and by using the same metrics to analyse model results. This report will focus specifically on three coastal evolution models used commonly for the Dutch coast: UNIBEST-CL+ (WL|Delft Hydraulics, 1994), PONTOS (Steetzel, et al., 1998) and LONGMOR (Van Rijn, 1998; Van Rijn, 2002; Van Rijn, 2005). The project is part of the KPP-B&O Kust 2012 programme carried out by Deltares for Rijkswaterstaat Waterdienst. The work carried out in this project was done by Deltares in cooperation with Arcadis (Arcadis project number C03041.002919).