Long term modelling of longshore sand transport and coastline changes along central COAST of Holland

This paper is focused on the simulation of longshore sand transport (LST) and associated coastline changes at the central coast of Holland with and without structures using 1D coastline models. Results of 2DH models are also given. Two contrasting sites are considered: 1) the large-scale beach plain south the long breakwaters of IJmuiden and 2) the large scale (mega) nourishment site south of The Hague. Both sites have a coastline that was brought significantly out of equilibrium as a result of the interventions, and the coastline changes involved are therefore very insightful for studying both the development towards the new equilibrium coastline orientation as well as the associated rate of coastline change and longshore transports involved. The LST-values were computed with the semi-empirical LST equation of Van Rijn (2014) and analyzed to better understand the actual drivers for coastal change along the coast of Holland coast and performance of the models. The models were forced with measured and simulated wave data from nearby offshore stations over a relatively long period of time between 1979 and 2021 (40 years). The 1D models provided good hindcast results of the long-term net coastal change at the Sand Motor and IJmuiden. The actual coastal curvature in the shadow zone of the long IJmuiden breakwaters was slightly under-represented, but could be calibrated with moderate adjustments. The effect of the wave shadowing zone in the lee of the IJmuiden breakwaters could be represented with sufficient accuracy either by using nearshore wave conditions (using the SWAN wave model) as well as with a much simpler engineering method for wave diffraction of Kamphuis (1992). For the Sand Motor site, the 1D model provided sufficiently accurate predictions for assessments of erosion along the tip, but the area south of the Sand Motor was not represented well because it is affected by the accelerating tidal current and the wave shadowing of the long breakwaters at Hoek van Holland (entrance to Rotterdam harbour). The estimated net annual longshore transport at the original (undisturbed) coastline at both sites are: about 50,000 m³/yr to north at IJmuiden (km 55–65) and about 100,000 m³/yr to north at the Sand Motor site (at km 109). Annual variations are very large at both sites as the contribution of waves from the south-western and northern sectors can vary considerably. Including the harbour breakwaters of IJmuiden, the net LST increases to 300,000 m³/yr to north at km 66 (south of IJmuiden) due to wave shadowing effects. Similarly after construction of the Sand Motor nourishment, the net LST-values at both flanks of nourishments changed drastically.