Nourishing the shoreface: observations and hindcasting of the Egmond case, The Netherlands

Abstract

Although beach nourishment policy in the Netherlands is generally successful in maintaining the 1990 shoreline, the efficiency of nourishments at several locations along the North Sea coast is unacceptably low. These locations are identified as “erosion hotspots”. Egmond aan Zee is such a hotspot. The bathymetric longshore irregularities make this coastal stretch highly susceptible to rip currents and horizontal circulations, which transport large volumes of (nourished) sand to the sea. In 1999, an alternative nourishment design was implemented. This consisted of a shoreface nourishment at the outer bar (at 7.5-m depth) combined with a nourishment of the beach behind it. High-resolution bathymetric surveys, carried out with the WESP, revealed that the shoreface nourishment resulted in a significant shoreward migration of the outer bar during 1999–2001. After 2 years, the system started to return to its natural dynamic equilibrium and lost the positive influence of the shoreface nourishment area: shoreward movement of bars and increase of bar height, which forms the first defense line of the mainland against the sea, and positive contribution to the sand budget, which increased as a result of the nourishment. After 3 years (May 1999 to April 2002), the sediment volume still had an increase of 477,500 m 3 (45% of the applied sand) relative to May 1999. The shoreface nourishment area did not have a direct effect on the beach. The shoreface nourishment area probably will have to be maintained to retain the sediment on the beach in the long run. In Egmond, the shoreface nourishment started to disappear before the sediment could reach the beach. The time scale of sediment feed to the beach zone probably is of the order of 5–10 years. The shoreface nourishment seems to function as a reef, creating a lee-side effect that influences both cross-shore and longshore sediment transport, resulting in two main hypotheses. Firstly, the longshore effect, in which large waves break at the shoreface nourishment, cause a calmer wave climate shoreward of the shoreface nourishment area and a reduction of the longshore current. The shoreface nourishment partially blocks the wave-driven longshore current. Secondly, the cross-shore effect, in which shoaling waves generate onshore transport. These hypotheses are supported by generated model results with both a process-based profile model, UNIBEST-TC, and a process-based coastal area model, DELFT3D-MOR. The UNIBEST-TC model was able to predict the detachment of the outer bar from the shoreface nourishment and, to some extent, the shoreward bar movements. In the lee of the shoreface nourishment area, the model found an increase in sediment, confirming the shielding effect of the shoreface nourishment area. The DELFT3D model found, as a result of the shoreface nourishment area, a decrease of the flow velocity and the wave height just shoreward of the shoreface nourishment area. The model was able to predict the similar trend in sedimentation/erosion pattern, although the intensities were smaller. In the lee of the shoreface nourishment area, a clear sedimentation was found. Erosion was found south and seaward of the shoreface nourishment area. The modeled profiles did not follow the measured profile changes. The separation of the shoreface nourishment from the outer bar and the shoreward migration of the bars could not be accurately modeled. Because the cross-shore transport caused by wave asymmetry is not taken into account in the standard version of DELFT3D, these results were in line with the expectations.