| dc.description.abstract | Climate change is expected to worsen coastal erosion in the future related to sea level rise, changes in wave climate and an increase in storm activity. Consequently, it is becoming more and more relevant to understand and quantify the present-day dynamics of the shoreline and its underlying drivers of change. In line with the dynamic coastline preservation policy adopted in the Netherlands since 1990, many sand nourishments have been carried out over the past 30 years to protect the Dutch coastline against erosion. These sand nourishments mainly comprised of beach and shoreface nourishments. However, knowledge is still limited of the effects of beach and shoreface nourishments on the dynamics of the shoreline and its (long-term) temporal trends, especially in the case of repeated sand nourishments. The latter being important given that nourishment efforts are likely to increase in the future due to rising sea levels. This study focused on a stretch of the Holland coast along Castricum, Egmond and Bergen, where in the northern part at Egmond and Bergen 23 beach nourishments and 10 shoreface nourishments have been performed. In the south near Castricum, on the other hand, beaches remained unnourished. The Python toolkit CoastSat was used to obtain time-series of cross-shore shoreline positions along the entire study area over a time period of 1985 – 2023. In this toolkit satellite images from Landsat 5-9 and Sentinel-2 are utilised to extract the instantaneous position of the shoreline. By analysing the temporal and spatial variability of the shoreline, this study shows that the repeated nourishments had a large influence on the long-term interannual trend of the coastline leading to considerable shoreline advance and an increasing trend of 2.00 m/year for the nourished coast at Egmond and Bergen, while the unnourished coast remained more or less stable. Shoreface nourishments, especially in combination with a beach nourishment, attributed most to the expansion of the shoreline (~50 m). Although no considerable shoreline sedimentation was observed at the scale of only one shoreface nourishment, over time the repeated shoreface nourishments led to a large cumulative effect with shoreline expansion becoming more pronounced over time. Beach nourishments only resulted in relatively minor sustainable shoreline advance (< 10 m) and were often completely eroded within 1 or 2 years. For individual beach nourishments typically between 10 and 40 m of shoreline progradation occurred after construction. It was found that the effect of the sand nourishments was very localised to the initial extent of the nourishments and did not exhibit large alongshore migration to the adjacent, unnourished coast. Only after 2016, sand also slightly started to spread to the adjacent coastlines, related to the cumulative effect of the shoreface nourishments. Furthermore, in terms of temporal variability, it was found that seasonality in shoreline position along the entire study area was very limited, the amplitude of the seasonality typically was less than 10 m. A distinct response of the shoreline to storm events was also limited. Future studies will have to tell if the shoreline progradation trend at Egmond and Bergen will continue under the current nourishment strategy or if, due to climate change, nourishment efforts will have to increase. | |
