IMIS - Marine Onderzoeksgroepen

Purpose
Nearshore nourishments have emerged as a widely used management strategy to reduce coastal retreat at locations with abundant sand resources. The largest nearshore replenishment project ever in Belgium was completed in 10/2023- 02/2024 in Knokke-Zoute on the coast of eastern Belgium. A volume of 1.2 million m3 of fine sand (D50:187 μm) extracted at the site of the new Terneuzen sea lock (The Netherlands) was placed on the nearshore to mitigate beach erosion.
Methods
High resolution bathymetric surveys were carried out to evaluate the performance of this intervention and to moni­tor coastal behaviour and evolution over a period of nearly 2 years.
Results
The placement performance of the large nearshore nourishment is calculated to be 58% due to the spreading of the fine sand during the supply operations. The beach did not directly benefit from the nourishment while accretion was observed in the Appelzak channel located seaward of the nourishment (from 0.37 to 1 km from the coastline). The seaward cross-shore sand transport processes dominated until 1 year after completion: sand was eroded on the shoreface part of the profile and transported to the seabed of the Appelzak channel by wave action. Following this phase, the sand loss continued but via alongshore transport induced both by oblique waves and tidal asymmetry. Although the nourished sand spread outside of the nourished area, most of it is still present in the regional coastal cell.
Conclusion
This study advances understanding of coastal responses to high-magnitude, fine-grained sand re-use interven­tions and provides a scientific basis for decision-making in the design of similar erosion-management operations in other coastal environments.