Sand extraction from a ridge might lead to erosional hotspots at the beach
In many coastal seas large-scale sand ridges occur, which result from interactions between waves, currents and the sandy bottom (morphodynamic self-organization). These ridges occur in water depths of 10-20 m, have horizontal length scales of kilometers and evolve on a timescale of centuries. Areas where these ridges are observed include the coastal shelves of Long Island and the North Sea (see figure 1).
Because of their large sand volume, these ridges are considered as potential sand resources for e.g. beach nourishments projects. However, extracting sand from these ridges might reduce the natural onshore sand transport to the nearshore zone. Moreover, removing parts of the ridge will reduce the natural protection against storms.
To better understand these impacts, the IMAU coastal group, in joint collaboration with the Politechnical University of Barcelona (UPC) and the University of Santander has developed a model that simulates the evolution of sand ridges, such that it accounts for wave shoaling and wave refraction processes.
The results show that following sand extraction, the ridge recovers within a few hundred years. During this period of recovery, higher waves might reach and erode the beach. Sand needed for this recovery comes mostly from deepening of the trough that is located downstream of the ridge. However, in case of sand extraction close to the shoreface, a significant amount of sand is imported from the nearshore zone. This leads to coastal erosional hotspots in the vicinity of the dredged area. Thus, sand extraction has negative consequences for beach stability, in particular when the dredging is carried out in the shallower waters of the inner shelf.
Abdel Nnafie, PhD student Coastal and Shelf Sea Dynamics
- Nnafie, A. [et al.] 2011. Formation of shoreface-connected sand ridges: effects of rigid-lid approach, quasi-steady approach and wave-topography feedbacks. A: River Coastal and Estuarine Morphodynamics. “Proceeding of the 7th IAHR Symposium on River, Coastal and Estuarine Morphodynamics”. Beijing, p. 2114-2123