The Sand Motor mega-nourishment is designed to slowly disappear. The predicted lifetime is ~20 years. Since the construction of the Sand Motor in 2011, sands have been eroding from the middle part and accreting at the adjacent beaches of Kijkduin and Monster. As a result the hook-shaped peninsula flattened to a nearly symmetric wave-shaped sand body, which is expected to continue the upcoming years. The flattening of such a shoreline perturbation or sandwave is known as diffusion. The diffusivity of the Sand Motor, a long-term process, can be explained by net sediment transport imposed by the wave climate and the shoreline orientation.
When waves enter the coast under an angle, they generate a longshore current. At the Sand Motor the longshore current is varying alongshore in rate and direction, because the angle of wave incidence at the shore is alongshore different (see figure below). This is the result of the curved coastline. The longshore current transports sediments, inducing changes of the morphology. When the wave direction changes, the direction of sediment transport changes and thus the morphology. To explain the long-term (i.e. several years) change of morphology, we need to know the dominant wave direction-which is southwest at the Holland coast. Thus waves enter nearly normal to the shoreline at the southern side, but obliquely at the tip of the Sand Motor. This results in a weak current at the southern side, while the current intensifies at the tip. These gradients in net longshore current induce gradients in net sediment transport, leading to erosion at the tip of the Sand Motor and accretion at the adjacent beaches.
Upcoming week we will report on morphologic change on shorter timescales, such as the migration of sandbars.
*Ashton, A. and A.B. Murray, 2006. High-angle wave instability and emergent shoreline shapes: 1. Modeling of sand waves, flying spits, and capes. Journal of Geophysical Research 111. F04011, doi:10.1029/2005JF000422.
*Ashton, A. and A.B. Murray, 2006. High-angle wave instability and emergent shoreline shapes: 1. Modeling of sand waves, flying spits, and capes. Journal of Geophysical Research 111. F04011, doi:10.1029/2005JF000422.