Nearshore Velocity Fluctuations at a Mega Scale Nourishment

Mega-scale, concentrated nourishments are nowadays proposed as mitigation for coastal erosion or to widen beaches for recreation. The intention of such concentrated nourishments is to spread their sediment in both alongshore and cross-shore direction. The mixing process of sediment and other suspended matter by hydrodynamic processes is of key importance for an evaluation and prediction of these coastal projects. Surf zone velocity oscillations and eddies on timescales of up to 30 minutes are contributing factors to this mixing. One of the questions herein is how the artificial altering of the natural profile affects the velocity fluctuations and spatial patterns. This paper will discuss the characteristics of low frequency velocity motions in a new man-made setting; the large Sand Engine nourishment protruding 1 km into the sea. Measurements will be presented from an alongshore array of 6 ADCPs which were deployed during a 6-week field campaign with the intention to investigate amongst others the effect of the wind sea climate and the strong (>0.5 m/s) alongshore tidal currents at the site. The observed magnitudes of both very low frequency (f<0.004Hz) and infragravity motions (0.004<f<0.04Hz) across the array will be discussed with their dependency on controlling parameters.