Field Observations of Tidal Flow Separation at a Mega-Scale Beach Nourishment

Abstract

Abstract:Large beach nourishments are often considered an efficient way to deal with coastal zone management issues, notably coastal erosion. Such large nourishments can act as a geometric perturbation, which might cause the tidal current to detach from the coastline. The Sand Motor is a mega-scale beach nourishment in the Netherlands. Field observations of the tidal flow field north of the Sand Motor are presented, showing evidence of flow separation and eddy formation during the flood phase of the tide. GPS-tracked drifters are transported offshore upon deployment near the tip of the nourishment. Flow convergence measured with nearshore current profilers is indicative of flow reversal at the lee side. Images of the temporal development of coherent patterns in time-averaged X-band radar backscatter align with in-situ observations of the flow field. Tidal flow separation at such large nourishments might have an impact on morphological development of the nourishment, as well as on swimmer safety.

Tidal flow separation at protruding beach nourishments

“…A sequence of these images can be found in Radermacher et al . [], but further analysis of the radar data is omitted here.…”

“…In the time series of alongshore flow velocity, the influence of these forcing mechanisms is not visible, except for the positive peak during the late flood phase. As noted before, comparison with X‐band radar imagery [ Radermacher et al ., ] suggest that these peaks coincide with the arrival of the freshwater front.…”

“…Time-averaged backscatter intensity images [e.g., Dankert et al, 2003;Haller et al, 2014] have supported a qualitative understanding of flow patterns around the Sand Motor. A sequence of these images can be found in Radermacher et al [2015], but further analysis of the radar data is omitted here.…”

A Mega-Nourishment (Sand Motor) Affects Landscape Diversity of Subtidal Benthic Fauna

“…In contrast to beach nourishments and regular shoreface nourishments, it has been shown that the Sand Motor has a size that is sufficient to significantly change the tidal currents. Field and model data presented by Radermacher et al (2015) and Radermacher et al (2017) show a large tidal eddy on either side of the Sand Motor. Grain size data presented by Huisman et al (2016) confirm the image, and it is also reproduced by the model (Luijendijk et al, 2017).…”

“…Regular shoreface nourishments are known to shift the area of wave breaking seaward and thereby modify longitudinal and crossshore sediment transport rates (van Rijn, 2011), but the effect is lasting for only a few years and little interference with tidal currents is usually observed because the nourishments are narrow and oriented in the direction of the tidal currents. In contrast, the Sand Motor has already been shown to significantly interfere with tidal currents, contracting the flow lines near the peninsula (Radermacher et al, 2015) and causing recirculating eddies at both sides in the residual flow. This influences nearby sediment sorting (Huisman et al, 2016) and diversifies the correlation structure between depth, waves, tidal current and sediment composition.…”