Tidal Flow Separation at the Sand Motor

This study investigates the existence and character of tidal flow separation at the Sand Motor, a mega-scale sand nourishment at the Delfland coast. Tidal flow contracts at the tip of the Sand Motor, downstream the tip the flow diverges and flow separation could occur. In case of flow separation the diverging streamlines cannot follow the contours of the coastline and the main flow will detach from the coastline. Flow separation causes an eddy downstream of the tip. The spatial variation in tidal flow is important to investigate, because the presence of tidal flow separation could be a hazard for swimmer safety. An advantage of the presence of tidal flow separation is the possible increased mixing of nutrients. By a literature study the first expectations about the location and period of tidal flow separation are done. Comparable studies to tidal flow separation are done in the past, which are relevant for this research. Two important conditions for tidal flow separation are an adverse pressure gradient and a developed boundary layer. From the literature study can be concluded that the location of the eddy generated by tidal flow separation is on the leeside of the headland. During the second half of the flood or ebb period tidal flow separation was expected, because then the two conditions are present. The core of this research is the field measurement campaign using GPS-tracked drifters. In total 15 measurement sessions are performed during both flood and ebb periods. The drifters recorded flow convergence and offshore flow near the tip of the Sand Motor, which possibly indicated the presence of tidal flow separation. The observed flow patterns are subsequently processed and characterized. To characterize the measurement sessions, the environmental conditions are researched. By observing radar images of the drifter sessions, the importance of the freshwater plume, discharged from the Rhine River, became visible. It interrupted tidal flow separation multiple times during drifter sessions. Also tidal flow separation and the associated eddy were observed in the radar images. Interpretation of the measurements shows that the wind conditions and the freshwater plume are the most important environmental conditions for the appearance or absence of tidal flow separation. By observing the results of several flood measurement sessions, the eddy seems to grow in time and gradually extends further offshore. The importance of the timing of the drifter sessions, to measure the tidal flow separation is tested with the numerical model D-Flow FM. The model is used to research the development in time of the flow separation and the associated eddy. Simulations show that the eddy travels from north to south during flood, which explains the measurements several flow patterns during flow separation. The simulations also show growth of the eddy between its generation and dissipation. The model is also used to research the adverse pressure gradient, by testing when the Bernoulli Effect is active at the tip of the Sand Motor. Around the peak of the high water the effect is present, causing an earlier presence of an adverse pressure gradient, which is an essential condition for tidal flow separation. This means that flow separation already could start around the peak of the high water during flood. Tidal flow separation could be a hazard for swimmers near the tip of the Sand Motor. If caught by the current during tidal flow separation, there is no easy escape opportunity and swimmers can be transported a couple of hundred metres offshore. Not only the tidal flow separation is a hazard for swimmer safety, but also the currents in the surf zone could be dangerous. These are even less predictable than tidal flow separation, so the general advice for swimmers would be not to swim near the tip of the Sand Motor.