M. Radermacher

Tidal flow separation at protruding beach nourishments

In recent years, the application of large-scale beach nourishments has been discussed, with the Sand Motor in the Netherlands as the first real-world example. Such protruding beach nourishments have an impact on tidal currents, potentially leading to tidal flow separation and the generation of tidal eddies of length scales larger than the nourishment itself. The present study examines the characteristics of the tidal flow field around protruding beach nourishments under varying nourishment geometry and tidal conditions, based on extensive field observations and numerical flow simulations. Observations of the flow field around the Sand Motor, obtained with a ship-mounted current profiler and a set of fixed current profilers, show that a tidal eddy develops along the northern edge of the mega-nourishment every flood period. The eddy is generated around peak tidal flow and gradually gains size and strength, growing much larger than the cross-shore dimension of the coastline perturbation. Based on a 3 week measurement period, it is shown that the intensity of the eddy modulates with the spring-neap tidal cycle. Depth-averaged tidal currents around coastline perturbations are simulated and compared to the field observations. The occurrence and behavior of tidal eddies is derived for a large set of simulations with varying nourishment size and shape. Results show that several different types of behavior exist, characterized by different combinations of the nourishment aspect ratio, the size of the nourishment relative to the tidal excursion length, and the influence of bed friction.

Applicability of video-derived bathymetry estimates to nearshore current model predictions

ABSTRACT Radermacher, M., Wengrove, M.E., Van Thiel de Vries, J.S.M., Holman, R.A., 2014. Applicability of video-derived bathymetry estimates to nearshore current model predictions. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 290–295, ISSN 0749-0208. In the framework of swimmer safety, coastal managers desire accurate nearshore current predictions obtained from numerical models. To this end, detailed and up-to-date bathymetry is a necessity. Remote sensing techniques for bathymetry estimation are a promising solution. The focus of this paper is to assess the performance of wavenumber-based bathymetric inversion using Argus imagery (also known as the cBathy algorithm) as a feasible input bathymetry for numerical models to make reasonable nearshore current predictions. Numerical flow simulations on a cBathy bed are compared to simulations on an in-situ surveyed bathymetry. Results demonstrate that simulated nearshore currents on a cBathy bathymetry have a root-mean-square error in the order of 10 cm/s (magnitude) and 40 degrees (direction) when compared to simulated currents on a surveyed bathymetry. In the intertidal zone cBathy should be combined with a different method for bathymetry estimation in order to decrease these errors.

Field observations of tidal flow separation at a mega-scale beach nourishment

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.

A check on the efficiency of an air-bubble screen using acoustic measurements and an artificial tracer

PurposeA working group, composed of the University of Genoa (Italy), responsible for monitoring the dredging of the Port of Genoa, and Royal Boskalis Westminster N.V. (The Netherlands), responsible for the dredging activities, carried out an experiment on closing a landfill area with a double air bubble-screen (ABS) to verify the efficiency both of this method of containing sediment and of the measurement method applied.Materials and methodsWithin the Port of Genoa dredging, which was started in 2009, the dredged material is used to fill two areas between existing quays. To contain the sediment to be placed into a third subbasin without interrupting the passage of the barges which discharge the sediment, the use of an ABS was proposed as a barrier to the spreading of the sediment. The experiment was carried out during a 4-day oceanographic campaign to verify, preliminarily, the results under different weather and sea conditions. An artificial tracer, rhodamine water tracing (RWT), was used in varying concentrations to simulate the spreading of the sediment, and an RWT sensor and an acoustic Doppler current profiler (ADCP) were used to measure the RWT distribution and verify the efficiency of the ABS.Results and discussionThe high turbidity in the port and the high rate of RWT dilution made it relatively difficult to detect the RWT. The RWT sensor sometimes measured higher values than expected probably due to the following: the recirculation of the tracer in current eddies, other sensible suspended matter and adsorption of RWT on suspended matter. The internal ABS could not retain all the RWT, possibly because it was much weaker than the external one, and because mitigating measures, like silt screens or ABSs, are not totally closed and should not be used if complete impermeability is required. The ABS introduction accentuated the natural separation between the basin and the channel water bodies creating a barrier between the two basins.ConclusionsAn ABS installed at the entrance to a confined area with low dynamics results in a circular vertical flow with the potential to retain suspended sediments within an enclosed area. The meteorological variability during the tests showed that a weak ABS could malfunction in the presence of strong wind condition. The results showed that the ADCP is a suitable instrument to highlight the ABS position and its effect on the surrounding hydrodynamics. Further research, using a sediment tracer, is required to study with greater detail the effects of the ABS on the water column.