The objective of the study described in this paper is to localize the transport path of suspended particulate matter (SPM) in the Dutch coastal zone in the southern North Sea. It is known that a large mass of SPM is transported northward from the Strait of Dover, which is however mostly hidden from satellite and other surface measurements. The study area is located at 80 km north of the Rhine-Meuse estuary mouth in the far-field plume of the region of freshwater influence (ROFI). We investigate the occurrence and persistence of a turbidity maximum zone (TMZ) in an area closer to the coast than studied in previous observational programs. Shipboard measurements of vertical profiles of SPM concentrations, density and current velocities with a high cross-shore spatial resolution are presented. A turbidity maximum zone is found at a distance between 0.5 and 3 km from the coast along 30 km of the coastline. Observed concentrations are shown to vary strongly within a tidal cycle, and also between contrasting meteorological conditions in terms of the springneap tidal cycle, the significant wave height and the wind force. Temporary stratification is observed during spring tides, and occurs on the ebb phase of the tidal cycle. Cross-shore transports at a transect perpendicular to the coast show an accumulation of SPM in the TMZ within one tidal cycle.Possible mechanisms for this accumulation close to the coast are discussed.2

The dynamics of suspended particulate matter (SPM) and chlorophyll-a from intratidal to annual time scales in a coastal turbidity maximum.

This study identifies and unravels the processes that lead to stratification and destratification in the far field of a Region of Freshwater Influence (ROFI). We present measurements that are novel for two reasons: (1) measurements were carried out with two vessels that sailed simultaneously over two cross-shore transects; (2) the measurements were carried out in the far field of the Rhine ROFI, 80 km downstream from the river mouth. This unique four dimensional dataset allows the application of the 3D potential energy anomaly equation for one of the first times on field data. With this equation, the relative importance of the depth mean advection, straining and nonlinear processes over one tidal cycle is assessed. The data shows that the Rhine ROFI extends 80 km downstream and periodic stratification is observed. The analysis not only shows the Van Oord, Rotterdam, The Netherlands important role of cross-shore tidal straining but also the significance of along-shore straining and depth mean advection. In addition, the nonlinear terms seem to be small. The presence of all the terms influences the timing of maximum stratification. The analysis also shows that the importance of each term varies in the cross-shore direction. One of the most interesting findings is that the data are not inline with several hypotheses on the functioning of straining and advection in ROFIs. This highlights the dynamic behaviour of the Rhine ROFI, which is valuable for understanding the distribution of fine sediments, contaminants and the protection of coasts.

Results are presented from campaigns carried out in March and May 2012 across the main channel of the Vlie Inlet in the western Dutch Wadden Sea. On both occasions current velocities and concentrations of suspended particulate matter (SPM) were measured at six stations over one tidal cycle. Concentrations are found to be high only during late ebb and early flood, implying a brief but intense export and import of SPM, respectively. Transport of water and SPM occurs predominantly over the southwestern part of the channel. The tidal prism in the channel varies in the range of 5–7 × 108 m3. Gross amounts of SPM transported during ebb and flood are 6–8 × 106 kg during the first campaign, but only half that amount during the second, possibly due to a different wind direction. The limitations encountered when calculating net effects over a tidal cycle are discussed, such as the fundamental indeterminacy in the duration of the tidal period itself.

Large amounts of suspended matter are transported through the Dutch coastal zone in the southern North Sea. Current estimates, based on budget studies, are in the order of 15 -20 Mton per year transported in northward direction, which should take place in a small strip of 5 -10 km wide. For this study we have performed a series of measurements on total suspended matter in an area in the most northerly extent of the Rhine region of fresh water influence. The measurements focused on observations both in the vertical and in the horizontal on the behavior of suspended matter in the nearshore zone up to 7 km from the shoreline. A peak in bottom concentrations is observed close to the coast along the coastal stretch. This hot spot location is found in the cross-shore direction at about 1.5 km from the coastline at a water depth of 15 m. Here, total suspended matter concentrations near the bottom exceed 200 mg/l. These peak concentrations have not been identified before and add to the suggestion that a large part of the northward suspended matter transport occurs very close to the coast.