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Impact of Density Gradients on Net Sediment Transport Into the Wadden Sea

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Impact of Density Gradients on Net Sediment Transport Into the Wadden Sea 566 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 38 Impact of Density Gradients on Net Sediment Transport into the Wadden Sea HANS BURCHARD Baltic Sea Research Institute Warnemünde, Rostock, Germany GÖTZ FLÖSER GKSS Research Centre, Geesthacht, Germany JOANNA V. STANEVA Institute for Chemistry and Biology of the Marine Environment, Carl von Ossietzky University, Oldenburg, Germany THOMAS H. BADEWIEN Institute of Physics, Carl von Ossietzky University, Oldenburg, Germany ROLF RIETHMÜLLER GKSS Research Centre, Geesthacht, Germany (Manuscript received 10 March 2007, in final form 26 June 2007) ABSTRACT This study tests the hypothesis that horizontal density gradients have the potential to significantly con- tribute to the accumulation of suspended particulate matter (SPM) in the Wadden Sea. It is shown by means of long-term observations at various positions in the Wadden Sea of the German Bight that the water in the inner regions of the Wadden Sea is typically about 0.5–1.0 kg mϪ3 less dense than the North Sea water. During winter this occurs mostly because of freshwater runoff and net precipitation; during summer it occurs mostly because of differential heating. It is demonstrated with idealized one-dimensional water column model simulations that the interaction of such small horizontal density gradients with tidal currents generates net onshore SPM fluxes. Major mechanisms for this are tidal straining, estuarine circulation, and tidal mixing asymmetries. Three-dimensional model simulations in a semienclosed Wadden Sea embayment ϭ Ϫ3 Ϫ1 with periodic tidal forcing show that SPM with sufficiently high settling velocity (ws 10 ms )is accumulating in the Wadden Sea Bight because of density gradients. This is proven through a comparative model simulation in which the dynamic effects of the density gradients are switched off, with the conse- quence of no SPM accumulation. These numerical model results motivate future targeted field studies in different Wadden Sea regions with the aim to further support the hypothesis. 1. Introduction but also because they absorb light (and thus hinder primary production). In areas of high SPM concentra- Wadden Sea as suspended matter sink tion like the Wadden Sea, pelagic primary production Interest in suspended particulate matter (SPM) has decreases with respect to the open sea due to high wa- increased during the last decades not only because con- ter turbidity (Van Beusekom et al. 2001). taminants like lead and organic pollutants [e.g., poly- For this reason, the SPM concentration in the Wad- chlorinated biphenyls (PCBs) or tributyltin oxide (TBT)] den Sea, its variability, and its transport has been in- bind to SPM rather than dissolve in the water phase, vestigated in the past 50 yr with many experimental studies and modeling exercises. One of the most inter- esting features is that any SPM satellite scene (see, e.g., Corresponding author address: Hans Burchard, Baltic Sea Re- search Institute Warnemünde, Seestraße 15, D-18119 Rostock, Fig. 1) shows a high SPM concentration inside and a Germany. low concentration outside the Wadden Sea. This well- E-mail: [email protected] known feature alone should give rise to an outward- DOI: 10.1175/2007JPO3796.1 © 2008 American Meteorological Society Unauthenticated | Downloaded 09/27/21 04:03 PM UTC JPO3796 MARCH 2008 BURCHARD ET AL. 567 Fig. 1. Total suspended matter in the German Bight taken by the Medium Resolution Imaging Spectrometer (MERIS) on board the European Space Agency (ESA) satellite Envisat 12 Aug 2003. Unauthenticated | Downloaded 09/27/21 04:03 PM UTC Fig 1 live 4/C 568 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 38 directed SPM flow due to turbulent dispersion. How- velocity, to a net export of fine-grained material. Thus ever, the Wadden Sea is more or less stable on a time one or more mechanisms must counterbalance or even scale of decades or even centuries. Measurements, on exceed the outward-directed turbulent dispersion of the other hand (Pejrup 1988a,b; Eisma 1993; Puls et al. SPM. 1997; Townend and Whitehead 2003), often show an For such a mechanism, several suggestions have been inward transport (resulting in a sea bottom rise of 0.07 made: to8mmyrϪ1) that may be altered by human influences 1) Settling lag (Postma 1954; Van Straaten and Kuenen like the construction of dikes, land reclamation, or coastal lagoon closure (Afsluitdijk in the Netherlands 1958; Postma 1961; Bartholdy 2000): Sedimented separating the Lake Ijssel from the North Sea, and the material is taken into suspension by the flood cur- Hindenburg and Rømø dams closing the Sylt–Rømø rent at a certain critical water velocity and trans- Bight to the south and north, respectively; see Fig. 2). ported onshore. Once the velocity falls below the Observational evidence shows that suspended matter critical value (because of changing tide or decreas- deposition is inhomogeneous in time and in space in- ing current velocity toward the mainland), the par- side the Wadden Sea. Suspended matter is accumulated ticles do not settle immediately, but take some time in summer but eroded during winter or during severe to reach the ground, moving farther inward than a storms (e.g., Lumborg and Pejrup 2005). The deposi- symmetric water velocity pattern would allow. The tion rate may be high on tidal flats but low or negative basic assumption of the settling lag mechanism (and in the tidal channels (Dyer 1994; Pedersen and Bar- also the second mechanism) is a nonzero sinking tholdy 2006). Thus, averages like annual accumulation velocity for SPM and an inwardly decreasing water rates are difficult to estimate. Moreover, the annual velocity that introduces the asymmetry. sediment budget can be largely determined by extreme 2) Scour lag (Postma 1954; Van Straaten and Kuenen storm events (see Lumborg and Pejrup 2005). A very 1958; Postma 1961; Bartholdy 2000): During ebb, a effective method therefore derives deposition rates higher current speed is required to erode the par- from sediment cores by measuring activities of radio- ticles from the bottom than the current speed at active elements. However, not many areas are suited which the particles have settled during the flood be- for this purpose because the Wadden Sea sediment is fore. This is due to the fact that critical shear stresses often reworked in the upper 10 cm by benthic macro- above which sedimentation ceases are typically fauna (Andersen et al. 2000). lower than critical shear stresses above which ero- Eisma (1993) estimated that the Wadden Sea sedi- sion takes place. Bartholdy (2000) has shown for the ment growth, in general, keeps pace with sea level rise northern Danish Wadden Sea that the scour lag is and land subsidence; he reports sedimentation rates of much more important than the settling lag. 10–28 mm yrϪ1 for the Dollart, Jade Bay, and Leybucht 3) Asymmetric tidal water level curve and current veloc- in the German Bight. De Haas and Eisma (1993) re- ities (Groen 1967; Dronkers 1986a,b): Lumborg and ported 1–2mmyrϪ1 (with variations up to 8 mm yrϪ1) Windelin 2003 explain in a qualitative way the ef- in the Dollart. Postma (1981) estimates that the tide fects of asymmetric current profiles on the sediment entering the Wadden Sea leaves 3.5 ϫ 106 tyrϪ1 of transport, and two asymmetries are taken into ac- suspended matter behind, which amounts to an annual count. First, the change in current velocity during average growth of 0.27 mm (with 8500-km2 area for the high water is much slower than that during low wa- Wadden Sea and 1500 kg mϪ3 density for the SPM). ter. Therefore, the suspended sediment has more Pejrup (1997) measured with 210Pb dating an accumu- time to settle down on the tidal flats than at low tide lation rate in the Sylt–Rømø Bight of 58 000 t yrϪ1, where water is mainly in the deeper tidal channels. 63% of which are due to the water exchange with the Second, the current velocity is higher during flood North Sea. With an area of 400 km2 for the bight, this than during ebb; thus the sediment is carried farther amounts to only 0.06 mm yrϪ1. Andersen et al. (2000) inward. Even if the tidal wave has a symmetric shape and Andersen and Pejrup (2001) measured rates of outside the Wadden Sea, it may be distorted when 5–12 mm yrϪ1 for a specific site in the Sylt–Rømø Bight entering the tidal channels. Lumborg and Windelin demonstrating that the accretion rates can differ by two (2003) argue that the tidal wave once entering one orders of magnitude inside the same basin. of the Wadden Sea Bights is distorted because of However, there is also some evidence of sediment decreasing water depths, resulting in higher current export: Flemming and Nyandwi (1994) argue that the speed during flood than during ebb. Each of these combined influence of sea level rise and the construc- effects may lead to inward transport of suspended tion of dikes leads, by an increase in the mean tidal matter. However, Ridderinkhof (2000) found that, Unauthenticated | Downloaded 09/27/21 04:03 PM UTC MARCH 2008 BURCHARD ET AL. 569 FIG. 2. German Bight and the positions of the measuring devices in the Wadden Sea. The German Bight image was taken by MERIS on board the ESA satellite Envisat. The insets are taken from Google Earth satellite scenes. On the right inset, the dams connecting the islands of Sylt and Rømø with the mainland can be clearly seen. Unauthenticated | Downloaded 09/27/21 04:03 PM UTC Fig 2 live 4/C 570 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 38 in the Ems-Dollard estuary, the velocity profiles are the mechanisms leading to net SPM transports down opposite to the ones reported from the western the salinity gradient (section 4).
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