Sediment dynamics in tidally dominated environments controlled by transport and turbulence: A case study for the East Frisian Wadden Sea

Abstract

[1] Various tide-related sediment transport mechanisms near a barrier island coast are addressed on the basis of analytical theory and numerical simulations carried out with a three-dimensional (3-D) numerical model. The theory proposed gives an explanation of the observed ''twin peaks'' in the concentration of suspended sediment; however, it does not adequately describe the whole variety of sediment responses. The major idea in this paper assumes that it is not only the change of the level of turbulence and advection individually but also the correlation between the two which controls the concentration of sediment. Because the horizontal gradients of sediment concentration are created not only by sources at the coast (e.g., fluxes from rivers) but also by the transport and turbulence, the system is highly nonlinear. Several simulations have been carried out aimed at revealing the nonlinear mechanisms in the sediment transport and individual contribution of turbulence and velocity field. An experiment in which the sediment model is driven only by turbulent kinetic energy fails to simulate most of the characteristic temporal and spatial sediment patterns. With another experiment it is demonstrated that the velocity gradient in the vertical gives an important mechanism shaping the patterns of temporal and spatial variability (and tidal asymmetries). Without shear diffusion, the sediment dynamics are largely governed by the ''displacement mechanism''; that is, the system is more linear. One practical conclusion from the numerical simulations is that using simplified (2-D) dynamics in sediment transport modeling along with externally prescribed turbulence characteristics could lead to large artifacts.

Hydrodynamics and sediment dynamics in the German Bight. A focus on observations and numerical modelling in the East Frisian Wadden Sea

“…Shortening the offset to 30 minutes reveals that the maximum acceleration of current occurs almost immediately after high and low water. This acceleration period increases substantially during neap tides, in particular during the flood phases, which is consistent with the earlier observations analysed by Stanev et al (2007a).…”

Hydrodynamics and sediment dynamics in the German Bight. A focus on observations and numerical modelling in the East Frisian Wadden Sea

“…Shortening the offset to 30 minutes reveals that the maximum acceleration of current occurs almost immediately after high and low water. This acceleration period increases substantially during neap tides, in particular during the flood phases, which is consistent with the earlier observations analysed by Stanev et al (2007a).…”

Residual Sediment Transport in Tidally Energetic Estuarine Channels With Lateral Bathymetric Variation

“…The sediment flux at the bed, , which is the bottom boundary condition for Equation , is based on well‐known parameterizations of erosion and deposition fluxes, E and D : (i) The erosion flux E is only nonzero when the bottom shear stress exceeds a critical value (Partheniades, 1965), that is, , where M 0 is the erosion coefficient, τ b is the bed shear stress, and τ ce is the critical shear stress for erosion. Here, M 0 is set to 5.0 × 10 −6 kg m −2 s −1 , which is comparable to the values suggested by several other studies (Sassi et al., 2015; Stanev et al., 2007; Staneva et al., 2009). The critical shear stress for erosion is calculated as , where τ ∗ ce is the critical Shields parameter: (Soulsby, 1997); (ii) The deposition flux D is only nonzero for bottom shear stresses smaller than a critical value and is limited by the near‐bottom sediment concentration (Krone, 1962), that is, , where c b is the near‐bed sediment concentration, and τ cd is the critical shear stress for deposition.…”

Acoustic and optical methods to infer water transparency at Time Series Station Spiekeroog, Wadden Sea

“…Therefore, mainly fine sediment concentrations are expected. A modelling study by Stanev et al (2007) showed different sediment concentrations and dynamics for fine SPM (mud, d mud = 63 µm; d: diameter) and sand (d sand = 200 µm) for this Wadden Sea area. Depending on the specific location, the dynamics of the different sediment types (fine or coarse) act differently dependent on the tidal signal.…”

Hydrodynamics and sediment dynamics in the German Bight. A focus on observations and numerical modelling in the East Frisian Wadden Sea

“…Shortening the offset to 30 minutes reveals that the maximum acceleration of current occurs almost immediately after high and low water. This acceleration period increases substantially during neap tides, in particular during the flood phases, which is consistent with the earlier observations analysed by Stanev et al (2007a).…”

Residual Sediment Transport in Tidally Energetic Estuarine Channels With Lateral Bathymetric Variation

“…The sediment flux at the bed, , which is the bottom boundary condition for Equation , is based on well‐known parameterizations of erosion and deposition fluxes, E and D : (i) The erosion flux E is only nonzero when the bottom shear stress exceeds a critical value (Partheniades, 1965), that is, , where M 0 is the erosion coefficient, τ b is the bed shear stress, and τ ce is the critical shear stress for erosion. Here, M 0 is set to 5.0 × 10 −6 kg m −2 s −1 , which is comparable to the values suggested by several other studies (Sassi et al., 2015; Stanev et al., 2007; Staneva et al., 2009). The critical shear stress for erosion is calculated as , where τ ∗ ce is the critical Shields parameter: (Soulsby, 1997); (ii) The deposition flux D is only nonzero for bottom shear stresses smaller than a critical value and is limited by the near‐bottom sediment concentration (Krone, 1962), that is, , where c b is the near‐bed sediment concentration, and τ cd is the critical shear stress for deposition.…”

Acoustic and optical methods to infer water transparency at Time Series Station Spiekeroog, Wadden Sea

“…Therefore, mainly fine sediment concentrations are expected. A modelling study by Stanev et al (2007) showed different sediment concentrations and dynamics for fine SPM (mud, d mud = 63 µm; d: diameter) and sand (d sand = 200 µm) for this Wadden Sea area. Depending on the specific location, the dynamics of the different sediment types (fine or coarse) act differently dependent on the tidal signal.…”

Hydrodynamics and sediment dynamics in the German Bight. A focus on observations and numerical modelling in the East Frisian Wadden Sea

“…Shortening the offset to 30 minutes reveals that the maximum acceleration of current occurs almost immediately after high and low water. This acceleration period increases substantially during neap tides, in particular during the flood phases, which is consistent with the earlier observations analysed by Stanev et al (2007a).…”

Residual Sediment Transport in Tidally Energetic Estuarine Channels With Lateral Bathymetric Variation

“…The sediment flux at the bed, , which is the bottom boundary condition for Equation , is based on well‐known parameterizations of erosion and deposition fluxes, E and D : (i) The erosion flux E is only nonzero when the bottom shear stress exceeds a critical value (Partheniades, 1965), that is, , where M 0 is the erosion coefficient, τ b is the bed shear stress, and τ ce is the critical shear stress for erosion. Here, M 0 is set to 5.0 × 10 −6 kg m −2 s −1 , which is comparable to the values suggested by several other studies (Sassi et al., 2015; Stanev et al., 2007; Staneva et al., 2009). The critical shear stress for erosion is calculated as , where τ ∗ ce is the critical Shields parameter: (Soulsby, 1997); (ii) The deposition flux D is only nonzero for bottom shear stresses smaller than a critical value and is limited by the near‐bottom sediment concentration (Krone, 1962), that is, , where c b is the near‐bed sediment concentration, and τ cd is the critical shear stress for deposition.…”

Acoustic and optical methods to infer water transparency at Time Series Station Spiekeroog, Wadden Sea

“…Therefore, mainly fine sediment concentrations are expected. A modelling study by Stanev et al (2007) showed different sediment concentrations and dynamics for fine SPM (mud, d mud = 63 µm; d: diameter) and sand (d sand = 200 µm) for this Wadden Sea area. Depending on the specific location, the dynamics of the different sediment types (fine or coarse) act differently dependent on the tidal signal.…”