Wave-induced Bed Shear Stress Research Articles

Constraining the amplitude of late Oligocene bathymetric changes in Western Ross Sea during orbitally-induced oscillations in the East Antarctic Ice Sheet: (1) Implications for glacimarine sequence stratigraphic models

Late Oligocene shallow glacimarine sequences recovered from western Ross Sea, Antarctica by the Cape Roberts (drilling) Project display orbitally-influenced cycles of advance and retreat of a laterally-extensive ice sheet across the continental shelf, in concert with changes in contemporary water-depth. During interglacial periods, when the glacier terminated on land, the coastline was largely ice-free and wave-influenced, and sediments accumulated in hydrodynamic equilibrium with the contemporary wave-climate. Here, we present estimates of paleobathymetry from intervals of three Milankovitch-duration glacimarine sequences (9, 10 and 11) that accumulated in open ocean conditions. We utilise an approach where the percentage of mud (<63 µm fraction) in bulk sediment is related to the wave-induced bed shear stress, and for a given wave climate, water depth (e.g. [Dunbar, G.B. and Barrett, P.J., 2005. Estimating palaeobathymetry of wave-graded continental shelves from sediment texture. Sedimentology 52, 253–269.]). Particle size-derived changes in paleobathymetry for the three late Oligocene sequences were between 20–40 and 60–90 m. These water depth changes are consistent with the magnitude of contemporary global eustatic sea-level changes of 30–40 m estimated from far-field continental margin and deep-marine ocean proxy records. On the basis of our bathymetric constraints we contribute to a conceptual stratigraphic model for shallow glacimarine sequences, whose depositional architecture is controlled by a combination glacier advance and retreat and changes in relative sea-level.

On the sensitivity of the sedimentary system in the East Frisian Wadden Sea to sea-level rise and wave-induced bed shear stress

The paper addresses the individual and collective contribution of different forcing factors (tides, wind waves, and sea-level rise) to the dynamics of sediment in coastal areas. The results are obtained from simulations with the General Estuarine Transport Model coupled with a sediment transport model. The wave-induced bed shear stress is formulated using a simple model based on the concept that the turbulent kinetic energy (TKE) associated with wind waves is a function of orbital velocity, the latter depending on the wave height and water depth. A theory is presented explaining the controls of sediment dynamics by the TKE produced by tides and wind waves. Several scenarios were developed aiming at revealing possible trends resulting from realistic (observed or expected) changes in sea level and wave magnitude. The simulations demonstrate that these changes not only influence the concentration of sediment, which is very sensitive to the magnitude of the external forcing, but also the temporal variability patterns. The joint effect of tides and wave-induced bed shear stress revealed by the comparison between theoretical results and simulations is well pronounced. The intercomparison between different scenarios demonstrates that the spatial patterns of erosion and deposition are very sensitive to the magnitude of wind waves and sea-level rise. Under a changing climate, forcing the horizontal distribution of sediments adjusts mainly through a change in the balance of export and import of sediment from the intertidal basins. The strongest signal associated with this adjustment is simulated North of the barrier islands where the evolution of sedimentation gives an integrated picture of the processes in tidal basins.

Wave friction factor as related to the Shields parameter for steady currents

The wave friction factor f w is an important dimensionless parameter used to estimate wave-induced bed shear stress which, together with current-induced bed shear stress, controls sediment transport in the marine and lacustrine environment. However, f w is either overestimated or underestimated by existing equations. The matter is complicated by the fact that different equations must be used for hydrodynamically smooth or rough flows, but the limit between these two boundary conditions is still poorly defined. Some equations also require estimated values for the Nikuradse roughness length and the wave boundary layer thickness, neither of which can be derived accurately. It is, therefore, not surprising that f w values as calculated by existing equations differ by as much as a factor of 3. In the present paper, f w is related to the Shields parameter β for unidirectional currents, which not only facilitates the direct comparison and summation of bed shear stresses, but also yields a more accurate equation valid for grains of different density in any fluid irrespective of the boundary condition.

Infiltration effects on sediment mobility under waves

Sediment mobility measurements with a horizontal sand bed under non-breaking waves are reported. Conditions include no seepage and steady downward seepage corresponding to head gradients up to 2.5. The results indicate that infiltration tends to inhibit sediment mobility for a horizontal bed of 0.2-mm quartz sand exposed to moderated wave induced bed shear stresses. The effect is weak for the parameter range of the present study. The two opposing effects of shear stress increase due to boundary layer thinning and the stabilizing downward drag are successfully accounted for through the modified Shields parameter of Nielsen [Nielsen, P., 1997. Coastal groundwater dynamics. Proc. Coastal Dynamics '97, Plymouth, ASCE, pp. 546–555] using coefficients derived from independent studies. That is, from the shear stress experiments of Conley [Conley, D.C., 1993. Ventilated oscillatory boundary layers. PhD Thesis, University of California, San Diego, 74 pp.] and the slope stability experiments of Martin and Aral [Martin, C.S. and M.M. Aral, 1971. Seepage force on interfacial bed particles. J. Hydraulics Div., Proc. ASCE, Vol. 97, No. Hy7, pp. 1081–1100].

Storm surge effects on a back-barrier tidal flat of the Danish Wadden Sea

The most severe storm passing Denmark in the 20th century occurred on 3–4 December 1999. During this event, waves, currents and suspended sediment concentrations were measured on a muddy tidal flat vis-a-vis the salt marsh of Skallingen in the inner part of the Hobo Dyb tidal area on the Danish west coast. Locally generated wave heights increased during the storm to Hs=0.38 m, with peak spectral wave periods of up to 3.6 s and wave-induced bed shear stresses reaching a maximum of about 2 N m–2. The highest current velocity measured over the tidal flat during the surge was 0.64 m s–1 (~1 N m–2). During the surge peak, the wind-induced water circulation close to the bed was about 0.2 m s–1 which is equivalent to a bed shear stress of about 0.2 N m–2. The concentration of suspended matter on the tidal flat increased from about 10 mg l–1 to about 200 mg l–1, and the mobile layer on the mud flat was removed. The estimated deposition on the salt marsh amounted to 133 g m–2 (0.15 mm) which is only about 50% higher than that of a previously monitored gale event, and corresponds to less than 10% of the yearly deposition at the site. It is suggested that deposition on the salt-marsh surface caused by storms or gales depends on the prevailing meteorological and hydrographical conditions as well as on the sequence of previous import and high-energy events. The results indicate that a single extreme storm can be of less importance for annual variations in salt-marsh deposition than more regularly occurring gales.

Modeling of coastal profile evolution on the abandoned delta of the Huanghe River

This paper presents a quantitative, dynamic model for simulating and predicting the evolution of the nearshore muddy profile on the abandoned Huanghe Delta, which has been undergoing severe erosion since the shift of the Huanghe River in 1855. The model computes the wave-induced bed shear stresses on the muddy profile, and therefore calculates the erosion rate. The sea level fluctuation caused by the oceanic tide plays an important role in redistributing the wave force and hence the sediment transport. This factor is introduced into the model. The steady-state nearshore muddy profile is simulated as a response to the average erosion rate of the local wave climate. However, at long time scales, the averaged erosion rate is significantly influenced by the surface sand armouring and the vertical increase with depth of cohesive sediment consolidation during coastal erosion, leading to an irreversible evolution of the muddy profile. The Effective Scouring Wave, derived from local wave climate, is proposed to quantitatively incorporate these time-variant geological influences. The evolution of the muddy profile is determined by the change in the Effective Scouring Wave climate. The eroding muddy coast exhibits a monotonic, concave upward profile, whose curvature increases as the coastal erosion continues. The maximum curvature of the profile lies below the mean low tidal level. As the erosion continues the maximum curvature moves down and shoreward. The position of the maximum curvature of the nearshore muddy profile can be used to estimate the evolutionary stages of the muddy coast. On the northern abandoned Huanghe Delta, the muddy profile approaches an equilibrium configuration when the maximum curvature reaches down to a water depth of about 6 m below the mean sea level. The model reveals a dynamic geomorphic system as a response to the abandonment on the Huanghe Delta. Similar patterns and evolutionary processes may be generalized to erosional muddy coasts in other settings.