Attention is given to the properties of sediment beds over the full range of conditions (silts to gravel), in particular the effect of fine silt on the bed composition and on initiation of motion (critical conditions) is discussed. High-quality bed-load transport data sets are identified and analyzed, showing that the bed-load transport in the sand range is related to velocity to power 2.5. The bed-load transport is not much affected by particle size. The prediction of bed roughness is addressed and the prediction of bed-load transport in steady river flow is extended to coastal flow applying an intrawave approach. Simplified bed-load transport formulas are presented, which can be used to obtain a quick estimate of bed-load transport in river and coastal flows. It is shown that the sediment transport of fine silts to coarse sand can be described in a unified model framework using fairly simple expressions. The proposed model is fully predictive in the sense that only the basic hydrodynamic parameters (dep...

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Unified view of sediment transport by currents and waves. I: Initiation of motion, bed roughness, and bed-load transport

Closed-Loop Turbulence Control: Progress and Challenges

Bed-load transport for steady flows and unsteady oscillatory flows

[1] The morphodynamic response of the nearshore zone of an embayed beach induced by wave groups is examined with a numerical model. The model utilizes the nonlinear shallow water equations to phase resolve the mean and infragravity motions in combination with an advection-diffusion equation for the sediment transport. The sediment transport associated with the short-wave asymmetry is accounted for by means of a time-integrated contribution of the wave nonlinearity using stream function theory. The two-dimensional (2-D) computations consider wave group energy made up of directionally spread, short waves with a zero mean approach angle with respect to the shore normal, incident on an initially alongshore uniform barred beach. Prior to the 2-D computations, the model is calibrated with prototype flume measurements of waves, currents, and bed level changes during erosive and accretive conditions. The most prominent feature of the 2-D model computations is the development of an alongshore quasi-periodic bathymetry of shoals cut by rip channels. Without directional spreading, the smallest alongshore separation of the rip channels is obtained, and the beach response is self-organizing in nature. Introducing a small amount of directional spreading (less than 2°) results in a strong increase in the alongshore length scales as the beach response changes from self-organizing to being quasi-forced. A further increase in directional spreading leads again to smaller length scales. The hypothesized correlation between the observed rip spacing and wave group forced edge waves over the initially alongshore uniform bathymetry is not found. However, there is a correlation between the alongshore length scales of the wave group-induced quasi-steady flow circulations and the eventual alongshore spacing of the rip channels. This suggests that the scouring associated with the quasi-steady flow induced by the initial wave groups triggers the development of rip channels via a positive feedback mechanism in which the small scour holes start attracting more and more discharge.

Morphodynamic modeling of an embayed beach under wave group forcing

Concentrations in oscillatory sheet flow for well sorted and graded sands

Phase lags in oscillatory sheet flow: experiments and bed load modelling

Shoaling short gravity waves at sea approaching the shore become asymmetric and are able to generate a net resulting sand transport in cross-shore direction (on-shore-offshore transport). The wave-related sand transport is still very difficult to predict due to the complexity of its underlying processes, which mainly take place in a thin layer near the sea bed in the wave boundary layer (thickness of order centimeters). The development of models for cross-shore sand transport heavily relies on experimental lab research, especially as taking place in large oscillating water tunnels (see, e.g., Nielsen, 1992). In oscillating water tunnels the near-bed horizontal orbital velocity, as induced by short gravity waves, can be simulated above fixed or mobile sandy beds (for a detailed description, see, e.g., Ribberink and Al-Salem, 1994). It should be realized that the vertical orbital flow and relatively small wave-induced residual flows as streaming and drift are not reproduced in flow tunnels. Research aimed at their contribution to the net sediment motion under surface waves is still ongoing (see Ribberink et al., 2000).

Sand Motion induced by oscillatory flows: sheet flow and vortex ripples

Field observations often show a considerable variation in mean grain size along the coastal profile. Under high waves in shallow water, bed ripples are washed out, and sheet flow becomes the dominant transport mode: large amounts of sand are transported in a thin layer close to the bed, i.e., the sheet flow layer. This paper focuses on grain size influences on transport processes in oscillatory sheet flow. Experiments were carried out in the Large Oscillating Water Tunnel (LOWT) of Delft Hydraulics, in which near-bed orbital velocities in combination with a net current can be simulated at full scale. Three uniform sands with different mean grain size were used. It was found that in contradiction to expressions found in literature, both the erosion depth and the sheet flow layer thickness are larger for fine sand (D 50 = 0.13 mm) than for coarser sand (D 50 ≥ 0.21 mm). Measured time-averaged velocity and concentration profiles indicate that the presence of a sheet flow layer leads to an increased flow resistance and to damping of turbulence and that these effects are stronger for a thicker sheet flow layer (i.e., for fine sand). These mobile-bed effects are analyzed further by comparing the measurements with the results of a one-dimensional vertical advection-diffusion boundary layer model. Simulating the mobile-bed effects in the model by introducing an increased roughness height and a reduced turbulent eddy viscosity showed that the roughness height is of the order of the sheet flow layer thickness and that turbulence damping increases for a decreasing grain size.

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Mobile-bed effects in oscillatory sheet flow

http://www.geo.uu.nl/fg/gruessink/PPT_files/HassanCE.pdf

Transport processes of uniform and mixed sands in oscillatory sheet flow

The sheet-flow layer, under a combination of waves and mean current, is typically a few millimeters thick and characterized by very high sediment concentrations. Small conductivity probes, capable of measuring high sediment volume concentrations, were employed to estimate sediment fluxes within the sheet-flow layer. Two probes, spaced 20 mm apart in the streamwise direction, were deployed upward through the sand bed in a water tunnel. Using correlation techniques, the sensors provided concentration information and estimates of particle velocity. These measurements yielded direct measurements of sediment flux. Two different sand sizes (120 and 320 μ) were investigated with mean velocities between 0.3 m/s and 0.5 m/s and oscillatory amplitudes between 0.9 m/s and 1.5 m/s that were created using monochromatic, sinusoidal waves of 7.2-s period. Mean sediment-velocity estimates, averaged over a wave period, were smaller than predictions of water velocity from a combined wave/current boundary layer model. On th...

Wael Hassan

Papers

Phase lags in oscillatory sheet flow: experiments and bed load modelling

Sand Motion induced by oscillatory flows: sheet flow and vortex ripples

Mobile-bed effects in oscillatory sheet flow

Transport processes of uniform and mixed sands in oscillatory sheet flow

Sand transport in oscillatory sheet flow with mean current