Wael Hassan

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

Sheet flow corresponds to the high velocity regime when small bed ripples are washed out and sand is transported in a thin layer close to the bed. Therefore, it is often assumed that sand transport in oscillatory sheet flow behaves quasi-steady: time-dependent transport rates are assumed to be instantaneously related to the near-bed orbital velocity. However, new experimental results show that even in sheet flow, phase lags between sediment concentration and near-bed velocity can become so large that they lead to a reduction of the net (wave-averaged) transport rate. A phase lag parameter is defined, which shows that phase lags become important for fine sand, high velocities and short wave periods. A semi-unsteady model is developed that includes the effects of phase lags on the net transport rate. New experiments were carried out in a large oscillating water tunnel with three different sands (D50=0.13, 0.21 and 0.32 mm) for a range of prototype, combined wave–current flow conditions. Measured net transport rates were compared with predictions of a quasi-steady model and the new semi-unsteady model. This comparison indicates that net transport rates are reduced if phase lags become important: the quasi-steady model overestimates the net transport rates and the semi-unsteady model gives better agreement with the data. Time-dependent measurements of velocities and concentrations show that the reduced net transport rates can indeed be explained by phase lag effects, because for tests with smaller net transport rates than predicted by the quasi-steady model, considerable phase lags between velocities and concentrations were observed, even inside the sheet flow layer. Verification of the semi-unsteady model against a larger data set confirms the occurrence of phase lag effects in oscillatory sheet flow. Also for the larger data set the semi-unsteady model yields better agreement with the data than the quasi-steady model.

Mobile‐bed effects in oscillatory sheet flow

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.

Modelling of Sand Transport with Graded Sands under Oscillatory Sheet-Flows

The present study focuses at improving our basic understanding of size- graded sand transport mechanisms under oscillatory sheet-flow conditions using different transport models. Moreover, the study aims at improving the performance of different transport model concepts for the description of both the rate and size- composition of the transported sand. Quasi-steady and intermediate models are investigated, in combination with a large dataset of oscillatory sediment transport measurements (sheet-flow conditions) as collected from two oscillatory water tunnels. A comparison is made between the measured net transport rates and transport rates per size fraction and the results from various transport formulas (Bailard 1981; Ribberink 1998; Dibajnia & Watanabe 1996; Dohmen-Janssen et al., 2002). First, the comparison is based on a 1-fraction approach i.e. the sand is modeled using D50 of the mixture. Second, a size-fraction approach is tested and developed further using the experimental dataset. Exposure corrections are proposed for fine and coarse size fractions in the sand mixture to improve model performance.