Experimental and Numerical study on Sheetflow Sediment transport under Skewed-asymmetric Waves and Currents

In recent years, sheetflow sand transport regime has attracted the attention of many coastal engineers and scientists as it is predominant in the surf zone. Sheetflow conditions develop when the near bed velocity is large enough to wash out sand ripples and transport sand in a thin layer with high sand concentration along the bed. This sand transport regime involves very large net transport rates and thus results significant changes of the beach topography. When waves propagate to the nearshore zone, their shapes gradually change primarily owing to the combined effects from wave shoaling, breaking, and nonlinear interactions. As waves enter the shallow water; their shapes evolve from sinusoidal to the pure velocity asymmetric waves (skewed waves) with sharp crests separated by broad, flat wave trough in intermediate water depths. As waves continue to shoal and break, they transform through asymmetrical, pitched-forward shapes with steep front faces in the inner surf, to a pure acceleration asymmetric waves (asymmetric waves) (pitched-forward) near the shore. In addition to the change of wave shapes, the interaction of nearshore waves and currents is also an indispensable hydrodynamic element in coastal regions. For example, the offshore-ward near-bottom current, referred to as undertow, develops to compensate the onshore flux caused by waves. This type of waves-currents interaction, however, is generally weak. In contrast, a strong interaction can be observed in the vicinity of river mouth. The existence of different wave shapes and their interactions with nearshore currents may lead to the different sediment transport behaviors. Many laboratory studies have been conducted in the oscillatory flow tunnels with sinusoidal flows, pure skewed and pure asymmetric flows. However, it is hardly found any experiment conducted with the combined skewed-asymmetric oscillatory flows and with strong opposing currents. Thus, new prototype scale laboratory tests (53 tests) using different wave shape conditions with and without the presence of strong opposing currents were performed. These experiments were motivated by the fact that most natural waves in surf zone produce mixed skewed-asymmetric oscillatory flows and sand transport at the river mouth is influenced by the interaction of nearshore waves and strong river discharge. Experimental results reveal that in most of the case with fine sand, the "cancelling effect", which balances the on-/off-shore net transport under pure asymmetric/skewed flows and results a moderate net transport, was developed for combined skewed-asymmetric flow. However, under some certain conditions (T > 5s) with coarse sands, the onshore sediment transport was enhanced by 50% under combined skewed-asymmetric flows. Additionally, the new experimental data under collinear oscillatory flows and strong currents show that offshore net transport rate increases with decreasing velocity skewness and acceleration skewness. Image analysis technique was employed to investigate major aspects of sediment transport under skewed-asymmetric flows and currents. Measured maximum erosion depths were found larger for shorter wave periods and for wave profiles with shorter time to maximum velocities. This suggested that faster flow acceleration could produce higher bed shear stress. In addition, the effect of flow acceleration is clearly seen in the near-bed sand particle velocities, with higher accelerations resulting in higher peak near-bed velocities. In a combined oscillatory-strong current flow, it is found that the presence of a strong steady current which results in larger ratio um/uw also increases the sheetflow layer thickness. It is because the appearance of currents in the opposite direction with waves could enlarge the available time length for flow erodes the sand bed and rises up sand to the maximum possible elevation. Thus, as a consequence it enlarges the sheetflow layer thickness. Taking into account the effects of mobile bed and the flow acceleration, empirical formulas have been proposed to estimate bed shear stress, the maximum erosion depth and the sheetflow layer thickness. Sand transport mechanism was investigated by comparing the bed shear stress and the phase lag parameter for each half cycle. The "phase lag parameter" was modeled as the ratio between the sheetflow layer thickness and the settling distance. By analyzing the temporal brightness distribution at different elevations which corresponds to the distribution of suspended sand concentration, it is precisely found that phase lag is considered to be significant once it value exceeds 0.9. In such circumstances, the so-called "cancelling effect", will occur. In contrast, in cases phase lag is small; the bed shear stress plays a more fundamental role as it causes an onshore enhancement for mixed shaped waves. A two phase flow model was employed to get further insight sand transport mechanism. Turbulent closure terms were modified to take into account the sand-induced stratification and a new criterion for non-moving interface was introduced. The simulated results agree well with observations. Analysis of forces acting on sand precisely shows that an increase of flow acceleration will increase applied forces on sand particles and hence the sand velocity travelling in the upper sheetflow layer. However, inside the pick-up region, due to high sand concentration, sand motions will be blocked by the intergranular stress and as a result it increases the bed shear stress. The two phase flow model also confirmed that the Nikuradse bed roughness which is often estimated as of the order of the sheetflow layer thickness appears to be corrected. Influences of mobile bed effects to the sheetflow structure were favorably discovered by the two phase flow model. Comparing with a fixed bed case, it is found that the variation of the unmovable bed over a wave cycle leads to an increase of eddy viscosity and thereby faster velocity damping in the upper boundary layer. In contrast, flow structure near the sand bed is much influenced by the high sand concentration in the sheetflow layer; resulting total different sand transport behaviors for the mobile bed. The importance of sand-induced stratification was also verified. Simulations including stratification effects reproduce better the relative transport contributions. It is also confirmed that the sand-induced stratification is an essential factor to maintain and keep sediment movements near the sand bed. The new net transport rate measurements were compared with several net transport rate models and found that those approaches fails to deliver an accurate prediction. The reason is pointed out due to the inappropriate estimates of the representative suspension height in their models. Thus the new estimation for sheetflow layer thickness was incorporated in a new net transport rate model, based on Watanabe and Sato's concept. The new model has been examined with comprehensive sheetflow experimental data and prediction skill over a wide range of hydraulics and sediment conditions shows that the new model fulfills for practical purposes and can be integrated into numerical morphodynamic models.報告番号: ; 学位授与年月日: 2012-09-27 ; 学位の種別: 課程博士 ; 学位の種類: 博士(工学) ; 学位記番号: ; 研究科・専攻: 工学系研究科社会基盤学専