The Role of Sand in Wave-supported Gravity Flows Over Primarily Muddy Seabeds PDF Download

Author: Zhuochen Han
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Languages : en
Pages : 128

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Book Description
Wave-supported gravity flows (WSGF) are one of the most crucial processes contributing to sediment transport across continental shelves with gentle bottom slopes. This dissertation studies the mechanisms of WSGF over primarily muddy shelves through laboratory experiments in an oscillatory water tunnel with a sediment bed of either 1% or 13% sand fraction. Physics behind this includes sand fraction effects on the dynamics of WSGF under equilibrium state, the role of sand in controlling sediment suspensions under equilibrium state, and key factors that affect the dynamics of WSGF during the bed adjustment period before the equilibrium state. First, sand fraction effects on the dynamics of wave-supported gravity flows in mud-dominant environments are investigated. Low and high energy regimes are differentiated based on a Stokes Reynolds number Re[Delta] [approximately equal to] 500. In the low energy regime, the sand fraction influences flow dynamics primarily through ripple formation; no ripples form in the 1% sand experiments, whereas ripples form in the 13% experiments that increase turbulence and the wave boundary layer thickness, [delta]m. In the high energy regime, small ripples form in both the 1% and 13% sand experiments and we observe high near-bed suspended sediment concentrations. The influence of stratification on the boundary layer flow is characterized in terms of the gradient Richardson number Rig. The flow is weakly stratified inside the boundary layer for all runs and critically stratified at or above the top of the boundary layer. In the lower energy regime, the sand content reduces the relative influence of stratification in the boundary layer, shifting the elevation of critical stratification, LB, from approximately 1.3[delta]m to 2.5[delta]m in the 1% and 13% experiments, respectively. In both sets of experiments LB [approximately equal to] [delta]m at the strongest wave energy, indicating a transition to strongly stratified dynamics. Second, sand particles control the sediment suspension over mud-dominant environments. High near-bed concentration and concentration gradient happen when the sand fractions suspend from the bed. The suspension of sand fraction in bed sediment mixture leads to the formation of a high suspended-sediment concentration layer. A modified sediment suspension criterion is created based on th Van Rijn (1984) and using the median particle size of only the sand fraction in the bed and is successful in predicting the necessary suspension that contributes to wave-supported gravity flow formation. This modified sediment suspension criterion provides a limited condition for the formation of wave-supported gravity flows. Finally, experimental results during bed adjustment periods are presented. The total bed adjustment before the equilibrium can be divided into three stages. Stage I, the initial adjustment stage, occurs in the first 30 - 40 min, where the non-uniformity of the bed introduces sediment redistribution and transient ripples start to form, and the bed elevation might increase or decrease. Stage II, the decreasing erosion stage, occurs during 30 - 90 min for 1% sand experiments or 40 - 105 min for 13% sand experiments, where the bed erosion rate is near a constant value. Stage III, the near-equilibrium stage, occurs in the final 15 min, where the bed erosion rate drops to near zero, upward transport flux is balanced with the settling flux, stratification is induced at the top of the wave boundary layer, and the system reaches an equilibrium state. Two different controlling mechanisms of the bed erosion and deposition are discussed: bed control, and stratification control. In our experiments, bed armoring due to the surface coarsening of the bed is observed, which decreases the bed erodibility. Transient ripples are observed in the total bed adjustment period, which elevates near-bed shear stress. The formation of transient ripples might slightly increase bed erosion. Stratification plays a limited role in the near-bed sediment system during the bed adjustment period but becomes important when the equilibrium state is reached.