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- Title
The Role of Sand in Wave Boundary Layers Over Primarily Muddy Seabeds: Implications for Wave‐Supported Gravity Flows.
- Authors
Han, Zhuochen; Horner‐Devine, Alexander R.; Ogston, Andrea S.; Hsu, Tian‐Jian
- Abstract
We performed laboratory experiments to investigate the influence of sand content on the dynamics of wave‐supported gravity flows in mud‐dominant environments. The experiments were carried out in an oscillatory water tunnel with a sediment bed of either 1% or 13% sand. Low and high energy regimes are differentiated based on a Stokes Reynolds number ReΔ ≈ 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, δm. 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 1.3δm to 2.5δm in the 1% and 13% experiments, respectively. In both sets of experiments LB ≈ δm at the strongest wave energy, indicating a transition to strongly stratified dynamics. Plain Language Summary: The transfer of fine sediment from river mouth to deeper marine environments is an important component of source‐to‐sink sedimentary systems. Sediment can be deposited along the shoreline, and/or on the continental shelf and ultimately may be deposited in deep ocean. However, mechanisms that move materials across the shelf are still unclear. Recent studies show that wave‐supported gravity flows (WSGF) might be one of the most crucial processes contributing to sediment transport across shelves with gentle bottom slope. Our work investigates the sediment particle size effects on the dynamics of WSGF, especially the sand fractions, in predominantly muddy shelves through laboratory experiments. We find that sand fraction affects the dynamics mainly through ripple formation. Under low wave conditions, a higher sand fraction contributes to formation of steep ripples, which enhances turbulence. Under strong wave conditions, sediment‐induced density stratification damps the turbulence and ripple effects are less important. In addition, stratification plays different roles vertically: near the bed stratification is weak due to the high shear, while stratification is critical above this high shear region. The impact of sand fractions can thus change the cross‐shelf sediment transport rate, which can further influence the deposition and associated biogeochemical processes. Key Points: Greater sand fraction enhances ripple formation and near‐bed turbulenceSediment‐induced stratification suppresses turbulence in high wave conditionsA weakly stratified layer is observed near the bed with a critically stratified layer above
- Subjects
OCEAN waves; OCEAN bottom; MARINE sediments; OCEAN circulation; OCEANOGRAPHY
- Publication
Journal of Geophysical Research. Oceans, 2021, Vol 126, Issue 5, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2020JC016621