Sediment‐Induced Stratification in an Estuarine Bottom Boundary Layer
Sediment‐Induced Stratification in an Estuarine Bottom Boundary Layer
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DOI：
10.1029/2019jc016022
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发表时间：
2020-08
影响因子：
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通讯作者：
Galen Egan;A. Manning;G. Chang;O. Fringer;S. Monismith
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文献类型：
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作者：
Galen Egan;A. Manning;G. Chang;O. Fringer;S. Monismith
我们对南旧金山弗朗西斯科湾的浅滩进行了实地观察，以研究沉积物引起的分层如何影响整个水柱的平均流量和动量与沉积物的混合。Vectrino剖面仪测量了近床流速和悬浮泥沙浓度剖面，我们使用这些剖面来计算湍流泥沙和动量通量的剖面。使用放置在整个水柱中的声学多普勒测速仪的数据计算额外的湍流统计数据。结果表明，泥沙诱导的分层，这是建立了强烈的近床波剪切，可以减少摩擦底部阻力的平均流。测量的湍流统计数据表明，这种阻力减小是由分层抑制近床湍流通量和减少湍流动能耗散引起的。然而，湍流沉积通量并不受沉积物引起的分层的限制。最后，我们比较了我们的结果，一个共同的模型参数化的特点分层通过一个稳定性参数修改湍流涡动粘度，并提出了一个新的无量纲参数，可能更适合代表分层时，建模振荡边界层流动。在河口，泥浆通过波浪和水流对河床施加压力的综合作用而被侵蚀。由于泥浆颗粒比水密度大，它们倾向于向下沉降，并在床附近形成高泥浆浓度流体的薄层。这被称为沉积物诱导分层。我们测量了南旧金山弗朗西斯科湾浅水区近床处的泥沙浓度和水流特性，以确定泥沙诱导的分层如何影响波浪、水流和泥浆之间的相互作用。我们发现，泥沙诱导的分层可以通过阻尼湍流使床感觉更平滑。这些结果对于理解和模拟沉积物在河口中的运动具有重要意义。
We took field observations on the shallow shoals of South San Francisco Bay to examine how sediment‐induced stratification affects the mean flow and mixing of momentum and sediment throughout the water column. A Vectrino Profiler measured near‐bed velocity and suspended sediment concentration profiles, which we used to calculate profiles of turbulent sediment and momentum fluxes. Additional turbulence statistics were calculated using data from acoustic Doppler velocimeters placed throughout the water column. Results showed that sediment‐induced stratification, which was set up by strong near‐bed wave shear, can reduce the frictional bottom drag felt by the mean flow. Measured turbulence statistics suggest that this drag reduction is caused by stratification suppressing near‐bed turbulent fluxes and reducing turbulent kinetic energy dissipation. Turbulent sediment fluxes, however, were not shown to be limited by sediment‐induced stratification. Finally, we compared our results to a common model parameterization which characterizes stratification through a stability parameter modification to the turbulent eddy viscosity and suggest a new nondimensional parameter that may be better suited to represent stratification when modeling oscillatory boundary layer flows. Plain Language Summary In estuaries, mud is eroded through the combined effect of waves and currents exerting stress on the bed. Because the mud particles are denser than water, they tend to settle downward and form thin layers of high‐mud‐concentration fluid near the bed. This is called sediment‐induced stratification. We measured sediment concentrations and flow properties near the bed in a shallow area of South San Francisco Bay to determine how sediment‐induced stratification affected the interactions between waves, currents, andmud.We found that sediment‐induced stratification canmake the bed feel smoother by damping turbulence. These results are important for understanding and modeling how sediment moves around in estuaries.