Bottom Boundary Layer
KeywordsContinental Shelf Continental Margin Turbulence Shear Bottom Boundary Layer Viscous Shear
In marine geosciences, the bottom boundary layer (BBL) refers to a layer of flow in the immediate vicinity of the solid sea bottom where the effects of viscosity are significant in determining the characteristics of the flow. The BBL was first discovered by Prandtl (1905) in aerodynamics and subsequently applied to other fluids moving on the surface of a solid body.
Starting upwards from the sea bed, the total thickness of the BBL is defined as the distance above the bottom at which the mean flow velocity equals to 0.99 U ∞, where U ∞ is the free-stream velocity of a layer that is in a geostrophic balance overlying the BBL. On top of the geostrophically balanced layer is a surface layer subjected to wind-wave mixing. When both the bottom micro-topography is uniform and the overlying flow is steady, the BBL can be easily quantified from the vertical flow structure. Various ways exist to estimate the thickness δ of the BBL under neutral conditions (e.g., Grant and Madsen, 1986; Nielsen, 1992). In general, the BBL thickness at continental margins is at the order of 5–50 m.
A thin inner layer just above the bottom where turbulence is inhibited by the presence of the solid boundary. The flow is controlled by molecular viscosity and the shear stress is consistent with the bottom shear stress.
An outer layer where turbulence shear dominates and viscous shear can be neglected.
A transitional layer where both the viscous shear and the turbulence shear are important.
As the shear stress is almost constant and fulfills Newton’s law of viscosity in the inner layer, flow velocity can thus be approximated by a linear form. However, this only applies to a hydraulically smooth bottom where bed roughness is too small to affect the velocity distribution. In hydraulically rough bottom, bed roughness is large enough to produce eddies close to the bottom and the inner layer may not be detectable. Upwards from the inner layer, the importance of molecular viscous decreases and turbulence gradually dominates the flow. Mean flow velocity in this transitional layer obeys the law of the wall and can be approximated by a logarithmic function. The outer turbulent layer takes up a majority (80–90 %) of the BBL. Flow characteristics of this layer mainly depend on the velocity difference with the external free flow (i.e., velocity defect) and the overall scale of the boundary layer.
Wind waves affect the BBL by imposing a wave boundary layer wherever the water depth is less than half of the wave length. The wave-induced oscillatory water motion is affected by the sea bottom within the wave boundary layer.
However, in a natural continental shelf, any definition of the BBL structure is not straightforward due to the influences of many factors (e.g., density stratification, internal waves, seabed topography). A practical indicator for the BBL is a thermohaline pycnocline observed in the water column (e.g., Stips et al., 1998; Perlin et al., 2005). Just above the sea bottom, there is a homogenous layer of temperature, salinity, and density, which indicates the inner layer and the transitional layer.
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