nearbed

Nearbed is a term used in fluid dynamics and sedimentology to describe the region of fluid flow immediately above a solid bed, such as the seabed, riverbed, or lakebed. The term is commonly written near-bed or near bed, though some texts use nearbed as a single word. The nearbed region is characterized by reduced mean velocity, large velocity gradients, and elevated turbulence relative to the outer flow. It is a zone where bed shear stress acts on sediments and processes such as entrainment, incipient motion, and bedload transport begin. The thickness of the nearbed region depends on flow conditions, sediment size, water depth, and viscosity, and is typically on the order of the boundary layer thickness or a few grain diameters near the bed.

In hydrodynamic models, the bottom boundary layer may be resolved to capture near-bed dynamics, or near-bed processes may be parameterized to estimate sediment entrainment rates. Studying near-bed dynamics uses instruments and techniques such as acoustic Doppler velocimetry, microstructure profilers, or image-based flow measurements. Near-bed processes influence coastal erosion, sedimentation patterns in rivers and reservoirs, and habitat availability for benthic organisms.

See also: boundary layer; bottom boundary layer; sediment transport; bed load; resuspension; turbidity.