The Hydrodynamic Response of the Sediment‐Water Interface to Coherent Turbulent Motions

Unique observations of the hydrodynamics across the sediment-water interface are used to quantify the instantaneous response of the interstitial fluid to the passage of coherent turbulent motions in the overlying flow. Over a range of permeability Reynolds numbers ReK=√Ku∗/ν (where K is the sediment permeability, u∗ is the shear velocity, and ν is the fluid viscosity), the passage of these turbulent motions create velocity fluctuations and momentum fluxes at the sediment-water interface to greatly exceed their mean values. Sweep motions are observed to penetrate into the sediment bed and induce instantaneous momentum fluxes that can be an order of magnitude larger than the mean bed shear stress. By penetrating into the sediment bed, the turbulent motions increase the effective roughness experienced by the flow and therefore the flow resistance. The properties of the mean flow alone are thus insufficient to describe the interaction of the overlying flow with the sediments.