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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 Re-K = root Ku(*)/nu (where K is the sediment permeability, u(*) is the shear velocity, and nu 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.

Plain Language Summary Biochemical and physical processes at the sediment-water interface (SWI) are strongly affected by the magnitude of the instantaneous flow velocity and flow-induced forces. As measurements of the flow across the SWI are complicated by the presence of the sediment bed, little is known about the influence of large turbulent motions on the flow at the interface. In this study, we use a rare experimental methodology of the flow across the interface to identify the response at the SWI to the passage of large turbulent motions in the overlying flow. We show that these large turbulent motions can penetrate across the SWI into the sediment bed and induce instantaneous interfacial velocities and forces that are significantly greater than their time-averaged values. By penetrating into the sediment bed, these large turbulent motions enhance the flow resistance of the sediments to the overlying flow.