The variation of flow and turbulence across the sediment–water interface

Abstract

A basic framework characterising the interaction between aquatic flows and permeable sediment beds is presented here. Through the permeability Reynolds number (Re K = √ Ku * /ν, where K is the sediment permeability, u * is the shear velocity and ν is the fluid viscosity), the framework unifies two classical flow typologies, namely impermeable boundary layer flows (Re K 1) and highly permeable canopy flows (Re K 1). Within this range, the sediment-water interface (SWI) is identified as a transitional region, with Re K in aquatic systems typically O(0.001-10). As the sediments obstruct conventional measurement techniques, experimental observations of interfacial hydrodynamics remain extremely rare. The use of refractive index matching here allows measurement of the mean and turbulent flow across the SWI and thus direct validation of the proposed framework. This study demonstrates a strong relationship between the structure of the mean and turbulent flow at the SWI and Re K . Hydrodynamic characteristics, such as the interfacial turbulent shear stress, velocity, turbulence intensities and turbulence anisotropy tend towards those observed in flows over impermeable boundaries as Re K → 0 and towards those seen in flows over highly permeable boundaries as Re K → ∞. A value of Re K ≈ 1-2 is seen to be an important threshold, above which the turbulent stress starts to dominate the fluid shear stress at the SWI, the penetration depths of turbulence and the mean flow into the sediment bed are comparable and similarity relationships developed for highly permeable boundaries hold. These results are used to provide a new perspective on the development of interfacial transport models at the SWI.

A Model for Mass Transport Across the Sediment‐Water Interface

“…Our current understanding of dispersive mass transport, on the other hand, typically relies on studies of unbounded porous media (e.g., Güss, ) where the local hydrodynamics and transport characteristics are spatially uniform (over scales larger than the typical particle diameter) and unaffected by the external flow above the SWI. At the SWI, however, spatial variability of the flow field is determined by both the local interstitial flow velocity and by the disturbances that originate from the overlying flow (e.g., Scalo et al, ; Voermans et al, ; Vollmer et al, ). Flow heterogeneity at the SWI can even be observed when the interface is considered to be in a fully laminar state and affect the dispersive mass transport across the interface (Huettel & Gust, ).…”

“…This range of hydrodynamic behavior is characterized by the permeability Reynolds number, . When (i.e., ), the boundary behaves as an impermeable boundary, where interfacial turbulence is absent and viscosity controls the hydrodynamics at the interface (Voermans et al, ). This limit implies molecular‐dominated interfacial mass transport ( ).…”

“…As such, for the interface is effectively highly permeable and mass transport across the SWI can be dominated by turbulence ( ). Therefore, Re K is a primary parameter in the description of interfacial fluxes (Voermans et al, ).…”

Determining Near‐Bottom Fluxes of Passive Tracers in Aquatic Environments

“…Equation also assumes that the SWI can be considered fluid dynamically impermeable and flow properties behave like those near a solid boundary. As Voermans et al () demonstrate, this is strictly true only when the permeability Reynolds number , where K is the sediment permeability. This is the case for observations we describe below.…”

Experiments and Numerical Analysis of Flow in an Open Channel with Gravel Bed

“…In the study of [9] they undertake a series of novel experimental observations in order to provide a framework for characterizing the hydrodynamic processes that determine mass and momentum transfer across the Sediment-Water Interface (SWI). The experimental data will reveal the variation of the mean flow and turbulence across the SWI as a function of a dimensionless permeability.…”