Pore water flow due to near‐bed turbulence and associated solute transfer in a stream or lake sediment bed

Abstract

A model of pore water flow induced by pressure fluctuations from a turbulent boundary layer flow over a permeable sediment bed is presented. The bed has a smooth or rough flat surface without bed forms. Pressure and velocity fluctuations that penetrate from the sediment/water interface into the sediment pore system and affect mass (solute) transfer are described as periodic in space and time. The amplitude (p0) is determined from a study of the turbulent kinetic energy balance for wall turbulence; the wave number (χ) and the period (T) are given as functions of the shear velocity (U*) based on the near‐bed coherent motions. The flow field in the sediment is described by the continuity equation and Darcy's law. Simulation results show that pore water velocity is faster when the shear velocity (U*) on the sediment bed and/or the permeability of the sediment bed (k) are increased. Accordingly, the exchange velocity of water or solute transfer rate across the sediment/water interface (V0) becomes larger when (U*) and (k) are increased. However, the penetration depth of pore water velocity fluctuations into the sediment bed becomes smaller when (U*) is larger, i.e., when the period of fluctuating pressure is short. Overall, this paper provides new and quantitative information on the enhancement of pore water flow in a flat sediment bed over which a turbulent current is flowing.