Near‐bed sediment concentration in gravel‐bedded streams

In this paper we present an analytical model for the concentration of suspended sediments in the proximity of the bed of gravel rivers. The model is applicable to high‐gradient streams carrying sands and finer sediment particles in suspension over poorly sorted gravel beds. The analysis is carried out in terms of equations which realistically describe the velocity profile of the carrying stream and the dispersion of heavy particles in turbulent shear flows over stable gravel beds. To test our hypotheses we verified the equations using available flume and river data and evaluated their response for various combinations of bed roughness and suspended load characteristics. Our results indicate that characterization of the near‐bed concentration can be formulated in terms of a velocity defect representation of the mean turbulent velocity profile and a sediment eddy diffusivity distribution developed for sand bed rivers. The near‐bed concentration is shown to be rather sensitive to variations in the gravel bed relative roughness, the depth at which the concentration is evaluated above the virtual bed surface, and the ratio between the settling velocity of the suspended sediment and the bed shear velocity.