Entrainment of bed sediment composed of very fine material

Abstract

AbstractIn an estuary bed composed of very fine sediment material, tidal currents cause active sediment transportation, bar morphology changes, and severe bank erosion. However, it is difficult to evaluate the erosion rate of a sediment bed composed of particles measuring five to several tens of micrometres using methodologies found in the available literature that estimate entrainment in the presence of fine sediment suspension. This paper proposes a new method to evaluate the erosion rate of a sediment bed composed of very fine material by introducing the concept of entrainment developed for density stratified flows. A series of flume experiments were conducted to investigate the erosion rates of bed material with water–sediment mixtures of different concentrations, sediment particle sizes, and overall Richardson numbers. The results of the flume experiments are compared with past theories and experimental results conducted for density stratified flows. The data show that the erosion rate of a water–sediment mixture is proportional to the inverse of the overall Richardson number, as proposed in previous studies on density stratified flows. In cases with a strong cohesion, the erosion rate can be evaluated using a modified Richardson number, introduced to evaluate the influence of viscosity on entrainment rates. The applicability of the proposed method is also tested by comparing the results of the present study with those of previous studies in terms of equilibrium suspended sediment concentration and size distributions of suspended sediment. The present results show that the proposed method can be used to evaluate not only the erosion rate of suspended sediment but also the characteristics of equilibrium transport processes.