|In the 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 bed sediment using methodologies developed for suspended sediment materials in previous researches. The present study proposes a new method to evaluate the erosion rate of sediment bed composed of silt-clay material by introducing the concept of entrainment for evaluating a mixing process of density stratified flow.Nineteen cases of flume experiments are conducted to investigate the erosion rates of bed material with different concentrations of water-sediment mixture, sediment particle sizes and Richardson numbers. The results of the flume experiments are then compared with the results of the experiments conducted for density stratified flows. The results are summarized in terms of the entrainment coefficient and the Richardson number. The results show that the erosion rate of bed sedimentntrainment coefficient can be evaluated using a modified Richardson number which is introduced to evaluate the influence of viscosity on entrainment rates.