Effect of Sediment Suspension on Turbulence Structure: Comparative Study of Cohesive and Non-cohesive Sediments

It is well known that the turbulence structure is significantly affected by density stratification. In many cases, the density stratification is caused by sediment suspension. Moreover, sediment suspension is also governed by turbulence structure. This study aims to investigate the effects of sediment suspension on turbulence structure. Due to the difficulties in directly measuring turbulence characteristics, a one-dimensional-vertical (1DV) sediment transport model is used in this study. Three cases of numerical experiments are tested (non-cohesive sediment; cohesive sediment; no sediment). To consider the flocculation process of cohesive sediment, a robust flocculation model previously verified with laboratory data is incorporated with a 1DV sediment transport model. It is found from this study that the turbulence intensity is decreased by sediment suspension (turbulence damping). The effect of density stratification is considered to be main mechanism of turbulence damping. It is also known in this study that cohesive sediment decreases the turbulence intensity less than non-cohesive sediment under the condition of same mass concentration. The low settling velocity and strong suspension of cohesive sediment are considered to cause the relatively weak effect on turbulence damping. In order to further investigate this finding, the intensity of density stratification is quantitatively calculated with Richardson number. The result shows that less stable condition of density stratification is caused by cohesive sediment. The less damped turbulence has the second effect to increase the upward suspension of cohesive sediment resulting in weakening turbulence damping again. The results of this study are numerically obtained by solving governing equations. Although it is worthy to study mechanisms of natural phenomena with a numerical model, governing equations include many assumptions and closures. Therefore, it is of great necessity to re-examine findings of this study through laboratory or field experiments.