Stratification effects by cohesive and noncohesive sediment

Abstract

This study focuses on stratification effects induced by the interaction between suspensions of fine‐grained cohesive and noncohesive sediment and the turbulent flow in estuarine and coastal environments. A concise literature review reveals that this interaction may cause an appreciable modification of the vertical profiles of velocity, vertical eddy viscosity/diffusivity, and Reynolds stresses. This interaction is further studied with a one‐dimensional vertical (1DV) numerical model, which includes the standard k‐ε turbulence model with buoyancy destruction terms. Application of this model to the experimental results by Coleman [1981] shows that the measured changes in the velocity profile can be explained entirely by sediment‐induced buoyancy effects. It is argued that when the carrying capacity for cohesive sediment is exceeded, a fluid mud layer is formed upon the deposition of the cohesive sediment flocs, contrary to the case with noncohesive sediment, where the depositing grains immediately form a rigid bed. Thus a two‐layer fluid system develops causing significant damping of the vertical mixing processes, decreasing the carrying capacity further. This positive feedback results in a catastrophic collapse of the turbulence field and the concentration profile. A scaling law for this saturation behavior is derived from classical stratified flow theory. This law is sustained with a series of numerical experiments with the 1DV model. It also predicts the suspended sediment concentrations observed in the Yellow River to the right order of magnitude. It is concluded that sediment‐induced buoyancy yields appreciable stratification effects at already moderate suspended sediment concentrations. In the case of cohesive sediment these stratification effects can result in a catastrophic collapse of the turbulent flow field and the vertical profile of suspended sediment concentration.