On the consolidation and erosion of cohesive sediments

Abstract

Modelling of cohesive sediments is of paramount importance for water quality issues: cohesive sediments control the light extinction and they are the major carriers for heavy metals. The concentration of cohesive sediments in open water systems is too a large extent controlled by erosion from and deposition to the waterbed. The permanent opening of the Haringvliet Sluices in the Netherlands, which is currently considered, may e.g. lead to resuspension of polluted cohesive sediments that have been deposited in the seventies after the closure of the tidal basin. The currently most widely used erosion models are poorly based on physical principles and lack the ability to describe erosion of the consolidated sediment bed at increased shear stresses e.g. during a storm. More advanced models do exist, but these are too slow for long term water quality modelling. A new modelling approach is needed that is both fast and accurate. In this graduation project a new (one-dimensional) vertical model is developed and tested within Delft3D-WAQ. The basic hypothesis is that the erosion rate E can be described as a function of a strength parameter, the soil density ?, and a load parameter, the bed shear stress ?b. The model consists of (1) an erosion module and (2) a consolidation module to provide for the soil density as a function of time and depth. A new erosion formulation is proposed that takes into account the probability distribution of the bed shear stress. It assumes that erosion occurs if the actual bed shear stress exceeds the yield strength of the water bed. For the actual bed shear stress a Rayleigh distribution is assumed. The yield strength can be derived from the cohesion and the density. Alternating erosion and deposition in open water systems leads to the presence of soft mud layers. The finite strain consolidation theory of Gibson [1967], which includes the effect of the density on the permeability, can be used to describe the density profiles of these soft mud layers. The Gibson equation is transformed to material co-ordinates to obtain fixed boundary conditions. The resulting non-linear partial differential equation is simplified by discarding the short-term advection terms, and solved by means of a Fourier series. This solution is fast and has successfully been validated on consolidation experiments in a settling column. Besides, an improved method is proposed to determine the consolidation parameters. The new water bed model has successfully been validated on short term (12-24 hours) erosion experiments performed in a laboratory flume after consolidation of several days. The combination of a simple parameterised consolidation module and the new erosion formulation looks promising. If the model parameters are associated with the physics of more experiments and some minor numerical problems are solved, a realistic model for long term simulations results. Furthermore, the new model corroborates the concept that erosion is governed by the tail of the probability distribution of the actual bed shear stress, i.e. by turbulent bursts