Erosion of a fluid mud layer due to entrainment

Abstract

A continuous transport cycle of mud material can be noticed in a natural water environment. Aggregation, settling, deposition, consolidation and erosion are typically interlinked. These processes are influenced by the cohesive properties of the mud and by the characteristics of its environment. Fluid mud is a highly concentrated near-bed sediment suspension with a sediment concentration between about 10 and 300 g/l, and can be formed by hindered settling or by the fluidization of the bed. Once formed, the fluid mud can be transported due to • horizontal pressure gradients, frictional and gravitational forces. • turbulence and instability of the interface between the fluid mud layer and the water layer above, resulting in mass transport from a non-turbulent layer to a turbulent layer. This process is defined as entrainment. This report concentrates on the process of entrainment by turbulent water flow. A quantitative measure for entrainment is the dimensionless entrainment rate E, which is the ratio of the entrainment rate ue (i.e. the entrained volume of fluid mud per unit area and per unit time) to a characteristic flow velocity. Dimensional analysis indicated that £ is a function of an overall Richardson number. From the literature it followed that the entrainment of fluid mud resembles the fresh/saline water entrainment process, though properties of the cohesive sediments may greatly influence the entrainment behaviour. Two numerical models have been used to predict the entrainment of fluid mud: an entrainment model describing the small scale behaviour (1) and the two-layer fluid mud model which considers mud transport on a larger scale (2). The results have been compared with experimental data or observations. (1) From the analysis of the integral entrainment model of Kranenburg (1994), it resulted that the values taken for the empirical coefficients involved and the assumptions made for the effects of viscous drag and side wall friction are satisfactory. The effect of consolidation and the related change from entrainment to floc erosion becomes apparent for large times. (2) The two-layer fluid mud model, developed by Delft Hydraulics, showed the importance of entrainment for mud transport. When applying a settling velocity, which varies with the sediment concentration, upward transports (due to entrainment) and downward transports (caused by settling) are much larger than in the case of a constant settling velocity. Also the results agree better with observations then. The major limitation of this model originated from the fact that in the model no differences in bed material were made and only neap tide was simulated instead of a neap tide - spring tide cycle. The incorporation of the integral entrainment model of Kranenburg into the two-layer fluid mud model will only be one step forward and further improvements are needed.