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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.

This paper describes results of the second part of a study on stratification effects by cohesive and noncohesive sediment. Winterwerp (2001) applied classical stratified flow theory implemented in a one-dimensional vertical numerical model (the 1DV POINT MODEL), showing that sediment-induced stratification effects may occur at already fairly small suspended sediment concentrations (i.e., a few 100 mg/L). We also discussed a basic difference between the behavior of cohesive and noncohesive sediment, which emerges as a result of the large water content of mud flocs. In this paper we elaborate further on the hydrodynamic description of the transport of fine suspended sediment by analyzing field and laboratory observations over a very large range of concentrations. We propose a sediment stability diagram to explain some features of hyperconcentrated flows, such as those observed in the Yellow River. We show that the behavior of hyperconcentrated flows is affected largely by hindered settling effects reducing the energy required to keep the sediment in suspension. The hydrodynamic description of sediment transport is used to predict capacity conditions as a function of a dimensionless stream power, i.e., U3/hgWs. This prediction agrees favorably with observations reported in literature covering four orders of magnitude in suspended sediment concentration.

Student project report, in cooperation with Smith-Warner International Ltd. (SWIL), Kingston, Jamaica. At this moment the shipping channels in Kingston Harbour, Jamaica, slowly accrete. When the harbour is expanded, the local and global sediment transport is likely to change. During this project it is investigated whether these changes are significant and if they will have a negative influence on the Kingston Harbour area. Also the increase of flood risk for the area surrounding Hunts Bay is investigated. This investigation is done by modeling the hydrodynamics of the Kingston Harbour area with MIKE21 and Delft3D, where after both modeling programs are compared to each other. For the input data for the models, research has been done concerning the boundary conditions. This data is gathered from several projects done in the past about other areas in the harbour and fieldwork in Hunts Bay. During the year, most of the wind comes from the east and south-east direction. There are also two mayor streams which debouch into Hunts Bay, namely the Sandy Gully and the Rio Cobre. Since there is only discharge known about the Rio Cobre (daily values from 1985 to 2010), only the Rio Cobre is taken into account. The maximum measured value was 563 m3/s (during hurricane IVAN) and the average value is about 12 m3/s. For the sediment input data some fieldwork is done in Hunts Bay to gather information about the type of soil. From this it is concluded that it is silt, which is confirmed after a lab research of the sediment. However these accurate soil properties couldn’t be implemented into the models due to the lack of time. During the fieldwork also a bathymetric survey was done, which showed that Hunts Bay is sedimented compared to the previously used bathymetric data, gathered from admiralty charts in 2000. Calibration of both models is done by comparing it with the measured water level and flow velocities underneath the Causeway Bridge. Since this is the only point where data was available for, the calibration kept global, and should be improved in the future. The modeling showed that most of the sediment transport into the shipping channel is caused by the high discharge of the Rio Cobre. Ivan showed the most extreme sedimentation and the biggest change due to the expansion. In the present situation the shipping channel is gradually silting, with two areas where the siltation is concentrated. With the first phase expansion these ‘mountainous’ areas will be much more concentrated. However it can be concluded that the changes in the sediment transport due to the first phase expansion are not significant and will not lead to more problems than there are without this expansion. For this problem a sediment trap is proposed. At first it was placed just eastward of the Causeway Bridge, but this didn’t solve the problem and it would be in the way for the phase two expansion. Therefore a sand trap is designed in Hunts Bay, just westward of the Causeway Bridge. This location is really effective, since it stores the sediment from the rivers. This solution prevents the shipping channel to silt. Again, since the lack of reference data, on the size of the pit nothing can be said.

The objective of this study is to determine large scale of fine sediment transport near coral reefs islands in the Singapore Strait. Coral reefs in the Singapore Strait face great pressure due to high sedimentation and turbidity, which cause decreasing of light penetration and of environment quality for the coral growth. Sedimentation and high level of turbidity are caused by large scale of dredging and land reclamations in the Singapore. The location of Singapore Strait between two major water systems, leads to complex tidal system of the strait. Tidal elevation is predominantly semidiurnal while current velocity is diurnal. Field data from several tidal stations and offshore observation points are utilized in order to analysis large scale of fine sediment transport. The representative of time series from each observation point during the southwest and northeast monsoon were selected based on the data availability and quality. Seasonal variation of current velocity shows eastward dominant flow during the SW monsoon and westward dominant flow during the NE monsoon. Residual flow plays more roles in the fine sediment transport than tidal asymmetry. Average residual flow ranges from 0.1 to 0.3 m/s with the peak 0.8 m/s in January. Wave action does not show significant impact to resuspension of fine sediment in deep water, but considerably cause fine sediment to resuspend at near surface. A turbidity level near coral reef islands corresponds to the fine sediment concentrations variation. High turbidity event occurs at all observation points and far from optimal level for coral growth. High turbidity event occurs more than weeks which is likely caused by resuspension of large sediment supply by strong currents.