Cohesive Sediment

Modelling the transport of fine sediments in the marine environment is a highly complex task with a broad field of application. Various processes take place and interact with one another to determine the movement of fine sedimentary particles in the aquatic domain.  Amongst these processes stratification significantly affects the vertical propagation of a sedimentary plume, especially in areas of freshwater influence, with the possibility of matter entrapment and accumulation along the interface; this possibility is higher the finer the sedimentary matter is. The proposed modelling approach describes the physical processes that take place in the water column (advection, dispersion, coagulation, settling, effects of stratification), at the boundary of the seabed (benthic shear) and the mass-exchanges between the two (deposition, consolidation, resuspension, erosion). The Fine Sediment Transport Model was applied in the gulf of Thermaikos (NW Aegean Sea), an area of significant freshwater and riverine sediment inflow, considering the major rivers of the domain as sources of particulate matter. The results were, generally, in agreement with available in-situ measurements of suspended particulate matter. In cases of significant deviations between simulation and measurement, the source of the error did not lie within the modelling approach, but was due to the considered input parameters. The investigation revealed the existence of transport and sedimentation patterns related to the riverine source of the material and the individual parts of the gulf. The sedimentation rates were estimated at 18, 7 and 3 mm/y locally at the outflows of Aliakmon, Axios and Pinios respectively, while the areas of high sediment accumulation coincided with locations where the benthic material ranges from very fine to fine silt.

A new model for calculating suspended sediment concentration of cohesive sediments is presented. Processes that govern the behaviour of cohesive sediments differ significantly from those governing non-
cohesive sediments. In addition to the physical complexities of cohesive sediment processes, chemical and biological aspects must be considered as well. All of these factors point to the need for a multi-disciplinary approach in studies that seek a better understanding of the transport of cohesive sediment. Several new concepts were incorporated in the proposed model which is capable of estimating the space and time variation of the concentration of cohesive sediment. First, the model incorporates the effect of electrochemical forces between cohesive sediment particles. Second, the wave friction factor is expressed in terms of the median particle size diameter in order to enhance the accuracy of the estimation of bed shear stress. Third, the erosion rate of cohesive sediments is also expressed as in a time-dependent form. None of the aforementioned phenomena have been previously included in mud transport models. Simulated suspended sediment concentration profiles are compared with field data collected from Vermilion Bay, Louisiana (Kemp, 1986). Comparisons between the numerical model results and field data indicate that the model performs satisfactory.