Vertical structure of estuarine fine sediment suspensions

This research investigated the physical characteristics (vertical structure) of estuarine fine sediment suspension profiles within a comprehensive descriptive framework. Suspension related mechanisms of erosion, entrainment, diffusion (in the presence of buoyancy stabilization), advection, settling and deposition were examined in this context. A vertical mass transport model developed from functional relationships between the above processes was used to explain some of the important physical characteristics.; Lutoclines, sharp steps (gradients) in the concentration profile, are regions where the local mixing rate is minimal. The mechanisms for their formation have been shown to be the non-linear relationships between (1) vertical diffusion and concentration gradient and (2) vertical settling and concentration. The effect of sediment settling is to further stabilize the lutocline layer, thereby making it much more persistent in high energy environments than other pychnoclines (e.g., haloclines). Application of the vertical transport model to data from settling column tests, wave flume resuspension tests, and estuarine field investigations provided reasonable predictive agreement for lutocline dynamics.; Fluid mud, a near-bed, high concentration layer with negligible structural integrity, results from high bed erosion or fluidization rates relative to upward entrainment fluxes and from rapid deposition. Sensitive pore pressure and total pressure measurements made in a laboratory flume have been used to demonstrate that waves, for example, provide one mechanism for fluid mud formation by rapid destruction of effective stress in the sediment bed. The upper interface of the fluid mud layer, by definition a lutocline, represents a local maximum in net downward settling flux (i.e., maximum settling minus diffusive flux). The fluid mud layer thus forms (and grows) from rapid deposition whenever the depositional flux at the bed exceeds the rate at which the sediment can develop effective stress (usually very low).; Fluid mud has been shown to be either horizontally mobile or stationary depending on the depth of horizontal momentum diffusion vertically downward into the high concentration layer. Fluid mud tends to occur over a density range between 1.01-1.1 g/cm{dollar}sp3{dollar} but due to the dependence on hydrodynamic action near the bed a precise definition cannot be made on the basis of density alone. (Abstract shortened with permission of author.)...