Development and validation of a three-dimensional morphological model

Fine sediment size (< 63 Ixm) is best measured by a sedimentation technique which records the whole size distribution. Repeated size measurement with intermediate steps of removal of components by dissolution, allows inference of the size distribution of the removed component as well as the residue. In this way, the size of the biogenic and lithogenic (noncarbonate) fractions can be determined. Observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 Ixm. The dynamics of sediment erosion, deposition, and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10-1xm grain size, .and noncohesive above that size. Thus silt coarser than 10 Ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed. Silt that is f'mer than 10 Ixm behaves in the same way as clay (< 2 !xm). Useful parameters of the distribution are the 10-63 Ixm mean size and the percentage 10-63 Ixm in the fine fraction. We cannot use size distributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of the flow, Ku), regions of high eddy kinetic energy (KE) must be avoided. At the present, such abyssal regions lie under the high surface K E of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circmpolar Current, and Brazil/Falldand currents in the Argenthe Basin. This is probably a satisfactory guide for the Pleistocene. With regard to the carbonate subfraction of the size spectrum, size modes due to both cocco!iths and foramlnlferal fragments can be recognized and analyzed, with the boundary between them again at about 10 lm. The flux of less than 10 lm carbonate, at pelagic sites above the lysocline, is another candidate for a productivity indicator.

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Sortable silt and fine sediment size/composition slicing: Parameters for palaeocurrent speed and palaeoceanography

Salt marshes are delicate landforms at the boundary between the sea and land. These ecosystems support a diverse biota that modifies the erosive characteristics of the substrate and mediates sediment transport processes. Here we present a broad overview of recent numerical models that quantify the formation and evolution of salt marshes under different physical and ecological drivers. In particular, we focus on the coupling between geomorphological and ecological processes and on how these feedbacks are included in predictive models of landform evolution. We describe in detail models that simulate fluxes of water, organic matter, and sediments in salt marshes. The interplay between biological and morphological processes often produces a distinct scarp between salt marshes and tidal flats. Numerical models can capture the dynamics of this boundary and the progradation or regression of the marsh in time. Tidal channels are also key features of the marsh landscape, flooding and draining the marsh platform and providing a source of sediments and nutrients to the marsh ecosystem. In recent years, several numerical models have been developed to describe the morphogenesis and long-term dynamics of salt marsh channels. Finally, salt marshes are highly sensitive to the effects of long-term climatic change. We therefore discuss in detail how numerical models have been used to determine salt marsh survival under different scenarios of sea level rise.

/pdf/numerical-models-of-salt-marsh-evolution-ecological-46978odo66.pdf

Numerical models of salt marsh evolution: Ecological, geomorphic, and climatic factors

http://www.vliz.be/imisdocs/publications/ocrd/267179.pdf

Erosion of mud/sand mixtures

Erosion behavior of soft cohesive sediment deposits has been investigated in laboratory experiments. Such deposits are representative of the top, active layer of estuarial beds. An experimental procedure involving layer by layer erosion under a range of bed shear stresses, τb, of successively increasing magnitude was utilized. Interpretation of the resulting concentrationtime data together with bed density profiles yielded a description of the variation of the bed shear strength, τs, with depth as well as an expression for the rate of surface erosion. In general, τs increased with depth and was also influenced by the type of sediment, bed consolidation period and salinity. The rate of erosion was found to vary exponentially with (τb-τs)1/2. In modeling estuarial bed erosion, it is essential to take these characteristics of τs and the rate of erosion into account.

Erosion of soft cohesive sediment deposits

The effects of shear stress, suspended sediment concentration, and shear strength of bed on the erosion rates of a cohesive bed in an open channel with salt water have been investigated. The deposition rates of suspended cohesive sediment and the patterns of bed erosion have been studied to a lesser extent. For the experimental range the erosion rates were found to be independent of the shear strength of the bed and the concentration of suspended sediment. They depend strongly on the bed shear stress. The minimum shear stresses for initiation of erosion were also found to be independent of the shear strength of bed. There seems to exist a critical velocity for the clay part of suspended sediment, above which all such sediment remains in suspension, whereas even for velocities slightly below this critical limit, the suspended clay deposits rapidly. Scouring occurred predominantly within a well defined narrow and relatively straight zone near the center of the flume.

Erosion and Deposition of Cohesive Soils

An experimental investigation, utilizing an apparatus consisting of an annular-rotating channel and a counterrotating ring, of the depositional characteristics of a fine, cohesive sediment revealed that after an initial period of rapid deposition, the sediment concentration approaches asymptotically an equilibrium value. The ratio of this equilibrium concentration to the initial concentration is nearly independent of initial concentration, and for given sediment properties and water chemistry depends only on the flow conditions. For the three water depths investigated, the ratio of equilibrium to initial concentration was found to be a single function of an average shear stress around the channel-section perimeter. A comparison of the size distributions of the parent material with the material retained in suspension when equilibrium was achieved indicated that greatest deposition occurs among the clay-size fractions, suggesting that the deposition is controlled predominantly by flocculation, and that the strength and size of the flocs exert a stronger influence on the deposition than does the initial particle weight. A silty-clay sediment with a mean particle diameter of 0.0009 mm was used in all experiments.

Emmanuel Partheniades

Papers

Erosion and Deposition of Cohesive Soils

Depositional Behavior of Kaolinite in Turbulent Flow

Closure to “Erosion and Deposition of Cohesive Soils”

Discussion of “Two‐Dimensional Model of Mud Transport” by Peter Cole and Gordon V. Miles (January, 1983)

Discussion of “ Cohesive Material Erosion by Unidirectional Current ” by J. William Kamphuis and Kevin R. Hall (January, 1983)