Sediment transport

Hydraulic dredging. Hydraulic dredging removes and transports sediment in a liquid slurry form. 

Specific research investigations into sediment/particulate transport, sediment/water/contaminant interactions, soil (unsaturated and saturated) contaminant fate and transport, and biological degradation processes were identified as priorities by the Exposure Assessment workshops. 

If an age model can be developed for a deep-sea sediment core, then the accumulation histories of the various sediment components can be recon-stmcted. Assuming that the age model is correct, the main uncertainty in the environmental significance of the reconstruction is then the potential for lateral sediment transport. Sediments can be winnowed from or focused to a given site on the seafloor. 

Channel facies sediments are transported mainly as bedload. The varying particle sizes found from bed to bed appear to represent different flow regimes. The ability of any given flow regime to transport sediment is determined by the boundary shear between the moving water and the movable sediment bed. The boundary shear is related to flow velocity through Newton s stress law ... 

One test of the method was conducted over a medium-sized iron-copper deposit near the southeastern coast of China. The ore body occurs in a skam near to the contact of granodiorite with interlayered marbles and homfels. The attitude of the ore body is controlled by the contact, which generally is steep. The strike of the ore zones is approximately east-west, plunging west, and their western contact is westerly convex. The top of the ore body, which may have been eroded, is now covered by 140 to 160 m of transported sediments. There is a thick soil cover the ground surface is quite flat and suitable for arable farming. 

Mineralisation beneath mixed eluvium and transported sediment... 

A survey was carried out on a grid covering an area of 13.6 km. Lines were 100 m apart and sample sites 20 m apart along each. There is a series of elongate H2S anomalies above the known ore zone, including where this is covered by transported sediments, and other anomalies indicate mineralisations within fractures and fissures (Fig. 9-10). 

FIGURE 6.7 Diagram of a single deck mechanical thickener for producing a clear supernatant and a solids enriched slurry from a pulp suspension in water. Slow-moving rakes transport sedimented solids toward thickened pulp outlet, and an exit valve for this stream allows control of the solid/liquid ratio. 

The physical transport of dissolved xenobiotics within aquatic systems may obviously take place under the influence of currents and tides. It has also been shown, however, that particulate matter and river sediment may be important vehicles, and that the dissemination of xenobiotics may also take place by subsequent diffusion from these transported sediments into the water column. An attempt will be made to illustrate the operation of these mechanisms. 

Most of the deep waters in contact with the abyssal sediments are a mixture of North Atlantic and Antarctic bottom-water masses. They are relatively saline and fairly cold (0.5-1.5°C). Currents at these depths typically are slow and do not impose shear stress of sufficient magnitude to transport sediments (Gardner etal., 1984 Demidova, 1999). 

There are many equations which allow calculation of sediment transport rate within a water body, or sediment flux (see for example Task Committee of Computational Modeling of Sediment Transport Processes, 2004 for a review). However, these equations tend to be for a uniform sediment distribution, which is far from the variable source supply of material seen in events when the majority of sediment is moving. It is also generally considered that a particular flow has a maximum capacity to transport sediment, although the concentration this relates to depends again on sediment characteristics. Hence tliere are examples in China where sediment concentrations can reach several tens of thousands of parts per million for very fine particles, whereas a flow may become saturated with sand-sized particles at far lower concentrations. Rivers are often considered to be either capacity- or supply-limited in terms of their sediment transporting dynamics. However, in practice for most rivers, most of the time, sediment transport is limited by a complex and dynamic pattern of sediment supply. 

The concepts of limited sediment supply to rivers and an upper limit to the capacity of a particular flow rate to transport sediment of a particular particle size distribution and density explain the phenomenon of hysteresis, where sediment concentration for a given flow rate differs on the rising and falling limbs of a flow event. There is thus no simple relationship between flow and concentration, and a sediment rating curve will inherently have wide error bars. 

In an early study of oceanic bottom water flow within the C-C F.Z., Johnson (1972) deployed free-fall bottom current meters in an area north of the Clipperton Fracture Zone where substantial sediment erosion was known to occur. The limited data showed that the bottom currents were generally slow (<10 cm see ) but fluctuated markedly due to a strong semi-diurnal tidal component. It was also established that the currents flowed mainly to the east with minor variations due to topographic effects. In addition, data from a 14-day record of bottom current measurements taken at 210 m above the sea floor revealed an averge bottom water flow of 2.0 cm sec in an ENE direction with peak velocities of up to 16.5 cm sec at semidiurnal periods. These data showed that peak velocities of bottom water transport were strong enough to erode and transport sediment in the area (Amos et al. 1977). 

Soil mineral matter consists of 50-90% of the dry weight of marsh soils (Nyman et al., 1990). Storm passage including hurricanes is a primary mechanism that delivers mineral sediments to Louisiana marshes (Reed, 1989 Roberts et al., 1989 Turner et al., 2006). Increased water levels and wave action in bays and lakes prior to frontal passage transport sediments from water bottoms to... 

Tides can also locally affect surface sediment distribution. Tides are particularly dominant in the East China Sea of the mouth of the Changjiang River and off southwestern Korea. In both instances tidal cmrents are sufficiently strong (l 2 knot(s)) to erode and transport sediments. The intrusion of the Taiwan Warm Water (TWW) between the Changjiang Coastal Water (CJCW) and the Jiangsu Coastal Water (JCW) results in a seaward transition from turbid (CJCW) to clear (TWW) to turbid (JCW) waters east of the Changjiang River Estuary. As a result, Changjiang River sediments tend to be transported to the south by the CJCW, while the sediments to the east of the TWW are predominantly Huanghe River sediments transported south by the JCW. 

Bagnold, R.A. 1962. Auto-suspension of transported sediment, turbidity currents. Proceedings of the Royal Society of London, Series A, 265 315-319. 

In the deposition process, when the velocity of the transporting medium becomes too low to transport sediment, the sediment falls out and becomes deposited. 

This chapter concerns morphologic change around and engineering of tidal inlets. The hydrodynamics at a tidal inlet, the water movement that transports sediment and determines inlet morphologic forms, could not be discussed due to space limitations. Also, advanced numeric modeling of inlets was not covered. Here, some elements of these subjects are presented for completeness. 

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