Sedimentation water flow velocity

Figure 2.13 Illustration ofthe relationship between the water flow velocity and the fate of sediment particles. At low velocities the larger particles sediment out if originally suspended, or, if originally residing in the sediment, they remain in place (lower right zone) note that this is the sedimentation velocity curve from Figure 2.12. At intermediate velocities...

The travel time for suspended load is controlled by the flow velocity and the distance to the basin outlet. Flow velocities do not change much downstream in a typical river system (Leopold, 1953) and typically range from 0.1 to several m/s. Hence, suspended load should be able to travel at least 10 to 100 km per day and the travel time for suspended sediment to traverse even the longest rivers in the world should be less than a season. Although some of the suspended load will be deposited in floodplains, the component of the suspended load that does not get sequestered in terrestrial depositional environments is delivered almost as fast as the water that it flows in. Bedload travels much more slowly. In mountain drainage basins, the velocity of individual bedload clasts is on the... 

You are engaged in laboratory flume experiments on transfer of dissolved oxygen into the sediments below the flowing water. The goal is to measure the sediment-water mass transfer coefficient and relate it to other parameters of the flow fleld. The flume is 20 m in length, with a depth between 3 and 10 cm and velocity between... 

Compared to the situation in lakes, the sediment-water interactions in rivers are more complex. Because the flow velocity is constantly changing, particles may either settle at the bottom or be resuspended and deposited again further downstream. In order to adequately describe the effect of these processes on the concentration of a chemical in the river, we would need a coupled water-sediment model with which the profile of the chemical along the river of both the aqueous concentration in the river and the concentration in the sediment bed are described. This is a task to be left to numerical modeling. We choose a simpler approach by approximating the net deposition of the particles and the chemicals sorbed to them as a linear process (see Eqs. 23-16 and 23-17) ... 

The physical properties of soils and sediments are particularly important in determining infiltration rates in soils as well as groundwater flow velocities. One feature of particular importance is the void space or porosity of soils, rocks, and sediments. Water is capable of moving from one void space to another in these materials thereby allowing the flow of water. Total porosity is defined mathematically by the equation ... 

Figure 6.8 Conceptual diagram of the different scales of the components of the benthic boundary layer (BBL). In bottom water above the sediment-water interface where the Eckman layer occurs as flow is affected by the rotation of the Earth and bottom friction, where w = friction velocity and / = Coriolis parameter the logarithmic layer predominates when the velocity profile is well described using a logarithmic function a viscous sublayer is formed by molecular viscosity a diffusive boundary layer forms, whereby solute transport is controlled by molecular diffusion. (Modified from Boudreau and Jprgensen, 2001.)...

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 ... 

A lake can also illustrate a theoretically valid, but not useful, control volume. Consider a control volume that comprised only the northern half of the lake the southern boundary of the control volume would then be a surface cutting vertically across the entire lake from the water surface to the lake sediments. Measurement of chemical transport across this boundary would be immensely difficult it would require detailed water flow measurements at an impossibly large number of sites, given that the speed and direction (i.e., the velocity) of water currents in a lake typically vary from place to place and time to time. Such a control volume would not simplify estimates of chemical inputs and outputs. 

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). 

Diffusion between the overlying water and the soil is an important assimilation process because it involves the transfer of toxic organics from a region of low density (the water column) to a region of high density (the sediment), where sorption can occur. Diffusion is particularly important in wetlands where long residence and low flow velocities are normal. 

Advective flux refers to the bulk flow of solids or pore water relative to an adopted frame of reference such as the soil-water interface of wetlands (Berner, 1980). Advection is associated with the flow of material with either the velocity of its own or the velocity of medium (water) through which the material is transported (Lerman, 1979). During advective transport, solutes are typically transported at the same velocity as water or air. Advective fluxes can include sediments and solutes carried in surface water flows, or solutes in groundwater and pore water flow. The flow of material of a given density p(M L ) with velocity f/(L T ) results in the advective sediment flux J as described in the following equation ... 

A field study conducted in December 1999 (123,124) confirmed the efficacy of this treatment process. The LAA discharges into North Haiwee Reservoir in a channel about 25 m wide, bordered on the eastern side by a small peninsula. Samples of overlying water, sediment, and sediment porewaters were collected in this channel and in a control site on the eastern side of the peninsula. In the channel, an orange-brown floe was visible floating in the water and was observed to settle near the banks, where flow velocity was reduced. In the water sample collected closest to the LAA discharge, the comparison between arsenic and iron concentrations in unfiltered samples (17 pg/L As and 1.9 mg/L Fe) and samples filtered through 0.45-pm membrane filters (3.3 pg/L As and 0.13 mg/L Fe) indicated that both As and Fe were largely associated with the particulate phase. In contrast, at the control site, the concentrations of arsenic in unfiltered and filtered samples were quite similar unfiltered samples contained 5.7 and filtered samples... 

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