Suspended load transport, sediments

Erosion of noncohesive sediments depends on the equilibrium suspended load transport capacity and bed load flux relative to local conditions. The mode of transport for a noncohesive particle class k depends on the magnitude of the local shear velocity relative to the particle settling velocity (Ws ) and Shields critical shear velocity for initiation of motion ( cr, fc7 m/s) ... 

The EEDC and SEDZLJ models are based on similar theory for cohesive and nonco-hesive sediment transport and representation of the sediment bed. Both models have the capability to simulate multiple cohesive and noncohesive particle size classes, as well as bed load transport and suspended load transport. There are some key differences between how these models implement sediment transport, however. EFDC calculates net erosion or deposition for a given particle size class at a given simulation timestep. In the case of noncohesive sediments, the net rate of water-bed sediment exchange is defined by the suspended and bed load transport capacities of the water... 

Rivers transport suspended sediments derived from the disintegration of basin surface layers. With reduced velocity, sediment is deposited in the river channel. The finest material is carried to the sea. It has been estimated that the average mechanical denudation rate for continents is 0.056 mm year (35). This is based on a total suspended load of 13.5 x 10 metric tons year (S). Presently, about two-thirds of the world s total suspended sediment load derives from Southern Asia and large Pacific Islands. Berner has estimated the increase in sediment loss in the U.S. and world since prehuman times to be approximately 200% (35). Current estimated erosion rate from the major land forms is provided in Table I. The relatively recent construction of large sediment trapping dams that normally caused sediment to be deposited in river valleys or transported to the ocean has drastically reduced sediment yields in great rivers. 

The material transported by rivers consists of dissolved ions (dissolved load), sediment suspended in the flow (suspended load), and sediment transported along the bed of the river (bedload). The total load and the proportion of the load represented by these phases varies widely among rivers in different environments. In particular, climate, topography, and erosion influence the amount and composition of riverine sediment loads. 

River transport of clay minerals into the ocean is spatially and temporally variable. The global annual suspended load of river sediment into coastal waters currently averages 12.6 X 10 ton. This flux is approximately 10% less than was delivered before humans began damming rivers. (One notable exception is the Mississippi River, whose sediment load has increased due to very high rates of soil erosion. The riverine sediments deposited in the mouth of the Mississippi River form one of the world s largest deltas.)... 

By trapping the sediment from used alpine waters, reservoirs reduce the transport of suspended load to residual flow reaches. In these sections with a reduced channel flow, the tractive force and shear stress is drastically reduced. This additionally reduces the bed load transport, which may then result in solid matter originating from unaffected tributary streams remaiiung in the main channel, thus significantly increasing the debris-flow hazard for episodic high water discharge [36]. 

Now, we need a boundary condition to determine /3i. This is difficult with suspended sediment profiles. We can develop a fairly good estimate of the distribution of suspended sediment once we have a known concentration at some location in the flow field. In the sediment transport field, bed load and suspended load are often discussed. The relation between the two, and some experience and measurements of both simultaneously, can be used to predict an equivalent suspended sediment concentration at the bed. Then, the relevant boundary condition of equation (E5.2.11) is... 

Sediments in rivers are transported as suspended load or bed load and are directly discharged into open oceans, coastal shelves, or remain stored in estuaries and rivers. 

Figure 7 Theoretical proportion of dissolved transport as a function of the concentration of riverine suspended sediments for three values of the ratio of concentration in the suspended load over concentration in the dissolved load. = 10, 100, and 1,000 correspond to the most mobile elements (Na, B, Re, Se, As, Sb), the intermediate elements (Cu, Ni, Cr, Ra) and highly immobile elements (Th, Al, Ti, Zr) respectively (see Figure 2).

Clastic sediments deposited in conduit systems can be conveniently be divided into five facies depending on the mechanism of deposition. The channel facies comprises most observed sediment piles and can be subdivided further depending on the objectives of a particular investigation. Diamicton facies and slackwater facies are deposited from suspended loads. Channel facies and thalweg facies are transported as bedload. The backswamp facies is defined to describe those residual in infiltrated clastic sediments that are deposited in place with little horizontal transport. 

Caesium is said to behave conservatively that is, the bulk of the radionuclide inventory is associated with the water phase and so transport processes are dominated by the bulk movement of sea water. Plutonium and americium, on the other hand, behave non-conservatively the bulk of their inventory is associated with sediments and the transport processes affecting sediments are very important to their behaviour. The proportion of each nuclide present in the water column as suspended particulate is a simple function of value and suspended sediment load (Sholkovitz, 1983), as indicated in Table 8.2. Thus, for the full range of sediment loadings, water column inventories of Cs are dominated by... 

In vegetated area, the shear velocity to govern sediment transport is evaluated by using equation 6 from the obtained depth-averaged velocity in horizontal 2D flow analysis, and it is applied to evaluate bed load transport rate, entrainment flux of suspended sediment and so on. 

For a free-surface slurry flow in an open conduit inclined to the horizontal, the transport of the suspended load is similar to that of a closed conduit. However, for the case of free-surface flow, there is no pressure differential across the length of the conduit, and direct application of Equation 27 is not meaningful. For sediment transport in open-channel flow, Yalin [57] and Novak and Nalhuri [58] showed that the Froude number is an important parameter that describes sediment transport, and is given as... 

After eroding from the sediment bed, particles will settle back to the bed or will be transported with the flow. The transport is generally apportioned into two categories bedload and suspended load. Bedload refers to the transport of particles that roll, bounce, or slide in a thin layer near the bed and remain in frequent contact with the bed. The bedload layer is on the order of a few particle diameters in thickness. Suspended load refers to the transport of particles within the water column without frequent contact with the bed. Particles transported as suspended load move at the same speed as the surrounding water, while particles transported as bedload move more slowly than the surrounding water. 

If the shear velocity exceeds the critical shear velocity and the particle settling velocity, eroded sediment will be at least partially transported as suspended load. For suspended load, the direction and magnitude of the water-bed sediment exchange flux (Jo, g/m /s) ultimately depends on the difference between the near-bed actual concentration (5 e, g/m ) and the near-bed equilibrium concentration (5eq, g/m ), as described by Equation 10.13. 

Similar to EFDC, the SEDZLJ model simulates erosion of sediments into suspended load or bed load depending on the applied bed shear stress and the sediment particle size distribution at the surface of the bed. Fine-grained (i.e., cohesive) sediments are assumed to be transported entirely with the suspended load and therefore do not contribute to the bed load flux. For noncohesive sediments, the ratio of suspended load to total load transport (qs/qt) for eroding sediments is estimated using the following relationship, which is based on flume data collected by Guy etal.(1966) ... 

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