Sedimentation impacts many aspects of the environment soil erosion, water quality, water supply, flood control, river regulation, reservoir lifespan, groundwater table, irrigation, navigation, fishing, tourism, etc. It has attracted increasing attention from the public and engineers in the field.Deposit and scour are common in rivers because of the difference between sediment load and the real sediment transportation capacity of flow. Deposition in river channels raises the elevation ofRiver beds. Consequently, it enhances the waterLevel at the same discharge, and increases the occurrence and the damage of floods. On the other hand, scour brings some safety problems for river training works, lowers water levels, and therefore affects water supply and navigation along rivers.

Erosion can be caused by the kinetic energy of raindrops impinging on the soil surface and by the mechanical force of surface runoff. Surface runoff is caused by heavy rainfall and snow water from spring thaw in the natural or artificial hydrographic network.Erosion caused by water is the most common, widespread and harmful type of soil erosion in the world. The main categories of this type of erosion are surface erosion and channel (or gully) erosion.

SEDIMENT TRANSPORT IN RIVERS

Bed material load and wash loadSediment is classified as either bed load or suspended load according to the patterns and laws of movement. It can also be classified as bed material load and wash load according to the particle size, its origin and effect in fluvial processes.The ratios of fine to coarse sediment in river bed material sediment are quite different.Sediment in river beds is often (but not always) composed of much coarser and much less fine sediment than moving sediment. There is always an exchange between coarse sediment and bed material during transport.Incoming coarse sediment may originate directly from the river bed of an upstream reach. It is directly supplied from the bed and therefore is called bed material load. In contrast, fine sediment, eroded and washed from upland watersheds, is transported through a channel over a long distance and is scarcely ever deposited in the channel; therefore, it is called wash load. Thus, the amount of coarse sediment carried by flow depends on sediment transport capacity and exhibits a well-defined relationship with the flow discharge. In contrast, the concentration of fine sediment depends only on the supply of sediment from the upstream reach, and no obvious relationship with flow discharge is found.

Bed load, saltation and suspended load

It should be pointed out that only bed material load, not wash load, is discussed here.At low flow, although some sediment moves in suspension, most sediment particles move in the form of sliding, rolling and saltation in a zone close to the bed surface with a thickness of 1 to 3 times the particle diameter. Such sediment is called bedload. This zone is called the bed surface layer. With increasing flow velocity, some particles are caught by turbulent eddies. Entering the main flow region, these particles are transported downstream by flow. Sediment supported by turbulent eddies and moving downstream in suspension is called suspended load.With a high level of shear stress, however, not only can the particles enter into motion on the bed surface, but also those in the subsurface layer of the bed can do so as well. This motion penetrates further into the bed in response to further increases in shear stress. The velocity of the moving sediment is significantlysmaller in a deeper bed. The sediment that moves in such a wayis called the laminated load.

Relative importance of bed load and suspended loadThe relative importance of bed load and suspended load dependson sediment size and flow velocity. For the same composition ofbed sediment, sediment slides, rolls or moves in saltation of flowvelocity is low. As velocity increases, part of the sediment iscarried into the main flow zone and becomes suspended load. Therest remains in the bed surface layer and moves as bed load, butthe thickness of the bed surface layer is augmented. Following stillfurther increases in flow velocity, the suspended load is greater,and it exceeds the bed load. In general, for ordinary river flows,sediment coarser than a certain diameter moves mainly as bedload, and sediment finer than that diameter moves mostly insuspension.