Deposition begins once the flow velocity falls below the settling velocity of a particle, which for a given particle size is less than that required for entrainment.
Settling velocity is closely related to particle size, so that the coarsest fraction in motion should be deposited first with progressively finer grains settling out as the flow velocity continues to fall.
The net effect is a vertical and horizontal (downstream and transverse) gradation of sediment sizes (Knighton, 1984).
The most common depositional feature is the flood-plain, formed from a combination of within-channel and overbank deposition, although many sedimentary forms are involved (Lewin, 1978), quoted from (Knighton, 1984).
During lateral channel migration, erosion of one bank is approximately compensated by deposition against the other, principally but not exclusively in the form of point-bars. With continuing migration, a point-bar is built stream ward and also increases in height through the deposition of sediment carried onto the bar surface by inundating flows.
The sediment deposited by overbank floodwaters comes from material carried in suspension, either as wash load (silts and clays) or the finer fractions of the bed-material load (fine and medium sand).
Since the transportation ability of the flow tends to decrease away from the channel margins, both the amount and mean size of deposited sediment should similarly vary. Also, as the flood recedes and flow velocities over the flood-plain decline, an upward fining of sediment may develop.
Flood-plains provide storage space for sediment as it moves through a drainage basin, the potential for which increases as they become wider with distance downstream. The flood-plain is an integral part of the fluvial system, whose deposits influence channel form through the composition of the channel boundary and therefore the type of material supplied to a stream.
Impacts of Sediment
One of the principal causes of degraded water quality and aquatic habitat is the depositing of eroded soil sediment in water bodies.
Excessive amounts of sediment resulting from natural or human-induced causes can result in the destruction of aquatic habitat and a reduction in the diversity and abundance of aquatic life.
Diversity and population size of fish species, mussels, and benthic (bottom-dwelling) macro invertebrates associated with course substrates can be greatly reduced if the substrates are covered with sand and silt.
Where sand or silt substrates have historically predominated, however, increased deposition may have little detrimental impact on benthic aquatic life.
Suspended sediment causes the water to be cloudy (turbid). Increased turbidity reduces light transmission (and hence photosynthesis), thereby reducing the growth of algae and aquatic plants, which can adversely affect the entire aquatic ecosystem.
Moreover, increased turbidity decreases the water’s aesthetic appeal and the human enjoyment of recreational activities.
If the river cross-section is sufficiently reduced by sediment build up, sedimentation can increase downstream flooding. In addition, some metal ions, pesticides and nutrients may adhere to sediment particles and be transported downstream.
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Increased sediment in surface-water bodies (e.g. rivers, lakes, and reservoirs) may have an economic impact on public water systems that use them as a source of drinking water.
High turbidity not only is aesthetically displeasing, but also interferes with disinfection of the water prior to it being pumped to customers. Communities whose water-supply source has been more turbid often invest millions of naira to upgrade their treatment faculties in order to remove the increased sediment load.
Excessive erosion can reduce the soil’s inherent productivity, whereas the associated sedimentation can damage young plants and fill drainage ditches, lakes, and streams.
Erosive processes can reduce farm income by decreasing crop yields and increasing maintenance costs for drainage systems. Additional erosion damages in both rural and urban areas include reduced ditches, and surface-water supplies.
Erosion is also an issue in areas of modern farming, where the removal of native vegetation for the cultivation and harvesting of a single type of a crop has left the soil unsupported. Many of these regions are near rivers and drainage.
Loss of soil due to erosion removes useful farmland, adds to sediment loads, and can help transport anthropogenic fertilizers into the river system, which leads to eutrophication, with negative consequences on aquatic lives.
Sediment control is a practice or device designed to keep eroded soil on a construction site or other places, so that it does not wash off and cause water pollution to a nearby stream, river, lake, or bay.
Sediment controls are usually employed together with erosion controls, which are designed to prevent or minimize erosion and thus reduce the need for sediment controls.
Coordinated efforts by federal and state agricultural programmes can help reduce the amount of soil erosion and sedimentation.
Farmers should be encouraged to utilize a variety of conservation practices to reduce erosion and limit sedimentation, such as conservation tillage methods, use of conservation buffers and riparian habitat, and establishment of conservation easements.
Water quality can be protected by controlling and minimizing erosion. Federal and state laws should provide some consideration for erosion and sedimentation control along streams and lakes during flood control, drainage, highway, bridge, and other stream- related construction projects.
Both the public and private agencies should be engaged in a variety of programmes for erosion control. Such programmes should include research projects, education programmes, technical assistance, regulatory measures, and cost-share financial assistance.
In a nutshell, preventive measures for reducing excessive sediment load in streams include:
Proper repair and maintenance of drainage ditches and levees.
Minimal disturbance of the stream banks.
Avoidance of structural disturbance of rivers.
Reduction of sediment excesses arising from construction activities.
Application of artificial and natural means for preventing erosion, and
Use of proper land and water management practices on the water-shed.
These preventive measures are preferred over remedial measures, which include;
Construction of detention reservoirs, sedimentation ponds, or settling basins.
Development of side-channel flood-retention basin and
Removal of deposited sediment by dredging.
In summary, erosion process exacerbates sedimentation problems; therefore, in designing any control measures for sedimentation problems, accounts must be taken of erosion control.
Sediment is naturally-occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice, and by the force of gravity acting on the particle itself.
There are several sources of sediment, which include eroded uncovered soils, construction sites, dredging and channelization of streams, grazing along river banks etc.
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Sediment is transported based on the strength of the flow that carries it and its own size, volume, density and shape. Sediment are most often transported by water, wind and glacier.
Some of the major impacts of sediment are pollution – increase in water turbidity, eutrophication, and disruption of aquatic ecosystems.
There are two broad categories of sediment control – preventive and remedial measures. Preventive measures are proactive, while remedial measures are reactionary.