Siltation

Siltation or siltification is the pollution of water by particulate terrestrial clastic material, with a particle size dominated by silt or clay. It refers both to the increased concentration of suspended sediments, and to the increased accumulation (temporary or permanent) of fine sediments on bottoms where they are undesirable. Siltation is most often caused by soil erosion or sediment spill.

Sometimes siltation is called sediment pollution, although that is an undesirable term since it is ambiguous, and can also be used to refer to a chemical contamination of sediments accumulated on the bottom, or pollutants bound to sediment particles. Siltation is the preferred term for being unambigiuous, even if not entirely stringent since it also includes other particle sizes than silt.

Siltation or Sedimentation
Siltation of a waterway

Causes

Dumping of faecal sludge into the river
Siltation caused by fecal sludge collected from pit latrines and dumped into a river at the Korogocho slum in Nairobi, Kenya.
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Siltation caused by raw sewage sludge and industrial waste in the New River as it passes from Mexicali to Calexico, California.
Jacuecanga Angra dos Reis Rio de Janeiro Brazil Brasfels
Siltation caused by sewage sludge from shipyardRio de Janeiro.

The origin of the increased sediment transport into an area may be erosion on land, or activities in the water.

In rural areas the erosion source is typically soil degradation due to intensive or inadequate agricultural practices, leading to soil erosion, especially in fine-grained soils such as loess. The result will be an increased amount of silt and clay in the water bodies that drain the area. In urban areas the erosion source is typically construction activities, since this involves clearing the original land-covering vegetation and temporarily creating something akin to an urban desert from which fines are easily washed out during rainstorms.

In water the main pollution source is sediment spill from dredging, from the transportation of dredged material on barges, and the deposition of dredged material in or near water. Such deposition may be made to get rid of unwanted material, such as the offshore dumping of material dredged from harbours and navigation channels. The deposition may also have as purpose to build up the coastline, artificial islands, or for beach replenishment.

Climate change also affect siltation rates.[1]

Another important cause of siltation is the septage and other sewage sludges that are discharged from households or business establishments with no septic tanks or wastewater treatment facilities to bodies of water.

Vulnerabilities

Muddy USGS
Silted river polluted by sediment.

While the sediment in transport is in suspension, it acts as a pollutant for those who require clean water. This includes uses such as for cooling or in industrial processes, and it includes aquatic life that are sensitive to suspended material in the water. While nekton have been found to avoid spill plumes in the water (e.g. the environmental monitoring project during the building of the Øresund Bridge), filtering benthic organisms have no way of escape. Among the most sensitive organisms are coral polyps. Generally speaking, hard bottom communities and mussel banks (including oysters) are more sensitive to siltation than sand and mud bottoms. Unlike in the sea, in a stream the plume will cover the entire channel, except possibly for backwaters, which is why fish will also be directly affected in most cases.

Siltation can also affect navigation channels, or irrigation channels. It refers to the undesired accumulation of sediments in channels intended for vessels, or for distributing water.

Measurement and monitoring

SediMeter sensor
A sensor for measuring siltation in situ.

One may distinguish between measurements at the source, during transport, and within the affected area. Source measurements of erosion may be very difficult, since the lost material may be a fraction of a millimeter per year. Therefore, the approach taken is typically to measure the sediment in transport in the stream, by measuring the sediment concentration and multiplying that with the discharge; for example, 50 mg/L times 30 m3/s gives 1.5 kg/s.

Also sediment spill is better measured in transport than at the source. The sediment transport in open water is estimated by measuring the turbidity, correlating turbidity to sediment concentration (using a regression developed from water samples that are filtered, dried, and weighed), multiplying the concentration with the discharge as above, and integrating over the entire plume. To distinguish the spill contribution, the background turbidity is subtracted from the spill plume turbidity. Since the spill plume in open water varies in space and time, an integration over the entire plume is required, and repeated many times to get acceptably low uncertainty in the results. These measurements are made close to the source, in the order of a few hundred meters.

Anything beyond a work area buffer zone for sediment spill is considered the potential impact area. In the open sea the impact of concern is almost exclusively with the sessile bottom communities, since empirical data show that fish effectively avoid the impacted area. The siltation chiefly affects the bottom community in two ways: The suspended sediment may interfere with the food gathering of filtering organisms, and the sediment accumulation on the bottom may bury organisms to the point that they starve or even die. Only if the concentration is extreme will it decrease the light level sufficiently for impacting primary productivity. An accumulation of as little as 1 mm may kill coral polyps.

While the effect of the siltation on the biota (once the harm is already done) can be studied by repeated inspection of selected test plots, the magnitude of the siltation process in the impact area may be measured directly by monitoring in real time. Parameters to measure are sediment accumulation, turbidity at the level of the filtering biota, and optionally incident light.[2]

Siltation of the magnitude that it affects shipping can also be monitored by repeated bathymetric surveys.

Mitigation

Key Biscayne Crandon Park beach
Resuspension of fines from a replenished beach causing siltation offshore

In rural areas the first line of defense is to maintain land cover and prevent soil erosion in the first place. The second line of defense is to trap the material before it reaches the stream network (known as sediment control). In urban areas the defense is to keep land uncovered for as short a time as possible during construction, and to use silt screens to prevent the sediment from getting released in water bodies. During dredging the spill can be minimized but not eliminated completely through the way the dredger is designed and operated. If the material is deposited on land, efficient sedimentation basins can be constructed. If it is dumped in relatively deep water there will be a significant spill during dumping, but not thereafter, and the spill that does arise will have minimal impact if there are only fine-sediment bottoms nearby.

One of the most difficult conflicts of interest to resolve, as regards siltation mitigation, is perhaps beach nourishment. When sediments are placed on or near beaches in order to replenish an eroding beach, any fines in the material will continue to be washed out for as long as the sand is being reworked. Given that all replenished beaches are eroding (or else they would not need replenishment), they will contribute to nearshore siltation almost for as long as it takes to erode away what was added, albeit with somewhat decreasing intensity over time. Since this leakage is detrimental to coral reefs, the practice leads to a direct conflict between the public interest of saving beaches, and preserving any nearshore coral reefs. To minimize the conflict, beach replenishment should only be done with sand not containing any silt or clay fractions. In practice the sand is often taken from offshore areas, and since the proportion of fines in sediments typically increases in the offshore direction, the deposited sand will inevitably contain a significant percentage of siltation-contributing fines.

It is desirable to minimize the siltation of irrigation channels by hydrologic design, the objective being not to create zones with falling sediment transport capacity, as that is conducive to sedimentation. Once sedimentation has occurred, in irrigation or navigation channels, dredging is often the only remedy.

References

  1. ^ U.D. Kulkarni; et al. "The International Journal of Climate Change: Impacts and Responses » Rate of Siltation in Wular Lake, (Jammu and Kashmir, India) with Special Emphasis on its Climate & Tectonics". The International Journal of Climate Change: Impacts and Responses. Retrieved 2009-11-16.
  2. ^ Siltation Monitoring Plan excerpt, retrieved 2010-07-11, http://lindorm.com/beaches/sedmon2.php
Akra Kaur Dam

Akra Kaur Dam, sometimes also referred to as Ankara Kaur Dam, is located near Gawadar in Balochistan, Pakistan. The dam was constructed in 1995 at a cost of $24 million to supply water to Gawadar and adjoining villages. It is the sole source of water supply to residents of the Gwadar District area. The dam stretches over an area of 17,000 acres (6,900 ha; 27 sq mi).In 2005, torrential rain in the area caused an overflow from the dam, inundating a number of villages and claiming at least 20 lives. In July 2012, reports emerged that the dam had dried up completely due to large-scale siltation. This has posed serious water supply challenges to local residents, including acute shortage of drinking water.

Chautauqua National Wildlife Refuge

The Chautauqua National Wildlife Refuge is located on the Illinois River in Mason County northeast of Havana, Illinois. It is managed by the U.S. Fish and Wildlife Service as one of the four Illinois River National Wildlife and Fish Refuges.

The refuge consists of 4,388 acres (17.8 km²) of Illinois River bottomland, nearly all of it wetland. The parcel is the former Chautauqua Drainage and Levee District, a failed riverine polder. In the 1920s, workers with steam shovels surrounded the levee district with a large dike in an attempt to create a large new parcel of agricultural farmland. The levee district proved to be financially unable to maintain the dike, however, and the Illinois River reclaimed the polder. The complex alluvial topography that had existed before this intervention was replaced by the broad shallow pool of Chautauqua Lake.

In 1936, the federal government acquired the 4,388-acre (17.8 km2) Chautauqua Drainage and Levee District parcel, including the dikes that enclosed the pool, and began to manage it for wildlife-refuge and flood control purposes. The flood-control aspects of this management have grown more challenging in the years since, as continued agricultural runoff and siltation of the Illinois River has made much of Chautauqua Lake shallower. On some shoreline strips of the lake, the silt has built up to the level of the lake surface, and an alluvial topography of sloughs and floodplain woodlands may be slowly re-establishing itself. However, many of the plant and animal species inhabiting the current Chautauqua Lake and Wildlife Refuge and adjacent Illinois River are nonnative and invasive species such as the Asian carp.

As of 2005, of the 4,388 acres (17.8 km²) of the Chautauqua National Wildlife Refuge, 3,200 acres (12.9 km²) were classified as an open pool, 800 acres (3.2 km²) were classified as "water and timbered bottomland", and the remaining 388 acres (1.6 km²) were classified as upland forest. The closest numbered highway is U.S. Highway 136 in Mason County.

A nesting pair of bald eagles was observed in the Chautauqua National Wildlife Refuge in the winter of 2005-06.The Cameron/Billsbach Unit is a detached section of the refuge located further north, in Marshall County, near Henry, Illinois. It covers an additional 1,079 acres (4.37 km²).

Colombian grebe

The Colombian grebe (Podiceps andinus), was a grebe (aquatic bird) found in the Bogotá wetlands on the Bogotá savanna in the Eastern Ranges of the Andes of Colombia. The species was still abundant in Lake Tota (3,000 metres (9,800 ft)) in 1945. The species has occasionally been considered a subspecies of black-necked grebe (P. nigricollis). It was flightless.The decline of the Colombian grebe is attributed to wetland drainage, siltation, pesticide pollution, disruption by reed harvesting, hunting, competition, and predation of chicks by rainbow trout (Oncorhynchus mykiss). The primary reason was loss of habitat: drainage of wetlands and siltation resulted in higher concentrations of pollutants, causing eutrophication across Lake Tota. This destroyed the open, submergent pondweed (Potamogeton) vegetation and resulted in the formation of a dense monoculture of water weed (Elodea).By 1968, the species had declined to approximately 300 birds. Only two records of this bird were made in the 1970s; one seen 1972, and the last confirmed record from 1977 when three birds were seen. Intensive studies in 1981 and 1982 failed to find the species and it is now considered extinct.

Dahla Dam

The Dahla Dam, also known as Arghandab Dam, is located in the Shah Wali Kot District of Kandahar Province in Afghanistan, approximately 40 kilometres (25 mi) north of the provincial capital Kandahar. Constructed in 1952, it is said to be the second largest dam in Afghanistan. As of 2019, the Afghan government is spending $450 million dollars on making the dam more useful. The project includes raising the dam's walls by 12 meters so its reservoir can hold nearly a billion cubic meters of fresh water and installing three turbines for the production of 22 megawatts (MW) of electricity.The Dahla Dam is built on the Arghandab River which flows over a length of 250 miles (400 km). Over the years its reservoir was subject to siltation, and its canal system reduced irrigation benefits. This necessitated undertaking rehabilitation of the dam which involved desiltation works and pertinent components of the project to improve the water delivery system; this component was completed during March 2012 with assistance provided by Canada. The second phase involved raising the height of the dam and the relevant dykes to compensate for the loss of storage in its reservoir due to siltation, and to achieve the full benefits of irrigation for which the dam was originally built.

Dams and reservoirs in Ethiopia

Ethiopia is called the water tower of Africa due to its combination of mountainous areas with a comparatively large share of water resources in Africa. Only a fraction of this potential has been harnessed so far, 1% at the beginning of the 21st century.. In order to become the powerhouse of Africa, Ethiopia is actively exploiting its water resources by building dams, reservoirs, irrigation and diversion canals and hydropower stations. The benefits of the dams are not only limited to hydropower. Many dams are multi-purpose dams that are also designed to provide water for irrigation, drinking water and flood control. However, hydropower is expected to be the main benefit of the dams.

Forebay (reservoir)

A forebay is an artificial pool of water in front of a larger body of water. The larger body of water may be natural or man-made.Forebays have a number of functions. They are used in flood control to act as a buffer during flooding or storm surges, impounding water and releasing in a controlled way into the larger waterbody. They may be used upstream of reservoirs to trap sediment and debris (sometimes called a sediment forebay) in order to keep the reservoir clean. This may entail the use of a forebay dam or pre-dam. They may also be used upstream of lakes to prevent siltation. Some forebays are used simply to create a natural habitat for flora and fauna, to counterbalance the environmental impact of a dam or reservoir. Forebays vary greatly in size depending on their situation and purpose.

Hirakud Dam

Hirakud Dam is built across the Mahanadi River, about 15 kilometres (9.3 mi) from Sambalpur in the state of Odisha in India. Behind the dam extends a lake, Hirakud Reservoir, 55 km (34 mi) long. It is one of the first major multipurpose river valley projects started after India's independence.

Hydroelectricity

Hydroelectricity is electricity produced from hydropower. In 2015, hydropower generated 16.6% of the world's total electricity and 70% of all renewable electricity, and was expected to increase by about 3.1% each year for the next 25 years.

Hydropower is produced in 150 countries, with the Asia-Pacific region generating 33 percent of global hydropower in 2013. China is the largest hydroelectricity producer, with 920 TWh of production in 2013, representing 16.9% of domestic electricity use.

The cost of hydroelectricity is relatively low, making it a competitive source of renewable electricity. The hydro station consumes no water, unlike coal or gas plants. The typical cost of electricity from a hydro station larger than 10 megawatts is 3 to 5 U.S. cents per kilowatt hour. With a dam and reservoir it is also a flexible source of electricity, since the amount produced by the station can be varied up or down very rapidly (as little as a few seconds) to adapt to changing energy demands. Once a hydroelectric complex is constructed, the project produces no direct waste, and in many cases it has a considerably lower output level of greenhouse gases than fossil fuel powered energy plants.

Ichamati River

Ichamati River (Bengali: ইছামতী নদী) (also spelt Ichhamati), is a trans-boundary river which flows through India and Bangladesh and also forms the boundary between the two countries. The river is facing the problem of siltation leading to thin flow of water in the dry season and floods in the rainy season. Experts are handling the situation and remedial matters are being discussed between the governments of India and Bangladesh.

Ikwelo River

Gweru River is a river in Midlands (Zimbabwe) Province of Zimbabwe.

Mackinaw River State Fish and Wildlife Area

The Mackinaw River State Fish and Wildlife Area is a 1,448-acre (586 ha) state park in Tazewell County, Illinois. It is operated by the Illinois Department of Natural Resources (IDNR). The Area dates to 1970, when a conservation group based in Bloomington, the Parklands Foundation, donated its acreage to the state.The Mackinaw River SFWA consists of more than two square miles of upslope on the south bank of the Mackinaw River, primarily wooded land with some upland meadows. There are also two sections of Mackinaw River bottomland that offer direct access to the river; each section is approximately 1⁄4-mile (400 m) wide. The Mackinaw River SWFA is primarily managed for the hunting of whitetail deer, although fishing and canoeing are also welcomed.

The Mackinaw River is a free-running river throughout most of its length, and therefore offers potential for the preservation of fish and shellfish species (particularly mussels) historically associated with the tallgrass prairie. However, most of the river's drainage is heavily utilized for crop farming, with its potential for erosion and consequent siltation. Sixty-six percent of the river's drainage is cropland. The Mackinaw River State Fish and Wildlife Area is an island of natural drainage into the vulnerable river, although it takes up less than 1.2% of the river's total watershed.

The nearest town to the Mackinaw River SFWA is Mackinaw, Illinois.

Magat Dam

Magat Dam is a large rock-fill dam in the island of Luzon in the Philippines. The dam is on the Magat River, a major tributary of Cagayan River. Construction of the dam started in 1975 and was completed in 1982. Magat Dam is one of the largest dams in the Philippines. It is a multi-purpose dam which is used primarily for irrigating about 85,000 hectares (210,000 acres) of agricultural lands, flood control, and power generation through the Magat Hydroelectric Power Plant.

The water stored in the reservoir is enough to supply about two months of normal energy requirements.The dam was constructed to last for 50 years but increased siltation and sedimentation in the reservoir, slash-and-burn farming, illegal logging and fish-caging resulted in the deterioration of the dam's watershed. The 1990 Luzon earthquake also contributed to the increased siltation in the Magat River system. Because of this, President Gloria Macapagal-Arroyo instructed various government agencies to create a rehabilitation plan to improve the lifespan of the dam system.

Ngondoma Dam

Ngondoma Dam is a dam on the Ngondoma River located 500 meters west of the Empress Mine Township in the Midlands Province of Zimbabwe. The dam is 67 kilometers northwest of the Kwekwe and 54 km southwest of Kadoma (by air).The Zhombe Communal Land lies to the east of the dam, and the Chief Chisina, Gokwe land is on its west. The dam belongs to the Zhombe East, Kwekwe District, and is one of the 1,620 dams in the Midlands. Dams in this province make up 17% of all dams in Zimbabwe and are managed by ZINWA (Zimbabwe National Water Authority).

Oyster River (New Hampshire)

The Oyster River is a 17-mile-long (27 km) river in Strafford County, southeastern New Hampshire, United States. It rises in Barrington, flows southeast to Lee, then east-southeast in a serpentine course past Durham to meet the entrance of Great Bay into Little Bay. The bays are tidal inlets of the Atlantic Ocean, to which they are connected by a tidal estuary, the Piscataqua River. The freshwater portion of the river is 14.1 miles (22.7 km) long, and the tidal river extends 2.9 miles (4.7 km) from Durham to Great Bay.The Oyster River reaches tidewater at the base of a dam in the center of Durham, just west of the river's crossing by NH Route 108. Due to siltation, the river is only fully accessible to motorized boats west of the Durham Water Plant for approximately three hours on either side of high tide. Boaters have noticed the increasing effect of siltation on navigation since 1998.

Pulangi Dam

The Pulangi IV Hydroelectric Power Plant, also known as the Pulangi Dam, is located on the Pulangi River near Maramag in Bukidnon province on the island of Mindanao in the Philippines. It uses two reservoirs, produced by damming the Pulangi River, to supply water to a run-of-the-river hydroelectric power plant; the power plant is capable of generating 255 megawatts (342,000 hp) of power. Construction began in 1982; the first two generators became operational in December 1985, with the last generator being commissioned in 1986.The upper (pondage) reservoir located at 7°47′11″N 125°1′25″E diverts water into a power channel which parallels the river until it reaches the lower reservoir (surge pool) at 7°42′56″N 125°1′25″E, 7.5 km (5 mi) to the south. At the lower reservoir, water is fed to each of the three 85 MW (114,000 hp) Francis turbine-generators via a penstock. The Pulangi IV power plant provides 23% of the hydroelectric power generated on Mindanao.Since operation, the reservoirs associated with the power plant have received an estimated 1,500,000 m3 (1,216 acre⋅ft) of sediment annually. Of the reservoir's combined 67,000,000 m3 (54,318 acre⋅ft) active capacity, 23,000,000 m3 (18,646 acre⋅ft) has been filled with silt. The siltation rate was almost 1 meter annually and caused the dam's reservoir to work at 50% capacity. The unexpected siltation threatens safe operation of the dams and power generation, in addition to drastically shortening the predicted operational lifespan of the dam. At first, the minimum and maximum water levels were raised and, in 2007, dredging work was performed around the head work of the upper reservoir's head. Selective dredging in the upper reservoir began in 2010, and continues as of 2011.The artificial lake created by the reservoir proper is called the Pulangi Lake or the Maramag Basin.

Reelfoot National Wildlife Refuge

Reelfoot National Wildlife Refuge is a part of the U.S. system of National Wildlife Refuges consisting of an area of Northwest Tennessee and Western Kentucky that consists primarily of a buffer zone around Reelfoot Lake, Tennessee's only large natural lake. It formed after the New Madrid earthquakes of 1811–1812 and is one of the Mississippi River Basin's richest locations for waterfowl, aquatic life, and other wildlife.

It covers 10,428 acres (4,220 ha) and comprises primarily lands adjacent to the lake that have not been included in the Tennessee State Park system. The refuge was established in 1941 and has been expanded on several occasions. Some of it consists of agricultural land that is leased to farmers, but they are required to use stricter conservation practices than were widespread when the same land was held in private ownership, primarily to lessen the siltation of the lake and surrounding bodies of water.

Salt River (California)

The Salt River is a formerly navigable hanging channel of the Eel River which flowed about 9 miles (14 km) from near Fortuna and Waddington, California, to the estuary at the Pacific Ocean, until siltation from logging and agricultural practices essentially closed the channel. It was historically an important navigation route until the early 20th century. It presently intercepts and drains tributaries from the Wildcat Hills along the south side of the Eel River floodplain. Efforts to restore the river began in 1987, permits and construction began in 2012, and water first flowed in the restored channel in October 2013.

Sedimentation

Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism. In geology, sedimentation is often used as the opposite of erosion, i.e., the terminal end of sediment transport. In that sense, it includes the termination of transport by saltation or true bedload transport. Settling is the falling of suspended particles through the liquid, whereas sedimentation is the termination of the settling process. In estuarine environments, settling can be influenced by the presence or absence of vegetation. Trees such as mangroves are crucial to the attenuation of waves or currents, promoting the settlement of suspended particles.Sedimentation may pertain to objects of various sizes, ranging from large rocks in flowing water to suspensions of dust and pollen particles to cellular suspensions to solutions of single molecules such as proteins and peptides. Even small molecules supply a sufficiently strong force to produce significant sedimentation.

The term is typically used in geology to describe the deposition of sediment which results in the formation of sedimentary rock, but it is also used in various chemical and environmental fields to describe the motion of often-smaller particles and molecules. This process is also used in the biotech industry to separate cells from the culture media.

Sukhna Lake

Sukhna Lake in Chandigarh, India, is a reservoir at the foothills (Shivalik hills) of the Himalayas. This 3 km² rainfed lake was created in 1958 by damming the Sukhna Choe, a seasonal stream coming down from the Shivalik Hills. Originally the seasonal flow entered the lake directly causing heavy siltation. To check the inflow of silt, 25.42 km² of land was acquired in the catchment area and put under vegetation. In 1974, the Choe was diverted and made to bypass the lake completely, the lake being fed by three siltation pots, minimising the entry of silt into the lake itself.

Aquatic ecosystems

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