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Sediment

The process of sedimentation is primarily a physical law related to biological phenomena pesanteur.Des can accelerate or reduce it, including efforts in ecological and biogeochemical cycles

Sedimentation depends on the context geomorphological, climatic, ecological and water velocity.

Specific fauna to sediment contributes to their nature, their mobility and bioavailability of elements they contain; particles, nutrients , or pollutants. Worms Polychaeta typical vases of estuaries contribute, but they also help to aerate the sediment

Pollutants can be accumulated in the food chain and leaving the sediment to re-contamination of other compartments of the ecosystem , through the bioturbation

Dam constructed to block sediment from the explosion of the volcano Mt St Helens on the Toutle River ( Washington State , USA )

The materials are deposited differentially according to their weight, feature used by gold miners to explore for sediment, where they are generally more of lead shot for hunting than gold

Sedimentation is the origin of sedimentary rocks, including for example chalk and oil play a major role in the carbon cycle

The sediment of any material can be transported by the movement of a fluid (air, water, ice) and is eventually deposited, forming a layer of solid particles on the bed or bottom of a watercourse or another liquid.

Summary

1 Origin and types of sediment
2 Transport of sediment
3 Coasts and shallow seas
- 3.1 Forms of seabed
4 The flow-cons
5 In France
- 5.1 The Grenelle of the sea and sediment
6 See also

Origin and sediment types

Physical particles, living organisms and their excreta settle permanently in freshwater, brackish and saline, or deposited in glaciers. They may be remobilized and transported elsewhere.
Sedimentation of origin ( wind ) increases with aridity and desertification. Sediment transported by wind, are minerals from soil erosion and rocks, volcanoes, sea spray, fires.
Periods of flooding of rivers and streams cause a greater amount of sediment, because the flows, stronger, have greater strength and greater erosive energy transport. The subsequent decline in water levels often creates large areas of new soil on floodplains.
The disappearance or regression of log jams natural, beavers and their dams can change the parameters of a watershed erosion and sedimentation downstream, along the line of a stream or river, or building dams, artificial reservoirs that trap in their large amounts of sediment and pollutants sometimes.
Soil erosion degraded by agriculture and urban soil leaching are a growing source of sediment in the channels. For example, in the network Nord / Pas de Calais, BNS estimated . The volume of sediment that BNS expects to extract between 2007 and 2027 is approximately 8.5 million m3 to 80% from the network lecture (large and main channels) .

The dunes and loess are the results of a sediment transport wind.
The moraines and tills are deposits of sediment have been transported by ice. The gravitational collapse also creates sediments such as talus and landslides as well as elements of karstology.
The lakes , deltas , seas and oceans sediments accumulate over long periods (marine transgression). The material can be terrigenous (from land) or marine (whose origin is marine). The sediments are the source of sedimentary rocks may contain fossils of the inhabitants of this volume of water once covered by layers of sediment. Their core is used to find the changing climate.

Sediment transport

River

When a fluid such as water leaks, it can take care of suspended particles. In quiet environment, the vertical velocity of sedimentation is the maximum speed, or limit, a falling particle. It is given by Stokes' law:

$w = \ frac {(\-rho_p \ rho_f) g ^ 2} {4.5 \ mu}$

where w is the vertical speed limit sedimentation, is the density (the subscripts and indicate and respectively), is the acceleration due to gravity, is the radius of the particle and is the viscosity fluid dynamics.

If the flow velocity is greater than the deposit, the aggregate continues downstream. As there are always different diameters in the flood, the largest settle ( settling ) while being able to continue to descend through such mechanisms as saltation (particle-wall collisions, by rolling or sliding, whose traces are often kept in rock solid) and can be used to estimate the velocity.

In streams with greater slope, coarser sediment can be transported. In the mountains, the diameter of larger rock particles transported up to several tens of centimeters. When this occurs sediment transport during a flood, there is an event thrust.

Contaminated Sediments

Downstream of populated areas, cultivated and / or industrialized, the sediment of rivers, canals, stations of water, estuaries and coasts as well as some lakes are often highly polluted. The pollution may be old (from medieval or ancient mines of metals, for example) and / or recent. Sometimes called "pollution stock, which may (again) become a" pollution flow "during floods, or following a mandrisation watercourse or after work contributing directly or indirectly (changing currents ) to their resuspension.
Sediment may also have been contaminated directly by voluntary or involuntary discharge of waste, particularly in the Baltic Sea and along the French coast, or in some lakes by dumping voluntary ammunition, explosives and waste diverse. Along the canals, depots sludge can sometimes act as points of releasing pollution.

Twin River

Any particle whose diameter is approximately greater than 0.7 mm will form visible topographic components and carved into the bed of the watercourse.

Environments key stakeholders

Environments are the main stakeholders:

Coasts and shallow seas

The second primary environment where the sediment can be suspended in a fluid is in the seas and oceans. It can come from rivers as before. In the middle of the ocean, are living organisms that are primarily responsible for the accumulation of sediment, because their shells sank to the bottom of the ocean after their death and it fossilized. Buried over millions of years these organic sediments are gradually transformed into combustible fuels such as coal or oil , depending on the duration of burial and the environment of the organic layers.

Forms of the seabed

The shape of the seabed is influenced by the tide.

The flow-cons

The fall of particles in a liquid causes cons-flow opposed their descent. These cons-flows have to slow the descent of the particles located above and slow sedimentation.

To highlight this phenomenon, an experience of settling in a tube can easily be performed (see the Boycott effect ).

France

The Grenelle of the sea and sediment

The COMOPS "sediment" in its report made in June 2010 notes that the law may limit the dumping of highly toxic sediment, but not ban it altogether "because of the dumping of sediment that exceeds the threshold are possible if this is the least detrimental to the environment "(principle of the London Convention).
He concluded that if the dredging would not in itself a contaminated sediment, it nevertheless induces "a remobilization of particle-polluted flows from the watershed." We must therefore "act upstream to prevent the flow pollutants at source by applying the principle of" polluter pays "on all the activities that contribute to sediment pollution, and encourage good practice for dredging and Immersion " .

The COMOPS request :

a "protocol" official "evaluation of the dangerousness of sediment (including their management on land)," after analyzing the validation tests currently in progress. "
Scientific assessments before any regulatory changes.
A first regulatory review (MEEDDM) is required before mid 2011, the "existing rules and its application in France and in the Member States concerned by this issue, including the disposal sites, managing onshore sediment monitoring of unregulated substances (but the Water Framework Directive requires taking into account), possible compensatory measures for impacts of dredging / disposal, and practices of any mixtures of dredged sediment.
Disposal sites should be better monitored,
We must take into account substances not yet regulated, consistent with the Water Framework Directive and the Strategic Framework Directive to the marine environment, especially in areas most susceptible. Products such as PAHs in particular may already be fixing thresholds (rem: this recommendation has not been a consensus within the Working Group)
We need a better deal (which by blanketing ...) and develop onshore sediment, which means better characterize (application guides are also needed)
It must both consider the limits of immersion and the development of routes for the disposal of sediments not immersible.
Update before the end of 2011 the rules for application on dredging through better coordination of water policies and BPI for examining files (dredging and sediment management on land), incorporating the new classification of BPI on waste ( Decree of April 13, 2010), assessing the scientific dangers of sediment on land, following better-risk transactions (eg "historical pollution and particularly sensitive sites"). The prefects are already - through their orders - to strengthen the monitoring of disposal sites, "" specifying the objectives of this monitoring, procedures for implementation and interpretation of authorization "...).

The COMOPS wishes Finally, an authorization procedure (instruction + public inquiry) "unique and comprehensive" for requests for permission to dredge disposal and / or management of land sediments, with clarification of procedures and responsibilities on the management of macro-waste emerged in the course of dredging operations.
We must understand and take into account the flow of pollutants during the dredging and dumping, and develop "global indices integrators of pollution levels in sediments", involving ESEARCHERS and stakeholders in the spirit of the Grenelle of the environment. "
Methodological guides and / or recommendations are to be published (including online) to:

- Monitoring and management of disposal sites at sea, on the basis of rules of interpretation of monitoring, and possibly thresholds and innovative approaches to monitoring (GEODE, 2011);

- Study of the impacts under pre-authorization of dredging and disposal at sea (GEODE, finalization of the guide for 2012);

- Evaluation of pressures and impacts of dredging, with comparative analysis of health and environmental impacts by mode of sediment management in 2012 as part of the implementation of DCSMM);

- The sdimatriaux (recovery of sediments in road engineering, coastal engineering and development (MEEDDM, 2011).

Support experimentation to develop recycling options for sediment and learn, with aid of MEEDDM and its public institutions in structuring and bringing together the development projects underway.
Promote good practice , the labels , specifications , technical guides and knowledge and experience feedback by increasing communication (animation, exchanges of experiences among state, ports, establishing scientific and technical associations Protection environment, with an annual public report and balance sheet multi-year annual day exchange and refund on dredging and dumping and sdimatriaux ...
Better involve actors at the technical group, and educate users (including ports) to the importance of reducing the volumes of macro-waste dumped at sea .

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Sediment

Summary

Origin and sediment types