Sediment

Types of sediments

Physical particles, living beings and their excrétats form a deposit in permancence in fresh water, brackish and salted, or settle in the Glacier S. They can be remobilized and transported elsewhere. The Sedimentation of origin (wind) does not increase with the aridification and the turning into a desert. The sediments transported by the wind, are minerals resulting from the erosion of the grounds and rocks, volcanos, spray, fires.

the dune S and the Lœss are results of a wind sedimentary transport.
the Moraine S and Till S are deposits of sediments having been transported by the ice. Gravitating collapses create also sediments like the slopes and the slips as well as the elements of Karstologie.

The Lake S, Delta S, Sea S and Océan S accumulate sediments for long periods (marine invasion). The material can be terrigenous (coming from the grounds) or marine (whose origin is marine). The sediments deposited are the source of sedimentary rocks which can contain Fossile S of the inhabitants of this volume of water formerly covered by the layers with sediments. Their coring makes it possible to know the evolution of the Climat.

The periods of Raw of the rivers and the rivers involve a greater quantity of sediments, because the flows, stronger, have a more important erosive force and a greater energy of transport. The subsequent fall of the water level often creates the vast wide ones of new ground on the easily flooded plains. The disappearance or the regression of the natural Embâcle S, the Beaver S and their stoppings can modify the erosive parameters of a catchment area and sedimentation downstream, just as the drain of a river or a river, or the construction of artificial Barrage S which imprison in their reserves of important quantity of sediments and sometimes of pollutants.

Transport of sediments

Cours d' water

When a fluid as the Eau runs, it can take care of suspended particles. The sedimentation test is minimal speed that a flood must have to transport, rather than to deposit, sediments, and is given by the law of Stokes :

$w= \ frac {(\ rho_p- \ rho_f) gr^2} {4.5 \ driven}$

where W is the speed of sedimentation , ρ is the Density (the indices p and F indicate respectively fluid particle and ), G is acceleration due to gravity, R is the ray of the particle and μ is the dynamic viscosity of the fluid.

If the rate of the flow is larger than that of deposit, the aggregate continues towards the downstream. As there are always different diameters in the flood, largest settle (Décantation) while being able to continue to go down by mechanisms like the saltation (collisions particle-wall, by bearing or slip, whose traces are often preserved in the solid rocks) and can be used to estimate the current velocity.

In the rivers to more important slope, coarser sediments can be transported. In mountain, the diameter of larger transported rock particles can reach several tens of centimetres. When this transport of sediments occurs at the time of a rising, one attends an event of haulage.

Polluted sediments

Downstream from the inhabited areas, cultivated and/or industrialized, the sediments of the rivers, channels, stations of water, Estuary S and littoral as of certain lakes are often very polluted. The Pollution can be old (exit of mines medieval or ancient of metals for example) and/or recent. One speaks sometimes about “ pollution of stock ”, which can (Re) become a “ pollution of flow ” in period of rising, or following a meandrisation of the river or following work contributing directly or indirectly (modification of the currents) to their handing-over in suspension.
Des sediments can also be polluted directements by the voluntary or involuntary rejection of waste, and in particular in the Baltic and on the French littoral or in certain lakes by the voluntary immersion of ammunition, explosives and various Déchet S. Along the channels, deposits of Boues of clearings out can sometimes constitute points of relarguage of pollution.

River beds

Any particle whose diameter is roughly larger than 0,7 mm will form topographic components visible and carved in the bed of the river.

Environments principal agents

The principal environments agents are:

Delta
alluvial Terrace
alluvial range
Meander S and River in braids
Lake of dead arm
Raised
sedimentary Basin

Not very deep coasts and seas

The second principal environment where the sediment can be in suspension in a fluid is in the seas and oceans. It can come from the rivers like previously. In the middle of the ocean, in fact the living organisms are mainly responsible for the accumulation of sediments, because their shell runs on the ocean floor after their death and fossilizes there. Hidden during million years, these organic sediments are gradually transformed into Combustible S fossils such as the coal or the Pétrole, according to the duration of hiding and the environment of the organic layers.

Forms of the marine bed

The form of the marine bed is influenced by the Marée.

Against-flows

The fall of particles in a liquid causes against-flows opposed to their descent. These against-flows cause to slow down the descent of the particles located above and slow down sedimentation.

To highlight this phenomenon, an experiment of sedimentation in a tube can easily be realized (see the effect Boycott).

See too

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