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| [[Image:Beach Stones 2.jpg|thumb|200px|{{center|[[Cobble]]s on a beach}}]]
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| [[Image:Leman img 0573.jpg|thumb|upright|200px|{{center|[[River Rhône]] flowing into [[Lake Geneva]]}}]]
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| [[Image:Spring Runoff in the Adriatic Sea.jpg|thumb|upright|200px|{{center|Sediment billowing out from Italy's shore into the [[Adriatic Sea]]}}]]
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| '''Sediment''' is a naturally occurring material that is broken down by processes of [[weathering]] and [[erosion]], and is subsequently [[sediment transport|transported]] by the action of wind, water, or ice, and/or by the force of [[gravity]] acting on the particle itself.
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| Sediments are most often transported by water ([[fluvial]] processes), wind ([[aeolian processes]]) and [[glacier]]s. Beach sands and [[stream channel|river channel]] deposits are examples of fluvial transport and deposition, though sediment also often settles out of slow-moving or standing water in lakes and oceans. Desert sand dunes and [[loess]] are examples of aeolian transport and deposition. [[Glacial]] [[moraine]] deposits and [[till]] are ice-transported sediments.
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| == Classification ==
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| Sediment can be classified based on its [[Particle size (grain size)|grain size]] and/or its composition.
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| ===Grain size===
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| {{main|Particle size (grain size)}}
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| {{see also|soil texture|Unified Soil Classification System}}
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| [[Image:Sediment in the Gulf of Mexico.jpg|thumb|Sediment in the [[Gulf of Mexico]]]]
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| [[Image:Sediment in the Gulf of Mexico (2).jpg|thumb]]
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| [[Image:Sediment off the Yucatan Peninsula.jpg|thumb|Sediment off the [[Yucatán Peninsula]]]]
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| Sediment size is measured on a log base 2 scale, called the "Phi" scale, which classifies particles by size from "colloid" to "boulder".
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| {| class="wikitable"
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| ! φ scale !! Size range<br>(metric) !! Size range<br>(inches) !! Aggregate class<br>(Wentworth) !! Other names
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| |-
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| | < -8 || > 256 mm || > 10.1 in || [[Boulder]]
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| |-
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| | -6 to -8 || 64–256 mm || 2.5–10.1 in || [[Cobble (geology)|Cobble]]
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| |-
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| | -5 to -6 || 32–64 mm || 1.26–2.5 in || Very coarse [[gravel]] || [[Pebble]]
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| |-
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| | -4 to -5 || 16–32 mm || 0.63–1.26 in || Coarse gravel || Pebble
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| |-
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| | -3 to -4 || 8–16 mm || 0.31–0.63 in || Medium gravel || Pebble
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| |-
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| | -2 to -3 || 4–8 mm || 0.157–0.31 in || Fine gravel || Pebble
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| |-
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| | -1 to -2 || 2–4 mm || 0.079–0.157 in || Very fine gravel || [[Granule (geology)|Granule]]
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| |-
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| | 0 to -1 || 1–2 mm || 0.039–0.079 in || Very coarse [[sand]]
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| |-
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| | 1 to 0 || 0.5–1 mm || 0.020–0.039 in || Coarse sand
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| |-
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| | 2 to 1 || 0.25–0.5 mm || 0.010–0.020 in || Medium sand
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| |-
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| | 3 to 2 || 125–250 [[micrometre|µm]] || 0.0049–0.010 in || Fine sand
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| |-
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| | 4 to 3 || 62.5–125 µm || 0.0025–0.0049 in || Very fine sand
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| |-
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| | 8 to 4 || 3.9–62.5 µm || 0.00015–0.0025 in || [[Silt]] || [[Mud]]
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| |-
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| | > 8 || < 3.9 µm || < 0.00015 in || [[Clay]] || Mud
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| |-
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| | >10 || < 1 µm || < 0.000039 in || [[Colloid]] || Mud
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| |}
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| ===Composition===
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| Composition of sediment can be measured in terms of:
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| * parent [[Rock (geology)|rock]] [[lithology]]
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| * [[mineral]] composition
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| * [[chemical]] make-up.
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| This leads to an ambiguity in which [[clay]] can be used as both a size-range and a composition (see [[clay mineral]]s).
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| == Sediment transport ==
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| [[Image:StoneFormationInWater.jpg|thumb|Sediment builds up on human-made breakwaters because they reduce the speed of water flow, so the stream cannot carry as much sediment load.]]
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| [[Image:Glacial Transportation and Deposition.jpg|thumb|Glacial transport of boulders. These boulders will be deposited as the glacier retreats.]]
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| {{main|Sediment transport}}
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| {{see also|Rouse number}}
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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. Stronger flows will increase the lift and drag on the particle, causing it to rise, while larger or denser particles will be more likely to fall through the flow.
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| === Fluvial processes: rivers, streams, and overland flow ===
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| ====Particle motion====
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| Rivers and streams carry sediment in their flows. This sediment can be in a variety of locations within the flow, depending on the balance between the upwards velocity on the particle (drag and lift forces), and the [[terminal velocity|settling velocity]] of the particle. These relationships are given in the following table for the [[Rouse number]], which is a ratio of sediment fall velocity to upwards velocity.
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| <math>\textbf{Rouse}=\frac{\text{Settling velocity}}{\text{Upwards velocity from lift and drag}}=\frac{w_s}{\kappa u_*}</math>
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| where
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| * <math>w_s</math> is the fall velocity
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| * <math>\kappa</math> is the [[von Kármán constant]]
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| * <math>u_*</math> is the [[shear velocity]]
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| [[File:Hjulströms diagram en.PNG|thumb|200px|{{center|[[Hjulström curve]]: The velocities of currents required for erosion, transportation, and deposition (sedimentation) of sediment particles of different sizes.}}]]
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| {| class="wikitable"
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| |- bgcolor="#efefef"
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| !Mode of Transport
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| !Rouse Number
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| |-
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| |[[Bed load]]
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| |>2.5
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| |-
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| |[[Suspended load]]: 50% Suspended
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| |>1.2, <2.5
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| |-
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| |[[Suspended load]]: 100% Suspended
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| |>0.8, <1.2
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| |-
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| |[[Wash load]]
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| |<0.8
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| |}
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| If the upwards velocity approximately equal to the settling velocity, sediment will be transported downstream entirely as [[suspended load]]. If the upwards velocity is much less than the settling velocity, but still high enough for the sediment to move (see [[Sediment transport#Initiation of motion|Initiation of motion]]), it will move along the bed as [[bed load]] by rolling, sliding, and [[Saltation (geology)|saltating]] (jumping up into the flow, being transported a short distance then settling again). If the upwards velocity is higher than the settling velocity, the sediment will be transported high in the flow as [[wash load]].
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| As there are generally a range of different particle sizes in the flow, it is common for material of different sizes to move through all areas of the flow for given stream conditions.
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| ==== Fluvial bedforms ====
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| {{main|Bedform}}
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| [[Image:Ripples mcr1.JPG|thumb|Modern asymmetric ripples developed in sand on the floor of the Hunter River, New South Wales, Australia. Flow direction is from right to left.]]
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| [[Image:Sinuous dunes mcr1.JPG|thumb|Sinuous-crested dunes exposed at low tide in the Cornwallis River near Wolfville, Nova Scotia]]
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| [[Image:Channel-StellartonFm-CoalburnPit.JPG|thumb|Ancient channel deposit in the Stellarton Formation ([[w:Pennsylvanian|Pennsylvanian]]), Coalburn Pit, near Thorburn, Nova Scotia.]]
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| Sediment motion can create self-organized structures such as [[ripple marks|ripple]]s, [[dune]]s, [[antidune]]s on the river or stream [[stream bed|bed]]. These bedforms are often preserved in sedimentary rocks and can be used to estimate the direction and magnitude of the flow that deposited the sediment.
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| ==== Surface runoff ====
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| [[Overland flow]] can erode soil particles and transport them downslope. The erosion associated with overland flow may occur through different methods depending on meteorological and flow conditions.
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| * If the initial impact of rain droplets dislodges soil, the phenomenon is called rainsplash erosion.
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| * If overland flow is directly responsible for sediment entrainment but does not form gullies, it is called "sheet erosion".
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| * If the flow and the substrate permit channelization, gullies may form; this is termed "gully erosion".
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| ==== Key fluvial depositional environments ====
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| The major [[fluvial]] (river and stream) environments for deposition of sediments include:
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| * [[River delta|Deltas]] (arguably an intermediate environment between fluvial and marine)
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| * [[Point bar]]s
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| * [[Alluvial fan]]s
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| * [[Braided river]]s
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| * [[Oxbow lake]]s
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| * [[Levee]]s
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| * [[Waterfall]]s
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| ===Aeolian processes: wind===
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| {{main|Aeolian processes}}
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| Wind results in the transportation of fine sediment and the formation of sand dune fields and soils from airborne dust.
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| ===Glacial processes===
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| [[Image:GLMsed.jpg|thumb|Glacial sediments from Montana]]
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| Glaciers carry a wide range of sediment sizes, and deposit it in [[moraine]]s.
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| ===Mass balance===
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| {{main|Exner equation}}
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| The overall balance between sediment in transport and sediment being deposited on the bed is given by the [[Exner equation]]. This expression states that the rate of increase in bed elevation due to deposition is proportional to the amount of sediment that falls out of the flow. This equation is important in that changes in the power of the flow changes the ability of the flow to carry sediment, and this is reflected in patterns of erosion and deposition observed throughout a stream. This can be localized, and simply due to small obstacles: examples are scour holes behind boulders, where flow accelerates, and deposition on the inside of [[meander]] bends. Erosion and deposition can also be regional: erosion can occur due to [[dam removal]] and [[base level]] fall. Deposition can occur due to dam emplacement that causes the river to pool, and deposit its entire load or due to base level rise.
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| == Shores and shallow seas ==
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| Seas, oceans and lakes accumulate sediment over time. The sediment could consist of ''terrigenous'' material, which originates on land, but may be deposited in either terrestrial, marine, or lacustrine (lake) environments; or of sediments (often biological) originating in the body of water. Terrigenous material is often supplied by nearby rivers and streams or reworked [[marine sediment]] (e.g. [[sand]]). In the mid-ocean, living organisms are primarily responsible for the sediment accumulation, their shells sinking to the ocean floor upon death.
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| Deposited sediments are the source of [[sedimentary rock]]s, which can contain [[fossil]]s of the inhabitants of the body of water that were, upon death, covered by accumulating sediment. Lake bed sediments that have not solidified into rock can be used to determine past [[climate|climatic]] conditions.
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| === Key marine depositional environments ===
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| [[Image:EolianiteLongIsland.JPG|thumb|right|[[Holocene]] [[eolianite]] and a carbonate beach on [[Long Island, Bahamas]].]]
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| The major areas for deposition of sediments in the marine environment include:
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| * [[Littoral]] sands (e.g. beach sands, runoff river sands, coastal bars and spits, largely [[sedimentary rock|clastic]] with little faunal content)
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| * The continental shelf ([[silt]]y [[clay]]s, increasing marine faunal content).
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| * The shelf margin (low terrigenous supply, mostly [[calcite|calcareous]] faunal skeletons)
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| * The shelf slope (much more fine-grained silts and clays)
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| * Beds of estuaries with the resultant deposits called "[[bay mud]]".
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| One other depositional environment which is a mixture of fluvial and marine is the [[turbidite]] system, which is a major source of sediment to the deep [[sedimentary basin|sedimentary]] and [[Abyssal plain|abyssal basins]] as well as the deep [[oceanic trench]]es.
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| Any depression in a marine environment where sediments accumulate over time is known as a [[Sediment trap (geology)|sediment trap]].
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| The null point theory explains how sediment [[deposition (geology)]] undergoes a hydrodynamic sorting process within the marine environment leading to a seaward fining of sediment grain size.
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| ==Environmental issues==
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| {{see also|Sediment transport#Applications}}
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| ===Erosion and agricultural sediment delivery to rivers===
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| One cause of high sediment loads from [[slash and burn]] and [[shifting cultivation]] of [[tropical]] forests. When the ground surface is stripped of vegetation and then seared of all living organisms, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have become erodible. For example, on the [[Madagascar]] high central [[plateau]], which constitutes approximately ten percent of that country's land area, most of the land area is devegetated, and gullies have eroded into the underlying soil in [[furrows]] typically in excess of 50 meters deep and one kilometer wide.{{Citation needed|date=December 2008}} This [[siltation]] results in discoloration of rivers to a dark red brown color and leads to fish kills.
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| 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 crop has left the soil unsupported. Many of these regions are near rivers and drainages. 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]].
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| == Dregs ==
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| Sediment in [[wine]], [[beer]], [[Turkish coffee]] or other beverages is known as ''dregs''.
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| ==See also==
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| * [[Bar (river morphology)]]
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| * [[Beach cusps]]
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| * [[Biorhexistasy]]
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| * [[Bioswale]]
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| * [[Decantation]]
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| * [[Deposition (geology)]]
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| * [[Erosion]]
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| * [[Exner equation]]
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| * [[Particle size (grain size)]]
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| * [[Regolith]]
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| * [[Sand]]
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| * [[Sediment precipitation]]
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| * [[Sediment trap (geology)|Sediment trap]]
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| * [[Sedimentary depositional environment]]
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| * [[Settling]]
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| * [[Surface runoff]]
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| ==References==
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| {{reflist}}
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| *{{Citation |first=Donald R. |last=Prothero |first2=Fred |last2=Schwab |title=Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy |publisher=W. H. Freeman |year=1996 |isbn=0-7167-2726-9 }}
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| *{{Citation |first=Raymond |last=Siever |title=Sand |publisher=Scientific American Library |location=New York |year=1988 |isbn=0-7167-5021-X }}
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| *{{Citation |first=Gary |last=Nichols |title=Sedimentology & Stratigraphy |publisher=Wiley-Blackwell |location=Malden, MA |year=1999 |isbn=0-632-03578-1 }}
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| *{{Citation |first=H. G. |last=Reading |title=Sedimentary Environments: Processes, Facies and Stratigraphy |publisher=Blackwell Science |location=Cambridge, MA |year=1978 |isbn=0-632-03627-3 }}
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| [[Category:Sediments]]
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| [[Category:Sedimentology]]
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| [[Category:Environmental soil science]]
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| [[Category:Petrology]]
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