Sediment

Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed of a body of water or other liquid. Sedimentation is the process of deposition by settling of a suspended material.

Sediment builds up on human-made breakwaters because they reduce the speed of water flow, so the stream cannot carry as much sediment load.

Sediments are also transported by the wind (eolian action) and glaciers. Desert sand dunes and loess (fine, silty deposits) are examples of eolian transport and deposition. Glacial moraine (rock debris) deposits and till (unsorted sediment) are ice-transported sediments. In addition, simple gravitational collapse, as occurs after the dissolution of layers of bedrock, creates sediments such as talus (slope formed by accumulated rock debris) and mountainslide deposits. Each sediment type has different settling velocities, depending on the size, volume, density, and shape of the material being deposited.

Seas, oceans, and lakes also accumulate sediment over time. The material can be terrestrial (deposited on the land) or marine (deposited in the ocean). Terrigenous deposits originate on land but may be deposited in either terrestrial, marine, or lacustrine (lake) environments.

Deposited sediments are the source of sedimentary rocks, which can contain fossils that were covered by accumulating sediment. Lake-bed sediments that have not solidified into rock can be used to determine past climatic conditions.

Sediment transport

Rivers and streams

If a fluid, such as water, is flowing, it can carry suspended particles. The settling velocity is the minimum velocity a flow must have in order to transport, rather than deposit, sediments, and (for a dilute suspension) is given by Stoke's Law:

$w={\frac {2(\rho _{p}-\rho _{f})gr^{2}}{9\mu }}$

where w is the settling velocity, ρ is density (the subscripts p and f indicate particle and fluid respectively), g is the acceleration due to gravity, r is the radius of the particle and μ is the dynamic viscosity of the fluid.

If the flow velocity is greater than the settling velocity, sediment will be transported downstream as suspended load. As there will always be a range of different particle sizes in the flow, some will have sufficiently large diameters that they settle on the river or stream bed, but still move downstream. This is known as bed load and the particles are transported via such mechanisms as saltation (jumping up into the flow, being transported a short distance then settling again), rolling and sliding. Saltation marks are often preserved in solid rocks and can be used to estimate the flow rate of the rivers that originally deposited the sediments.

Early applications of mathematical modeling of sediment transport in riverine systems were observed in the late 1970s. One such application was conducted by Santa Cruz County for the San Lorenzo River to study erosion from surface runoff and the resulting turbidity and bedload transport to downstream reaches. This work was used to analyze effects of land use practices in this drainage basin.

One of the main causes of riverine sediment load siltation stems from slash and burn treatment 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 been rendered erosive; for example, on the Madagascar high central plateau, comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of vegetation, with gully erosive furrows typically in excess of 50 meters deep and one kilometer wide. Shifting cultivation is a farming system which sometimes incorporates the slash and burn method in some regions of the world. The resulting sediment load in rivers flowing to the west is ongoing, with most rivers a dark red brown colour. The accumulation of these fine particulates in the water also lead to massive fish kills, as they cover fish eggs along the bottom floor.

Surface runoff

Surface runoff water can pick up soil particles and transport them in overland flow for deposition at a lower land elevation or deliver that sediment to receiving waters. In this case the sediment is usually deemed to result from erosion. If the initial impact of rain droplets dislodges soil, the phenomenon is called "splash erosion". If the effects are diffuse for a larger area and the velocity of moving runoff is responsible for sediment pickup, the effect is called "sheet erosion". If there are massive gouges in the earth from high velocity flow for uncovered soil, then "gully erosion" may result.

Fluvial Bedforms

Any particle that is larger in diameter than approximately 0.7 millimeter (mm) will form visible topographic features on the river or stream bed. These features, known as bedforms, include ripples, dunes, plane beds, and antidunes. Again, bedforms are often preserved in sedimentary rocks and can be used to estimate the direction and magnitude of the depositing flow.

Key depositional environments

The major fluvial (river and stream) environments for deposition of sediments include the following:

1. Deltas: River deltas, which are arguably intermediate between fluvial and marine environments, are landforms created by the buildup of sediment at the "mouths" of rivers and streams, that is, at places where they reach the sea. Delta shapes depend on how the water flows, how the current changes, and the amount of sediment being carried. <also from Wiki>.
2. Point bars: They are the result of an accumulation of gravel, sand, silt, and clay on the inside bank of a bend of a river. They demonstrate a characteristic semi-ellipse shape because of the way they are formed, with larger sediment forming the base, and finer particles making up the upper part of the point bar. Point bars contribute to size and shape changes of a meander (bend) over time.
3. Alluvial fans: These are fan-shaped deposits formed where a fast-flowing stream flattens, slows, and spreads typically at the end of a canyon onto a flatter plain. <from wiki>
4. Braided rivers: They consist of a network of small channels separated by small and often temporary islands called braid bars. Braided streams are common wherever a drastic reduction in stream gradient causes the rapid deposition of the stream's sediment load. Braided channels are also typical of river deltas and peneplains. <from wiki>
5. Oxbow lakes: These are lakes cut off from nearby streams or rivers after wide meandering (or bending) of a body of water into low-lying plains. A combination of deposition and rapid flow work to seal the meander, cutting it off from the original body of water it was formerly connected to. <reworded from Wiki>
6. Levees: These are natural or artificial embankments or dikes that border the perimeter of a river. They have a wide earthen base and taper at the top. Natural levees occur as a result of tidal waves or sharp meandering of a river. Artificial levees increase the velocity of the flow of water as well as prevent flooding. <reworded from Wiki>

Shores and shallow seas

The second major environment where sediment may be suspended in a fluid is in seas and oceans. The sediment could consist of terrigenous material 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.

Marine Bedforms

Marine environments also see the formation of bedforms, whose characteristics are influenced by the tides or currents.

Key depositional environments

The major areas for deposition of sediments in the marine environment include:

1. Littoral sands (e.g. beach sands, coastal bars and spits, largely clastic with little faunal content)
2. The continental shelf (silty clays, increasing marine faunal content).
3. The shelf margin (low terrigenous supply, mostly calcareous faunal skeletons)
4. The shelf slope (much more fine-grained silts and clays)
5. Beds of estuaries with the resultant deposits called "bay mud".

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 and abyssal basins as well as the deep oceanic trenches.

See also

• Bioswale
• Decantation
• Settling
• Surface runoff
• Erosion

Footnotes

Credits

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