راسب

الحصى على شاطئ

River Rhône flowing into Lake Geneva

Sediment billowing out from Italy's shore into the Adriatic

الرواسب Sediment هي الجسيمات التي يمكن نقلها عن طريق تدفق مادة سائلة، إلى أن تترسب في نهاية المطاف. واسطة نقل الرواسب عادة تكون المياه أو الرياح أو الأنهار الجليدية. الرمال على الشاطئ و رواسب قنوات الأنهار هي أمثلة على عملية الترسب بالمياه في حين ان الغبار و كثيب هي أمثلة على عملية الترسيب نتيجة حركة الرياح. الركام الجليدي هو مثال على الرواسب التي تنتقل بفعل الأنهار الجليدية و حركة الجليد.

Sediments are most often transported by water (fluvial processes), but also wind (aeolian processes) and glaciers. Beach sands and 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.

 التصنيف

 حجم الحبيبات

المقالة الرئيسية: حجم الحبيبات

انظر أيضاً: بنية التربة and نظام موحد لتصنيف التربة

الرواسب في خليج المكسيك

رواسب شبه جزيرة يوكاتان

φ الوزن	حجم النطاق (metric)	حجم النطاق (إنش)	إجمالي الفئة (وينتوورث)	أسماء أخرى
< -8	> 256 mm	> 10.1 in	Boulder
-6 to -8	64–256 mm	2.5–10.1 in	Cobble
-5 to -6	32–64 mm	1.26–2.5 in	Very coarse gravel	حصاة
-4 to -5	16–32 mm	0.63–1.26 in	Coarse gravel	حصاة
-3 to -4	8–16 mm	0.31–0.63 in	Medium gravel	حصاة
-2 to -3	4–8 mm	0.157–0.31 in	Fine gravel	حصاة
-1 to -2	2–4 mm	0.079–0.157 in	Very fine gravel	Granule
0 to -1	1–2 mm	0.039–0.079 in	Very coarse sand
1 to 0	0.5–1 mm	0.020–0.039 in	Coarse sand
2 to 1	0.25–0.5 mm	0.010–0.020 in	Medium sand
3 to 2	125–250 µm	0.0049–0.010 in	Fine sand
4 to 3	62.5–125 µm	0.0025–0.0049 in	Very fine sand
8 to 4	3.9–62.5 µm	0.00015–0.0025 in	Silt	طين
> 8	< 3.9 µm	< 0.00015 in	Clay	طين
>10	< 1 µm	< 0.000039 in	Colloid	طين

 Shape

Schematic representation of difference in grain shape. Two parameters are shown: sphericity (vertical) and rounding (horizontal).

The shape of particles can be defined in terms of three parameters. The form is the overall shape of the particle, with common descriptions being spherical, platy, or rodlike. The roundness is a measure of how sharp grain corners are. This varies from well-rounded grains with smooth corners and edges to poorly rounded grains with sharp corners and edges. Finally, surface texture describes small-scale features such as scratches, pits, or ridges on the surface of the grain.

 Form

انظر أيضاً: Sphericity

Form (also called sphericity) is determined by measuring the size of the particle on its major axes. William C. Krumbein proposed formulas for converting these numbers to a single measure of form, such as

${\displaystyle{\displaystyle\psi_{l}={\sqrt[3]{\frac{D_{S}D_{I}}{D_{L}^{2}}}}}}$

where ${\displaystyle{\displaystyle D_{L}}}$, ${\displaystyle{\displaystyle D_{I}}}$, and ${\displaystyle{\displaystyle D_{S}}}$ are the long, intermediate, and short axis lengths of the particle. The form ${\displaystyle{\displaystyle\psi_{l}}}$ varies from 1 for a perfectly spherical particle to very small values for a platelike or rodlike particle.

An alternate measure was proposed by Sneed and Folk:

${\displaystyle{\displaystyle\psi_{p}={\sqrt[3]{\frac{D_{S}^{2}}{D_{L}D_{I}}}}}}$

which, again, varies from 0 to 1 with increasing sphericity.

 Roundness

المقالة الرئيسية: Roundness (geology)

ملف:Rounding.gif

Comparison chart for evaluating roundness of sediment grains

Roundness describes how sharp the edges and corners of particle are. Complex mathematical formulas have been devised for its precise measurement, but these are difficult to apply, and most geologists estimate roundness from comparison charts. Common descriptive terms range from very angular to angular to subangular to subrounded to rounded to very rounded, with increasing degree of roundness.

 Surface texture

Surface texture describes the small-scale features of a grain, such as pits, fractures, ridges, and scratches. These are most commonly evaluated on quartz grains, because these retain their surface markings for long periods of time. Surface texture varies from polished to frosted, and can reveal the history of transport of the grain; for example, frosted grains are particularly characteristic of aeolian sediments, transported by wind. Evaluation of these features often requires the use of a scanning electron microscope.

 التركيب

Composition of sediment can be measured in terms of:

• Parent rock lithology
• Mineral composition
• Chemical make-up

This leads to an ambiguity in which clay can be used as both a size-range and a composition (see clay minerals).

 نقل الرواسب

تتراكم الرواسب من بفعل الإنسان لتكون حاجزا للأمواج للتقليل من تدفق المياه فبالتالي لايمكن للتيار أن يحمل مايصل من الرواسب.

Glacial transport of boulders. These boulders will be deposited as the glacier retreats.

المقالة الرئيسية: Sediment transport

انظر أيضاً: Rouse number

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.

 العمليات النهرية: الأنهار والجداول، والتدفق البري

 حركة الحبيبات

طريقة النقل	Rouse Number
Bed load	>2.5
Suspended load: 50% Suspended	>1.2, <2.5
Suspended load: 100% Suspended	>0.8, <1.2
Wash load	<0.8

 Fluvial bedforms

المقالة الرئيسية: Bedform

تموجات حديثة غير متماثلة في الرمل على أرض نهر هنتر، نيو ساوث ويلز، استراليا. اتجاه التدفق من اليمين إلى اليسار.

Sinuous-crested dunes exposed at low tide in the Cornwallis River near Wolfville, Nova Scotia

Ancient channel deposit in the Stellarton Formation (Pennsylvanian), Coalburn Pit, near Thorburn, Nova Scotia.

 عمليات ريحية: الرياح

المقالة الرئيسية: عمليات ريحية

 عمليات جليدية

رواسب جليدية من مونتانا

 Mass balance

المقالة الرئيسية: معادلة إكسنر

 الشواطئ والبحار الضحلة

 مفتاح بيئات الترسيب البحرية

Holocene eolianite and a carbonate beach on Long Island, Bahamas.

 قضايا بيئية

انظر أيضاً: نقل الرواسب § تطبيقات

 Erosion and agricultural sediment delivery to rivers

One cause of high sediment loads is 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 to form distinctive gulleys called lavakas. These are typically 40 meters (130 ft) wide, 80 meters (260 ft) long and 15 meters (49 ft) deep. Some areas have as many as 150 lavakas/square kilometer, and lavakas may account for 84% of all sediments carried off by rivers. This siltation results in discoloration of rivers to a dark red brown color and leads to fish kills. In addition, sedimentation of river basins implies sediment management and siltation costs. The cost of removing an estimated 135 million m³ of accumulated sediments due to water erosion only is likely exceeding 2.3 billion euro (€) annually in the EU and UK, with large regional differences between countries.

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.

The Sediment Delivery Ratio (SDR) is fraction of gross erosion (interill, rill, gully and stream erosion) that is expected to be delivered to the outlet of the river. The sediment transfer and deposition can be modelled with sediment distribution models such as WaTEM/SEDEM. In Europe, according to WaTEM/SEDEM model estimates the Sediment Delivery Ratio is about 15%.

 Coastal development and sedimentation near coral reefs

Watershed development near coral reefs is a primary cause of sediment-related coral stress. The stripping of natural vegetation in the watershed for development exposes soil to increased wind and rainfall and, as a result, can cause exposed sediment to become more susceptible to erosion and delivery to the marine environment during rainfall events. Sediment can negatively affect corals in many ways, such as by physically smothering them, abrading their surfaces, causing corals to expend energy during sediment removal, and causing algal blooms that can ultimately lead to less space on the seafloor where juvenile corals (polyps) can settle.

When sediments are introduced into the coastal regions of the ocean, the proportion of land, marine, and organic-derived sediment that characterizes the seafloor near sources of sediment output is altered. In addition, because the source of sediment (i.e., land, ocean, or organically) is often correlated with how coarse or fine sediment grain sizes that characterize an area are on average, grain size distribution of sediment will shift according to the relative input of land (typically fine), marine (typically coarse), and organically-derived (variable with age) sediment. These alterations in marine sediment characterize the amount of sediment suspended in the water column at any given time and sediment-related coral stress.

 Biological considerations

In July 2020, marine biologists reported that aerobic microorganisms (mainly), in "quasi-suspended animation", were found in organically-poor sediments, up to 101.5 million years old, 250 feet below the seafloor in the South Pacific Gyre (SPG) ("the deadest spot in the ocean"), and could be the longest-living life forms ever found.

 انظر أيضا

• Bar (river morphology)
• Beach cusps
• Biorhexistasy
• Bioswale
• Decantation
• تعرية
• Exner equation
• Particle size (grain size)
• Regolith
• رمل
• Sediment precipitation
• Sediment trap
• Sedimentary depositional environment
• Settling
• Surface runoff
• Deposition (geology)

 المصادر

• Prothero, Donald R.; Schwab, Fred (1996), Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy, W. H. Freeman, ISBN 0-7167-2726-9 
• Siever, Raymond (1988), Sand, New York: Scientific American Library, ISBN 0-7167-5021-X 
• Nichols, Gary (1999), Sedimentology & Stratigraphy, Malden, MA: Wiley-Blackwell, ISBN 0-632-03578-1 
• Cambridge, MA, H. G. (1978), Sedimentary Environments: Processes, Facies and Stratigraphy, Blackwell Science, ISBN 0-632-03627-3