1.1 Grain size 1.2 Composition
2.1.1 Particle motion
2.2 Aeolian processes: wind
3 Shores and shallow seas
3.1 Key marine depositional environments
4 Environmental issues
5 See also 6 References
φ scale Size range (metric) Size range (inches) Aggregate class (Wentworth) Other names
< -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 Pebble
-4 to -5 16–32 mm 0.63–1.26 in Coarse gravel Pebble
-3 to -4 8–16 mm 0.31–0.63 in Medium gravel Pebble
-2 to -3 4–8 mm 0.157–0.31 in Fine gravel Pebble
-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 Mud
> 8 < 3.9 µm < 0.00015 in Clay Mud
>10 < 1 µm < 0.000039 in Colloid Mud
Composition 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).
displaystyle textbf Rouse = frac text
displaystyle w_ s
is the fall velocity
is the von Kármán constant
displaystyle u_ *
is the shear velocity
Hjulström curve: The velocities of currents required for erosion, transportation, and deposition (sedimentation) of sediment particles of different sizes.
Mode of Transport 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
If the upwards velocity is 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
Initiation of motion), it will move along the bed as bed load by
rolling, sliding, and 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.
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.
Modern asymmetric ripples developed in sand on the floor of the Hunter River, New South Wales, Australia. Flow direction is from right to left.
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.
If the initial impact of rain droplets dislodges soil, the phenomenon is called rainsplash erosion. If overland flow is directly responsible for sediment entrainment but does not form gullies, it is called "sheet erosion". If the flow and the substrate permit channelization, gullies may form; this is termed "gully erosion".
Key fluvial depositional environments The major fluvial (river and stream) environments for deposition of sediments include:
Deltas (arguably an intermediate environment between fluvial and marine) Point bars Alluvial fans Braided rivers Oxbow lakes Levees Waterfalls
Aeolian processes: wind
Main article: Aeolian processes
Wind results in the transportation of fine sediment and the formation
of sand dune fields and soils from airborne dust.
Glaciers carry a wide range of sediment sizes, and deposit it in moraines. Mass balance Main article: Exner equation 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 change the ability of the flow to carry sediment, and this is reflected in the 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. Shores and shallow seas Seas, oceans, and lakes accumulate sediment over time. The sediment can 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, the exoskeletons of dead organisms are primarily responsible for sediment accumulation. Deposited sediments are the source of sedimentary rocks, which can contain fossils 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 climatic conditions. Key marine depositional environments
The major areas for deposition of sediments in the marine environment include:
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
Any depression in a marine environment where sediments accumulate over
time is known as a sediment trap.
The null point theory explains how sediment deposition undergoes a
hydrodynamic sorting process within the marine environment leading to
a seaward fining of sediment grain size.
Bar (river morphology)
Particle size (grain size)
^ Fernandez, C.; Wu, J. Q.; McCool, D. K.; Stöckle, C. O. (2003-05-01). "Estimating water erosion and sediment yield with GIS, RUSLE, and SEDD". Journal of Soil and Water Conservation. 58 (3): 128–136. ISSN 0022-4561. ^ Van Rompaey, Anton J. J.; Verstraeten, Gert; Van Oost, Kristof; Govers, Gerard; Poesen, Jean (2001-10-01). "Modelling mean annual sediment yield using a distributed approach". Earth Surface Processes and Landforms. 26 (11): 1221–1236. doi:10.1002/esp.275. ISSN 1096-9837. ^ "A step towards a holistic assessment of soil degradation in Europe: Coupling on-site erosion with sediment transfer and carbon fluxes". Environmental Research. 161: 291–298. 2018-02-01. doi:10.1016/j.envres.2017.11.009. ISSN 0013-9351.
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