Sediment is any particulate matter that can be
transported by
fluid flow, and which eventually is
deposited.
Sediments are most often transported by water (
fluvial processes) transported by 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.
Classification
Sediment can be classified based on its
grain size and/or its
composition.
Grain size
Sediment size is measured on a log base 2 scale, called the "Phi"
scale, which classifies particles by size from "colloid" to
"boulder".
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:
This leads to an ambiguity in which
clay can be
used as both a size-range and a composition (see
clay minerals).
Sediment transport

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

Glacial transport of boulders.
These boulders will be deposited as the glacier
retreats.
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.
Fluvial Processes: Rivers, streams, and overland flow
Particle Motion
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
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.
\textbf{Rouse}=\frac{\text{Settling velocity}}{\text{Upwards
velocity from lift and drag}}=\frac{w_s}{\kappa u_*}
where
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
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.
Fluvial bedforms
Sediment motion can create self-organized structures such as
ripples,
dunes,
antidunes on the river or stream
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.
Surface runoff
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.
- 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
Wind results in the transportation of fine sediment and the
formation of sand dune fields and soils from airborne dust.
Glacial Processes
Glaciers carry a wide range of sediment sizes, and deposit it in
moraines.
Mass balance
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.
Shores and shallow seas
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.
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:
- Littoral sands (e.g. beach sands,
runoff river sands, coastal bars and spits, largely clastic with little faunal content)
- The continental shelf (silty clays, increasing marine faunal content).
- The shelf margin (low terrigenous supply, mostly calcareous faunal skeletons)
- The shelf slope (much more fine-grained silts and clays)
- 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.
Environmental Issues
Erosion and agricultural sediment delivery to rivers
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. This results in discoloration
of rivers to a dark red brown color and leads to fish kills.
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.
See also
References
- Donald R. Prothero and Fred Schwab, Sedimentary Geology: An
Introduction to Sedimentary Rocks and Stratigraphy, W. H.
Freeman (1996), ISBN 0 7167 2726 9.
- Raymond Siever, Sand, Scientific American Library, New
York (1988), ISBN 0 7167 5021 X.
- Gary Nichols, Sedimentology & Stratigraphy,
Wiley-Blackwell, Malden, MA (1999), ISBN 0 6320 3578 1.
- H.G. Reading, Sedimentary Environments: Processes, Facies
and Stratigraphy, Blackwell Science, Cambridge, MA (1978),
ISBN 0 632 03627 3.