An alluvial fan is a fan-
or cone-shaped deposit of sediment
crossed and built up by streams. If a fan is built up by debris
flows it is properly called a debris cone
or colluvial
fan. These flows come from a single point source at the apex
of the fan, and over time move to occupy many positions on the fan surface.
Fans are typically found where a canyon draining from mountainous terrain emerges
out onto a flatter plain,
and especially along fault-bounded mountain
fronts.
A convergence of neighboring
alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial fa
Formation
As a stream's gradient decreases,
it drops coarse-grained material. This reduces the capacity of the channel and
forces it to change direction and gradually build up a slightly mounded or
shallow conical fan shape. The deposits are usually poorly sorted. This fan shape can also be
explained with a thermodynamic justification: the system of sediment
introduced at the apex of the fan will tend to a state which minimizes the sum
of the transport energy
involved in moving the sediment and the gravitational potential of material in the
fan. There will be iso-transport energy lines forming concentric arcs
about the discharge point at the apex of the fan. Thus the material will tend
to be deposited equally about these lines, forming the characteristic fan
shape.
In arid climates
Alluvial fans are often found in desert areas
subject to periodic flash floods from nearby thunderstorms
in local hills. The
typical watercourse
in an arid climate has a
large, funnel-shaped basin at the top, leading to a narrow defile, which opens out into an alluvial fan at
the bottom. Multiple braided streams are usually present and active
during water flows.
Phreatophytes
are plants that
are often concentrated at the base of alluvial fans. They have long tap roots 30 to 50 feet
(9.1 to 15.2 m) to reach water that has seeped through the fan and hit an
impermeable layer, sometimes collecting in springs and seeps. These stands of shrubs cling to the soil at their bases and
often form islands of habitat for many animals as the wind blows the sand around the bushes
away.
In humid climates
Alluvial fans also develop in
wetter climates. In Nepal the Koshi River
has built a megafan covering some 15,000 km2
(5,800 sq mi) below its exit from Himalayan
foothills onto the nearly level plains where the river traverses into India before joining the Ganges. Along the
upper Koshi tributaries, tectonic forces elevate the Himalayas
several millimeters annually. Uplift is approximately in equilibrium with
erosion, so the river annually carries some 100 million cubic meters (3.5
billion cu ft) of sediment as it exits the mountains. Deposition of this
magnitude over millions of years is more than sufficient to account for the
megafan.
All along the interface between
the Indo-Gangetic Plain and the Himalaya in India, Pakistan, Nepal and Bhutan the outermost, lowest Siwalik foothills
are built of poorly consolidated sedimentary rocks that have eroded into a
wide, continuous alluvial apron called Bhabhar in Hindi and Nepali.
Despite overpopulation on the plains, this bhabhar zone is highly malarial and has
remained largely uninhabited.
In North
America, streams flowing into California's
Central Valley have deposited smaller
but still extensive alluvial fans. Such as that of the Kings River flowing out of the Sierra Nevada creates a low divide,
turning the south end of the San Joaquin Valley into an Endorheic
basin without a connection to the ocean.
Flood hazards
Alluvial fans are subject to
flooding and can be even more
dangerous than the upstream canyons that feed them. Their slightly convex perpendicular
surfaces cause water to spread widely until there is no zone of refuge. If the
gradient is steep, active transport of materials down the fan creates a moving
substrate that is inhospitable to travel on foot or wheels. But as the gradient
diminishes downslope, water comes down from above faster than it can flow away
downstream, and may pond to hazardous depths.
In the case of the Koshi River,
the huge sediment load and megafan's slightly convex transverse surface
conspire against engineering efforts to contain peak flows inside manmade
embankments. In August 2008 high monsoon flows
breached the embankment, diverting most of the river into an unprotected
ancient channel and across surrounding lands with high population density. Over a million people were
rendered homeless, about a thousand lost their lives and thousands of hectares
of crops were destroyed. The Koshi is known as the Sorrow of Bihar for
contributing disproportionately to India's death tolls in flooding, which
exceed those of all countries except Bangladesh.
In the Solar System
Alluvial fans are also found on Mars descending from
some crater rims over their flatter floors. Observations of fans in Gale crater
made by satellites from orbit have now been confirmed by the discovery of fluvial sediments
by the Curiosity rover.
Alluvial fans have been observed
by the Cassini-Huygens mission on Titan
using the Cassini orbiter's Synthetic Aperture radar (SAR)
instrument. These fans are more common in the drier mid-latitudes at the end of
methane/ethane rivers where it is thought that frequent wetting and drying
occur due to precipitation, much like arid fans on Earth. Radar imaging
suggests that fan material is most likely composed of round grains of water ice
or solid organic compounds about two centimetres in diameter.
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