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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Alluvial fan | 4/4 | https://en.wikipedia.org/wiki/Alluvial_fan | reference | science, encyclopedia | 2026-05-05T07:19:43.613390+00:00 | kb-cron |
=== Flood hazards === Alluvial fans are subject to infrequent but often very damaging flooding, whose unusual characteristics distinguish alluvial fan floods from ordinary riverbank flooding. These include great uncertainty in the likely flood path, the likelihood of abrupt deposition and erosion of sediments carried by the flood from upstream sources, and a combination of the availability of sediments and of the slope and topography of the fan that creates extraordinary hazards. These hazards cannot reliably be mitigated by elevation on fill (raising existing buildings up to a meter (three feet) and building new foundations beneath them). At a minimum, major structural flood control measures are required to mitigate risk, and in some cases, the only alternative is to restrict development on the fan surface. Such measures can be politically controversial, particularly since the hazard is not obvious to property owners. In the United States, areas at risk of alluvial fan flooding are marked as Zone AO on flood insurance rate maps. Alluvial fan flooding commonly takes the form of short (several hours) but energetic flash floods that occur with little or no warning. They typically result from heavy and prolonged rainfall, and are characterized by high velocities and capacity for sediment transport. Flows cover the range from floods through hyperconcentrated flows to debris flows, depending on the volume of sediments in the flow. Debris flows resemble freshly poured concrete, consisting mostly of coarse debris. Hyperconcentrated flows are intermediate between floods and debris flows, with a water content between 40 and 80 weight percent. Floods may transition to hyperconcentrated flows as they entrain sediments, while debris flows may become hyperconcentrated flows if they are diluted by water. Because flooding on alluvial fans carries large quantities of sediment, channels can rapidly become blocked, creating great uncertainty about flow paths that magnifies the dangers. Alluvial fan flooding in the Apennine Mountains of Italy have resulted in repeated loss of life. A flood on 1 October 1581 at Piedimonte Matese resulted in the loss of 400 lives. Loss of life from alluvial fan floods continued into the 19th century, and the hazard of alluvial fan flooding remains a concern in Italy. On January 1, 1934, record rainfall in a recently burned area of the San Gabriel Mountains, California, caused severe flooding of the alluvial fan on which the towns of Montrose and Glendale were built. The floods caused significant loss of life and property. The Koshi River in India has built up a megafan where it exits the Himalayas onto the Ganges plain. The river has a history of frequently and capriciously changing its course, so that it has been called the Sorrow of Bihar for contributing disproportionately to India's death tolls in flooding. These exceed those of all countries except Bangladesh. Over the last few hundred years, the river had generally shifted westward across its fan, and by 2008, the main river channel was located on the extreme western part of the megafan. In August 2008, high monsoon flows breached the embankment of the Koshi River. This diverted most of the river into an unprotected ancient channel and flooded the central part of the megafan. This was an area with a high population density that had been stable for over 200 years. Over a million people were rendered homeless, about a thousand lost their lives and thousands of hectares of crops were destroyed.
=== Petroleum reservoirs === Buried alluvial fans are sometimes found at the margins of petroleum basins. Debris flow fans make poor petroleum reservoirs, but fluvial fans are potentially significant reservoirs. Though fluvial fans are typically of poorer quality than reservoirs closer to the basin center, due to their complex structure, the episodic flooding channels of the fans are potentially lucrative targets for petroleum exploration. Alluvial fans that experience toe-trimming (lateral erosion) by an axial river (a river running the length of an escarpment-bounded basin) may have increased potential as reservoirs. The river deposits relatively porous, permeable axial river sediments that alternate with fan sediment beds.
== See also == Alluvium – Loose soil or sediment that is eroded and redeposited in a non-marine setting Floodplain – Land adjacent to a water body which is flooded during periods of high water Placer deposit – Accumulation of valuable minerals formed by gravity separation River delta – Silt deposition landform at the mouth of a river Subaqueous fan – Type of sediment deposit Tectonic influences on alluvial fans
== Notes ==
== References ==