What is an alluvial fan in the French Pyrenees?
An alluvial fan in the French Pyrenees fans outwards from a narrow canyon emerging from an escarpment. This accumulation takes the shape of a shallow cone section with its apex at the sediment source.
An alluvial fan in the French Pyrenees fans outwards from a narrow canyon emerging from an escarpment. This accumulation takes the shape of a shallow cone section with its apex at the sediment source.
Alluvial fans vary greatly in size, ranging from only a few meters across to as much as 150 kilometers wide. The Kosi River fan in India exemplifies a gigantic stream-flow-dominated megafan covering some 37,000 square kilometers.
Record rainfall on the 1st of January 1934 caused severe flooding of the Montrose and Glendale fan. Over a million people were rendered homeless when the Koshi River breached its embankment in August 2008.
Debris flow fans occur in all climates but are common where source rock is mudstone. Saturation of clay-rich colluvium by locally intense thunderstorms initiates slope failure.
An alluvial fan forms where a confined feeder channel exits a mountain front onto open ground and loses carrying power. If a river exits without any reduction in flow it creates an alluvial plain instead.