Alluvial fans are depositional forms associated with the change of slope along a stream. This occurs when a stream exits a mountains dominated areas to flow in a plane surface such as an alluvial plain. The morphological development of alluvial fans is most often controlled by torrential flows heavily loaded in sediments and debris. Flood risks on these forms can not be looked at using usual hydrological approaches. Aggradation and avulsion process play a critical role in evaluating the flooding risk on an alluvial fan.
Thomas Buffin-Bélanger
Section de géographie, Université du Québec à Rimou
ski, 300 allée des Ursulines, Rimouski, Québec, Canada,
Bernard Hétu
Section de géographie, Université du Québec à Rimou
ski, 300 allée des Ursulines, Rimouski, Québec, Canada,
An alluvial fan is a triangle-shaped deposit of gravel, sand, and even smaller pieces of sediment, such as silt. This sediment is called alluvium.
Alluvial fans are usually created as flowing water interacts with mountains, hills, or the steep walls of canyons. Streams carrying alluvium can be trickles of rainwater, a fast-moving creek, a powerful river, or even runoff from agriculture or industry. As a stream flows down a hill, it picks up sand and other particles—alluvium.
The rushing water carries alluvium to a flat plain, where the stream leaves its channel to spread out. Alluvium is deposited as the stream fans out, creating the familiar triangle-shaped feature.
The narrow point of the alluvial fan is called its apex, while the wide triangle is the fan's apron. Alluvial fans can be tiny, with an apron of just a few centimeters spreading out from the trickle of a drainpipe. They can also be enormous. Over time, water flowing down the Koshi River in Nepal, for example, has built up an alluvial fan more than 15,000 square kilometers (almost 5,800 square miles) wide. This "megafan" carries alluvium from the Himalaya Mountains. * from National Geographic .
In order to log this EarthCache, you must answer the following questions :
1- How many paces do you walk from the top of the Alluvial fan to the bottom?
2- Estimate the degree of the slope of the alluvial fan.
3- Estimate the degree of the slope on the NORTH side of the Alluvial fan and compare it to the south side.
4 -Optional question. With your log, post a picture showing your GPS in front of the alluvial fan.
Send your answers to : david.butland@globetrotter.net