Physics Photo of the Week

October 1, 2010

Rope-like erosion
This is a photo of a dirt and gravel one-lane road in rural northern Vermont after a night of fairly intense rain.  The photo is looking up a short hill and was taken before any cars had driven on the road since the rain.  The rain was rapid enough that the flowing water down and off the road formed a number of rope-like channels, very similar to a river delta. 

Often severe rainstorms badly erode country roads by forming steep gulleys in the middle of the road.  However, this road had recently been graded with a slight crown or rise along the center line.  The crown helps divert the water tow
ard the sides of the road and prevent deep eroded gullies.

Such roping drainage patterns are typical of alluvialfans formed at the exit of mountain canyons.  The NASA photo at right shows a large alluvial fan (55 km x 55 km) at the edge of theTaklimakan Desert in China’s XinJiang Province.  There is a narrow canyon that exits onto the the desert floor in the lower right.  Rainwater containing silt and particles flows onto the desert plain, depositing the silt forming mini-dams.  The mini-dams then divert the water left or right thus forming a delta as well as rope-like drainage patterns similar to the pattern in the country road above.

It is very dangerous to live and develop communities on or near alluvial fans.  The canyons that feed the fans are often subject to flash flooding from rainstorms in mountains that drain through the narrow openings into the desert floor.  The outflow often flows with great velocity as well as along unpredictable channels. 

With your child's sandbox, a playschool sand table, or a beach you can experiment with erosion patterns quickly.  The last photo here shows erosion channels formed by letting rainwater drain out of a boat that is pulled onto the sand.  The erosion formed by the drain flow created a deep channel.  Barriers build up, and the water becomes diverted into many rope-like channels.  An alluvial fan is seen midway down the beach as well as deep gullies at the boat's drain outlet and at the water's edge on the right.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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