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 toward
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.
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.
Photo of the
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 firstname.lastname@example.org.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.