Physics Photo of the Week

October 19, 2007

Leaf litter fronts
During a heavy rain on August 26, 2007, the pine needles and leaf litter on the road
had bunched into these parallel windrows about 20-30 cm apart.  Wilbur the basset serves as a good scale for the picture. 

The heavy rain had flowed in a steady sheet down the road in the same direction earlier in the day.  Of course the rain washed the leaves, pine needles, and small sticks down the road as well.  These windrows are the result of shear flow.  However, the "fluid" doing the flow is not the rain water but the layer of leaves and sticks as the litter is washed along the road.  This is analogous to the wind gusts and water flow bunches shown in the PPOW for October 5, 2007.  In each of these cases, the fluid (air, thin sheet of water, or leaf litter) exhibits a shear flow.  The upper layer flows faster than the bottom layer due to friction with a fixed surface.  As a result material builds up into the "fronts" as the top layer piles up material above the slower moving bottom layer.  The fronts were "frozen" in position after the rainwater subsided.

Another example of water "flow fronts" can be seen in water flowing down the spillway of Beetree Dam near Warren Wilson College in the photograph at right.  Here the upper layer of water falls faster than the water in contact with the concrete sluiceway and bunches into the fairly uniformly spaced "fronts".

Similar effects are manifest in cloud formations (another photo someday) weather storm patterns, and astrophysics where there is a velocity shear in the flow of stellar material. 

The next Physics Photo of the Week will be published in two weeks - November 2, 2007, due to the fall break at Warren Wilson College.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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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