Wet sand is common in coastal dune environments. Waves on the beach,
rain, and melting snow and ice make surfaces moist. The moisture
is an important control on how much sand moves by wind.
Moist sand has much more
resistance to movement by wind than dry sand. Moisture increases
cohesion between sand grains, raising the threshold wind velocity.
In general, the wetter the sand, the greater the cohesion and the
more resistant the sand is to the forces of the wind. Above relatively
low levels of moisture (roughly 4%), even very strong winds are
unable to move the sand grains.
Sand pillars illustrate differences in cohesion between wet and
dry sand. Moisture in the pillars holds the sand grains together
enough to support nearly vertical edges. The pillars exist because
wind removed dry sand and left the moist sand behind as pillars.
Note mechanical pencil in photo for scale. (Hoffmaster State Park
in December 2001.)
sand dries out as water evaporates or drains into the ground. Drying
rates depend on atmospheric conditions, the movement of moisture
through the sand, and other factors. Strong winds speed up evaporation.
When sand grains at the surface dry out (even though the sand beneath
them is still wet), they can be moved by the wind. Impacts of moving
grains with the wet surface can put other grains into motion.
Wind does move
sand from wet surfaces--as long as the right combination of wind
velocity, evaporation, sand grain size and moisture content is present.
Total amounts of sand transport on wet surfaces are much lower than
the effects results of similar winds blowing over loose dry sand