HOW ARE DUNES AND RIPPLES FORMED?

Each wind produces changes to the surface of the sand. It takes a 10 mph (16 kph) wind to move fine, dry sand. During a sandstorm, it may seem as if the sand is blowing thousands of feet into the air. Your view of the distant mountains may be obscured. Don’t be fooled! The dunes are not entirely airborne! What you see are dust particles being carried aloft because they are smaller and lighter than sand grains. Seldom does dune-sized sand itself rise more than 6 ft. (2 m) above the ground, it is too heavy.

Actually, most sand travels within about 18 in. (46 cm) of the ground in a jumping movement called saltation. A windborne grain bounces a short distance before crashing into the sandy surface sending other grains into flight. The process continues as the wind carries the new grains a little farther, crashing them, in turn, into the surface and other grains along the way. Sand grains too heavy to blow in the wind creep along the surface of the sand, while smaller ones are truly airborne above the dunes. As a consequence, the entire sand surface flows or creeps along.

Have you been comparing the texture, size and color of sand in various sections of the dunes? Some differences will be more obvious if you have the chance to visit both the tall dunes by Highway 190 and the low ones by the picnic area. The soft, fine sand by the picnic area is carried farther by the wind than the coarser material found in the taller dunes.

Smaller grains move farther than large ones. As a result of this sorting of sand grains by variable winds, the larger grains accumulate in small ridges forming ripples. Size and shape of these formations are related to specific interactions between sand and wind. The windward side of a dune generally has more dramatic sculpturing. Examine an area with interesting ripples. Wind speed, direction, duration, and the amount of sand are important factors that have been measured and used to determine the size and spacing of dune ripples. Just as these ripples form, the dunes themselves respond to changes in each of these factors.

	{ Speed		{ Size
WIND	{ Duration	+ SAND	{ Uniformity = RIPPLE FORMATION
	{ Direction		{ Amount

SUMMIT ANGLE OF REPOSE SLIPFACE WINDWARD ANGLE OF REPOSE FOC 34°

All dunes may be described by the above traits. Variations in shape and size of individual dunes are determined by the mathematical values applied to each of the traits mentioned in the discussion of ripple formation. Another feature of interest is the dune shape. Among these dunes you may find examples of this classic textbook model and its variations.

Plants on the playa in between the dunes

Note that the downwind side of the dune, the slipface, is the steeper one. Fine dry sand can only pile up so high before it breaks away and slides down the slipface as an avalanche to rest at the more stable angle of repose, about 34 degrees. We create miniature avalanches as we walk along the crest of a steep dune.