The Basics of Dune Formation
There are three processes by which eolian (wind blown) sand forms dunes - saltation, surface creep, and suspension. The most important of which is saltation wherein sand grains bounce as they are moved by wind. As they bounce along, they displace other sand grains and settle to form ripples. Given enough time and steady winds these ripples can turn into ridges and eventually dunes.
As a ridge becomes taller, wind-blown sand is more likely to be deposited on the upwind side and forms a gradual slope. The downwind side becomes steeper with a slope of 30% or more. The steep downwind sides are called slip faces. As sand builds up on the crest of these ridges it eventually slides down the slip face and the dune slowly moves in the direction of the wind. Precipitation can cause wet sand to cascade down these slip faces like an avalanche. Both wet and dry sand avalanches can be dangerous. Dinosaurs and pickup trucks have been trapped in them.
The area between dunes is called the interdunal area. The dunes have the same effect as a snow fence in that they cause finer particles of silt and clay to be deposited on the downwind side. Subsequently the interdunal soils have more water holding capacity and vegetation can flourish there. There are numerous wetlands among the interdunal areas of the Nebraska Sand Hills.
Nebraska researchers have studied active dune fields around the world in order to understand how the Nebraska Sand Hills were shaped. The number of defined dune types has grown since H.T.U. Smith postulated that there were three basic types. As scientists have learned more about the mechanisms of dune formation, the level of detail in their description has become more refined.
James B. Swinehart (1998) in the Nebraska Sand Hills Atlas lists eight dune types found there. Researchers have gone so far as to quantify the area covered by each type. The eight types are as follows:
Sand sheet low relief - sheet-like with surface of very low relief
Sand sheet moderate relief - sheet-like with surface of moderate relief
Parabolic - "U" shaped, horns anchored by vegetation and pointed upwind
Barchan - crescent, horns pointed downwind
Barchanoid ridge - row of connected crescents
Dome-like - circular or elliptical mound
Domal-ridge - row of connected mounds
Linear - symmetrical ridge
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