Go Home U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Sand Dunes Computer animations and paper models By Tau Rho Alpha*, John P. Galloway*, and Scott W. Starratt* Open-file Report 98-131-A - This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this program has been used by the U.S. Geological Survey, no warranty, expressed or implied, is made by the USGS as to the accuracy and functioning of the program and related program material, nor shall the fact of distribution constitute any such warranty, and no responsibility is assumed by the USGS in connection therewith. * U.S. Geological Survey Menlo Park, CA 94025 Comments encouraged tralpha @ omega? .wr.usgs .gov [email protected] [email protected] (gobackward) <j (goforward) Description of Report This report illustrates, through computer animations and paper models, why sand dunes can develop different forms. By studying the animations and the paper models, students will better understand the evolution of sand dunes, Included in the paper and diskette versions of this report are templates for making a paper models, instructions for there assembly, and a discussion of development of different forms of sand dunes. In addition, the diskette version includes animations of how different sand dunes develop. Many people provided help and encouragement in the development of this HyperCard stack, particularly David M. Rubin, Maura Hogan and Sue Priest. This report was enhanced by reviews from Peter Stauffer and Warren Yeend. Table of Contents for paper version of this report. Title page page 1 Description of report page 2 Teachers guide page 4 Paper models of sand dunes and a sand dune crossection model page 16 0*0 (go backhand) <~j L^ (go forward) Description of Report Requirements for using the diskette version are: Apple Computer, Inc., HyperCard 2.2 software and an Apple Macintosh computer with an internal disk drive. If you are using System 7 or above, we recommend having at least 8 MB of physical RAM with 4.5MB of memory available for HyperCard. The animation is accompanied by sound. If no sound is heard, change the memory of HyperCard to 4500K and ensure that the control panel "Sound," which is in the "Control Panels" folder under the "Apple" menu, has the volume set to at least 2. To change the memory available to HyperCard, quit this stack. Highlight the HyperCard program icon and choose "Get Info" from the File Menu. Change the "memory requirements" to 4500K and start this stack again. To see the entire page (card size: MacPaint), select "Scroll" from the "Go" menu and move the hand pointer in the scroll window. If you are experiencing trouble with user-level buttons, select "message" from the "Go" menu. Type "magic" in the message box and press return. Three more user-level buttons should appear. The date of this Open File Report is March 30, 1998, OF 98-131-A, paper copy, 64 p.; OF 98-131-B, 3.5-in. Macintosh 1.4-MB high-density diskette. This stack (report) can be down loaded from self-extracting archives, that are on the U.S. Geological Survey's Learning Web site. The URL address is: < http://www.usgs.gov/education/animations/ > To order this report, contact: USGS ESIC Open-File Report Section or call (303) 202-4200. Box 25286, MS 517 Denver Federal Center Denver, CO 80225-0046 [ Back to table of contents ] ^ 127025 Teachers Guide: Page 1 of 19 General Introduction to Dunes: Many areas of the world have specific landforms that result from processes associated with wind. Wind-related processes and deposits are also termed eolian processes or deposits (aeolian from the Latin word Aeolus, meaning god of the winds). A principle eolian depositional landform is a sand dune. Sand dunes can develop in any environment in which loose particles of sand are exposed to wind action and are free to migrate and accumulate. Most people think mat dunes only form in desert or coastal environments. But dunes can be found in any environment where there is a steady wind regime, a dry (less than 10 inches of rainfall annually) environment, and a supply of loose surface material which is small enough to be transported by the wind. In general sand dunes are not found wet inviroments as wet sand grains adhere (stick) to each other and are not easily moved by wind.Thus a dry/cold windy climate (such as desert or polar environments) favors the formation of dunes. Wind can also move snow. Deposits of wind-driven sand and snow are called niveo-eolian deposits. These deposits occur on an annual basis in cold-climate environments and consist of layers of consolidated snow, which is often recrystallized into small pellets intermingled with layers of sand. A typical sand dune is created by the transport of sediment (small loose rock particles) by the wind and involves the interaction of the wind and the ground surface. Sand is defined as particles with a size between 2 mm to 0.0625 mm (2 mm equals 0.078 inches). Wind moves sand near the surface, usually within 1 meter of the ground, and carries it only a short distance. Very small particles are transported in suspension by the wind and stay in the air for a relatively long distance. Silt-clay size sediments, particles less than 0.0625 mm, are usually deposited downwind and are called loess. These small sized particles play a minor role in the development of dunes. Dunes begin to form when winds carrying sediment encounter an obstacle that slows the wind. The reduced wind velocity causes the coarsest (largest) fraction of the sediment to fall back to the ground. This build- up of sand creates a larger obstacle that constitutes a wind break, causing more deposition. ( Back to table of contents ] 4 Teachers Guide: o Page 2 of 19 If the wind is not strong enough particles of sand may be pushed or rolled along the surface. This process is known as surface creep. Wind speed needs to be greater than 10 miles per hour (16 km) to put sand grains into motion. Sand is transported up the stoss side (upwind side) [see" typical dune" illustration below] of the dune in a series of short-distance jumps called saltation. The sand grains are transported a short distance by the wind then fall back to the ground surface hitting other sand grains in the process causing the sand grains to become airborne. This process of saltation continues as long as the wind is blowing. Upon reaching the top of the dune the sand then rolls down the slipface or the lee ward side of the dune. This continuous process of wind erosion from a source area, the transport of the sand up the stoss side and the deposition of sand on the slip face results in a net downwind migration of the dune. Thus sand dunes grow by several processes: 1) saltation and/or wind ripples on stoss slopes, 2) trapping of sand in the slipface, and 3) overtaking of one dune by another. Stoss side (upwind side) ^<^S?3^^ Slipface Wind Dune Migration «_ «. i . ... ...» . ^. *»* _ _ "V"_ Typical Sand Dune A typical dune is 3 to 100 meters high and asymmetricl. The slip face is usually twice as steep as the windward face (stoss slope) of the dune forms an angle relative to a horizontal ground surface, usually around 33 degrees. This is called the angle of repose. The stoss slope is usually 10 degrees from a horizontal ground surface. Sand grains deposited on the slipface of dunes form avalanche-like deposits which look like overlapping tongues which become thinner towards the top of the dune. The avalanching of the sand is a result of the pversteepened slipface which is beyond the angle of repose in dry sand. Movement of Sand grains sc>-£ """' Wind Stoss side Saltation Teachers Guide: S"1 Page 3 of 19 DUNE CLASSIFICATION Sand dunes may be classified in various ways based on: 1) size and shape of the dune(s), 2) the environment in which they are occur, coastal, desert, or polar, 3) their growth stages, and degree of internal complexity, and 4) the wind direction responsible for there formation. A simple geomorphological classification groups dunes on the basis of their shape, number of slip faces, and the wind directions that forms them and results in six major groups; 1) Transverse, 2 ) Oblique, 3) Longitudinal, 4) Barchan, 5) Star, and 6) Parabolic. Model 1 - Transverse dunes Transverse dunes occur where there is a plentiful supply of sand. In plan view, transverse dunes form straight to very gently curving ridges. The dunes are aligned perpendicular to the prevailing wind. The winds that form transverse dunes come from two directions, usually 75 to 90 degrees to the dune axis. There is a close relationship between the dune height, width and spacing of transverse dunes. Sediments of transverse dunes are dominated by grainfall and grainflow cross-strata deposited on the lee face and preserved as the dune migrates downwind. Wind ripple laminae form a thin set of strata that parallel the slope of the stoss slope and dune crest. Transverse dunes represent approximately 40 percent of the dunes found in the sand seas world wide. Model 2 - Oblique dunes Oblique dunes are similar to transverse dunes in that they form straight to very gently curving ridges.