Transcript sand
Lecture 2 Sand dunes and winds affecting formation • Sand dunes movement into populated areas, agricultural lands and cover roads and train tracks. Northeast China China Namibia Sand dunes is a worldwide phenomena that cover a wide range of land with particular threat to the arid and semi arid lands such as the Arabian region. In the Arabian region, high speed wind cause a “haboob” which is an intense dust storm carried on an atmospheric gravity current spreading sands to distances far away from its original location. Also, wind blowing cause sand dunes to spread and move from the desert to populated lands causing economic and social damages and give priority for fixation and stabilization . Igneous rocks weathering Mechanical weathering Solubilization Chemical weathering Sand Soluble salts silt نواتج عمليات التعرية الميكانيكية للصخور النارية سواء بواسطة الرياح أو األمطار أو الثالجات.1 Sand dunes are formed based on availability of sand, space and wind. continental/Inland sand dues are caused when sand movement is Inland Coastal/beach sand dunes are formed when waves break sea rocks and the broken grains move with wave currents to the shores and accumulate on the beach overtime. Wind impact is sever in arid lands because of the extreme environment (long dry season, high temperature, low vegetation cover, low precipitation, high speed winds) Wind role in sand Transport Winds can transport sand grains less than 0.5 mm in diameter from a place to another (exterior sands) Exterior sands are fine yellow colored grains. Local sands are coarse and dark colored grains, originated from sandy rocks also from the disintegration of silt plains that exposed as a result of vegetation loss. The definition of sand dunes The accumulation and precipitation of sand grains (varies in their size from few millimeters to 1 micron) via wind action. Factors affecting sand dunes movement 1. 2. 3. 4. 5. Wind direction Wind speed and timing Size of sand grains Land topography around sandy lands Vegetation density and rain 1. Wind in conjunction الرياح المجتمعة: wind directions are combined and move in the same direction with some minor angle deviations. Aufrere (1931) classification of primary wind 1. 2. 3. 4. Wind in conjunction الرياح المجتمعة: wind directions are combined and move in the same direction with some minor angle deviations. Winds in opposition الرياح المتعامدة: winds are in complete opposite direction to each other. Incidental winds الرياح العرضية: Winds are bi-directional, different angles and directions and are divided into a large number of types depending on the terrain variations. Multidirectional winds الرياح متعددة االتجاهات: multible wind patterns, here primary winds has more than 2 directions. The mechanisms of sand movement Sand is moved from a place to another via 3 mechanisms: 1. Suspension transform 2. Saltation transform 3. Surface creep The mechanisms of sand movement The mechanisms of sand movement Saltation transform: Sand grains (0.5-0.1mm) rises above the surface of the ground and then fall again. Suspension transform: Grained sand with smaller sizes (less than 0.1mm) remain suspended in the air, after rising from the ground and may carry over long distances from their source and fall again when winds weakened or collide against obstacle. Surface creep: large sized rough grains heavy to be picked up by wind move on the ground and creep on the surface