Blowout on Tibetan Plateau: morphology and sediments
Luo wanyin, Wang zhongyuan, Lu junfeng, Qian Guangqiang, Dong zhibao
Cold and Arid Regions Environmental and Engineering Research Institute, Lanzhou, China
Blowout becomes one of the main aeolian geomorphic process in alpine meadow regions on Tibetan Plateau with the global warming. However, compared with coastal and desert regions, less attention has been given to the wind erosion landforms in alpine regions. The aim of this work is to present the results of morphology and deposition characterizes of the blowouts in alpine meadow regions in Gonghe basin on Tibetan Plateau as revealed by aerial images and other field GPS surveys. Blowout mainly develops on gently sloping alpine meadows with trough or saucer shaped morphology. According to its size, blowout in Gonghe basin on Tibetan plateau can be divided into five categories：micro, small, medium, large and mega-blowouts. The depth of large and mega-blowout is always between 10m and 20m, while the largest can reach 25m depth. The size of the blowouts in Gonghe basin far exceeds than the sandy grassland and coastal blowouts in previous reports. The length-width ratio of micro blowout is smallest, but it gradually become larger during the blowout develops larger till to reach its maximum in medium size, and begins to decreasing in the after expansion stages. Axial-erosion is the mainly mode in the early evolution stages of blowout, while lateral erosion of the wall is the dominant expansion way when the development of blowout achieved certain scale. Most of the grain size frequencies curves of the sand samples at different location in the blowout have the characteristics of multi peaks with the first modal number varied between 1.67φto 2.32φ. The sorting of surface sediments became poorly, the range of grain size parameter is enlarged and the rate of grain size variation became more rapidly as the expansion of blowout scale. The percentage of coarse and very coarse sand increased especially in the middle-lower part of windward slope and the bottom of deflation basin in the large blowouts. Axial and deep erosion rate of blowouts become weaker because the accumulation of coarser sediments at its deflation bottom. The grains size distribution of mobile dune appeared at the downwind areas of mega-blowout is similar to the inside deposition of the blowout.Thawing subsidence of the permafrost active layer under freezing and thawing conditions destroys the integrality of alpine meadow layer as global warming. This provide the original induced force for the formation of blowout in alpine area. The increase of grazing capacity and human activity are the driving force for blowout evolution in alpine area. Fine grain size sands carried by wind from the blowouts deposited in the downwind sward becomes the important sand source material to the desertification. Blowouts evolution is the original force of the desertification on Tibetan Plateau.