Study On The Grain Erosion And Control Strategy In The Southwest Dry Valley

Grain erosion is defined as the phenomenon of detachment of grains from bare rocks, flow of grains down slope, and accumulation at the toe of the mountain, forming a deposit fan. The prerequisites of grain erosion development is bare rocks. Bare rocks were caused by avalanches, landslides and human activities, which were very common in the Xiaojing River basin. The Wenchuan Earthquake, occured in Sichuan on May 12, 2008, caused numerous rockfalls, avalanches and landsildes. Consequently grain erosion occurred throughout the earthquake area in dry valley of Min River. The process of grain erosion, and control strategies to limit it are studied by field investigations and experiments in the dry valley of Xiaojiang River basin in southwest China.Grain erosion occurs in the Xiaojiang River Basin mainly on south facing slopes with altitude ranges from 800m to 2000m. In general, grain erosion occurred mostly in finestone, phylite, slate, shale and limestone rocks. A grain erosion site mainly consists of three sections: a top where surface grain erosion occurs with the slope angle in the range of 50°-70°; a middle where grain flow occurs with a slope angle about 40°; and a bottom where deposit fan forms with the slope angle about 35°. Relatively uniform grain size and lack of large stones are common features of grain erosion, which are much different from those of landslides and avalanches. The rate of grain erosion in the Xiaojing River basin is mainly between 1 to 10 cm/yr. Grain erosion causes flying stones and can injure humans, and results in great damages on vegetation and numerous slope debris flows.The develope process of grain erosion is impacted by the combined action of climate, geology and landform. The foremost influence factor is climate, especially the variation of temperature and humidity, and the solar-radiation. Rocks are broken down to grains under the action of insolation and temperature change. Then, wind blows the grains from the bare rock down slope. An experiment was conducted to study the relation of grains blown down by wind with wind speed. Experimental results showed that the amount of grains blown down by wind per area of rock surface per unit time is proportional to the fourth power of the wind speed. However, the size of the grains blown down by wind increases linearly with the wind speed.Several experiments proved that grain erosion can be controlled if the bare rocks are covered with a layer of clay suspension with moss. Two drought-enduring moss species were collected from local and neighboring areas and mixed with mixed with the river sediment and humus, and evenly scrawled or sprayed to the bare rocks. The moss layer protected the rocks from insolation and mitigated the effects of temperature change, thus effectively mitigated grain erosion.