长江上游泥石流轻度危险区、基本无危险区

LIGHTER DANGER AND NEARLY WITHOUT DANGER DISTRICTS OF DEBRIS FLOW IN THE UPPER REACHES OF CHANGJANG RIVER

长江上游泥石流轻度危险区（属Ⅴ＿ａ级区）分为１５个小区，基本无危险区（Ⅴ＿ｂ级区）分为４个小区．全区跨６省，面积３０．３０万ｋｍ￣２．本区泥石流自然危险度级别以Ｅ级为主，经济发展程度级别多为５级（经济待发展区）．区内泥石流活动微弱，危害轻，但有个别人为泥石流造成的重灾点．本区的重点防范对象是人为泥石流．

The lighter danger district of debris flow is Ⅴ_a-District with actual and potential capacity oflight harm or threatening; nearly without danger district of debris flow is Ⅴ_b-District with ncarlyno harm or threatening.Ⅴ_a-District is divided into 15 parts. The area is 27.05×10￣4km￣2 and27% of the area of the upper reaches of Changjiang Revir.Ⅴ_b-District is divided in to 4 parts.The arca is 3.25 × 10￣4km￣2 and 3% of the area of the upper reaches of Changjiang Revir. Thedistrict is distributed over 6 provinces(see Table l).In Ⅴ_a-District ,the activities of debris flowate mainly feeble and feebler,the level of economic development is lower and there are mainlvawaiting development district(area is 17.12 × 10￣4 km￣2). The landform indexes are mainly 4 grade and 5 grade(the relative height h of statistic unitis 1 000-500m and<500m respectively).In the ravine,cutting is shallower and the dynamicconditions of debris flow which is provided by landform is little. Most geologic indexes are 5grade(the product S of fault length and strata weathering coefficient in statistic unit<0.10)and 4 grade(S=0. 15-0.10);The length of main fauIts is 108m /km￣2(in Ⅴ_a-District)and23m /km￣2(in Ⅴ_b-District).Most climatic index is 4 grade, The geologic and landform conditionsand climatic conditions are not favourable to the formation of debris flow.There is none debris flow activity in Ⅴ_b-District. Damages of debris flow are very light andits activity appears only in local area in small scale and low frequency in Ⅴ_a-District,but thehazards of man-made debris flow eaused by reservoir dam burst,may happen. Therefore,man-made debris flow should be prevented emphatically.