长江上游泥石流较轻度危险区

LIGHT DANGER DISTRICT OF DEBRIS FLOW IN THE UPPER REACHES OF CHANGJIANG RIVER

区内共有２０个小区，面积３０．６３万ｋｍ＿２．本区地跨８省区．区内有１４个小区泥石流自然危险度级别为Ｃ级（面积１１．９１万ｋｍ￣２），６个小区泥石流自然危险度级别属Ｄ级（面积１８．７２万ｋｍ￣２）．本区有４座县城受泥石流威胁或危害，除此之外泥石流危害程度一般较轻．

Light danger district of debris flow is Ⅳ- District with actual and potential capacity of harmor threatening. The district is divided into 20 parts, distributed over 8 provinces and autonomousregion(see Table 1). The area is 30.63× 10￣4 km￣2 and 31% of the area of the upper reaches ofChangjiang River.In the district,the activities of debris flow are mostly feebler , the level of economic develop-ment is lower and there are mainly awaiting dcvelopment districts. In the district,landform indexes are mainly 4 grade(the relative height h in statistic unit=1000-500m).The geologic indexes are mostly 4 grade(the product S of fault length and strataweathering coefficicnt in statistic unit=0. 25-0.10)and 5 grade(S<0.10). The length ofmain faults is 164m/km￣2. Most climatic index is l grade, In the geologic and landform condi-tions al1d climatic conditions ,only the later is more favourable to forming debris flow.Combination of natural conditions is only favourable to feebler activity of debris flow in thedistrict. Therefore, active characteristics of debris flow are as follows:scales are small the fre-quency is lower and light damages. In the destrict,there are 4 county towns which are damagedand threatened by debris flows and should be controlled majorly. Man-made debris flow should beprevented especially in populous area of Sichuan Basin and its edge zone.