长江上游泥石流中度危险区

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

区内共有２１个小区，面积２３．０８万ｋｍ＾２７分布在８省区内，泥石流自然危险度指标级别以Ｂ级和Ｃ级为主，泥石流活动强度属强烈一中等，本区泥石流活动强度和危害程度不一。区域泥石流危害程度＜＜部分单沟泥石流危害程度，区内有１３个县市所在地受泥石流威胁或危害。本区泥石流防治已有一定基础，需进一步加强。

Middle danger district of debris flow is Ⅲ-grade district which debris flow has actual orpotential middle harmful ability. The district is divided into 21 parts ,having area of 23.08×10￣4km￣2(see Table 1)and being 23% of the area of the upper reaches of Changjiang River.In the district, debris flow activities mostly are middle danger and the level of economic de-velopment is uneven.There are many awaiting development distriets.The formative conditions of debris flow are full in the district,Most landform indexes are 2grade(the relative height h of statistic units=3 000-2 000 m) and 3 grade(h=2000-1000m). Geologic indcxes are mostly 4 grade(the product S of fault length and cofficien t ofstrata weathering in statistic units= 0.15-0.10). The latter shows the most part of the districtis not in the centre location of structure movements , but in the affectioned areas. The length ofmain faults is 236m/km￣2. Most climatic indexes are 3 grade and 4 grade. Therefore , geologicand landform conditions and climatic conditions are favourable to middle activity of debris flow. The active characteristics of debris flow in the district are as follows:1.inconsistencybetween strong and weak degrees of debris flow activity; 2. inconsistency between active andendangerous degrecs of debris flow;3. regional comprehensive endangerous degree of debris flow《endangerous degree of the singe debris flow ravine;4.the potential endangerous power ofdebris flow is great;5.the action of human activities in debris flow formation is strengthening dayby bay;6.the harm and the threat of debris flow will incrcase with the development of economyand construction.There are 13 county towns which suffered from debris in the district. Therefore, theprevention and control of debris flow should be further stren8thened.