Analysis Of Status-Quo Of Collapsing Hill Erosion In Anxi County And Investigation On Its Control

Collapsing hill erosion is the most serious type of water loss and earth erosion in South China, while Anxi County ranks the first in Fujian province in collapsing hill erosion due to its long history, wide area, serious degree and great harzard. Based on an all-around investigation on the collapsing hill erosion in Anxi County, the author analyses in a deep-going way the present characteristics of this erosion in the county, such as its special distribution, shape and active state, and discusses its control measures so as to provide the basis for a scientific decision on the policy for reducing the erosion hazard, improving the ecological environment and realizing a sustainable development, and meanwhile to provide reference to the control of the collapsing hill erosion in those similar regions in the country.The investigation results show that Anxi County has 4744 hill collapse events and an erosion area of 906.39hm~2, ranking the first in the province and accounting for 54.19% of the total number and 34.82% of the total area of the province respectively. Its hill collapse density reaches 1.55 event /km~2 and its average erosion modulus is 66.1 thousand t/km~2.a;AII 24 towns (townships) in the country have hill collapse develop, with a trend of gradually decreasing from southeast to northwest, which is highly identical in special distribution to that of granite. The hill collapse has complex and diversified shapes, with mixed (accounting for 1/3 of the total number), ladle and arc types as the main; with small sized events in a large number (accounting for 40.37% of the total number) and large sized events having a great erosion area (accounting for 53.58%of the total area). The county now has atogether 3366 active collapsing hills with an erosion area of 657.30hm2, accounting for 70.95% of the county total number and 72.52% of the county total area . 67.55% of the hill collapse events have an average depth of less than 40m and 69.67% have a gully width of less than 50m.The widely distributed granite parent rock in the region provides a plentiful substance source for the hill collapse development, the inherent rock characteristics of granite are the inner factor for producing the hill collapse and the rainfall, especially the great storm, is the important cause for producing the collapsing hill erosion. The fracture belt in Fuan-Nanjing geological structure has created favorable conditions for the further development of hill collapse. The hill-dominant topography and between-hill basin are also favorable for retaining the rock weathering products, which can form a deep and thick weathering shell, and the human irrational activity has once intensified the occurrence of the collapsing hill erosion.On this basis, Anxin County has classified the areas that have been attacked by the collapsing hill erosion into strong, medium and light ones and has proposed the corresponding control measures. Guanqiao and Longmen townships in the southeast of the county are classified into strong eroded areas which need major prevention and treatment measures, where a comprehensive control will be taken, including mainly intensifying the treatment, combining the engineering measure with the planting measure and taking small drainage area as units. For medially eroded areas which is the secondary major control region, planting measure will be taken as a major policy and engineering measure as the support. For lightly eroded areas, the general policy is to take the prevention and protection as the main measure, to intensify the area enclosing and make supplementary planting so as to bring into full play the selfrestoration function of the ecological system.For the treatment of the single hill collapse event with serious e(?)sion, the author has put forward a systematic treatment thought according to the hill collapse development characters and physiognomic structure, and also the specific configuration of the measures for treating the water-collecting slope, collapsed material, gully bottom (channel) and alluvial sector, which has been proved to be feasible in the practice.