Analysis On Influences Of Debris Flow At Near-dam Reservoir Bank Of Wudongde Hydropower Station

Debris flow has been paid close attention as one kind of hazards around the world. It often causes large damage to ecological environment, and human's production, property and life, for its Fast velocity, surprise breakout,severe damage.The major hazard of Debris flow to water conservancy and hydropower project facilities mainly presente in the following: (1) Destroying and Silting hydropower station, water channel and construction zone directly. (2) Transporting large quantity of sediment,accelerating reservoir sedimentation , reducing the effective storage capacity, reducing flood regulation and decreasing the generating capacity. (3) Debris flow takes an adverse effect on hydropower projects by transporting large amount of mud into the Main River, uplifting the river bed ,and blocking water intake station.Debris flows is active in Wudongde hydropower station reservoir area, and large-scale debris flows have ever generated for many times. It is necessary to investigate the characteristics and formation conditions of debris flow to forecast the possibility of debris flow occurrence, outbreak scale as well as debris flow hazard, and to analyze the potential impacts of debris flows having on normal operation of Wudongde hydropower station.Debris flow is a product which is influenced by geological and meteorological factors; its formation must have three basic conditions, that is, a lot of loose fragmental materials, steep topography and rapid and heavy downpour. Wudongde area has the basic conditions of the development of debris flow because of its special environmental geolo- gical background and natural conditions: 1) The stratigraphical lithology and tectonical conditions contribute to the formation of loose fragmental materials. The main lithology exposed in Wudongde area is ancient landslide body and the Jurassic, Cretaceous sandstone, mudstone, this kind of structure that weak layers and hard layers emerge alternately contributes to the instability of rock mass. In construction, Wudongde Station belongs to the Yangtze tectonic quasi-western edge of the platform Kangdian axis. Kangdian axis is a north-south axis to the long-term activities with uplift. In the long history of strong western geosyncline activities, fold and faults is developed. So the slope deposits are widely distributed under inside and outside geology stress which provided a lot of loose solids for debris flow. 2) The terrain of study area in Jinshajiang region is high and steep, its relative elevation is over 1000m -2000m, which provides power conditions for the occurrence of debris flow, and developed channels in the hill provides flow conditions for debris flow. 3) The meteorological conditions in this area is conducive to the formation of debris flow. On the one hand, plenty of precipitation in this area will not only provide a liquid-phase material for debris flow but also provide the impetus conditions for the pooling of solids, and sharp rain is the triggering factor of the occurrence of mud-rock flow, On the other hand, large temperature difference in the area causes the inside and outside of rock heated unevenly which accelerates the weathering and collapse of rock surface. This favorable climate condition is conductive to the formation of loose solids. In short, a variety of special conditions in this area is conducive to the formation of debris flow.Viewed from the landscape patterns of debris flow, there are gully- type debris flow and the valley-type debris flow in Wudongde region, viewed from the nature of fluid of debris flow, the type of debris flow in this area is viscous debris flow. In this paper, author selected five large-scale developed debris flow gullies to study which have potential impact on the dam of power station. Systematically expound developing environment and developing characteristics of debris flow close to the dam area; according to the characteristics of debris flow and using the latest research results at this stage, author calculates the debris flow velocity, impact force, peak flow, total flow characteristics so on. According to the actual situation in the region, author selects L1 :the maximum amount at one time, L2 occurrence frequency of debris flow (times / century), S1-drainage area, S2- length of main ditches, S3- maximum relative altitude in drainage area, S4- incising density, S5- bending coefficient of major ditch bed, S6- length proportion of debris supplement section, S7- maximal quantity of rainfall in 24 hour, S8-population density, S9- vegetation coverage rate(%) of 11 kinds risk factors, and author uses entropy method combined with AHP method to calculate the factor weight, taking the debris flow risk as final target, source conditions, terrain conditions and predisposing factor conditions as criterion layer , risk factors as index layer, author establishes secondary level model and evaluate debris flow risk using multi-level fuzzy evaluation model. Study the difference between the multi-layer fuzzy comprehensive evaluation and one- layer fuzzy comprehensive evaluation. The results show that the results of two methods reflects the same trend, Xiabaitan debris flow gully, Zhugongdi debris flow gully belong to high risk level; Shangbaitan debris flow gully and Yindi debris flow gully belong to severe risk level; Shenyu River belongs to slightly risk level, relatively in line with the actual situation. At the same time, according to the context factors, author forecasts debris flow accumulation scope (the risk scope)and the largest accumulation of debris flow range, the results show that except for the potential threat of accumulation patterns and scope to the normal operation of power station to some extent, the debris flows will not have direct significant impact on the dam of power station which provides reference for power plant design, construction and power plant desilting. The main impact of debris flow on power station reflects on scouring and silting, sediment transportation, reducing the storage capacity and so on.The study results show that the results of the regional debris flow risk assessment using fuzzy comprehensive evaluation method are in line with the site investigation. The assessment model and criteria are accurate which provide references for similar mud-rock flows in other area. It has a theoretical value and practical significance.