Risk Assessment And Study On Prevention Of Debris Flows Near The Dam Of Wudongde Hydropower Station Based On Catastrophe Theory

Debris flow is a kind of common geological disaster occurred in the mountains.Widely distributing and offen occurring, debris flow is the worldwide geologicaldisaster and attracts widespread concern and attention. China is a country with a lot ofmountains and about two-thirds of the total land area is mountain. Therefore, ourcountry is one of countries suffering from severe and frequently debris flows,especially the Southwest of our country. The Southwest locates a transition zonebetween the first step and the second step, where elevation suddenly changes, tectonicfaults developments, the geomorphological type is the combination of high mountainsand deep valleys, and the climate is the monsoon climate with distinct wet and dryseasons. These are all favourable circumstances for the formation and development ofdebris flows in the Southwest. After5.12Wenchuan Earthquake, debris flowsoccurred more and more frequently in the areas affected by the earthquake. Therefore,risk assessment and the prevention research have great academic significance andpractical value.Debirs flows near the dam of Wudongde hydropower station were interpretedform SPOT5remote sensing images in this paper. Combining interpretation and fieldinvestigation, the geological setting and present status of the debris flows near thedam of Wudongde hydropower station were studied. Ten impact factors wereselected to evaluate the hazard degree of these debris flows, and three indices were slected to evaluate the vulnerability of these debris flows. Moreover, risk degree ofthese debris flows were assessment based on the hazard and vulnerability assessments.Based on catastrophe theory, initiation mechanism were analysed by establishing thecatastrophe model of debris flow. For different factors which influencs the iniation ofthe debris flows, different control measures were proposed. A method is proposed toforecast the rainfall threshold for debris flows in the areas where there are lessstatistical data. Through force analysis of the loose deposits on the gully bed, thecritical water depth for the initiation of debris flows is calculated according toMohr-Coulomb failure criterion. Then, the rainfall threshold is calculated throughanalysis the mechanism of runoff yield and confluence. By the method proposed inthis paper, the rainfall thresholds for the five debris flows near the dam of Wudongdehydropower station were forecasted. The conclutions obtained in this paper are asfollowing:1. There are five debris flows near the dam of Wudongde hydropower station,Xiabaitan, Shangbaitan, Zhugongdi, Yindigou and Canyuhe. The hazard degree ofXiabaitan, Shangbaitan, Zhugongdi and Yindigou are all moderate, and the hazarddegree of Canyuhe is slight. The vulnerability of these debris flows are all moderate,and the risk degree of these debris flows are also moderate.2. The catastrophe model of debris flow is agreed with the cusp model. Debrisflow can be expressed by the potential function of the cusp model in which saturationis state variable and gully slope and resistance are control variables. The loosendeposits can suddenly become debris flow when the control variables are in the cusparea.3. Through analysis the catastrophe model of debris flow we draw a conclusionthat debris flows can be prevented by controlling the saturation of loosen deposits,change the gully slope and resistance. The drainage areas of Xiabaitan, Shangbaitanand Zhugongdi are small and the gully beds are straight, therefore, these three debrisflow can be prevented by reducing the resistance of the gully beds. The drainage areasof Yindigou and Canyuhe are large, therefore, these two debris flow can be preventedby increasing the resistance of the gully beds, such as building check dam, grille dam, retaining dam and building belt dike, fish mouth and sheepfold type dividing dike inthe accumulation zone.4. The critical rainfall for debris flows near the dam of Wudongde hydropowerstation is between134.08mm and151.34mm.1h rainfall intensity which maybeinduce debris flows is between54.08mm and71.34mm.