Research On Formation Conditions And Forecast The Dangerous Area Of Debris Flow In The Zaduo County Along Lan Cang River

Our study area is located at Zhaqu River, the upper Langcangjiang River, Zaduo County, on the Tibetan Plateau. The study area is affected by the uplift of Tibet, where the tectonic movements are active, full of faults, folds, and gorges, and intense incision, and strong freezing-thawing. The study area belongs to continental semi-arid mountain steppe climate with intense rainfall causing debris flows easily happenning in the branch gullies of Zhaqu, which bring about a great loss and threats to the life and property of local residences and building facilities, and destroy the local ecological environment. Therefore, the formation, forecasting the their risking area and putting forward prevention projects can not only offer effective theories for regional disaster prevention and reduction, but also provide other similar regional debris flows forecasting with reference and experience.In our study, debris flows along Zhaqu in Zaduo County was classified and associated triggers were analyzed. Most of the debris flows in our study area were triggered by torrential rainfall. Most of the debris flows are gully-type, diluted, and still in the period of development. Most of the area of the debris flows is less than 5 km2 with a “V” cross section, a funnel or leaf shape in the plane section. The differences of the main longitudinal slope are quite large, the two slopes are steep, and most of the differences of the river are about 100~500 m; most of the sources of the debris flows are weathered rocks with fissures. The experiment of freezing and thawing of rocks shows that the wave velocity of the longitudinal wave, CT value and tensile strength tend to be less with the increasing of the freezing and thawing, which worsen the integrity of the rocks, lower the tensile strength and be broken, finally form the unconsolidated debris. The debris inflow into the channel, to be the main source of the debris flows. Rainfall and melt-water make up the hydrodynamic factors. And the rainfall is the major factor, the melt-water as one of the important factors in the hydrodynamic factors also plays a key role in the formation of the debris flows.With the combination of the field work and remote sensing for the morphology of the debris flows, it concludes four types of the debris flows, fan-shaped, broom-shaped, superposed-shaped and tongue-shaped. It can be seen from the analysis in the MATLAB software about the maximum length, thickness of the debris cone, and topographic and sources factors that, there is an exponent relationship among the maximum length, width and area of the debris cone, the relative differences of the height of the slope, the length and the storage of the main gully. It infers that the existent debris cone was formed after several depositions with the final torrential of 21.7mm~38. 3mm per day, and analyzes the influence of vegetation upon the formation and the scale of the debris flows. The influence of vegetation upon the formation, development and recession is limited. The formation and scale of the debris flows cannot be determined by the vegetation; therefore, the vegetation factor will not be regarded as the factor of the debris flows scopes. Finally, the river area, relative differences of the river and the storage of the source of the debris flows were regarded as the three factors. A forecasting model for the longest and widest debris cone in the study area was established with multiple regression analysis with data from twenty nine debris flows, and the errors have met the original forecasting resolution after self-testing. Another five typical debris flows data in the study area were imported to test the model, to analyze the reasons of producing errors, and finally showed that errors from the forecasting model was objective and reasonable. In the end, two prevention policies were put forward for the specific debris flow—Zuoqingsi Debris flow. The policy of blocking first and integrated with drainage, on the basis of obscure assessment analysis, is the optimal prevention policy, which contains debris dams, check dams, unilateral fences and drainage canals.