Characteristics And Control Techniques For Debris Flow In Beishan Of Longnan City

Longnan city, located in the southeast of Gansu province, is a typical mountain town, Wenchuan earthquake hardest hit area, and is one of the most earthquake-prone areas of geological disasters in our country. Due to its complex geological conditions, Longnan region is prone to disasters and has a poor disaster-bearing capacity. Longnan is situated in the western China, with less annual rainfall but concentrated rainfall, and more rain and large annual evaporation, which leads to a more loose and weathered rock and soil mass in Longnan area. As the Wenchuan. earthquake disaster areas, debris flow fully in post-earthquake is abundant of loose materials, plus the poor geological environment, the debris flow is developed here. Longnan city, adjacent to the north mountain, with the development of city, continues to occupy the top of the northern mountain slope and channels. The north mountain is a frequency-occurring district of geological disaster, where there are9debris flow gullies of various sizes such as guojia gully, guesthouse gully and so on. Debris flow channels in the north mountain are long and narrow, whose slope gradients are big, so there is less section for blocking dam project.After the Wenchuan earthquake, Longnan governments have taken some measures to control the geological disasters in the north mountain, and have made some achievements. On-site inspection found that the major measure taken for the control works of the north mountain is debris dam, and some channels are equipped with drainage projects. However, limited by the specific local geological environment and the rainfall conditions, existing engineering failed to fundamentally remove the threats that geological disasters in the north mountain posed to the urban areas.This thesis will conduct a detailed study on the geographic environment and geologic conditions in Longnan urban areas and the north mountain based on data access, field inspection and experimental research. Through the research on the features of the debris flow and its induced conditions, it is thought that all the debris flow gullies belong to typical rainstorm debris flow gullies, and the rainfall directly determines the occurrence of debris flow and affects its size and hazard rating. Through the quantitative analysis of the features of the debris flow gullies in the north mountain, the debris flow is confirmed as viscous debris flow, which has strong destructive power. Combined with the existing prevention and control project with the characteristics of topographic and geographic conditions of channels, the high cross drainage system has been proposed as a countermeasure to prevent and control debris flow.Water is an important induced factor to debris flow. It is found that water can control the debris flow in Longnan city. In order to fundamentally remove the threats of debris flow in Longnan urban areas, we take the measures to control the conditions that debris flow may form water source this time, making the catchment peak discharge in the gully be smaller than that in the gully when the debris flow erupts in a critical rainfall, to reach the solving thoughts of separation of water and sand. The high cross drainage systems have been adopted as treatment schemes, which are arranged in a gully debris flow formation zone, catchment slope and gully water clean areas. Draining out the rainfall in the debris flow formation areas through the channels during a rainstorm, so that making the catchment runoff in the debris flow gully be smaller than that in the gully during a critical rainfall, to reach the purpose of separation of water and sand. Furthermore, to reach the water conditions that control the formation of debris flow, thus the possibility of occurrence of debris flow will be reduced.