| With the development of our knowledge on global change research and our understanding on disaster prevention, the assessment and management of debris flow hazard have got more and more attention from the international community. There is a quick development on the assessment and prediction of debris flow disaster risk in small watershed in China, however, less systematic research on this topic has been done. For the assessment of risk and the forecasting of threat range of the debris flow hazard, there are still some problems. The assessment of the debris flow hazard in small catchment is also not enough and often too simple in the regions where major debris flow hazard occurs frequently, and especially in the regions affected by earthquake. And, the existing system of evaluation techniques and management model are often not suitable for the disaster governance and region reconstruction after a strong earthquake, which greatly limited the further development of the assessment and monitoring, early warning and prevention work of a major debris flow hazard. As a result, the local government will not have a strong scientific basis and the necessary technical support in economic development planning and other aspects of disaster prevention and mitigation. Therefore, to carry out the assessment and prediction methods research of debris flow hazard in small catchment will have a great practical significance in earthquake-affected regions. In this study, through a large number of field observation and physical model testing in a small catchment, we try to have a systematic analysis on the physical basis of debris flow to obtain the accurate value of the parameters and to define the critical conditions for the numerical model. Using SINMAP model, we will have an analysis of the slope stability, and we will do the water flow process simulation using SCS model, and also, using FLO-2D numerical model, we try to have a simulation of the erosion process under different boundary conditions. The main results and conclusions are as follows:To begin with, movement process of debris flow was conducted and simulated through physical model of movement of debris flow. Moreover, the movement process of debris flow was simulated under different boundary conditions, in different bulk density of debris flow, and different total initiation mass of debris flow. After then, the sensitive parameters from the experiments were obtained by the above results. On this basis, the impact of slope, motion coefficient, bulk density and different total initiation mass in flowing area of debris flow on the movement and deposition process of debris flow were discussed. For instance, the ultimate burial depth of viscous debris flow was more than that of dilute debris flows under ideal boundary conditions. This phenomenon also occurred under non-ideal boundary conditions. In addition, the instantaneous burial depth in the movement of debris flow under ideal boundary conditions was much more than that under non-ideal boundary conditions. Moreover, the instantaneous infiltration water pressure increased gradually and the instantaneous earth pressure decreased progressively when viscous debris flow was transforming into dilute debris flow. When the slope increased, movement time decreased and flowing velocity increased, but instantaneous infiltration water pressure and instantaneous earth pressure would reduce.Secondly, the slope instability of small catchments in different types of debris flow was simulated using the SINMAP model. The results indicated that the very unstable region of debris flow mainly focused on weathered rock and steep slopes area, and the main reason of unstability is that collapse and landslides developed in this area. In addition, the total unstable area of debris flow in Qingshui gully was 0.83km2, which accounting for 38.25% to the total area of catchment. Among them, very unstable area and unstable area was 0.15km2 and 0.32km2, which accounting for 6.91% and 14.75% to the total watershed area, respectively. However, the very unstable region of viscous debris flow mainly focused on soil-landslide area, and this is consistent with actual observation and investigation.Moreover, runoff volumes and depth at different return periods and different precipitation were obtained using the Soil Conservation Service-curve number (SCS-CN) approach in ArcGIS software. Runoff parameters of small watersheds in the study area were obtained using spatial analysis technology of GIS. Then, runoff amount and runoff depth in typical watersheds were estimated. These results are reliable and consistent with actual observation. Additionally, cumulative flow diagrams at different return periods and different precipitation in Qingshui gully and Hanlin gully were drawn by analyzing the infiltration depth, runoff depth and runoff amount in typical watersheds. For example, the simulated results indicated that the maximum runoff depth is 46.6mm and the total amount of runoff is 7.1 ×104m3 in Qingshui gully and the maximum runoff depth is 50.3mm and the total amount of runoff is 137×104m3 in Hanlin gully when precipitation is 54.2mm. Moreover, the maximum runoff depth is 77.7mm and the total amount of runoff is 13.3×104m3 in Qingshui gully and the maximum runoff depth is 81.6mm and the total amount of runoff is 237×104m3 in Hanlin gully when precipitation is 85.6mm. Runoff capacity changes with different land use types in the same HYDGRPs followed by mountain upland, moderate cover grassland, woodland, canals, urban land and rural residents. Similarly, runoff depth also changes with different soil texture in the same land use type, such as moderate cover grassland, followed by silty clay, silty clay loam, silt, loam, and gravel soil. This approach and result can provide a reliable scientific basis for simulating debris flow deposition, assessment and prediction of the risk range in typical small catchments.Finally, the FLO-2D numerical model was used to simulate the movement of debris flow under different return periods, different frequencies and different types of debris flow. The results indicated simulated risk scope of debris flow was similar to actual observation and investigation. FLO-2D numerical model has preferable effectiveness and practicality. Simulation results show that Qingshui gully, a typical viscous debris flow, return periods for the 500-year flood event, the maximum velocity is up to 6.5m/s, the maximum impulsive force is 28×104KN, the maximum deposited depth of debris flow is 6.2m, and the risk range is about 0.1km2. Hanlin gully, a typical dilute-viscous debris flow, return periods for the 500-year flood event, the maximum velocity is up to 10.5m/s, the maximum impulsive force is 99.8x 104KN, the maximum deposited depth of debris flow is 12.4m, and the risk range is about 0.61km2. By analyzing the prediction of the risk scope and disaster chains in typical small watersheds, the risk scope of dilute debris flow increases with return periods; however, the risk range of viscous debris flow changes slightly with return periods. FLO-2D numerical model can be used to simulate the movement process of debris flow because of its effective data processing, spatial analysis, model analysis, calculation and visualization features.The prediction results of the disaster chains in typical small watersheds indicated that avalanche, landslide and slope debris flow are the disaster chains in Qingshui gully. Particularly under extreme precipitation and other earth forces, it would eventually develop a disaster chain of gully-debris flow. However, the prediction results of the disaster chains in Hanlin gully are cracks, collapse, and landslide and slope debris flow. It also develops a disaster chain of flood-debris flow under the extreme precipitation and other earth forces. Additionally, geological environment, infrastructure, farmland and Hanlin township government are the main objects of threats. |
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