Spatial Prediction and Hazard Assessment Models of Hillslope Debris Flows at Village-Side Hillslope in Eastern Taiwan

This study develops the spatial prediction and hazard assessment models of hillslope type debris flow (HDF) to enhance the prevention and early warning of the HDF disaster to the villages. Induced by serious earthquakes and extreme weather conditions, HDF occurred frequently on the villages-side slopeland in Eastern Taiwan. The small scale HDF are often prone into high damage, because those slopeland is adjacent to the village. Based on this, to develop the spatial prediction and hazard assessment models of HDF is improving the safety of the residents.

This study uses the slope unit concept to establish the proper topographic unit for the spatial analysis. Fisher’s discriminant method is applied to develop the HDF spatial prediction model which consisted in 7 factors achieved from the slope units of metamorphic geology area in Eastern Taiwan. 27 HDF and 27 landslide events were adopted to develop the spatial prediction model, the model as following:

y=-1.144X₁-0.993X₂-0.049X₃+0.622X₄+0.353X₅+0.57X₆+0.478X₇

In above equation, y is the discriminant function, X₁ is the Average width of watershed, X₂ is the Average gradient of the initiation region, X₃ is the form factor of the initiation region, X₄ is average width of the initiation region, X₅ is the Depression ratio of the initiation segment1, X₆ is the depression ratio of the transport segment DRT, X₇ is the Gradient ratio of the initiation region. If the discriminant function y is greater than 0, a HDF is identified, otherwise a shallow landslide slope is identified. The results showed the overall correct estimation ratio is 88.2% and 85% verification ratio have been achieved in this study. 

The prediction model was then applied to 8 villages in study area, and the results show that 15 HDF have been caught in a total of 19 HDF in 8 village. The capture rate is about 79% and the overall capture rate of HDF and landslide unit is also 85%. In overall, the results show a good applicability of the prediction model in the metamorphic rock.

The project further draw up the hazard assessment model and comparing the result to the real HDF events which investigated by the field survey in 8 villages. Results showed that the potential of real HDF events were mostly classified in medium and high potential levels. Among them, there are 15 HDFs classified in medium and high potential in 19 HDFs. Concluding the results of the potential analysis, the result show a good application tendency in this research.