Abstract:

The Three Gorges is located in the eastern Sichuan Basin. Running across the Daba Mountain and western Hubei mountains，the Three Gorges Reservoir area has complex terrain and geological condition. It is one of the most serious regions of landslides disasters in China due to the large amount rainfall. Land slide，rock fall，and mud flow are the main geological hazards in the Three Gorges Reservoir area. Among them，land slide occurs most frequently. With the economic development in the Three Gorges Reservoir area，the loss because of landslide becomes more serious. Generally，the geological hazards are observed to be induced by heavy rain and continuous rain. On basis of the daily precipitation data and land slide data collected during 1971-2003，the relationship between the landslide and rainfall amount，intensity and duration days are analyzed by estimating the valid precipitation using the exponential decline and gradient descending calculation. Thus the thresholds of precipitation accumulation at different guarantee rate of different scale land slide are obtained. The thresholds are supposed to be of certain significance in predicting land slide based on shortterm precipitation forecast. The results show that the rainfall thresholds for medium and large scale landslides are larger than those of small scale landslides under the same occurrence of landslide guarantee rate in a similar geological environmental condition. When the occurrence of landslide guarantee rate is less than 50%，the rainfall thresholds for medium and large scale landslides are significantly higher than that for small landslides. However，with the improvement of the guarantee rate，difference of the precipitation thresholds becomes smaller. Since the Three Gorges Project has started to operate since 2004，the subsequent changes in seasonal water level fluctuation would change the rock structure of reservoir，which may possibly lead to the change in frequency of landslides. Therefore，in this study the relationship between landslide and precipitation amount is analyzed in the period before the Three Gorges Reservoir impoundment. The issues of whether the rule of geological disaster occurrence changed or not after the impoundment of reservoir and whether the relationship between landslides and rainfall are to be further studied