Study On The Regulations Of Debris Flow On Eastern And Western Sides Of Lujiang River Valley-One Section Of Nujiang River

Debris flow on eastern and western sides of Lujiang River (one segment of Nujiang River) was investigated by surveying external-force geological disasters, internal-force geological disasters, and potential underground karst collapse. Based on evaluating the disasters qualitatively, we focused on the dynamic spatial-temporal variation of debris flow.We surveyed sedimentary characteristics of the debris-flow accumulations and basic conditions for debris-flow formation. Mathematical statistics were used to analyze water catchment areas and longitudinal slope variations. We also employed optically stimulated luminescence (OSL) dating technique to determine ages of diluvial deposits. Combining the evaluation indexes of developing characteristics, we revealed the regional and staged characteristics of debris flow development. It was found that:(1) Our study could provide basic materials for preventing and controlling the geological disasters in the study area by investigating the historic disasters and summarizing distribution laws and development conditions for different disaster types. There were five external-force geological disasters in the study area, namely landslide, collapse, debris flow, unstable slope, and active gully. Internal-force geological disasters were mainly due to geothermal anomaly, which is controlled by structure, in accordance with the direction of regional structure. Two sites of potential karst collapse were found, which can be dangerous to engineering.(2) Spatial-temporal variations of debris flow development on both sides of Lujiang River were analyzed using detailed field survey and statistics of geomorphic parameters in different debris flow catchments and flowing areas. Total longitudinal slope of the western side of Lujiang River was 9.68% averagely, and that of the eastern side was 13.43%. Although incision is a little deeper on the western side, longitudinal slope is apparently less than the eastern side. Modern debris flow mainly developed on the eastern side of Lujiang River. The difference between the eastern side and the western one was significant. Debris flow development moves from the western to the eastern, where the development had differences between south and north, with movement from north to south.(3) A good foundation for building time series of debris flow development on both sides of Lujinag River was laid by using OSL technique to date the debris flow deposits at different section planes, and providing age data of the debris flow development in the study area. OSL dating technique for 21 samples indicates that there are three stages of debris flow development on the sides of Lujiang River. The first one is before middle-Pleistocene. The second is between late middle-Pleistocene and late Pleistocene (141.68±7.93Ka—20.06±1.83Ka). And the third is after Holocene (10.83±0.39 Ka—). The eastern side was primarily developed. And(4) Elementary preventing and controlling strategy was suggested by assessing the hazard of debris flow in the study area. We also recommended monitoring the obvious developed and big debris flow regularly, e.g. on the sides of Malang River, Baiyan River, and Mangle River.