A Study Of Evolutionary And Dynamic Numerical Simulation For Gully-Shaped Debris Flow

Debris flow is a common type of geological disaster in the west of China.It usually has the characteristics of sudden outbreak,ferocity,strong destructive force and mass disaster.In view of the serious harm caused by gully debris flow,it is of great practical significance to evaluate and predict it.Due to the complexity of formation conditions of gully debris flow disaster,the evaluation and prediction of debris flow is always a scientific problem to be solved.It can be seen that further understanding of the formation conditions and dynamic characteristics of gully debris flow is of great significance for the evaluation,prediction and control of this kind of debris flow.Taking baishuizhai gully in Nanxin town of Maoxian County as a typical example,the formation conditions and characteristics of gully type debris flow are studied by means of data collection,field investigation,engineering geological analysis and numerical simulation.The geomorphic evolution of the watershed is analyzed qualitatively and quantitatively,and the development characteristics and evolution history of ancient debris flow are summarized Based on the evolution of ancient debris flow,using remote sensing technology to divide the evolution law of material source,comprehensively evaluate and predict the development trend of gully type debris flow and put forward corresponding engineering control measures.The main achievements are as follows:（1）The debris flow watershed is in a sector shape,the watershed area is 68.2km²,the longitudinal length of the main gully is 12.8km.The main provenance of the debris flow in the survey area is from landslides,the static reserve of solid matter in the debris flow is 27295.18×10⁴m³,the dynamic reserve which may involve in debris flow activities is 774.13×10⁴m³,mainly distributing in the left ditch and the middle and lower reaches of the main gully.The safety rainfall R value of the survey area is safety rainfall and the water source is mainly from rainfalls.（2）Based on the quantification analysis calculating the entropy value,the survey area may form debris flow in terms of topography and landforms.The entropy value of the main gully is H=0.67,the stage of geomorphologic evolution is old age;the entropy value of the big ditch is H=0.36,the stage of geomorphologic evolution is maturity stage.（3）The survey area is an old debris flow gully,which happened at least IV debris flow.Among that,Phase I debris flow was the largest,speculating the volume was approximate 925.10×10⁴m³;the latter phases had lower scales,speculating the volumes were 388.30×10⁴m³,108.62×10⁴m³ and 45.13×10⁴m³,respectively;the volume of most recent"Jul.10^th"debris flow is speculated to be 19.29×10⁴m³.This gully at least experienced one time of formation-development-degradation-cease evolution,indicating it’s a debris flow gully in the decline stage.（4）The following results were obtained using the CFX module of ANSYS software:when the rainfall is once-in-a-decade（P=10%）,the highest velocity in Baishuizhai Gully debris flow is 11.39m/s,the average velocity is 4.32m/s and the velocity in the gully entrance is 1.28 m/s;when the rainfall is once-in-two-decades（P=5%）,the highest velocity in Baishuizhai Gully debris flow is 13.18m/s,the average velocity is 4.67m/s and the velocity in the gully entrance is 1.39m/s;when the rainfall is once-in-half-a-century（P=2%）,the highest velocity in Baishuizhai Gully debris flow is 14.78m/s,the average velocity is 5.96m/s and the velocity in the gully entrance is 1.98m/s;when the rainfall is once-in-a-century（P=1%）,the highest velocity in Baishuizhai Gully debris flow is 15.91m/s,the average velocity is 6.73m/s and the velocity in the gully entrance is 2.53m/s.The velocity of the gully entrance in case of once-in-a-century rainfalls is basically consistent with the value calculated by diluted debris flow formula.（5）The quantitative score of susceptibility for debris flow in Baishuizhai Gully is100,indicating that debris flow probability is high.Baishuizhai Gully still has the internal provenance and topographic conditions for another debris fall disaster;as continuous deterioration of global climate and environment,the intensity and frequency of extreme climate events will become more often;as a result,it is still possible for larger rainfalls in the survey area and for debris flow disasters which become more severe as rainfalls increase.（6）In case a once-in-a-century debris flow outbreak in Baishuizhai Gully,the probability of completely blockage of Minjiang River is relatively high;as for once-in-half-a-century,once-in-two-decades and once-in-a-decade debris flow,the probability is very low.Recommendations for engineering management measures:take comprehensive management,including obstruction,dredging,bioengineering,etc.