| The Diaolougou gully is located at Magengzi Town,Lebo County,Sichuan Province,and on the left bank of the Xisujiao River,a tributary of the Jinsha River.The debris flow first occurred on August 9,2002,and a heavier event reoccurred on May 7,2015,causing 8,dozens injuries,and huge economic losses deathes at the town of Magengzi.At present,there are still a large amount of debris material in the Diaolou gully,showing a high probability of recurring debris flow in the infure.The Diaolougou debris flow poses a serious threat to the life,property of more than 250 people in 40households,and other infrastructures in the town of Magengzi,with potential economic losses of approximately 80 million yuan.Due to its severe hazard,it is necessary to study the development characteristics and potential run out area of debris flow in the Diaolougou Gully,which will contribute to the hazard evaluation and hazard engineering mitigation design of the debris flow.In this work,through field exploration,engineering geological mapping,laboratory testing,and geological analysis,the development characteristics,disaster-inducing mechanism,and development trend of debris flow in the Diaolougou gully are investigated in detail.Then,the potential run out range of the debris flow is quantitatively analyzed with the help of Flo-2d numerical simulation method.Basded on preious findings,the engineering mitigation strategy is proposed.The main conclusions of this thesis are as following:(1)Heavy rainfall and steep topography are the main factors controlling the formation of debris flow in the Diaolougou Gully.The area of the Diaolougou Gully is0.85km~2,while the length of the main channel is 1.6km,and the average longitudinal slope of the valley is about 504.4‰.The total volume of debris material in the gully is investigated as 40.07×10~4m~3,while the volume of the mobile debris material that may involved in future debris flow activity is about 10.56×10~4m~3.Usually there is no water flow in the gully.Due to the steep topography,the Diaolougou Gully is able to collect rainfall quickly,and therefore flood often occurs in the summer.At the same time,the lower gully is densely convered by huge rock blocks,forming cliffs somewhere,in which the gully blocking may take effect and make the debris flow size increase exponentially.(2)The debris flow activity in the Diaolougou gully has a high frequency.In the recent two decades,there have been two large-scale debris flows,which occurred in August 2002 and May 7,2015 respectively.The debris flow activity of Diaolougou Gully is characterized as triggered by short-term heavy rainfall,short duration,and high rock block content.The debris flow in 2002 and 2015 respectively lasted for about 15minutes and 10 minutes,and their runout debris volume were about 3000m~3 and5000m~3,respectively.The 2015 debris flow initated in 10 minutes after a short-term heavy rain(10 minutes of rainfall is 24mm),and the rock content in that runout debris deposition is about 60%.(3)Using fuzzy analytic hierarchy process(ahp)to probability calculation of diaolou gully liability,compared with the 32%probability of diaolou gully is extremely prone,medium is used the probability of 32%,mild prone probability of 37%.This result and in accordance with the standard of evaluation results more close to the results of fuzzy comprehensive evaluation in the form of probability,the conclusion is more accurate than the specification,more suitable for application in disaster prevention and mitigation and risk assessment.(4)The unit weight of the debris flow in the main channel of the Diaolougou Gully is 1.72t/m~3s,the sediment correction coefficient is 1.611,and the debris flow velocity in the main channel is between 5.14 and 9.12 m/s.Under four different-frequency(P=1%,2%,5%,10%)rainstorms,according to the empirical methods in the code,the peak flow of debris flow at the mouth of the Diaolougou Gully are 90.796,81.442,68.614,and 58.755 m~3/s,respectively;the total amount of debris flows are 11004.46,9870.74,8315.96,and 7121.1 m~3/s,respectively;the amount of runout materials are4801.95,4307.23,3628.78,and 3107.39 m~3,respectively.(5)Combined with the peak flood flow rate lines under four storm frequencies(P=1%,2%,5%,and 10%),the numerical simulation of the debris flow runout in the Diaolougou Gully was conducted using the Flo-2d software.The accumulation of the“5.7”debris flow in 2015 was roughly consistent with the Flo-2d numerical simulation result under the P=2%rainstorm condition.In the short-duration rainstorm(10 minutes for heavy rain and debris flow activity)condition,the total volume,the runout volume of debris flow,and the area,the maximum width,the maximum longitude distance,the maximum deposition thickness of the accumulation fan gradually increased with the decrease in the frequency of rainstorms.Besides,through comparing the results calculated through Flo-2d and empirical formulas in the code,it was found that the values obtained from the Flo-2d simulation was larger than that calculated by the empirical formula for the total volume,runout volume of the debris flow,and maximum deposition width of accumulation fan.(6)Under long-duration heavy rainfall(rainfall frequency P=1%,debris flow duration is 1 hour),both the Flo-2d numerical simulation and empirical formula calculation determined that it is highly probable the Xisujiao River could be blocked by the runout debris material from the Diaolougou Gully.The Flo-2d numerical simulation method can predict more accurate parameters of debris flow accumulation fan especially when the debris accumulation process was influenced by the thither slope.(7)Combined with the numerical simulation results of debris flow runout using the Flo-2d,a reasonable engineering mitigation design for the Diaolougou Gully was proposed,including reinforcement of boulders in the lower main channel,slot dam,seat com-tooth dam and drainage groove near the gully mouth. |
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