Study On The Activity Mechanism And Initiation Criterion Of Typical Gully Debris Flow In Wudu District

Debris flow activity mechanism and initiation criterion are hot issues in debris flow disaster research.Longnan city Wudu district is located in the southeast of Gansu province.It is located at the intersection of Qinba mountain area,loess plateau and Qinghai-tibet plateau.The Wudu area has been one of the most vulnerable areas in China due to complex terrain,loose debris and frequent extreme rainfall.There are dozens of active debris flow structures in Wudu area.From the perspectives of geomorphological form,source type,activity and hazard objects of debris flow gully,Niwan gully and Macao gully were selected as research objects in this paper.On the basis of field investigation,numerical simulation method was adopted to study the formation mechanism of debris flow and the critical rainfall of debris flow outbreak.Among many debris flow simulation software,flo-2d and RAMMS are highly recognized by many scholars.Among them,flo-2d software requires many parameters,and the results are rich in details.However,the input of debris flow material source is too simple to reflect the material source conditions.RAMMS software can add material sources for debris flow simulation,but RAMMS simulation requires few parameters and the simulation results are rough.Based on this,this paper studies the Flo-2d and RAMMS two types of software,the simulation respectively for 10 years,20 years,50 years,in 100 in a heavy rain,under the condition of two characteristics of the debris flow activities,as well as the two channel required critical rainfall in debris flow,and start on the basis of comparison with each other,integrated to determine the activity characteristics of the two flow gully and start the criterion.The results show that:1)According to the comparative analysis of flow velocity of debris flow under natural working conditions,the maximum flow velocity of Macao gully obtained by Flo-2d is basically between 5-7m/s,and the flow velocity of Niwan gully is within 7-9m/s.In the RAMMS simulation results,the flow velocity of mudflow in Macao gully is between 9-12m/s,and that of Niwan gully is within 11-13m/s.It can be seen clearly that the RAMMS simulation results are greater than Flo-2d,with a difference of about 80%.Compare two accumulated debris flow gully depth,in Flo-2 d simulation results,the maximum stack depth of the Macao gully is 5-8m or so,Niwan gullu is 7-9m,while RAMMS results in two gully depth respectively is 9 to 11 m and 11 to 15 m,can be seen from the results of two software,Niwan gully stack depth than the Macao gullu and RAMMS simulation result more than Flo-2 d simulation results,the software simulation results is about 50%.2)In the dam condition,Flo-2d simulation shows that the maximum flow rate of the debris flow in the two gullies varies little,but the flow rate decreases significantly after the debris flow passes the blocking dam.In the RAMMS results,the maximum flow rate of the two gullies is basically 9-12 m,and the decrease is about 10-20%.In the blocking condition simulated by Flo-2d,the accumulation depth is better.The debris flow in front of the blocking dam is in an obvious accumulation state.The maximum accumulation thickness of Machou gully and Niwan gully is about 14 m and 15 m respectively.In RAMMS,the maximum accumulation depth after treatment is between 9-13 m and 13-17 m respectively,and the variation is about 20% compared with the natural working condition.3)The critical rainfall of the debris flow in the study area was analyzed,and the initial rainfall and the rainfall of the debris flow in the study area under natural conditions were obtained.The starting rainfall of the two ditches was the same(2.6mm/h),while the outfall gully rainfall was 28.17 and 24.18mm/h,respectively4)through comparative analysis of simulation results,it can be seen that RAMMS software is more suitable for debris flow with a large number of landslide sources in the basin,while flo-2d software is more suitable for debris flow with loose surface debris as its source.