The Research On Formation Mechanism And Mitigation Measures Of Large-scale Debris Flow In The Wenchuan Earthquake Area

The large-scale debris flow that occurred after the huge Wenchuan earthquake has caused tremendous damage.One of the main reasons lies in the fact that the underlying mechanism behind the formation of the post-earthquake severe debris flow still remains unrevealed,leading to weak effects of consequent countermeasures and improper prevention measures.Therefore,exploring the formation and countermeasures of post-earthquake large-scale debris flow in Wenchuan area is of great significance for both achieving instructions to prevention measures and the development of theoretical research on debris flow.In this thesis,we focus on Yingxiu-weizhou town,the area located in a section of the minjiang river valley,where the heavy disasters with a lot of large-scale debris flows occurred after wenchuan earthquake.Based on the distribution and active characteristics of large-scale debris flows in the research area,the formation mechanism and developing tendency of post-earthquake large-scale debris flow were discussed in detail.The prevention mode of post-earthquake of the large-scale debris flow were studiedThe following conclusions have been drawn based on the results of this study:(1)The study area is on both sides of the Minjiang river valley from Yingxiu upstream to the seat of Wenchuan county——Weizhou,which is located in the east of Wenchuan county.The fast down-cutting of the Minjiang riverbed caused the steep terrain on both sides of the riverbed,and 76.8%of the study areas are hillsides or steep slopes,which provide advantageous terrain condition for debris flows.As a part of Longmenshan nappe tectonic belt,the area is located between the central fault and the back range faul.The main strata are magmatic rocks in Neoproterozoic and early Paleozoic,metamorphic rocks in Sinian,and a small amount of metamorphic rocks in Silurian.Due to the influence of tectonic action,the magmatic rocks and metamorphic rocks are cleavage,the fractures are developed,and the rock mass is broken.The neotectonic movement is characterized by the rapid increase of regional crust and the accompanying faulting.The study area was divided into two zones with distinct climate features:the mountain humid subtropical monsoon climate and warm temperate continental semi-arid monsoon climate bounded by the Supodian of Ginkgo country.The rainfall in the area increased from north to south,and the characteristics of drought in the north and South were obvious,and the heavy rainfall in flood season was concentrated,which offer the rainfall condition for debris flows.(2)After the Wenchuan earthquake,the geologic hazards in the study area have undergone fundamental changes.Larger-scale and high-frequent debris flow hazards displaced the landslides as the major hazard.In this study,a large-scale debris flow can be defined by indexes such as the total discharge of one debris flow(>100,000m3),the peak discharge(>100m3/s),the death toll(>10),the direct economic losses(>5 million Yuan),the threat number(>500)and possible economic losses(>5 million Yuan)and so on.Gullies satisfying any one or more of the above-mention conditions can be confirmed as large scale debris flow gullies.According to the investigation and assessment,54 gullies were distributed in the study area,46 of which are large scale debris flow gullies.Half of these gullies distribute at the both sides of the Minjiang river,and the distribution density along the main stream of Minjiang river is as high as 0.85/km.The debris flows in the studied area have the characteristics of relatively small drainage basin,short and steep main gullies and wide distribution.distributions of debris flow varied with different strata.The debris flow drainage area occupied 99.7%of the outcropped area of magmatite in Sinian.The number changes to 89.7%and 84.9%by the gneiss in Paleoproterozoic and magmatite in Neoproterozoic,respectively.The gneiss strata in Mesoproterozoic has a least proportion of 55.7%.Because the seismic intensity of the study area is generally high(? degrees or more),the surface damages were generally serious.Consequently,there is little debris flow distribution difference in different intensity areas.The large scale post-earthquake debris flows in the studied area showed the following characteristics:the landslide sources for debris flows increased rapidly and initiated concentrated,break points of the landslide dams increased.The critical rainfall of debris flows decreased significantly.The post-earthquake debris flow is not only substantially increased in number,but also high-frequent and group-occurring.(3)Due to the influence of the devastating Wenchuan earthquake,the debris source increased dramatically,with a significant change in microtopography and the channel conditions.The hydraulic condition leading to debris flows also decreased significantly,namely the critical rainfall intensity that trigger a debris flow decreased distinctly than pre-earthquake.The aforementioned changed conditions that triggered debris flows in the earthquake-stricken area result in remarkable changes of debris sources which also lead to the change of debris flows disaster-causing mechanism.According to the investigation,analysis and conclusion,the disaster-causing models can be concluded as"block-bursting type","down-cutting type" and "landslide-recharging type".Block-bursting is one of the main factors which caused the dramatical increase in scaleof debris flow.Large scale is a key characteristic of block-bursting type debris flow,for instance,the debris flow occurred on 11th July in Qipan gully.A mass of debris deposited in gully caused by earthquake tends to form debris flow due to the rainfall erosion.A cascade effect of sediments block-bursting can also happen as a consequence of down-cutting type debris flow,for example,the debris flow occurred on 3rd July in Gaojia gully.The collapses and landslides caused by Wenchuan earthquake brought large amount of loose deposits,which could easily trigger debris flows with the help of heavy rainfall.Hence the retrogressive erosion of landslide caused the landslide-recharging type debris flow with very large scale and serious damage,such as the debris flow occurred on 14th August in Huoshao gully.(4)Based on the theory of fuzzy matter-element analysis,we have made the risk assessments on large-scale debris flow in the study area by selecting the clogging degree of channel,gradient of gully watershed,integrity coefficient of watershed,total moveable reserves debris per unit area,moveable reserves of landslide per unit area,accumulation moveable reserves of gully bed per unit channel length,lithology and average annual precipitation as main factors.Considering the fact that the formation of different types of debris flows including block-bursting type,down-cutting type and landslide-recharging type may be dominated by different factors,the standard source model of risk assessment has been established by appropriately selecting the dominating factors.The assessment results show that there were 21 gullies with high-risk,17 gullies with medium risk and 8 gullies with low risk,corresponding to 45.65%,36.96%and 17.39%of the total,respectively.Among the 23 landslide-recharging type debris flow gullies,30.44%were high risk,34.78%were medium danger and 34.78%were low risk.Among the 12 down-cutting type debris flow gullies,58.34%were high risk,41.66%were medium risk.Among the 11 block-bursting debris flow gullies,63.64%were high risk,36.36%were medium risk.Additionally 83%of large-scale debris flow gully is moderately high risk in the studied area.According to the assessment results,the criticality of block-bursting type,down-cutting type and landslide-recharging type debris flow gullies decreases successively,which is strongly linked to disaster-causing model.(5)For the studies on the counter measure system of large-scale debris flow after the earthquake,we focus on three typical cases:Qipan gully,a block-bursting type debris flow gully,Gaojia gully,a down-cutting type debris flow gully and Shaofang gully,a slide-recharging type debris flow gully.the counter measures of large-scale debris flow have been discussed respectively.First,for the block-bursting type debris flow,it regards eliminating the blocked and combining discharge as a guiding ideology.A comprehensive engineering measures have been used to eliminate the hidden channel blockage patient and reduce the scale and hazards of debris flow,which included 3 check dams,1 pile dam,1 silt dam,a gabion material structure and a drainage canal.Second,for the down-cutting type debris flow,it regards the blocked mainly and discharge auxiliary as a guiding ideology.In order to achieve the control of channel incision and reduce the possibility of blocking the main river by the debris flow,2 grille dams in the midstream and downstream of gully,the drainage engineering and diversion dike engineering in the mouth of gully were adopted.Finally for the t landslide-recharging type debris flow,it regards loose sediment retaining measures and cutting off supply of sediment and discharge auxiliary as a guiding ideology,and adopts a pattern which included checking dam group in upstream,pile anti plate and rib foot bottom protection in midstream,2 pile dam and 1 dams of natural siltation of solid bed pressure foot in downstream and aqueduct across the tunnel G213 ".(6),the countermeasures and models have been constructed,according to three kinds of different disaster modes.Debris flow triggered by blocked-break is related to the microtopography of gully,and channel blockage is an important cause of the this type of debris flow.So the key prevention of blocked-break debris flow are dredge channel,keeping off energy accumulation and controlling the scale.For the debris flow triggered by blocked-break type,it regards eliminating the blocked,controlling scale,and energy dissipation as a guiding ideology,and takes a pattern which governs the energy primarily as well as water and sediments.Therefore,the countermeasures include energy dissipation,soil treatment and water treatment.The priority sequence of countermeasures is energy dissipation,soil treatment and water treatment.The main formation mechanism underlying the vertical erosion debris flow is erosive by water,so the key measure of preventing to this debris flow is controlling the erosion rate.For the debris flow triggered by vertical erosion type,it regards cutting off the resource of water and stabilizing ditch bed and bank slope as a guiding ideology,and adopts a pattern which governs water firstly as well as sediments and energy.Moreover,the ditch bed,slope drainage and water blocking measures are adopted to prevent the erosion of the slope surface to form the pull trough,and the measures of stabilizing the soil and stabilizing the bed are carried out to increase the ability of resisting the impact of water.The order of priority of the main engineering measures is the water control project and soil treatment project,energy dissipation.The key to the formation of landslide recharge debris flow is that the advantage of source can be maintained.Artificial intervention for landslide source are used to prevent the landslide source to collapse.For the debris flow triggered by landslide recharge type,it regards loose sediment retaining measures and cutting off supply of sediment as a guiding ideology,and adopts a pattern which governs sediments primarily as well as water and energy.Moreover,centering on the project of soiltreatment(landslide and debris source),the water control measures are adopted tocooperate with the soil treatment project.The order of priority of the main engineering measures is the soil treatment project,water control project and energy dissipation.According to the investigation on the formation of different debris flow disaster mode of governance projects and the inspection and analysis of the effect of the implementation of the study area after the earthquake debris flow control,we found that most of the countermeasures have a good operation.Therefore,water treatment,soil treatment and energy dissipation works efficiently,which means the preventions achieved the expected goal.