The transition of hyper-concentrated flow and debris flow after earthquake is the key problem involved in debris flow engineering control.Based on the analysis of debris flow in Moutuo Gully,this paper studies the importance of source supplement and micro-topography in the transition of hyper-concentrated flow and debris flow.The terrain and landform are analyzed firstly;secondly,the density is used to analyze the transition character between hyper-concentrated flow and turbulent debris flow;and finally,landscape step-pool effect method and investigation method are applied to study the transition process and mechanisms.The result shows that(1) the initiation model of debris flows in Motuo Gully includes surface erosion,gully erosion,blocking,outburst and debris flow;(2) the formation of microtopography(step-pool) is resulted from waste slag and collapse or landslide;(3) the transition of hyper-concentrated flow and debris flow is closely related with the solid volume in one surge.

Large spoil tips from reconstruction works as a result of the Wenchuan Earthquake in China are new debris flow hazards to the human society. However, there is a lack of detailed comparative study on debris flow initiation in different spoil materials. This paper describes a series of tests and analyses on debris flow characteristics (initiation, scale and mechanism) at six sites with limestone and sandstone materials near the Dujiangyan area. Research shows the limestone spoil contains debris flow prone clay content with high concentration of montmorillonite (highly expandable). In addition, limestone spoil is of such a low permeability that water mainly concentrates in the upper surface layer. Those factors make it easy for the increase of pore water pressure, decline of internal friction and conhesion force, leading to the occurence of large debris flows. In contrast, the sandstone spoil is less problematic and causes no major debris flow threats. Based on our research on the mechanism, the“stereometric drainage”method is sucessfully applied to control limestone spoil debris flows.

Bedding rock slope is a typical rock mass structure which influences the stability of engineering rock mass and leads to geological hazards. Several methods of slope stability analysis,namely,Sarma Method,Imbalance Thrust Force Method,and Linear Sliding Surface Method,are compared in this paper. With the bedding rock slope at the bridge foundation of Shujiacao Bridge of Yichang-Badong Highway as an example,the rock slope stability was analyzed by these three methods. The analyzed results were compared with the actual rock mass structure and damage of this bedding rock slope. The comparison shows that: the result by Sarma method consists well with the actual situation of slope failure,it is more suitable for analyzing bedding rock slope with joint fissures; whereas the calculation result of Imbalance Thrust Force Method is larger,and Linear Sliding Surface Method is conservative and smaller.

In order to find out the genesis of the extraordinary debris flow disaster which took place in Hefangou gully of Hunan Province,where is a wet area of low seismic intensity,on June 10th in 2011,the drought grade of Hefangou Basin before the disaster was determined and the effect of lasting drought on the physical and mechanical characters of the granite residual soil was analyzed on the basis of field investigation and laboratory test.Besides,the rainfall quantity and time required to saturate the granite residual soil was calculated.Then the genesis of the debris flow was obtained: the vertical layered characteristic of the granite residual soil determined that the soil can easily start from the surface with high content of fine particles;early long duration drought cracked the soil structure and speeded up the soil weathering along the part with high content of fine particles near ground surface,which increased the permeability of the soil;later heavy rainfall of short duration caused the rainwater to infiltrate along the cracks on the soil surface with fine soil particles moving,enriched and then formed the impermeable layer of soil,the strength of the soil decreased;with the continued heavy rainfall,the pore water pressure above the impermeable layer in the upper soil soared to some extent,the soil failed and was liquefied to form the debris flow.

以19条泥石流沟源区75个砾石土试样的颗分数据为基础,应用分形理论研究泥石流沟源区砾石土粒度组成特征,并对粒度分形特征与土体颗粒组成的关系进行了讨论。结果表明:1.粒度分布具有分形特征,以一重分形特征为主;2.一重分形特征表明土体各粒组的含量连续分布性较好;二重分形表明土体各粒组的含量连续分布相对较差,某一粒组含量存在突变;3.所研究的沟道平均粒度分维值Da在2.45～2.78间,且平均粒度分维值Da随粘粒含量Pc增加逐渐增大,具有一重分形特征的沟道二者满足关系式Da=0.108ln（Pc）+2.511。最后探讨了研究源区砾石土粒度分形特征的意义。

Check dam is one of the most effective structures to control debris flows.In Huashiban gully,control measures includes civil engineering works and biological works,where there are 13 check dams which make up cascade check dams,mainly distributing in the debris flow form-circulate region.The type of gravity dams are concrete structure,with both axial ends embedded in the bedrock for about 0.5～1 m.Based on the comparison of the differences on gradient of the ditch,stability of the slope on both sides and erosion rate of debris flow between pre and post construction of the check dams,this paper shows the application of check dams on blocking and storing loss solid mass,stabilizing the ditch bed and bank slope,decreasing gradient of the gully,lowering debris flow velocity and its kinetic energy,and uplifting the base level of debris flow erosion in upstream.According to our research,check dams in Huashiban gully block solid mass 2.76×104 m3,only 0.50% of the total material source(575.1×104 m3),and 3.3% of the short-term activity soil source(85×104 m3),which indicate blocking is not the main function.Check dams stabilize the bed 4.37×104 m3,accounting for 0.76% of the total material source,and 5.2% of the short-term activity content of debris flow.On the other hand,Check dams stabilize the slope 134.07×104 m3,accounting for 23.3% of the total material source,leading to the decreasing of debris flow source.Gradient of the deposition area has declined to 60%～75%.For debris flows at the same frequency,the velocity,peak flux and erosion rate goes down,sediment transportation reduces by 35%～57%,and erosion base level in the upper ditch was uplifted by 0.3～0.5 m.On the whole,debris flows are translating from viscous flows to turbulent flows,and then high sand content flows.

By the site investigation of the debris flow disaster Sanyanyu gully,the first-hand data of the disaster was obtained.The mud-rock flow velocity,flow,impact of Sanyanyu gully(Dayu gully and Xiaoyu gully) were analyzed and calculated based on these collected data.The calculation results show that mud-rock flow velocity and flow reach the scale of super debris flow,and the impact is much greater than protection engineering design of the gully.At last,the specific suggestions on disaster prevention are given based on these studies.

Two giant debris flows occurred in Bayi gully in Dujiangyan County,respectively on August 13 and 18,2010,affecting seriously local people＇s daily production and life.Based on field investigation data obtained from Bayi gully,the causes and the dynamic characteristics of the debris flows was analyzed,and a detailed risk assessment were carried out.The study results show that the loose material source,rainfall amount and topographic conditions of Bayi gully were contributed to the initialization of debris flows.The debris flows in Bayi gully were giant in magnitude and high in risk.The study provides a reliable basis for the debris flow prevention and mitigation in Bayi gully.

The authors obtained the related data from the Sanyanyu debris flow gully through field investigations of super-large debris flow hazards in Zhouqu Couty,Gansu Province.This paper analyzed and calculated the impact of the Sanyanyu gully(Dayu gully and Xiaoyu gully) based on the collected data(debris flow dynamics important parameters).These results of parameter calculation and parameter design of prevention projects have a certain reference function for parameter design of prevention projects and are of important significance for post-disaster reconstruction of a disaster area.It was found that the impact force of the circulation zone is greater than that of the accumulation zone,the impact force of massive stones in the accumulation zone shows a trend to increase first,and then decrease,and reflects monotone linear relationship with the diameter of massive stones.The massive stones in the accumulation zone are prismatic,pyramidic and rhombic in shape.At last,we made a brief assessment on the calculated results according to the Liziyida debris flow gully,and proposed a specific advice on disaster prevention based on these studies.