Experimental Study On Blocking Effect Of Debris Flow At Bayonet Segments Microgeomorphology

Blocking effect is a common phenomenon in the process of debris flow movement.When the flow of debris flow is not enough to overcome its resistance movement,the blocking effect of debris flow is produced only by the accumulation of flow to overcome the resistance.The confluence of main and branch gullies,the abrupt change of terrain in the gullies and the collapse and slide accumulation in the gullies can cause the debris flow to block and collapse locally,and its macroscopic manifestation is the short-term cutoff and matrix flow in the gullies.The natural debris flow gully is a series of bends and straight sections with the same width and width.Especially under the geological conditions of strong tectonic activity and soft and hard lithology,it is more conducive to the formation of the micro landform of bayonet.The combination of wide and narrow bayonet topographic features is more likely to cause local blockage and collapse of debris flow,which will produce the amplification effect of debris flow and cause more serious harm.Debris flow blocking coefficient is a characteristic parameter that represents the blocking effect of debris flow,and is also a key parameter for calculating debris flow with the matching method in the code for investigation and design of debris flow prevention and control engineering.In the current specifications,the number of bayonets is often used as the basis for judging the degree of debris flow blockage,which is obviously empirical.The influence of the nature of debris flow,the movement characteristics of debris flow and the micro geomorphology of bayonets on the bayonet blockage effect and blockage coefficient is not considered,resulting in greater uncertainty in the calculation of debris flow,thus affecting the effect and operation safety of debris flow prevention and control projects.（1）This paper attempts to explore the critical conditions of debris flow blockage and collapse under different bayonet topography and debris flow characteristics through model tests,and analyzes the amplification effect of debris flow in bayonet segments.It has important theoretical significance and application prospects for improving the technical regulations of debris flow disaster prevention and mitigation and improving the understanding of debris flow movement and accumulation process.The main contents and results of the study are as follows:（2）Through the field investigation of typical bayonets in Huatoujian gully,it is found that the genetic type of bayonets is closely related to geological structure,formation lithology and landslide mass;The form type of the bayonet is mainly limited by the geological conditions of the basin,and there are obvious differences in the formation and evolution of debris flow in different regions.The overall performance is that the section index of the bayonet decreases from top to bottom,and the band narrowing and widening degree increase.V-shaped and trapezoidal bayonets occur in the longitudinal valley structural section,while U-shaped bayonets occur in the transverse valley section;Accumulation type bayonet is an important type of V-type bayonet formation in longitudinal valley,which often evolves into trapezoidal bayonet under the influence of subsequent flow or debris flow.（3）Taking debris flow in Huatoujian Gully as the prototype,the paper analyzes the main influencing factors of the blocking effect of debris flow at the bayonet,determines the reasonable test similarity scale,puts forward the test model,the preparation of model sand and the production method of the generalized model of the bayonet,and puts forward the standard test process and parameter measurement method.（4）When there is no additional loose material source mixing near the bayonet,the flow attenuation occurs,and the flow ratio Q_pr<1,the peak flow of debris flow declines;For the accumulation type bayonet,due to the mixing of additional deposits,the volume ratio concentration of debris flow and the total amount of debris flow at one time increased.In the three tests,there were two flow amplification effects,that is,the flow ratio Q_pr>1.（5）When the debris flow flows through the bayonet segments,if there is no supply of loose material sources in the gully,the flow attenuation overflow will occur,the peak flow of the debris flow will be delayed,and the flood peak hydrograph will be flattened;When the debris flow passes through the bayonet segments,the debris flow ratio Q_pr of bayonet segments can be expressed as:（?）（6）When the debris flow flows through the bayonet segments,it shows an obvious phenomenon of silting upstream and scouring downstream,accompanied by the coarsening and screening of the bed surface.Moreover,the scale of sediment,bed coarsening and sieving degree in the upstream of the bayonet segments are closely related to the geometric shape of the bayonet,the texture of debris flow and the longitudinal slope of the gully.