Critera Of Blocking Large River By Debris Flow And Diaster Reduction Countermeasures In Highways Along Rivers

With global climate change anomalies and growing human social and economic activities,extreme weather and geological events increase cont-inually.In particular,debris flow is very active in the four years after the Wenchuan earthquake,and very large scale debris flow breaks out several times.The practice in flood season shows that debris flow is one of the most active disaster species, and debris flow blocking the river has become a frequent disaster.This paper faces the grim situation of large debris flow taking place constantly in China in the extreme excitation conditions and the state of disaster forming environment changing rapidly,faces the problem of debris flow disaster prevention and mitigation in the mountainous area con-struction.Based on the background of debris flow blocking river at many points in the Min Jiang River from Yingxiu to Wenchuan and Zhouqu、Qing-ping large debris flows, the thesis analyzes the mechanism of debris blocking the river and disaster chain effect, by the methods of field survey, theoretical analysis, model test and numerical simulation, to provide a scientific basis for the town and major infrastructure to defense the large-scale disasters caused by debris flow blocking river.Mainly for the following work and res-earch results have been achieved.(1) This paper studies the typical river-blocked case based on field surveys and literature data collection.The features of the river-blocked by debris flow are proposed.There are three river-blocked types:total blocking, obstruct blocking, and local blocking.Obstruct the potential dam blocking occurs mainly in high-velocity turbulent debris flow, or smaller bulk density viscous debris flow, whose characteristics are debris flow always move under the water, silts the riverbed, changes the river profile, and causes flooding disasters in the form of upstream backwater. Causing the river blockage in the form of barrier dam occurs mainly in viscous (or high concentration) debris flow.The main characteristics of partial blockage in the general situation are that barrier dam makes the flow become narrow, changes the channel, and affects the flood discharge capacity.But when block-up coefficient is greater than 0.8, the debris flow push the water flow, which can cause the different bank erosion.Debris flow total blocking event is not common,but once occ-urred,barrier lake will cause upstream flood disaster,dam-break will cause downstream extraordinary flood, even make the main river diversion in the wide valley segment which causes the serious disaster in a bigger range.(2) In the Minjiang River, most of events are partially damming the main river as a barrier dam. Using flume experiments, reappears blocking larger-iver by debris flow with high density viscous debris flow when intersection angle is 90 degrees. Reveals the characteristics of the process of debris flow into the main rivers and the mechanism of blocking the main river. Focus on the features of partially blocking the main river as a barriers dam, The exp-erimental results shows that the thrust force of the follow-up debris flow is a main dynamic force driving the debris flow in the main river; Total volume of debris flows has important effects on damming large river, which become serious with the volume of debris flow increasing; In the process of debris flow blocking river, when damming coefficient R increasing, velocity of main river firstly increases and then decreases, when R less than 0.8, the lost material of debris flow is very little, head-water level is increasing linearly; When R more than or equal to 0.8, the lost material of debris flow become serious, head-water level is increasing exponentially.(3) Based on the dynamic equation of the debris flow at different slo-pes,water and sediment dynamic processes,and the interaction between ba-rriers dam of debris flow and main river were considered,and a model of the runout distance of debris flow in big river was established.In view of the phenomenon that the material erosion is very serious when debris flowing into big river,the erosion coefficient is obtaineding using the test dates of flume experiment.Based on the total amount equation of debris flow was blocking given by Bifan ZHOU,a modal of the total amount of the big river blocked by debris flow was considered in hight of the erosion coefficient. The criterion of debris flow blocking big river waspresented in terms of dynamic process and total amount of debris flow.(4) Based on the importance of the blocking coefficient R for forecast-ting hazard and the model test data, the expressions of blocking coefficient R is derived, which can be used to estimate the degree of debris flow barrier dam blocking the river, and also as another criterion of partial blocking river by debris flow.(5) According to the characters of the submerged disk, the equation of the head-water level is given by the broad-crested weir theory.the equation for describing the movement of debris flow head is derived based on density current theory; Carries out univariate analysis on the affecting factors of the movement of debris flow head, the analysis show that the debris flow visc-osity and shear stress have no significant effect on the velocity of the debris flow head, and the coefficient of circle resistance is the greatest;The relat-ionship is established between the runout distance of debris flow head in big large river and time, on this basis, the formula of the minimum amount of solids material is given when submerged dam blocks the river.(6) According to the disaster mode of debris flow barrier dam blocking the river.The flow field of the main river with different block coefficients was analyzed by the numerical simulation method according to this disaster type. The order degree of the main river was studied based on information entropy principle. When the block coefficient R was in the range of 0.8 to 0.9, the entropy of the main river decreased rapidly, and the velocity of the water flow was fast. The block coefficient, R>0.80, was taken as the criterion of the local blocking of the river by the debris flow.Verified by the river-blocked event caused by the debris flow along Minjiang River between Yinxing to Yingxiu section, the results of numerical simulation compared with the field data well. The criterion of the disaster at the opposite bank due to river-blocked caused by the debris flow is validated. Then proposed block diagram of the criterion and flows debris effect.The equation of calculate the value of head-water level is given, which propose a method for calculate the value of scopes of back up water level.(7)For disaster effects of different modes of blocking the river,approp-riate mitigation countermeasures are proposed. For total blocking mode,ele-vating the line elevation in submerged area should be first considered. If subjected to other conditions it cannot be implemented, consider limiting the flow rate and total amount of debris flow into the main river, in order to reduce or eliminate debris flows blocking rivers. According to the partly damming main river, the main disaster is erosion the different bank, the main engineering measures to protect the bank are diversion project and support structure and so on, to the obstruct blocking just consider raising the line elevation.