泥石流局部阻塞大河的特点及判据研究

刘翠容，杜 翠

(西南交通大学土木学院，四川成都610031)

0 Introduction

After the Wenchuan earthquake，debris flow disaster has increased significantly.According to the latest statistics，released in earthquakes hit debris flow gully in 967，a total of over more than 1，000 debris flow occurred.In the rainy season，debris flow became one of the most important disaster types and has a greatest influence on the earthquake disaster area.Due to the small debris flow critical rainfall has started after the earthquake，large scale，high speed，bulk density，carrying large of damming rivers by debris flow was increasing.As boulders and other new features，disasters event relating blast debris blocking Minjiang River is shown in Fig.1.

Fig.1 Map of debris flow blocking the upper reaches of Minjiang River图1 映秀镇上游岷江河谷段群发性泥石流航拍图

At present，the domestic scholar primarily focus on events of completely blocking of the main river by debris flow［1-6］.He Yiping，Chen Deming，and Dang Chao studied the impact factors by using flume experiment and create the criteria of completely blocking the river by debris flow［1-4］.These results based on damming streams by debris flow，which are critical phenomenon studies based on the energy balance are not suitable for the case of damming large river(partly and completely).The disasters caused by damming the large river have increased.Therefore，how to establish universal criteria for damming large river(partly and completely)is a new scientific problem.In the Minjiang River，most of events are partial blocking of the main river as barriers dam.Using flume experiments，studies on the law of the big river blocking by debris flow is conducted，and the criterion of big river blocking by debris flow is presented in terms of dynamic process and total amount of debris flow.

1 Experimental studies on partial blocking of rivers by debris flow

This test is mainly focused on local blocking of the main river by debris flow in the Minjiang River basin.By changing the amount volume of debris flow，different degrees of debris flows blocking the main river course is reproduced which reveal that variation of flow and role of flow on the debris flow dam，barrage of debris process to push the other side of the body.

1.1 Experimental overview

Model test is carried out in the Hydraulics Laboratory Hall of Southwest Jiaotong University.Laboratory equipment includes water recycling systems，tank systems and measuring systems.Water recycling system consists of underground reservoirs，pumps，V-notch weir，flume，desilting basin and drains.The features of main channel are as follows:2 m for width，15 m for height，0.75 m for total depth，5‰ for longitudinal slope.The features of main channel are as follows:debris-flow groove 0.33 m for wide，0.65 m for deep，4.8 m for long.The slope is adjustable to simulate the flow of debris flows.The second part of the groove is hopper;their export is attached to the first part and set a bayonet，the capacity is approximately 1.50 m3.Test apparatus is shown in Fig.2 and Fig.3.

Fig.2 Debris-flow groove图2 泥石流支槽

Fig.3 Experimental device图3 试验主水槽

1.2 Experimental procedure

There were lots of researches on the experiment of debris flow abroad［4-6］.But debris flow model test of similar theory is in developing stage.At present，sediment-flow of similar theory applies and follows Froude similarity criteria.In the analysis of Minjiang River basin，width of undistorted model is selected as 100 m of river，geometric scale is 50 and velocity scale is 7.According to 2 metre-wide test sinks of geometrically similar，and based on actual samples of debris flows in the Minjiang River basin，geometric scale is 50，maximum particles is 26.5 mm，sediment size gradation of debris flow is shown in Fig.4.

Fig.4 Sediment size gradation of debris flow图4 试验泥石流颗粒级配曲线

This test on partial blocking of main river by debris flows as a prototype is carried out，the intersection angle is 90°，unit weight of debris flow is 2.2 t/m3.In the course of the experiment，the depth of main river is 19.5 cm，the discharge of main river is 152 L/s，the river bed slope keeps constantly，changing the amount of debris flow to reveal the characteristics and laws of a barrage dam of debris flow in the process of pushing the other side of the body.Experimental data are presented in Table 1.

Table 1 Experimental data表1 试验数据

1.3 Result and analysis

Table 1 presents the analysis of video information and experimental data.It indicates that in the process of the barrage dam of debris flow to push the other side of the body，with the damming coefficient R increases，rivers flow and role of the main river flow in the dam of debris flow show non-linear change.The analysis is as follows:

1)The change in velocity of main rivers is nonlinear.When barrage/barrier dam is formed by debris flow，as the damming coefficient R increases，the main river is slowly narrowed and flow cross section area is gradually reduced.This causes velocity of main river Vmainto increase at first and then decrease，as shown in Fig.5.When R is less than 0.85，the velocity increases，when R equal to 0.85，the velocity comes up to maximum，and when R is more than 0.85 and less than 1，the velocity decreases rapidly.

2)The rate of loss of material in barrage dam is non-linear.According to the analysis of experimental data，extent of the blocking main river by debris flow is closely related to total of the debris flows，the more total of the debris flows have，the more extent of the blocking main river is.Blocking coefficient increases as the total of the debris flows increases.At the same time，in the process of the barrage dam of debris flow to push the other side of the body，as the blocking coefficients R increases，the lost material of debris flow is non-linear change，as shown in Fig.6.When R is less than 0.8，the lost material M is changes slowly with blocking coefficients，the barrage dam remains previous characteristics of piled wastefull stacked fans.When R is more than and equal to 0.8，the rate of loss of material is large，changes quickly with damming coefficients，characteristics of piled waste is been severely damaged.

Fig.5 Experimental measuring velocity change with block coefficients R curve图5 实验实测速度随堵塞系数R变化曲线

Fig.6 Drain-clogging factors of barriers dam change with block coefficients R curve图6 堰塞坝流失量随堵塞系数变化曲线

2 Criterion based on the interaction between barriers dam of debris flow and main river

2.1 Interaction mechanism between barriers dam of debris flow and main river

By the results of the model experimental test，it is found that in the process of the barrage dam of debris flow pushing the other side of the body as the debris flow moves across the river forming a barrier dam，the main river is slowly narrowed.During the process，flow structure of the main river becomes complex and flow velocity change.At the same time，with increase in blocking coefficient，main river flow on the barriers dam’s role also changes.When R is less than 0.8，there is large increase in velocity but the flow turbulence intensity is not too big.Eroding force of water flow against the weir dam is less than the resistance of solids thrust，debris flow head is the primary water flow’s resistance effects，flow resistance effects of debris flow migration distance in the main river，thus blocking the river level.At this point，the vortex caused by the dam itself responsible scour of barriers dam downstream on the head，but the scouring weaker，barriers dam complete，still remain at a constant accumulation characteristics.When R is more than 0.8，single-wide flow and velocity of the main river maximizes，reaches threshold velocity of some solid particles，at this point，water flow’s scouring of barriers dam is dominated，and resentencing，the major impact on blocking of the river to scour barriers dam.Large amounts of particulate matter are taken away，due to the high speed jet on barriers dam head strong erosion，high speed jet on weir dam strong scouring of the head，taking large amounts of particulate matter，barriers dam of debris flows is severely damaged，the losing dramatically increases.

2.2 Criterion of local blocking the main river as a barriers dam

The critical conditions for dam-forming，mainly include discharge ratio between tributary and mainstream，the confluent angles，total amount of debris flow，width of mainstream，resistance of debris flow against erosion and the slope of mainstream.Debris flow meeting the main dam of the river damming will be formed after blocking the main river.It must meet the following criteria，that is mudslides with enough energy in the main river to promote migration;debris of body length and height increase enough to offset the rise in water levels in the upper reaches of the river，on a request in the amount of debris flows;debris flow and main river intersection angle is too small，so most of the debris flow movement along the main river downstream，a small part of confluence of debris flow deposition cannot form blocking the river.Right-angle intersection is discussed.Weir dam blocking the main river of the following conditions is discussed.

First of all，based on the dynamic equation of the debris flow at different slopes by T.Takahashi［7］，a model of the run-out distance of debris flow in big river is established:

where kais the coefficient of active earth pressure，ka=tan2(45°－φ/2)，θuis the slope angle at valley mouth，θ is the slope angle of river，g is gravitational acceleration，γwis the unit weight of water(kN/m3)，CDis the resistance coefficient，which is related to Reand shape of subjects(0.5≤CD≤2.0)，n is the roughness parameter，γsis the unit weight of debris flow(kN/m3)，cduis the volume concentration ratio of debris flow，huis the mud high of through debris flow(m)，vuis the velocity of debris flow at valley mouth(m/s).

L is calculated and compared with main river width B by Equation(1).If block coefficients R=L/B ＜0.8，it indicated by foregoing analysis that at this point，the losses can be neglected.Total amount of debris flow into the main river V is at need total amount of debris flow V1for blocking main river by barriers dam.The formula proposed by Zhou Bifan for calculating the total amount of debris flow into the main river is as follows［8］.

where L is the run-out distance of the debris flow，φsis the friction angle of debris flow，Hwis the depth of main river。

If block coefficients R=L/B ＞0.8，it indicated by foregoing analysis that at this point，total amount of debris flow into the main river V must includes the losses，total amount of debris flow into the main river V should include that need total amount of debris flow V1for blocking main river by barriers dam and the losses of barriers dam V2，V=V1+V2.

Loss coefficient is defined as the ratio between the total amount of debris flow into the main river and the total amount of barriers dam of debris flow，that is r=(V1+V2)/V1.The loss coefficient can be used to quantitative description of the scouring effect of barriers dam.It is affected by factors such as velocity of main river v，water-level difference between upstream and downstream，depth of main river，physical characteristics of the debris flow(γs，φs，d50)and flux of debris flow.

Blocking rivers of debris flow would have to meet condition for total amount of solid matter:

According to the Equation(6)，determining the loss coefficient is the key to determine the total amount of debris flow.

It is found that damming main river by barriers dam become serious and the lost solid material increases with the volume of debris flow increasing.When R is more than 0.8，the losses of total amount should be included in calculating the total amount of debris flow.Based on three experimental data of large amount of loss，the loss coefficient is calculated as shown in Table 2.

Table 2 Model experimental data表2 水槽模型试验数据

Based on the Table 2，the loss coefficient is connected with the damming coefficient，when R=L/B＞0.8，the loss coefficient is equal to 1.25.

In conclusion，blocking main rivers by debris flow would have to meet two conditions for migration distance and total amount of solid matter.The criterion of local blocking the main river as a barriers dam is presented as follows:

where L is calculated by Equation(1)，(2)，(3).

3 Variation

July，10/2013，Niumian gully debris flow located at Yingxiu，the length，width and height of debris flow fans are 130，60 and 7 m，the volume is 5×105m3，and the damming coefficient is 2/3.The friction angle of debris flow is equal to 40°.The loss coefficient is equal to 1.Substituting above data into Equation(7)，then，

In summary，the result is less than the total volume of the Niumian gully debris flow，and then the E-quation(7)is available.

4 Conclusion

1)The model experiment results indicate that，the more total of the debris flows have，the more damming coefficient is;as the blocking coefficients R increases，the velocity of debris flow and the lost material of debris flow are non-linear change.

2)Applies patterns of criterion for local blocking the main river as a barriers dam are that:first，based on the Equation(1)，(2)and(3)，debris flow’s migration distance in main river is calculated，and it is divided by the wide of river to get damming coefficient.Then according to the damming coefficient，the calculation equation is selected to determine the total amount of debris flow of material required blocking the main river.For certain a debris flow，when the migration distance and total material meets requirements，the patterns of big river blocking by debris flow is presented.

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