Dynamic Effect Of Debris Flow Grain Size Distribution

Debris flow is a special event of a large number of high concentration viscous water-debris mixed flow across the channel.The characteristics of grain composition are not only related to the gradation of the source soil,but also affected by the movement process and sedimentary environment.The distribution characteristics of grains carried by debris flow reflect the mode of sediment movement and dynamic conditions,and grain size analysis is one of the most basic methods to study sediment deposition.However,related studies show that the characteristics of grain size distribution(GSD)is not only an important index to reflect the sedimentary environment,but also can well reflect the dynamic conditions of debris flow.When the topographic conditions change little,the movement characteristics and sediment accumulation characteristics of debris flow are largely determined by the composition of grains,the effects of initial topographic factors(such as alluvial fan slope,channel slope and width,etc.)and volume concentration can be ignored.The typical debris flow has the characteristic of wide gradation,which shows that it is a special fluid which is different from the general hyperconcentrated flow.In order to study the influence of grain composition on debris flow,the typical characteristics of grain composition must be fully understood.Therefore,it is necessary to find more accurate comprehensive parameters to describe the distribution characteristics.However,limited to the development of existing theories and experimental techniques,the granular effect in the dynamic process of debris flow is still not fully understood,and the influence of grain composition has not been well considered.In this paper,based on a large number of field monitoring data of debris flow,combined with the scale distribution formula of grain size,the movement characteristics of surge and the dynamic characteristics of grains are analyzed.Taking the natural debris flow slurry as the research object,the rheological properties of natural debris flow slurry with different GSD were tested by using the ball measurement system of rotary rheometer.Dynamic rotation experiments were carried out to study the dynamic evolution of grain composition and its effect on mobility.In laboratory experiments,combined with theoretical analysis,the influence of grain configuration on pore water pressure and the positive effect on debris flow acceleration are obtained.Finally,some new methods for evaluating the properties of debris flow are obtained by integrating the data.The main results of this paper are as follows:(1)The GSD characteristics of debris flow are found outThe characteristics of soil grain composition determine its strength characteristics,the dynamic change of coarse and fine grains leads to the fluctuation of soil strength,and the non-uniformity of grain distribution leads to the randomness of soil failure,which is directly related to debris recharge.When the recharge is mainly composed of coarse grains(large D_c),it has a strong erosion transport capacity.Most of the debris flow is mainly a stream flow at the beginning,and the debris flow with high solid volume concentration is formed through the process of slope collapse recharge and entrainment along the event path.The flow regime of debris flow varies with the change of grain composition,and the separation and adjustment of grains will occur due to the difference of velocity in debris flow.From a large number of surge monitoring data in the main gully,it can be found that the fluctuation relationship between debris flow density and the upper bound of dynamic characteristic parameters such as flow depth,velocity and flow discharge satisfies the power function relationship,which means that the surge with a certain density has a certain parameter limit and increases with the increase of density.The superposition of grains is essentially caused by the recharge of source and the superimposed of surges,and this process can be well reflected by the parameter of GSD(?,D_c).There is a good power function relationship between different grain sizes of debris flow:P(D_i)=KP(D_j)ⁿ,on the one hand,it shows that the grain gradation is relatively good,and continuous gradation,on the other hand,it also points out that any grain may affect the properties of the whole fluid.(2)The relationship between GSD parameters and rheological parameters was establishedThe rheological properties of slurry with different upper limit grain size of different prototype debris flow are significantly different,and the slurry samples with the same solid volume concentration will show different rheological properties due to different GSD.The rheological behavior of debris flow usually satisfies the Herschel-Bulkley model,its consistency coefficient varies randomly,and the power index(HB index)is a constant of about 0.45.Only at high shear rates,the behavior of the fluid is similar to that of the Bingham material.Both yield stress and effective viscosity are closely related to GSD parameters and can be accurately calculated by combining the formula of granular effect expressed by?(power law)and D_c(exponential function).This means that there is an interlocking relationship between fine and coarse grains in debris flow.The debris flow material shows obvious shear thinning characteristics,and the power law form of apparent viscosity which satisfies the shear rate:?_a=p?^-q,p changes significantly with the change of experimental group and upper limit grain size,but q changes little,the average value is 0.94 and the standard deviation is 0.026.This means that q is not sensitive to coarse grains,so it is a characteristic index which only depends on the content of fine grains in the slurry.These results are helpful to estimate the rheological properties of real flow and establish dynamic equations.(3)The process of grain adjustment under dynamic conditions and the mechanism of its influence on fluid properties are revealedUnder the action of dynamics,the grain composition of debris flow will undergo a certain evolution process,so that it will be alienated into a special fluid different from source soil and accumulation.The GSD parameters between the source soil and the deposition(?,D_c)show a disordered random state,while the fluid shows a power function relationship.When a certain dynamic action is applied to the grain composition and reaches a certain time,the relationship between?and D_c gradually changes with time and tends to be stable,while the fluid shows good mobility,and the specific surface area and deposit morphology tend to be stable.In addition,the separation and adjustment of grains in the process of movement,especially the separation and accumulation on the surface of debris flow,on the one hand,will produce different forms of load on the lower fluid,on the other hand,it will increase the relative fine grain content of the lower fluid.This will also promote the generation and maintenance of excess pore water pressure,and then enhance the mobility of debris flow.(4)The estimation method of related dynamic characteristic parameters is established.Based on the scale distribution formula of grain size,the empirical formulas of density and velocity are established.In addition,the dynamic parameters of debris flow have a certain upper bound value restricted by grain composition,and the relationship between this value and dynamic parameters such as density and velocity satisfies the power function relationship.based on this relationship,the possible magnitude and movement characteristics of debris flow can be judged.According to the statistical range of GSD parameters of flows with different properties,a debris flow disaster assessment and prediction method based on GSD parameters can also be provided.