Effects Of Lateral Confinement On The Propagation And Depositional Characteristics Of Granular Flow

Based on the statistics and classification of many rock avalanches,the physical models of rock avalanches with different lateral confinement were simplified for the establishment of granular flow experiments.Three types of physical modeling experiments with full-lateral confinement,half-lateral confinement and non-lateral confinement were designed and conducted to study the propagation,deposit morphologies and internal structures including inverse grading and retention of initial stratigraphy of the granular flow.High-speed photography and tilt photogrammetry methods are conducted to obtain the data of granular flow emplacement and deposit morphologies.And quantitative data of typical deposit structures are obtained by incision and sampling of the granular flow deposits.The obtained data were then processed through particle image velocimetry(PIV),reconstruction of 3D models of the deposit,by which the emplacement and deposit features of the granular flow under different confinement were investigated and compared systematically.The main conclusions are as follows:(1)According to the lateral confinement of topography,rock avalanches can be divided into three types,including valley depositional type,valley-plane depositional type and planar spreading type.Statistic shows that the flow mobility of the valley depositional type and valley-plane depositional type are higher than those of planar spreading type.According to the orthogonal analysis,the variances of the two variables,volume and fall height,are 4.35×10~7and 8.01×10~7 respectively,indicating that the impact of the fall height on the distance of rock avalanche movement is greater than the volume.(2)Under all experimental conditions,the velocities of the leading edge of the granular flow can be divided into four stages:an acceleration stage,a deceleration stage due to the impact with the hinged point,an increasing stage caused by momentum transfer,and depositional stage.Under all confinement conditions,the velocities of the leading edge increase with the increase of grain size.For granular flow with different lateral confinement,the leading-edge velocities of the full-lateral and half-lateral confinement is significantly higher than that of the non-lateral confinement granular flows.The displacement cloud images and transverse velocities of the granular flows show that the lateral spreading of the granular flow is the most significant under the non-lateral confinement condition,followed by the half-lateral confinement condition,while the transverse velocity tends to 0 and no lateral spreading developed for granular flow with full lateral confinement.(3)Experimental results show that there are significant differences in the morphology of the granular flow deposits at different lateral confinements.Under the full-lateral confinement condition,the granular flow deposits are banding shape along the longitudinal direction of the flume,while under the half-lateral confinement condition,the granular flow deposits are"elliptical"or"bull-head"shapes,and under the non-lateral confinement condition,the deposit are divergent and fan-shaped.The flow-like morphologies of the deposits under all lateral confinements are most prominent for granular flow with finer grain sizes.Under the full-lateral confinement condition,the flow-like morphologies are mainly characterized by the transverse ridges,lateral levees and tongue shaped lobes.In addition to transverse ridges,the typical flow-like morphologies such as hummocks and longitudinal ridges,are also developed in the half and non-lateral confinement granular flow deposits.(4)The characteristic parameters of grain size segregation and retention of stratigraphy are obtained based on the results of sampling-sieving and sampling-counting methods,respectively.The results show that the degrees of inverse grading and retention of stratigraphy show opposite variation trends in the deposits.For the same granular flow deposit,the degree of the retention of stratigraphy in the middle part of the deposit is the highest,while the degrees of grain size segregation is the lowest.With the weakening of lateral confinement,the segregation process is strengthened,and the degrees of the retention of stratigraphy tend to decrease.(5)Through the statistical analysis of the motion and deposition parameters of the granular flow under different lateral confinements,it is found that the effects of lateral confinement on the propagation and deposition of the granular flow is significant,and confines the range of the granular flow to a certain extent.When other conditions are the same,the area of the deposits with non-lateral confinement is the largest,showing the largest impacting ranges,followed by half-lateral and full-lateral confinement.However,from the perspective of flow velocity and impacting distance,the influence of granular flows under half lateral confinement and full lateral confinement is greater than that of the granular flow with non-lateral confinement.