The Effect Of Check Dam On Erosion And Deposition Of Debris Flow Gully

Check dam is one of the important engineering measures to prevent debris flow.The influence of multiple debris flow accumulating sedimentation on the channel morphology after the construction of check dam is not clear;the check dam beyond the designed service life has the risk of burst,and the impact of the process of removing the check dam on the channel scour and deposition morphology has not been studied yet.How to start and transform the retained sediment after removing the dam,and How to calculate the amount of transformation under the action of multiple times debris flow needs to be determined through experimental research.Based on the field investigation,theoretical analysis and simulation test,this paper mainly studies the influence of the construction and demolition of check dams on the erosion and deposition of the upstream channel and the regulation effect of the check dam on the material energy of debris flow under the condition of full reservoir.The following research results are obtained:(1)Based on the study of the influence of the construction of check dams on the deposition of the upstream channel,the evolution model of gully deposition under the cumulative action of multiple debris flows is establishedIt is found that the diluted debris flow deposited in dam bank centrally,and the deposition evolution model of diluent debris flow in the upstream channel of the check dam is established.For the diluted debris flow,the deposition slope is negatively related to the dam height and positively related to the sediment discharge,and the density of debris flow mainly affects the deposition form,but has no significant effect on the deposition slope.For step check dams,the sediment stops in the upstream dam preferentially.With the increase of the amount of sediment discharge,the deposition slope of the downstream dam approaches or even exceeds that of the upstream dam.Debris flow deposited in upstream dam bank centrally,while deposited in downstream dam bank dispersedly.Based on the test result,a calculation method of the reduction coefficient of the deposition slope considering the height of the dam and the amount of sediment discharge is proposed.(2)Based on the study of the start-up law of sedimental body in dam bank after the removal of check dam,a calculation method of cumulative starting rate considering the removed height of the dam and the amount of sediment discharge is proposed.The start-up of sedimental body in dam bank is mainly affected by two kinds of erosion: Hydraulic Erosion and instability damage.The evolution of the shape of sedimental body in dam bank can be divided into four stages: overall retrogressive erosion,undercutting,trenching development and quasi equilibrium shape.the cumulative starting rate of the sedimental body is positively related to the removed height,and positively related to sediment discharge.The effect of debris flow density on the cumulative starting rate is not significant.Under the condition of step check dams,when the downstream dam is removed first,the sediment passing dam increases rapidly to 3-5 times of that before removed;when the upstream dam is removed first,the sediment passing dam of the sand dam remains at the level before removed due to the buffering effect of the downstream dam,and the sediment conveying is stable.According to the test result,a calculation method of cumulative starting rate considering the removed height of the dam and the amount of sediment discharge is proposed.(3)Based on the study of the regulation law of debris flow material and energy by check dam under the condition of full reservoir,a calculation method of sediment trap coefficient considering channel erosion and deposition is proposed.The energy dissipation effect of the check dam and sedimental body in dam bank on debris flow is mainly hydraulic energy dissipation,which can be further divided into: flip flow energy dissipation,bucket flow energy dissipation and underflow energy dissipation.Under the condition of full reservoir,the debris flow discharge reduction coefficient is negatively related to the height of dam,negatively related to the density of debris flow,and the impact of sediment discharge on the debris flow discharge reduction coefficient is not significant;the flow velocity reduction coefficient of debris flow is negatively related to the height of dam,and the impact of sediment discharge and the density of debris flow on the flow velocity reduction coefficient is not significant.The sediment trap coefficient is positively related to the height of check dam,and negatively related to sediment discharge.The effect of debris flow density on the sediment trap coefficient is not significant.The debris flow density reduction coefficient is negatively related to the height of check dam,and positively related to sediment discharge.The effect of debris flow density on the debris flow density reduction coefficient is not significant.According to the test result,a calculation method of sediment trap coefficient considering channel erosion and deposition is proposed.