挟沙水流对堰塞坝溃决过程的影响机制试验研究OA

Objective In situations involving mountain floods,intense rainfall-induced erosion,and the breaching of clusters of landslide dams that lead to up-stream soil and water loss,the inflow to landslide dams often becomes a sand-laden flow.Compared to clear water inflow,sand-laden flows produce more complex infiltration,erosion,and deposition processes,substantially influencing both the dam breaching process and the peak flow rates.This study investigates the mechanisms through which the sediment concentration and particle size of sand-laden flows influence dam breaching processes and examines the erosion-deposition dynamics and seepage-clogging phenomena induced by sand-laden flows during landslide dam breaching. Methods Five sets of model experiments were conducted using a flume measuring 5 m in length and 0.4 m in width.Clear water inflow served as the control group,while the effects of different volumetric concentrations of sand-laden flow(0.01 and 0.03)and various maximum sediment sizes(0.006 5,0.125 0,and 0.500 0 mm)on the dam breaching process were investigated.The primary method involved pre-experimental testing to ad-just the composition of the sand-laden water flow,supplemented by thorough mixing in the water supply tank and the flow stabilization tank to achieve a stable and target sand concentration in the dam body area.The design of the landslide dam model was based on the range of dimension-less parameters related to dam height,dam volume,and reservoir volume derived from a database of landslide dam cases,and the material selec-tion referenced empirical data from the 2008 Tangjiashan landslide dam.The experiments primarily used water level gauges to record the breach outflow process,while high-resolution cameras and grid systems monitored changes in the erosion rates of the landslide dams.Timed quantitative sampling methods tracked variations in sediment concentration.In addition,sampling,drying,and sieving techniques collected data on the mois-ture content and grain size distribution of the residual dam.The main steps of the experiment included material preparation,placement of the mov-able bed,layered construction of the dam body,execution of model tests,recording of experimental data,and subsequent sample analysis. Results and Discussions The failure mode of the landslide dams in this series of experiments was consistently classified as overtopping failure,and the breaching process was primarily characterized by layered erosion accompanied by slope instability.The breach evolution process was di-vided into two stages:the initiation stage and the development stage.During the breach initiation stage,headward erosion predominated,and the dam height showed little noticeable reduction.The breach development stage was characterized by a rapid decrease in dam height,and peak flow rates were reached early in this stage.The water level in front of the dam reached its peak at the end of the breach initiation stage,whereas the peak flow rate occurred early in the breach development stage,and a secondary peak developed due to upstream slope instability that partially blocked the breach.The concentration of sand-laden flow in the dam area was inversely correlated with the breach flow rate,whereas the concen-tration measured at the end of the flume area showed a positive correlation.The mean particle size of the residual dam decreased longitudinally within the dam area and remained relatively stable in the downstream movable bed area.Comparative tests of sand-laden flows with different volumetric concentrations(Tests 1～3)showed that as the initial volumetric sand concentration increased from 0 to 0.03,the peak breach flow rate increased from 3.2 to 3.6 L/s,the time to peak increased from 76 to 90 s,the maximum water level in front of the dam increased from 24.6 to 26.9 cm,the residual dam height increased from 6.5 to 8.2 cm,and the final average breach width increased from 18.8 to 20.6 cm.The analysis indi-cated that higher sediment concentrations in the flow enhanced the erosion capacity during the breach development stage;however,the deposi-tional layer formed during the process influenced the downward erosion rate during both the breach initiation stage and the late breach develop-ment stage.Comparative tests of sand-laden flows with different maximum particle sizes showed that as the maximum particle size increased from 0.006 5 to 0.500 0 mm,the peak breach flow rate slightly increased from 3.3 to 3.4 L/s,the time to peak increased from 79 to 90 s,and the maximum water level in front of the dam increased from 26.0 to 26.5 cm.The residual dam height remained relatively constant at approximately 7.5 cm,and the final average breach width also showed minimal variation at approximately 19.5 cm.The analysis indicated that the particle size of the sand-laden flow had minimal influence on the erosion rate of the landslide dam,mainly because the effect of particle size on the viscosity of the flow became negligible when particle sizes exceeded 0.1 mm.The comparative analysis of clear water inflow(Test 1)and sand-laden flows(Tests 2～5)showed that during the breach initiation stage and the late stage of breach development,low-velocity and high-concentration sand-laden flows generated strong depositional effects.These effects caused a slight reduction in erosion rates,an increase in the maximum water level in front of the dam,and a greater residual dam height.In contrast,during the early to middle stages of breach development,high-velocity sand-laden flows demonstrated greater erosive capacity and reduced deposition,which was reflected in increased erosion rates and higher peak outflow.In addition,sand-laden flows during the dam breaching process produced a pronounced clogging effect,which resulted in a multilayered structure within the dam body composed of depositional layers,retention layers,and original layers.The dense structure of the retention layer reduced the permeability of the sand-laden flow,lowered the position of the saturation zone on the breach slopes,and increased the scale of slope failures. Conclusions The experimental results indicate that,compared to clear water,the viscosity coefficient of sand-laden flow is higher,which en-hances its erosive capability and also intensifies deposition.As the concentration increases,the viscosity of the sand-laden flow increases mark-edly,resulting in higher erosion rates,larger breach sizes,and greater peak outflow rates.As the particle size increases,the viscosity coefficient of the sand-laden flow exhibits negligible variation,leading to only minor changes in erosion rates and peak outflow rates.The seepage-clogging ef-fect refers to the accumulation of sand particles within the surface layer of the dam body during the infiltration of sand-laden water flow,accom-panied by the retention of these particles inside the dam.This retention layer decreases the infiltration rate,which lowers the saturated zone along the side slope of the breach.This change results in an increased scale of collapses on the side slope.