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Experimental Study on the Amplification Effect of Residual Layers on the Destructiveness of Surge-type Debris Flows
LIU Yun-hui, SONG Dong-ri, FENG Lei, LIU Jia
Journal of Changjiang River Scientific Research Institute ›› 2026, Vol. 43 ›› Issue (4) : 148-157.
PDF(6752 KB)
PDF(6752 KB)
Experimental Study on the Amplification Effect of Residual Layers on the Destructiveness of Surge-type Debris Flows
[Objective] To investigate the drag reduction effect of residual layers in periodic debris flows, we quantitatively revealed the amplification effect of the impact of subsequent waves due to the presence of residual layers. We propose evaluation indicators centered on the impact force ratio F* (amplification of subsequent wave impact relative to the first wave) and the momentum ratio R (amplification of total momentum flux relative to the head momentum flux), aiming to elucidate the dynamic mechanisms responsible for the increased destructiveness of subsequent waves. [Methods] Using a mesoscale flume, we examined debris flows with three different solid contents (40%, 50%, and 60%). Sensors measured flow height, normal stress, shear stress, and pore water pressure. Considering that the movement of subsequent waves over residual layers can be approximated as quasi-steady hydraulic jumps, we calculated unit width impact forces from post-jump velocities and water depths based on momentum conservation. Dimensionless indicators F* and R were constructed to quantify the amplification effects of multi-wave impacts and intra-wave momentum due to residual layers. [Results] (1) Higher solid content resulted in thicker residual layers, which tended toward a quasi-equilibrium state of erosion and deposition under multiple wave actions. (2) The presence of residual layers altered the flow regime of subsequent waves, with the initial wave typically exhibiting the highest Froude number (Fr). Subsequent waves showed an overall decrease in Fr, indicating a shift from inertia-dominated to gravity-dominated flow control. Increased solid content significantly reduced liquefaction and mobility, as indicated by decreased liquefaction degree λ with increasing solid content, suggesting enhanced effective stress and reduced fluidity. This change influenced the interaction intensity between subsequent waves and residual layers. (3) Residual layers significantly amplified the impact forces of subsequent waves, showing an “increase then stabilize” trend. Under all solid contents tested, impact forces generally exhibited a “rapid increase followed by stabilization”, consistent with the thickening and stabilization process of residual layers. Impact force ratios F* were greater than 1 for subsequent waves, indicating amplified peak impact forces under identical release conditions. (4) Momentum analysis revealed that R stabilized in later waves, aligning with the trend of F*. As R>1 indicated total momentum flux exceeding head momentum flux, the share of momentum carried by thicker residual layers drove stronger impacts of subsequent waves, especially pronounced under higher solid contents and thicker residual layers. [Conclusion] (1) In periodic debris flows, the formation and stabilization of residual layers constitute the primary processes leading to enhanced destructiveness of subsequent waves. Even if release conditions are identical for each wave, significant increases in impact loads can occur due to residual layer influences. (2) The indicator system F* (for comparing external manifestations across waves) and R (characterizing internal momentum distribution within a single debris flow wave) provides a concise assessment framework with clear physical meanings: residual layers amplify impacts by contributing “hidden momentum”, thereby influencing destructiveness amplification. (3) Engineering practices focusing solely on the first wave’s impact for protective structure verification may underestimate the destructiveness amplification effects of multi-wave events. It is recommended to consider the influence of residual layers in designing check dam scales, dam heights, and safety factors.
debris flow / residual layer / flow regime / destructiveness / impact / dynamic momentum / amplification effect
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Recent advances in theory and experimentation motivate a thorough reassessment of the physics of debris flows. Analyses of flows of dry, granular solids and solid‐fluid mixtures provide a foundation for a comprehensive debris flow theory, and experiments provide data that reveal the strengths and limitations of theoretical models. Both debris flow materials and dry granular materials can sustain shear stresses while remaining static; both can deform in a slow, tranquil mode characterized by enduring, frictional grain contacts; and both can flow in a more rapid, agitated mode characterized by brief, inelastic grain collisions. In debris flows, however, pore fluid that is highly viscous and nearly incompressible, composed of water with suspended silt and clay, can strongly mediate intergranular friction and collisions. Grain friction, grain collisions, and viscous fluid flow may transfer significant momentum simultaneously. Both the vibrational kinetic energy of solid grains (measured by a quantity termed the granular temperature) and the pressure of the intervening pore fluid facilitate motion of grains past one another, thereby enhancing debris flow mobility. Granular temperature arises from conversion of flow translational energy to grain vibrational energy, a process that depends on shear rates, grain properties, boundary conditions, and the ambient fluid viscosity and pressure. Pore fluid pressures that exceed static equilibrium pressures result from local or global debris contraction. Like larger, natural debris flows, experimental debris flows of ∼10 m³ of poorly sorted, water‐saturated sediment invariably move as an unsteady surge or series of surges. Measurements at the base of experimental flows show that coarse‐grained surge fronts have little or no pore fluid pressure. In contrast, finer‐grained, thoroughly saturated debris behind surge fronts is nearly liquefied by high pore pressure, which persists owing to the great compressibility and moderate permeability of the debris. Realistic models of debris flows therefore require equations that simulate inertial motion of surges in which high‐resistance fronts dominated by solid forces impede the motion of low‐resistance tails more strongly influenced by fluid forces. Furthermore, because debris flows characteristically originate as nearly rigid sediment masses, transform at least partly to liquefied flows, and then transform again to nearly rigid deposits, acceptable models must simulate an evolution of material behavior without invoking preternatural changes in material properties. A simple model that satisfies most of these criteria uses depth‐averaged equations of motion patterned after those of the Savage‐Hutter theory for gravity‐driven flow of dry granular masses but generalized to include the effects of viscous pore fluid with varying pressure. These equations can describe a spectrum of debris flow behaviors intermediate between those of wet rock avalanches and sediment‐laden water floods. With appropriate pore pressure distributions the equations yield numerical solutions that successfully predict unsteady, nonuniform motion of experimental debris flows.
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Hazardous natural flows such as snow-slab avalanches, debris flows, pyroclastic flows and lahars are part of a much wider class of dense gravity-driven granular free-surface flows that frequently occur in industrial processes as well as in foodstuffs in our kitchens! This paper investigates the formation of oblique granular shocks, when the oncoming flow is deflected by a wall or obstacle in such a way as to cause a rapid change in the flow height and velocity. The theory for non-accelerative slopes is qualitatively similar to that of gasdynamics. For a given deflection angle there are three possibilities: a weak shock may form close to the wall; a strong shock may extend across the chute; or the shock may detach from the tip. Weak shocks have been observed in both dense granular free-surface flows and granular gases. This paper shows how strong shocks can be triggered in chute experiments by careful control of the downstream boundary conditions. The resulting downstream flow height is much thicker than that of weak shocks and there is a marked decrease in the downstream velocity. Strong shocks therefore dissipate much more energy than weak shocks. An exact solution for the angle at which the flow detaches from the wedge is derived and this is shown to be in excellent agreement with experiment. It therefore provides a very useful criterion for determining whether the flow will detach. In experimental, industrial and geophysical flows the avalanche is usually accelerated, or decelerated, by the net effect of the gravitational acceleration and basal sliding friction as the slope inclination angle changes. The presence of these source terms necessarily leads to gradual changes in the flow height and velocity away from the shocks, and this in turn modifies the local Froude number of the flow. A shock-capturing non-oscillating central method is used to compute numerical solutions to the full problem. This shows that the experiments can be matched very closely when the source terms are included and explains the deviations away from the classical oblique-shock theory. We show that weak shocks bend towards the wedge on accelerative slopes and away from it on decelerative slopes. In both cases the presence of the source terms leads to a gradual increase in the downstream flow thickness along the wedge, which suggests that defensive dams should increase in height further down the slope, contrary to current design criteria but in accordance with field observations of snow-avalanche deposits from a defensive dam in Northwestern Iceland. Movies are available with the online version of the paper.
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Multiple debris-resisting barriers have been commonly used worldwide to mitigate debris flows in drainage lines. However, a well-developed methodology to assess the mobility of debris flows with consideration of the obstruction of the barriers does not exist. A free-field debris-flow condition that omits the presence of multiple debris-resisting barriers is commonly considered in design, although the effects of the barriers could be critical in determining the dynamics of the landslide debris including debris velocity and debris thickness. This paper proposes a staged debris mobility analysis that accounts for the effects of multiple debris-resisting barriers. The staged analysis adopts solutions of a depth-averaged debris mobility model. The input parameters of the analysis have been established from field data and laboratory test results. Rigorous numerical simulations of debris flows intercepted by multiple debris-resisting barriers have also been undertaken using the three-dimensional finite element program LS-DYNA to provide results for benchmarking the output of the staged analysis.
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Many different expressions have been proposed for the effective stress principle for porous media, but none has clearly been acknowledged as being the correct one. It should be possible to determine the correct expression, because this is fundamental principle of mechanics. Following review of the candidate expressions proposed for soil, concrete and rock, a comprehensive expression is derived involving the factors observed in experiments on 'artificial rock'. In this new expression a distinction is made between the compressibilities of grains and skeleton due to total stresses and pore pressures. This general and more comprehensive expression is then tested against the experimental evidence. The expression may be specialized for (a) granular materials with separate grains with contact points and (b) solid rock with interconnected pores. The new expression for the effective stress contains most of the previously proposed expressions as special cases. Based on results of compression tests on quartz (with hard grains) and on gypsum (with soft grains) it is shown that Terzaghi's effective stress principle works well for most geotechnical applications, but significant deviations occur at very high stresses.
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本研究试验阶段在中国科学院东川泥石流观测研究站开展,感谢中国科学院东川泥石流观测研究站提供的技术支持和良好工作条件。
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