Slit Dams Research Articles

Intercepting and impact resistance property of dry particle flow by PFC3D-based slit dam

Intercepting and impact resistance property of dry particle flow by PFC3D-based slit dam

CFD-DEM numerical investigation of the effects of water content and inclination angle on interactions between debris flows and slit dam

Debris flows, composed of sediment–water mixtures, can pose significant risks to downstream populations and infrastructure in mountainous regions. Slit dams have been demonstrated as being effective measures for decreasing the destructivity of debris flows. Reliable evaluations of the interactions between debris flows and slit dams are challenging if the complicated component characteristics of debris flows, such as the water content, are not considered. Here, a multi-scale study of debris flows impacting a slit dam is conducted via computational fluid dynamics and discrete element method (CFD-DEM) simulations to understand the influences of the water content (controlled by solid volume fraction in this study) and the channel inclination angle on flow–slit dam interactions. The results indicate that both the initial solid volume fraction and the channel inclination angle have significant effects on the impact kinematics of flow–slit dam interactions and regulation functions of the slit dam: (i) For gentle slopes, the deposition of debris flows is primarily influenced by flume basal friction, and the normalized peak total impact forces have little difference for a given inclination angle, whereas for steep slopes, an overtopping can occur, further result in the increasing of the normalized peak total impact forces with the increase of solid volume fraction. (ii) The run-up height and retention efficiency are also analyzed to provide a scientific reference for optimizing the design of slit dams.

Large wood remobilization in Asakura (North Kyushu, Japan): Adapting strategies to climate change and rural population depletion

In the aftermath of the heavy rainfall events of 2017 in North Kyushu, Asakura (Japan), driftwood-related disaster risk has proven to be as real as it was in 1933, when wood-laden debris-flow invaded the city centre of Kobe City near Sannomiya station. Despite a temporary truce obtained thanks to extensive Sabo dam constructions, climate change, the return of forest on agricultural land due to rural exodus and an ageing population are once again increasing driftwood hazard and disaster risk. In the face of these novel challenges, new management strategies may need to be considered, and the present contribution aims to propose a new approach to driftwood hazards and disaster risk. For this purpose, the present contribution is analysing the 2D hydrodynamic of scenario-based floods on driftwood deposited after the 2017 heavy-rainfall event in Asakura. The boundary conditions for the simulation were generated using UAV photogrammetry calibrated against existing DEM data to generate a DEM at 25 cm horizontal resolution and an orthophotographs at 5 cm resolution. The results show that with the new river configuration (post-2017), an instantaneous peak flow of 90 m3/s is necessary to flood the areas where the driftwood has stopped with a water depth of >40 cm. This is 9 times the mode discharge calculated from the inlet geometry. The majority of the driftwood is thus not an immediate hazard, and leaving the wood instead of removing it should be considered as a management strategy whenever it is feasible. Recent research in Western Europe and Northern America has already shown the importance of driftwood for biodiversity and wildlife, and combining both environmental and hazards and disaster risk objectives may be a solution. For this purpose, the authors propose that medium-size ditch and artificial “abandoned channels” could be created in order to trap the wood in the floodplain, so that the cost from removing the wood can be alleviated, horizontal trapping could complement vertical trapping at slit dams, and the local wildlife could benefit from the wood decomposition in the floodplain.

Influence of trapping efficiency on the pile-up geometry of granular flows behind slit dams

The post spacing of slit dams is a key parameter that controls the trapping efficiency of these open-type countermeasures. In this study we conduct flume experiments of quasi-monodisperse spherical particles passing through slits to investigate the relationship between the trapping efficiency and the pile-up geometry, as well as the impact of the latter on the interaction between granular flows and slit dams. The ratio of the slit width relative to the particle size b/d is varied while the flume inclination is held constant. Tests reveal that at a critical ratio b/d∼2.3 trapping is most unstable. The trapping efficiency influences the geometry of the granular pile-up that deposits behind the dam after it has been jammed. When >52% of the granular mass is trapped (occurring at b/d≤2.3), the height of the final deposit changes with the trapping efficiency, whereas below this threshold value (when b/d≥3.1)the flow-wise length becomes more sensitive to the trapping. Impact force measurements further shed light on the mechanisms relating the trapping efficiency and the geometry and their effect on the total force fluctuations.

Hydro-mechanical simulations aid demand-oriented design of slit dams for controlling debris flows, debris avalanches and rock avalanches

Slit dams are widely used for managing multiphase geophysical flows with diverse objectives. However, quantitatively linking dam configurations and flow properties to design indices remains particularly challenging for multiphase flows. This study performs systematic hydro-mechanical simulations of slit dams in arresting debris flows, debris avalanches, and rock avalanches, using a coupled computational fluid dynamics and discrete element method (CFD-DEM). Our high-fidelity simulations reasonably capture essential physics observed in experiments. Furthermore, unified design diagrams are compiled from multiple perspectives to quantitatively link flow properties (fluid contents and Fr conditions) and spillway width to crucial design indices, including overspilling dynamics, downstream momentum reduction ratio ζ, and retention efficiency. The results reveal that: i) Fr exhibits nonlinear correlations with these design indices that vary significantly during the impact regime transitions from pile-up to runup, due to the resulting changes in the size and shape jammed and mobilized domains; ii) Both ζ and retention efficiency negatively correlate with fluid content, Fr, and spillway width, with different priorities; and iii) Fluid contents and Fr jointly govern overspilling dynamics, while increasing spillway width can effectively reduce retention efficiency by changing trap patterns, including clogging, blockage and self-cleaning traps. This study and its findings contribute deep insights into multiphase flow-dam interactions and offer a unique physics-based dataset to facilitate the demand-oriented design of slit dams for controlling various anticipated flows.

Dynamics of granular debris flows against slit dams based on the CFD–DEM method: effect of grain size distribution and ambient environments

AbstractEarth surface flows in nature, like debris flows and rock avalanches, have threatened people’s safety and infrastructure during past decades. Though grain size distribution (GSD) has been acknowledged as a crucial characteristic in granular material behaviour, its coupled effects associated with environments on engineering structures such as the slit dam remain unclear. To bridge the gap, this paper reveals the coupled effect of the GSD and ambient environments (i.e. slope angles and saturation conditions) on avalanche/debris flows’ impact on the slit dam using a Computational Fluid Dynamics/Discrete Element Method (CFD–DEM) model. To describe strain-dependent rheological characteristics of debris fluids, the Herschel–Bulkley–Papanastasiou model is implemented in the finite volume method framework. A power grain size distribution law is considered to quantify GSDs, in which a fractal parameter takes charge of GSD types. After model verification with experimental/theoretical results, the impact force against slit dams, granular dynamics and final deposit patterns under a series of ambient circumstances are presented. Taking advantage of the CFD–DEM method, the impact force and kinetic energy induced by fluid and solid phases are discriminated. The contribution of solid and fluid phases to both impact force and dynamics appears to be dependent on GSDs. Accordingly, compared with saturated avalanche flows (i.e. debris flows), slit dams result in higher retaining efficiency when confronted with dry avalanche flows. Regarding a narrow diameter range used in analyses, the grain diameter ratio is then enlarged up to eight to reveal the potential size effect. As for the coupled role of GSDs and slope angles, in contrast to slope angles, the influence of GSD on avalanche flow interaction with slit dams is much smaller. Additionally, provided a narrow diameter range, the effect of GSDs on impact force can be partially attributed to the change in average grain diameter. After presenting the significance of ambience and GSDs to avalanche/debris flows, a series of parametric studies around the effect of fluid grid size, particle shape and the initial porosity of granular samples are discussed, aiming to advance the understanding of their influence in the interactions between debris flows and the slit dam.

Characteristics of debris flow impact on a double-row slit dam

Characteristics of debris flow impact on a double-row slit dam

Discrete meta-analysis of slit dam intercepted particle flow based on PFC3D

Discrete meta-analysis of slit dam intercepted particle flow based on PFC3D

Keynote lecture. Impact dynamics of debris flow against slit dam: Experimental and numerical investigation

Debris flows are gravity-driven phenomena common in mountainous regions that are hazardous to downstream facilities. To mitigate the impacts of these disastrous processes, structural countermeasures such as slit dams are constructed in gullies and along mountain slopes. Existing studies on the impact dynamics of debris flows against slit dams typically focus only on the flow characteristics but fail to take the geometry of the structure into account. Here we develop an analytical model, derived from the momentum approach, that allows for the estimation of the runup height and impact load of debris flows on slit dams. The model is validated against discrete element simulations and small-scale flume experiments. It is found that the runup height is controlled by both the Froude number and slit size. The proposed analytical model can predict the runup height well within a certain range of Froude numbers. Results from experiments further reveal that the fontal dynamic pressure is sensitive to the flow properties whereas the peak dynamic pressure is strongly affected by the slit size.

A simplified numerical model for evaluating sediment control by open-type sabo dams in the Joganji River basin

The present study proposes a method to estimate sediment runoff by introducing a dam function of the relationship between inflow sediment and sediment runoff through a slit dam. The model can process rainfall runoff, sediment yield and runoff of a mountainous basin, and the model is applied to the upper reaches of the Joganji River basin, which is known for its huge amount of sediment runoff and intense bed variation because of the sediment yield caused by the earthquake in 1858. The performance of the calculations of sediment control of the slit dam is evaluated by the model. The result indicates that sediment deposition is significantly changed by sediment runoff. The proposed method can be expected to evaluate sediment transport with sabo dams on a basin scale.

Model Experiment Exploration of the Kinetic Dissipation Effect on the Slit Dam with Baffles Tilted in the Downstream Direction

Slit dams can eliminate the risk of particle overload accumulation, which can be safer in controlling debris flow compared with a completely closed dam. In attempting to better use the energy dissipation effect of particle collision and reduce the impact of the dam body, referring to the traditional slit dam, this paper proposed one with tilted baffles in the downstream direction. Discrete element simulation and several flume model experiments were carried out herein to verify the advantages and explore the applicable conditions of this tilted baffle slit dam, in which the particle trapping efficiency and the change law of impact force of the tilted baffles under the conditions of different inclined angles, opening sizes, and particle sizes were studied. The results show that: 1. when the inclination angle is 30° ≤ θ ≤ 45°, the tilted baffles can dissipate more particle kinetic energy than the transverse baffles; 2. the maximum impact force and trapping efficiency of the tilted baffles decrease with the increase in the width diameter ratio b/d, with the opening width b of the slit to the particle diameter d; 3. with the given particle size of 6 mm ≤ d ≤ 14 mm, the range that the tilted baffles can effectively intercept the particles flowing down is 0 ≤ b/d ≤ 4, and it reaches the ideal interception state near 1 ≤ b/d ≤ 2, where, relatively, the impact force is weak, and the interception efficiency is high.

Empirical equation for the assessment of the effect of sediment concentration on the discharge coefficient of slit dams

Empirical equation for the assessment of the effect of sediment concentration on the discharge coefficient of slit dams

Effect of slit size on the impact load against debris-flow mitigation dams

Structural countermeasures such as slit dams and check dams are widely installed in mountainous regions to mitigate debris-flow hazards. However, current approaches adopted to estimate debris-flow impact load only depend on the flow properties without considering the effect of structural geometry. To better understand the effect of slit size on the impact load experienced by debris-flow mitigation dams, a series of small-scale debris flows impact tests on modelled slit dams and check dams are conducted in an instrumented flume. Measurement of the flow velocity, depth, impact load, total basal normal stress, and basal pore-fluid pressure enable a comprehensive grasp of the impact details. Tests reveal that the peak frontal impact pressure is largely unaffected by the slit size of structural countermeasures but is sensitive to the debris-flow properties. However, the slit size obviously influences the peak force experienced by the structures. A critical relative slit size (ratio of slit size to the maximum particle diameter in the debris flow) of 3.6 is determined wherein slit dams can effectively mitigate debris-flow hazards. In addition, a simplified bilinear pressure distribution model is proposed for debris-flow impact load estimation of the slit dams and check dams.

Analysis on the damage causes of sabo dam with one open cross section at upstream of Fuxing bridge in Lacus Creek

Lacus Creek is one of the important tributaries of the Laonong River upstream of the Gaoping River, which is an important river in southern Taiwan. After Typhoon Morakot in 2009, severe sediment disasters occurred, which affected the security of local revitalization, and the government has since invested in numerous projects to reduce the impact of sediment and flood disasters. In 2017, the Kaohsiung City government built a sabo type dam upstream of Fuxing Bridge in Lacus Creek; however, it was severely damaged by a heavy rain less than a year later. To investigate the cause of the damage, site investigations, concrete sampling tests, hydrology analysis, and reviews of the planning and design methods, construction process, etc. have been conducted, in order to understand the cause of the problem, avoid repeating mistakes, and clarify the responsibility of the administrative agency. The investigation and research found that many recent cases of torrent management have considered the transverse structures as isolated wildlife corridors, reduce the height of the transverse structures, create gentle slopes of the downstream surfaces, add fish passages, cut (openings) export slits, etc.; however, after setting up these environmentally friendly facilities, their effect and impact have not been explored in depth. This research survey found that, although the frequency of heavy rain events is not high in recent years, the current iterative impact of continuous rainfall may exceed the original design requirements. If the overflow mode with a single export opening is added, the scouring energy will be concentrated, and this additive effect is likely to cause the foundation of the sabo dam to lose bare space, and finally, the dam will be damaged. Therefore, it is recommended that when designing these torrent transverse structures, try to avoid using the method of a single narrow opening: it is better to increase the number of export openings (like a slit dam), expand the size of a single export opening, or adopt a fully enclosed dam design.

Numerical study of granular debris flow run-up against slit dams by discrete element method

Run-up of granular debris flows against slit dams on slopes is a complex process that involves deceleration, deposition, and discharge. It is imperative to understand the run-up mechanism and to predict the maximum run-up height for the engineering design and hazard mitigation. However, the interaction between granular flows and slit dams, which significantly affects the run-up height, is still not well understood. In this study, an analytical model based on the momentum approach was derived to predict the run-up heights of granular debris flows. A numerical investigation of granular debris flow impacting slit dams using the discrete element method (DEM) was then conducted. The influence of the Froude number (NFr) and the relative post spacing (R) on run-up height were studied. This study illustrates that the analytical model based on the momentum approach can predict the run-up heights well within a certain range of Froude numbers. There is a critical value of relative post spacing (RC): within the critical value, the maximum run-up height is insensitive to the relative post spacing; once R exceeds the critical value, the maximum run-up height decreases rapidly as the relative post spacing increases.

A model for simulation of debris flow sedimentation in slit detention-dam reservoirs

Debris flow is a natural phenomenon that requires appropriate structural measures such as detention dams to reduce hazards resulting from it. The objective of this paper is to present a two-dimensional numerical model to investigate debris flow deposition in the reservoir of the detention slit dams. The governing equations include shallow water equations considering the bed level variation, sediment continuity, and debris-flow transport equations. Shallow water equations were discretized using the MacCormack scheme and then Banihabib Scheme was applied for solving sediment continuity equation. The governing equations were solved in an iterative procedure and as a result, flow properties and bed level variations were obtained in silt dam reservoir. The model performance was evaluated against experimental and numerical models in two aspects including the outflow hydrograph and reservoir bed level variations or trapping efficiency. The model performed quite well in outflow hydrograph simulation, and the comparison of the bed level variations or trapping efficiency illustrated that the simulated values were acceptable. A sensitivity analysis of the parameters was carried out to analyze the influence of parameters variation on outflow and sedimentation volume. The results of this sensitivity analysis revealed sedimentation is more sensitive to the parameter variation than outflow hydrograph. Generally, the proposed numerical method can provide insights on the debris flow deposition in slit dam reservoir, which could be useful for slit dam design.

The force exerted by granular flows on slit dams

Boulders often accumulate at the fronts of fast-flowing steep creek hazards. To trap these boulders, slit dams are commonly installed along steep creeks. An outstanding challenge when designing such dams lies in estimating the load exerted by a cluster of boulders: Clusters may exhibit discrete or continuum loading behavior, depending on their size. In this study, a physical flume test was carried out and the obtained results were used to calibrate a discrete element method (DEM) model. The DEM was then used to carry out a parametric study as a function of grain size, slit width and the channel slope. Results reveal that for continuum-like flows, the pressure near the slit of a slit dam can be almost double than elsewhere on the slit dam, as quantified using a newly proposed dimensionless ratio. For discrete flows, impact forces near the slit are more than double than elsewhere, thus representing a critical case for slit dam design. The concentration of forces near the slit necessitates designing for loads 2–3 times that of the rest of the slit dam to avoid wearing. (If wearing occurs, the slit width will increase, along with the volume of discharge, a potentially catastrophic scenario.) Furthermore, two dimensionless weighting coefficients that distinguish pressure from continuum-like flows and discrete impacts are proposed and evaluated. These coefficients can be incorporated into existing analytic expressions used by engineers.

Experimental study on the regulation function of slit dam against debris flows

The authors received financial support from the National Natural Science Foundation of China (grant no. 11672318), the Youth Innovation Promotion Association, CAS, and the Chinese Academy of Sciences (CAS) Pioneer Hundred Talents Program, and from the research grant T22-603/15-N provided by the Research Grants Council of the Government of Hong Kong SAR, China.

Nondimensional Simulation-Based Regression Formulas for Slit Dam Design in Mountain Rivers

AbstractCompact regression formulas were developed through one-dimensional (1D) unsteady flow numerical model simulations for the analysis of slit dam (SD) hydraulic behavior in torrents. The formu...

An experimental study on controlling post-earthquake debris flows using slit dams

Post-earthquake debris flows often break out in groups frequently, which are usually caused by the abundant loose solid materials that produced by earthquake. Slit dams represent a practical and effective kind of countermeasure for controlling the post-earthquake debris flow. Flume experiments were carried out to study the interaction mechanism and the effect of slit dams on the post-earthquake debris flows. The results showed that affected by the slit dams, some certain types of deposits formed on the upstream. The steeper the flume slope, as well as the greater the width and the density of slits, the easier the lateral deposit became “V” shaped. Otherwise, the lateral deposit was more likely to be “–‒” shaped. When the flume slopes were 12°, 16° and 20°, the profiles of the deposits would be long-shallow type, short-thick type and short-shallow type, respectively. The slope of the deposition first decreases and then increases with the flume slope increasing within a certain range. The slit dam can trap the coarse sand and discharge the fine sand. The maximum attenuation rate can reach 44.4%. The effect of this capacity gradually weakens as the flume slope is increased. When the width or the density of slits is smaller, the greater the rate of decrease in peak sand discharge and the greater the effect of peak cutting will be. The reduction in the sediment storage rate is likely due to the increase in the width and the density of the slits. With the increase in flume slope, the sediment storage rate first increases, then decreases, which reaches a maximum value when the flume slope is 16°.