SPH-DEM Modeling of Debris and Mud Flows

<p>For the design of mitigation measures knowledge of debris-flow impact forces, usually estimated based on hydrostatic, hydrodynamic, or combined approaches, is essential. As these approaches are based on Newtonian fluids, they must be adjusted by empirical correction factors to account for the solid-fluid nature of debris flows. The values for the correction factors shown in the literature vary over a wide range and several studies showed a clear dependence with the Froude regime of debris flows.</p><p>To better understand the correction factors and to be able to calculate them using parameters that describe the flow behaviour a total of 32 experiments were conducted in the course of the project “Debris flow impact forces on bridge super structures (DEFSUP)”, funded by the Austrian Science Fund (FWF). Two different material compositions, different water contents as well as a total impact and a bypassing of the measuring block were tested.</p><p>The experimental setup designed within the project consists of a 4 m long semi-circular channel with a diameter of 300 mm and an inclination of 20°. The material is released from a rectangular reservoir in a dam-break scenario and accelerated with zero roughness on a length of 1.2 m and transferred to the semi-circle profile. The subsequently introduced roughness with a grain diameter of 1-2 mm generates a stationary phenomenological debris flow until it hits the measuring setup. With a starting volume of 50 kg, flow heights between 8 and 12 cm and velocities from 0.8 to 2.2 m/s were achieved according to the material composition and different water content. With these different mixtures a Froude-range from 0.6 to 3.6 was covered. In addition, normal stresses and pore water pressures were measured at the exact same point.</p><p>A detailed analysis of the measured impact forces together with the above mentioned measured parameters showed that the hydrodynamic correction factor is a constant mainly corresponding to the liquification ratio of the debris-flow mixture. Hence, the hydrodynamic correction factor can be regarded as a drag coefficient and seems to depend mainly on the internal friction of the flowing medium. At low Froude numbers measured impact forces exceed even a full momentum transfer if the mean bulk density is used for the calculation. This indicates that the impact forces can no longer be described by the hydrodynamic approach alone. For this reason, an additional pressure term based on a hydrostatic approach is considered in the combined concept. This additional pressure term depends on the dynamics of flow (Froude number) and can be modelled via a dynamic earth pressure coefficient.</p><p>The findings from these experiments contribute to a better prediction of debris-flows impact forces in terms of their material composition and flow behaviour.</p>

Exploring debris flow deposit morphology in river valleys: Insights from physical modeling experiments

Debris flows are classical two-phase flows that can be enhanced by entraining multi-grain sizes of sediments from the bed as they rush down steep slopes, in which particle segregation is related to assessing the potential hazards. However, understanding the characteristics and fluid-particle interaction mechanisms remains challenging. Here an existing depth-averaged two-phase continuum flow model is further improved by incorporating the effects of pore-fluid pressure and bed sediment conditions on erosion. To demonstrate its reliability, we compare numerical solutions with measurements of thickness, front location, and bed deformation in two sets of USGS large-scale experimental debris flows over erodible beds. The following physical understandings are obtained. First, the positive effects of pore-fluid pressure and coarse bed materials on erosion rates are numerically reproduced. Moreover, an additional mechanism for this phenomenon has been revealed. Specifically, debris flows on steep slopes are likely to fall into a high shear stress regime, under which conditions the sediment transport capacity always takes a maximum value and is independent of the sediment size. Therefore, the sediment settling velocity that is proportional to the sediment size affects the erosion rate directly. Second, we probe into the non-dimension number and energetics of the debris flows to find it necessary to incorporate water-sediment and particle-particle interactions into reproducing the debris flow processes. Third, two kinds of mechanisms for particle size coarsening in the head region of the debris flow are resolved: on the one hand, they can be incorporated and retained there if the debris flow acquires sediment from the bed in transit due to considering the hiding/exposure mechanisms and on the other hand, they can migrate to the head by preferential transport. Furthermore, a series of idealized tests were conducted to explore the factors contributing to the segregation of particles within a debris flow. The longitudinal particle segregation was reproduced by incorporating the shear-induced non-uniform vertical distributions of velocity and sediment concentrations, the visco-inertial rheology, as well as the grain-size heterogeneity into the modelling. Sensitive analysis shows that the transport of fine particles is more inhibited by the interaction of the flow, contributing to the larger transportation velocity of the coarse particle. We further observed that the water content, the slope, and the particle size would have positive effects on the longitudinal size segregation in the head region, contrasting with the negative effects of the flow viscosity. These factors affecting the segregation ratio are attributed to the changes in the ratio of the Reynolds Number of the flow between fine and coarse particle.

<p>Guillaume Meyrat, Brian McArdell, Ksenyia Ivanova, Perry Bartelt</p><p>WSL Institute for Forest, Snow and Landscape Research, 8903 Birmensdorf, Switzerland</p><p> </p><p><strong>Keywords</strong>: Debris flows, multi-phase models, dilatancy, shear stress, density distribution</p><p> </p><p>To implement an accurate numerical tool to simulate debris flow hazard is a longstanding goal of natural hazard research and engineering. In Switzerland the application of numerical debris flow models has, however, been hampered by many practical and theoretical difficulties. One practical problem is to define realistic initial conditions for hazard scenarios that involve both the rocky (granular solid) and muddy (fluid) material. Still another practical problem is to model debris flow growth by entrainment [1]. These problems are compounded by theoretical uncertainties regarding the rheological behavior of multi-phase flows. Recent analysis of debris flow measurements at the Swiss Illgraben test-site [2] (shear and normal stresses, debris flow height) show that the shear force, and therefore the entire debris flow behavior, is largely influenced by the debris flow composition, i.e. the amount of solid particle and muddy fluid at any specific location within the debris flow body (front, tail, etc.). The debris flow composition is, in turn, determined by the initial and entrainment conditions for a specific event. As a consequence, we have concluded that the very first step to construct a robust numerical model is to accurately predict the space and time evolution of the solid/fluid flow composition for any set of initial and boundary conditions. To this aim, we have developed a two-phase dilatant debris flow model [3, 4, 5] that is based on the idea that the dispersion of solid material in fluid phase can change over time. The model is thus able to predict different flow compositions (rocky fronts, watery tails), using shallow-water type mass, momentum and energy conservation equations. This helps to predict when the solid phase deposits, and when muddy fluid washes and channel outbreaks in the runout zone can occur. The parameters controlling the evolution of debris flow density and saturation have been derived by direct comparison to the full-scale measurements performed at the Illgraben test site.</p><p> </p><p><strong>References</strong></p><p><strong> </strong></p><p> </p>

Evolution of debris flow properties and physical interactions in debris-flow mixtures in the Wenchuan earthquake zone

The Wasatch Front north of Salt Lake City has a recurrent history of debris flows. Two distinctly different types of climatic events—summer thunderstorms and rapid spring snowmelts—have triggered debris flows. Heavy autumn rains, exceptional winter snowpacks, and cool and extended springs followed by sudden warming characterized weather during 1983 and 1984. As a result of these climatic events, large amounts of water were rapidly added to the landscape, resulting in several hundred landslides, many of which subsequently mobilized into debris flows and (or) hyperconcentrated floods. Reconnaissance during June of 1983 revealed many hillsides with recently active landslides that had moved slightly but had not mobilized, or had only partially mobilized, as debris flows. The presence of these partly-detached landslides prompted an investigation of the potential for future debris flows. Wieczorek and others (1983) developed a technique for evaluating the potential for debris flows and hyperconcentrated floods to reach the canyon mouths, based on three assumptions: 1) debris flows would occur in response to a large influx of water to the soils, either from a thunderstorm or rapid snowmelt; 2) the partly-detached landslides were the most likely sources for future debris flows; and 3) debris-flow material incorporated from channels would be proportional to that from landslide sources. The climatic and debris-flow events of 1984 were sufficiently similar to those of 1983 that we could evaluate these assumptions. Slightly fewer debris flows occurred in 1984 than in 1983, consistent with a slightly lower snow-pack water content and slightly slower rate of melting. Debris flows occurred near the rapidly receding snowline, apparently as a result of the large influx of water into the soils there. On an areal basis, partly-detached landslides identified in 1983 were statistically significant as sources for debris flows in 1984. In the few documented cases, the contribution of material scoured from channels was variable but very high and appeared significantly influenced by the geomorphic condition of the stream channel.

Introduction Debris flows are climate change-driven geohazards that pose a significant threat to life and the built environment. These flows are unconfined, in which the water-laden mass comprising of soil, gravel, boulders, and trees surges down steep hill slopes and channelises into streams, trapping everything that encounters them. These hazardous flows impact the engineering and defence structures, transportation systems, and the environment. State of Knowledge Various researchers have performed numerous laboratory-to-field scale experiments to understand debris flow's dynamic morphological and rheological properties. The state-of-the-art approach to evaluating the effect of multiple factors, such as water content, pore water pressure, grain size distribution, topography, solid volume fraction, and basal friction parameters on the initiation mechanism, dynamics, and morphology, has evolved rapidly in the past few decades. Objective The present paper discusses the progress in the identification of rheological and morphological characteristics of the debris flow and the advancement in debris flow experimentation. Approach and Scope This review will help the readers get a consolidated overview of the recent advancements in debris flow experiments. The study complements the previous studies by identifying the latest developments in debris flow experiments, such as the effects of material and geometric parameters on flow dynamics, mechanisms of evolution of excess pore water pressure in saturated and unsaturated bed conditions, entrainment mechanisms, and the classification of experiments based on the type and size of experimentation. Discussion and Relevance The characteristics of debris flow are also discussed based on different phases in debris flows, such as the initiation phase, dynamic phase, morphological characteristics, and impact on the built environment.

A reconnaissance of 36 tributaries of the Colorado River indicates that debris flows are a major process by which sediment is transported to the Colorado River in Grand Canyon National Park. Debris flows are slurries of sediment and water that have a water content < 40% by volume. Debris flows occur frequently in arid and semiarid regions. Slope failures commonly trigger debris flows, which can originate from any rock formation in the Grand Canyon. The largest and most frequent flows originate from the Permian Hermit Shale, the underlying Esplanade Sandstone of the Supai Group, and other formations of the Permian and Pennsylvanian Supai Group. Debris flows have reached the Colorado River on an average of once every 20 to 30 yr in the Lava-Chuar Creek drainage since about 1916. Two debris flows have reached the Colorado River in the last 25 yr in Monument Creek. The Crystal Creek drainage has had an average of one debris flow reaching the Colorado River every 50 yr, although the debris flow of 1966 has been the only flow that reached the Colorado River since 1900. Debris flows may actually reach the Colorado River more frequently in these drainages because evidence for all debris flows may not have been preserved in the channel-margin stratigraphy. Discharges were estimated for the peak flow of three debris flows that reached the Colorado River. The debris flow of 1966 in the Lava-Chuar Creek drainage had an estimated discharge of 4,000 cu ft/sec. The debris flow of 1984 in the Monument Creek drainage had a discharge estimated between 3,600 and 4,200 cu ft/sec. The debris flow of 1966 in the Crystal Creek drainage had a discharge estimated between 9,200 and 14,000 cu ft/sec. Debris flows in the Grand Canyon generally are composed of 10 to 40% sand by weight and may represent a significant source of beach-building sand along the Colorado River. The particle size distributions are very poorly sorted and the largest transported boulders were in the Crystal Creek drainage. Reworking of debris fans by the Colorado River creates debris bars that constrain the size of eddy systems and forms secondary rapids and riffles below tributary mouths. (See also W89-09239) (Lantz-PTT)

Abstract. This article presents MassMov2D, a two-dimensional model of mud and debris flow dynamics over complex topography, based on a numerical integration of the depth-averaged motion equations using a shallow water approximation. The core part of the model was implemented using the GIS scripting language PCRaster. This environment provides visualization of the results through map animations and time series, and a user-friendly interface. The constitutive equations and the numerical solution adopted in MassMov2D are presented in this article. The model was applied to two field case studies of mud flows on torrential alluvial fans, one in the Austrian Tyrol (Wartschenbach torrent) and the other in the French Alps (Faucon torrent). Existing data on the debris flow volume, input discharge and deposits were used to back-analyze those events and estimate the values of the leading parameters. The results were compared with modeling codes used by other authors for the same case studies. The results obtained with MassMov2D matched well with the observed debris flow deposits, and are in agreement with those obtained using alternative codes.

Constructing multiple barriers along the predicted flow paths has been recognised as an effective engineering measure to intercept large volumes of debris flows. In the field, barriers usually have a basal clearance, which is an opening between the channel bed and the base of the barrier, to allow small discharges pass through the barrier and prevent the accumulation of stream loads. However, the current design of basal clearance is still based on an empirical criterion. The objectives of this research are to investigate the effects of basal clearance on the interaction between debris flow and multiple rigid barriers and to propose and verify a new analytical framework for flow impact on multiple rigid barriers with basal clearance. Findings from this study can serve to improve the current design guidelines on multiple-barrier system. The newly proposed analytical framework consists of an impact equation for barrier with a basal clearance, discharge equations to estimate flow properties of the basal discharge and a landing equation to estimate the flow properties after flow lands on the basal discharge. To verify the proposed framework, physical and numerical modelling were adopted. A series of dry granular flow tests has been carried out with a 5-m flume model to investigate the fundamental impact behaviour of idealised uniform particles on a rigid barrier with basal clearance. A discrete element model was calibrated by the physical tests and a series of numerical parametric studies has been conducted with varying barrier heights and barrier spacing. Furthermore, due to scale dependent nature of debris flow, a 28-m flume facility has been newly developed to investigate the impact dynamics of debris flow on dual rigid barriers with basal clearance. It is found that basal clearance can be designed to optimise barrier size and minimise maintenance work for a multiple-barrier system. Basal clearance with a ratio between clearance height and flow depth, H&lt;sub&gt;c&lt;/sub&gt;/h&lt;sub&gt;i&lt;/sub&gt;, ranging from 0.5 to 1.0 with Fr = 7 can reduce the impact force and runup height of two-phase debris flow by up to 55% and 43%, respectively. The effectiveness of basal clearance on mitigating impact can be characterised by Friction number, N&lt;sub&gt;fric&lt;/sub&gt;, of the flow. A larger N&lt;sub&gt;fric&lt;/sub&gt; represents a higher shear resistance among particles that would resist the flow to be discharged and lead the flow tend to be redirected to parallel to the barrier. N&lt;sub&gt;fric&lt;/sub&gt; of dry granular flow that is by up to 55 times higher than the two-phase flow leads the basal clearance about 30% less effective on reducing the impact force and runup height of dry granular flow. Characteristic particle size, d&lt;sub&gt;max&lt;/sub&gt; that represents the largest particle of the debris material, is found capable of determining the occurrence of jamming at the basal clearance. Basal clearance height H&lt;sub&gt;c&lt;/sub&gt;/d&lt;sub&gt;max&lt;/sub&gt; ≤ 2.0 and H&lt;sub&gt;c&lt;/sub&gt;/d&lt;sub&gt;max&lt;/sub&gt; ≥ 3.0 may lead and not lead jamming at the clearance, respectively, and could be adopted for a barrier to regulate debris flow without losing barrier retention and without retaining any debris, respectively. The proposed impact equation can well estimate both dry granular flow and debris flow impact on a rigid barrier with a basal clearance. The proposed landing equation can provide an upper bound for flow depth and velocity near the landing location. However, with an increasing flow distance, the decrease of downstream flow depth may lead that only the discharge equations are needed to estimate the flow properties.

Climate change implies a general increase in global temperature, increased frequency and intensity of rainfalls. The combination between increased rainfall intensity and raise in temperature is likely to result in increases in frequency, severity and spatial spread of landslide hazard. Particularly, debris flows can become more common in many parts of the world. Debris flows are a mixture of granular materials and water which behaves as a moving continuum as a result of gravity. The sliding mass has high water content and hence a large speed and can remove and/or destroy lives, buildings and infrastructures along its way. This poses a significant threat to human life, health, well-being and financial loss for individuals, businesses and society. This article presents the results from laboratory tests of model debris flows hitting against deflection walls as a preventative measure. The purpose is to study the effect of using deflecting walls in mitigating debris flow hazard. In particular, the study examines the dependency of the run-up height of the debris mass after hitting the walls on the deflection angle and the inclination angle of the walls. Some shortcomings of the existing model are also discussed in order to improve future studies.

The design of appropriate mitigation measures to protect from landslides require the knowledge of the physical and mechanical processes governing their propagation. This knowledge can derive from experience, carefully verified and modeled through specific rheological models that take into account the main physical and mechanical properties referred to normal stream flows, hyperconcetrated flows, mudflows, debris flows, rock avalanches and their multiphase nature (flow like or granular).To predict the speed evolution and distance travelled by fine-grained material flows, an analytical (sliding block) model is proposed. The model takes into account (i) the (curved) geometry of the sliding surface; (ii) the mass variation due to possible erosion or deposition processes; iii) the interstitial pressures and, in particular, the evolution of the excess pore water pressures (generation, initially due to several phenomena and during the motion to the slope curvature, coupled to undrained and oedometric conditions; dissipation, due to consolidation process).The governing ordinary differential equation has been numerically solved. The role played by the main model parameters on the kinematics of rapid fine - grained material flows is evaluated through parametrical analyses and the range of their admissible values is investigated and defined. Model validations using laboratory measurements and analysing some documented cases are finally developed and carried out.

Numerical investigation of debris materials prior to debris flow hazards using satellite images

Deep-water deposits consisting mainly of massive sand are commonly identified as deposits of turbidity currents (i.e., turbidites). Speculation has risen in recent years as to whether some of these massive sandy deposits could have instead been deposited by debris flows. This possibility is explored here by examining the flow mechanics of sand-rich subaqueous gravity flows by means of laboratory experiments. In these experiments, sandy gravity flows were generated when well-mixed slurries of sand, clay, and water were released into a tank filled with tap water and allowed to flow under gravity over a slope that declined from 4.6° to 0.0°. The observed flow mechanics and resulting depositional features were strongly tied to the “coherence” of the debris flows (i.e., the ability of the slurry to resist being eroded and broken apart by the shear and pressure undergone by the flow). Low water content and high clay content resulted in strongly coherent debris flows, whereas high water content and low clay content resulted in weakly coherent flows. As little as 0.7 to 5 wt% of bentonite clay or 7 to 25 wt% of kaolinite clay at water contents ranging from 25 to 40 wt% was required to generate coherent gravity flows. Weakly coherent and moderately coherent flows produced significant, low-concentration subsidiary turbidity currents, and their deposits developed coarse- tail grading, water-escape structures, and minor increases in thickness at the base of the slope. Strongly coherent debris flows commonly hydroplaned and generated only minor subsidiary turbidity currents. Their deposits were structureless and ungraded, commonly showing tension cracks, compression ridges, water-escape structures, detached slide blocks, and a significant increase in thickness at the base of the slope. Application of distorted geometric scaling suggests that many aspects of these experiments appropriately scale up to the field scale of natural submarine debris flows.

Catastrophic gravity driven flows such as debris flows, mud flows, and flow slides cause major economic damage and loss of human life. Numerical models are important prediction tools which can help to design contention, channeling, and routing structures. Their accuracy is limited by that of the rheological model they implement. Bingham models provide a reasonable good compromise between accuracy and simplicity, and they have been frequently used to describe the rheology of hyperconcentrated flows, mudflows, and debris flows. Their use in depth integrated models presents a difficulty. In order to obtain the bottom friction term, one has to obtain the roots of a third order polynomial. This paper describes a depth integrated model for flows of Bingham fluids. Bottom friction is obtained using an approximation which provides good accuracy and saves 20% of CPU time.