산사태 발생은 인명 및 재산 피해를 유발할 뿐만 아니라 도 로 및 주요 시설물의 직·간접적 피해를 일으킨다. 특히, 산지 부근에 위치한 문화재는 산사태 발생시 매우 큰 피해를 입는 다. 그 예로 2002년 8월 태풍 ‘루사’로 인해 산사태가 발생하 여 전라북도 남원시 산내면 대정리에 위치한 실상사 백장암 의 백장암 3층 석탑(석조문화재 국보 제10호)과 백장암 석등 (보물 제40호)이 피해를 입었다(Song, 2007). 산사태를 유발시키는 가장 큰 인자로는 강우와 지진을 들 수 있다. 그중 지진으로 인한 산사태는 대규모 지진에 의해 발생되는 경우가 주를 이루지만, 강우에 의한 지층포화조건이 수반된다면 소규모 지진에서도 산사태 발생은 충분히 가능하 다. 그 예로 2006년 2월 필리핀 레이테(Leyte) 남부 지역에서 발생한 산사태를 들 수 있다. 레이테 산사태는 집중호우로 포 화된 지반에 규모 2.6의 소규모 지진이 발생하여 대규모 산사 태로 1,100여명의 사망자가 발생하였다(Evans et al., 2007; Kim, 2011). 우리나라 역시 기후변화에 의한 돌발성 집중호우 와 1988년 이후 꾸준히 증가하고 있는 지진 발생빈도(Fig. 1) 를 살펴볼 때 더 이상 지진에 의한 산사태의 안전지대가 아니 라는 것을 알 수 있다. 본 연구에서는 지진으로 인한 사면의 변위량 분석에 사용되 Abstract

[1] Characteristics of the transition from supercritical to subcritical regime (hydraulic jump) in laminar flow of a Herschel-Bulkley fluid on an horizontal plane are examined. The set of equations governing the flow is presented, recovering earlier results for a Bingham fluid. The determination of the flow conditions downstream, given those upstream, involves solution of a nonlinear algebraic system in three unknowns; an approximate analytical formula for conjugate depths is derived since no exact solution can be obtained in closed form. The relative error thus introduced is then evaluated numerically; its maximum value is shown to be less than a few percent. The behavior of flow depth, bottom shear stress and critical depth are established and illustrated as function of the flow behavior index; the critical depth and the ratio of conjugate depths increase with the behavior index and until the bottom shear stress exceeds a certain value, and then decrease afterwards.

[1] Aggregation of data collected in 28 controlled experiments reveals reproducible debris-flow behavior that provides a clear target for model tests. In each experiment ∼10 m3 of unsorted, water-saturated sediment composed mostly of sand and gravel discharged from behind a gate, descended a steep, 95-m flume, and formed a deposit on a nearly horizontal runout surface. Experiment subsets were distinguished by differing basal boundary conditions (1 versus 16 mm roughness heights) and sediment mud contents (1 versus 7 percent dry weight). Sensor measurements of evolving flow thicknesses, basal normal stresses, and basal pore fluid pressures demonstrate that debris flows in all subsets developed dilated, coarse-grained, high-friction snouts, followed by bodies of nearly liquefied, finer-grained debris. Mud enhanced flow mobility by maintaining high pore pressures in flow bodies, and bed roughness reduced flow speeds but not distances of flow runout. Roughness had these effects because it promoted debris agitation and grain-size segregation, and thereby aided growth of lateral levees that channelized flow. Grain-size segregation also contributed to development of ubiquitous roll waves, which had diverse amplitudes exhibiting fractal number-size distributions. Despite the influence of these waves and other sources of dispersion, the aggregated data have well-defined patterns that help constrain individual terms in a depth-averaged debris-flow model. The patterns imply that local flow resistance evolved together with global flow dynamics, contradicting the hypothesis that any consistent rheology applied. We infer that new evolution equations, not new rheologies, are needed to explain how characteristic debris-flow behavior emerges from the interactions of debris constituents.

Z. GHADAMPOUR, N. TALEBBEYDOKHTI, M. R. HASHEMI, A. H. NIKSERESHT AND S. P. NEILL Dept. of Civil and Environmental Engineering, Shiraz University, I. R. of Iran Email: Zahra_ghadampour@yahoo.com Dept. of Civil and Environmental Engineering, Environmental Research and Sustainable Development Center, Shiraz University, I. R. of Iran Dept. of Water Engineering, Shiraz University, I. R. of Iran; School of Ocean Sciences, Bangor University, Menai Bridge, UK Dept. of Mechanical Engineering, Shiraz University of Technology, I. R. of Iran School of Ocean Sciences, Bangor University, Menai Bridge, UK

We introduce a GPU-accelerated simulation tool, named Modeling on Shallow Flows with Efficient Simulation for Two-Phase Debris Flows (MoSES_2PDF), of which the input and output data can be linked to the GIS system for engineering application. MoSES_2PDF is developed based on the CUDA structure, so that it can well run with different NVIDIA GPU cards, once the CUDA vers. 9.2 (or higher) is installed. The performance of the MoSES_2PDF is evaluated, and it is found that the present GPU-CUDA implementation can enhance the efficiency by up to 230 folds, depending on the PC/workstations, models of GPU card and the mesh numbers in the computation domain. Two numerical examples are illustrated with two distinct initial inflow conditions, which are included in two modes of MoSES_2PDF, respectively. In the numerical example of a large -scale event, the 2009 Hsiaolin event, the results computed by two distinct NVIDIA GPU cards (RTX-2080-Ti and Tesla-V100) are found to be identical but tiny deviation is figured out in comparison with the results computed by the conventional single-core CPU-code. It is speculated to be caused by the different structure in the source codes and some float/double operation. In addition to the illustration in GIS system, the computed results by MoSES_2PDF can also be shown with animated 3D graphics in the ANSI-Platform, where the user can interact with 3D scenes. The feasibility, features and facilities of MoSES_2PDF are demonstrated with respect to the two numerical examples concerning two real events.

We have performed a series of laboratory experiments that clarify the nature of the transition between fluid‐mud and grain‐flow behavior. The surface velocity structure and the speed of the nose of debris flows in channels with semicircular cross sections were measured with several cameras and visual tracers, while the mass flow rate was recorded using a load cell at the exit chamber. Other rheological tests were used to calculate independently the yield strength and matrix viscosity of the debris‐flow mixture. Shear rates were varied by nearly an order of magnitude for each mixture by changing the channel radius and slope. Shear rates were significantly higher than expected (6–55 s−1), given the modest slopes examined (10.7°–15.2°). The large values were primarily a result of the concentration of shear into narrow bands between a central nondeforming plug and the sidewall. As a result, the shear rate of interest was calculated by using the width of the shear band and the plug velocity, as opposed to the flow depth and front velocity. The slurries exhibited predominantly fluid‐mud behavior with finite yield strength and shear‐thinning rheologies in the debris‐flow body, while frictional behavior was often observed at the front, or snout. The addition of sand or small amounts of clay tended to make the body of the flows behave in a more Bingham‐like fashion (i.e., closer to a linear viscous flow for shear stresses exceeding the yield stress). The addition of sand also tended to accentuate the frictional behavior at the snout. Transition to frictional grain‐flow behavior occurred first at the front, for body friction numbers on the order of 100. Similar behavior has been observed in an allied field site in the Italian Alps. In the experiments, it was hypothesized that the snout‐grain‐flow transition was a result of concentration of the coarsest material at the flow front, reduced shear near the snout, and loss of matrix from the snout to the bed. Regardless of the frictional effects at the snout, flow resistance in the body was nearly always regulated by yield‐stress and shear‐thinning properties, with no discernible boundary slip, despite volumetric sand contents in excess of 50%.

We give a theory of permanent roll waves on a shallow layer of fluid mud which is modelled as a power-law fluid. Based on the long-wave approximation, Karman’s momentum integral method is applied to derive the averaged continuity and the momentum equations. Linearized instability analysis of a uniform flow shows that the growth rate of unstable disturbances increases monotonically with the wavenumber, and therefore is insufficient to suggest a preferred wavelength for the roll wave. Nonlinear roll waves are obtained next as periodic shocks connected by smooth profiles with depth increasing monotonically from the rear to the front. Among all wavelengths only those longer than a certain threshold correspond to positive energy loss across the shock, and are physically acceptable. This threshold also implies a minimum discharge, viewed in the moving system, for the roll wave to exist. These facts suggest that a roll wave developed spontaneously from infinitesimal disturbances should have the shortest wavelength corresponding to zero dissipation across the shock, though finite dissipation elsewhere. The discontinuity at the wave front is a mathematical shortcoming needing a local requirement. Predictions for the spontaneously developed roll waves in a Newtonian case are compared with available experimental data. Longer roll waves, with dissipation at the discontinuous fronts, cannot be maintained if the uniform flow is linearly stable, when the fluid is slightly non-Newtonian. However, when the fluid is highly non-Newtonian, very long roll waves may still exist even if the corresponding uniform flow is stable to infinitesimal disturbances. Numerical results are presented for the phase speed, wave height and wavenumber, and wave profiles for a representative value of the flow index of fluid mud.

We give a theory of permanent roll waves on a shallow layer of fluid mud which is modelled as a power-law fluid. Based on the long-wave approximation, Karman’s momentum integral method is applied to derive the averaged continuity and the momentum equations. Linearized instability analysis of a uniform flow shows that the growth rate of unstable disturbances increases monotonically with the wavenumber, and therefore is insufficient to suggest a preferred wavelength for the roll wave. Nonlinear roll waves are obtained next as periodic shocks connected by smooth profiles with depth increasing monotonically from the rear to the front. Among all wavelengths only those longer than a certain threshold correspond to positive energy loss across the shock, and are physically acceptable. This threshold also implies a minimum discharge, viewed in the moving system, for the roll wave to exist. These facts suggest that a roll wave developed spontaneously from infinitesimal disturbances should have the shortest wavelength corresponding to zero dissipation across the shock, though finite dissipation elsewhere. The discontinuity at the wave front is a mathematical shortcoming needing a local requirement. Predictions for the spontaneously developed roll waves in a Newtonian case are compared with available experimental data. Longer roll waves, with dissipation at the discontinuous fronts, cannot be maintained if the uniform flow is linearly stable, when the fluid is slightly non-Newtonian. However, when the fluid is highly non-Newtonian, very long roll waves may still exist even if the corresponding uniform flow is stable to infinitesimal disturbances. Numerical results are presented for the phase speed, wave height and wavenumber, and wave profiles for a representative value of the flow index of fluid mud.

Volcanic hazards assessments for critical facilities, such as nuclear power plants, often must consider the potential effects from phenomena that have very low likelihoods of occurrence. The International Atomic Energy Agency recently released a Technical Document that provides detailed guidance on conducting practical volcanic hazards assessments that include rare phenomena, using both deterministic and probabilistic approaches. This guidance develops a graded approach to the analysis, and provides a range of model complexities for assessing hazardous volcanic phenomena. Although this guidance focuses on the safety of nuclear installations, it is applicable to other sites that warrant consideration of low likelihood volcanic hazards that have potentially high consequences.

Underwater landslides are a common source of tsunamis in coastal areas. Theoretical and experimental studies on this problem, however, are scarce because of the difficulties in modeling and observing a submarine slide and the associated tsunami. To study this problem, we present a numerical model that simulates the coupling of a Bingham plastic mudslide on a gentle uniform slope with the surface waves which it generates. A formulation of the dynamics of the problem is presented, where the landslide is treated as an incompressible Bingham plastic flow and the water motion is assumed irrotational. The long-wave approximation is adopted for both water waves and the mudslide. Dispersion of waves and potential turbulent mixing are not considered. The resulting differential equations are solved by a finite difference method. We present the numerical results for successive profiles of the mud surface, the horizontal velocity distributions of the slide, the evolution of the surface elevations, and the distributions of the particle velocity of the water motion. Three major parameters dominate the magnitude of the waves (if the volume of the mud is fixed): the density of mud, the yield stress of the mud, and the depth of water at the mudslide site. Because of the yield stress, the slides stop on the slope when the shear stress exerted on the bottom become smaller than the yield stress. The Bingham plastic behavior of the mud significantly reduces the extent and the speed of the mudslide and also the magnitude of the surface waves generated. Comparison of the solution of the Bingham plastic mudslide model with a snow flow test is presented. The solution of a viscous fluid model has been derived as a special case of the present solution.

To simulate debris-flow behaviour from initiation to deposition, we derive a depth-averaged, two-phase model that combines concepts of critical-state soil mechanics, grain-flow mechanics and fluid mechanics. The model's balance equations describe coupled evolution of the solid volume fraction, m, basal pore-fluid pressure, flow thickness and two components of flow velocity. Basal friction is evaluated using a generalized Coulomb rule, and fluid motion is evaluated in a frame of reference that translates with the velocity of the granular phase, vs. Source terms in each of the depth-averaged balance equations account for the influence of the granular dilation rate, defined as the depth integral of ∇⋅vs. Calculation of the dilation rate involves the effects of an elastic compressibility and an inelastic dilatancy angle proportional to m−meq, where meq is the value of m in equilibrium with the ambient stress state and flow rate. Normalization of the model equations shows that predicted debris-flow behaviour depends principally on the initial value of m−meq and on the ratio of two fundamental timescales. One of these timescales governs downslope debris-flow motion, and the other governs pore-pressure relaxation that modifies Coulomb friction and regulates evolution of m. A companion paper presents a suite of model predictions and tests.

To predict mudflow dynamics a numerical model based on shallow-water equations is developed. Previous work has shown that mudflow behavior can be well represented by a Herschel-Bulkley model. In a laminar regime, generally observed in practice, a wall friction force expression has been theoretically deduced for steady uniform flows. For unsteady flows the wall friction force is assumed to be equal to the resistance of a steady uniform flow with the same depth and mean velocity. The numerical model parameters, directly deduced from Herschel-Bulkley model, are determined independently by rheological measurements. Because of the lack of precise field data, experimental results obtained on a physical model are used in order to validate the numerical model. The main features of the experimental transient flows carried out in a laboratory flume are, for some identified conditions, in fairly good agreement with numerical model predictions, without any additional parameter fitting.

This report is an update of the working manual part of 'The hydraulic engineering characteristics of estuarine muds' Report No SR77, December 1986, made in the light of research conducted at Hydraulics Research Ltd (HR) and invited comments received from end users in the industry. A considerable research programme has been undertaken by HR since the publication of Report No SR77 and valuable information has been gained in respect of the behaviour of mud during tidal cycles (Ref 2), the deposition of sediment from flowing water (Ref 20), the consolidation of weak mud beds (Ref 26), the effect of sand on the consolidation and erosion processes (Ref 27) and the response of mud beds under waves (Refs 32 and 33). These findings have been incorporated into this revised manual. Report No SR77 was widely circulated to consulting engineers, contractors, academics and staff at HR, with a request for their views on the reports technical content, style and usability. Approximately half of the recipients replied with helpful and, in many cases detailed comments. Overall, the general impression was positive and encouraging. This report has been drafted with these comments taken into account wherever possible. This report summarises, in an engineering form, the main processes of cohesive sediment behaviour, namely, deposition, consolidation and erosion. The data presented are intended to show the practicing engineer which parameters are important in each of the processes and to enable broad estimates of the rates of deposition, consolidation and erosion to be made based on a limited knowledge of the field conditions. The behaviour of cohesive sediment does vary considerably in quantitative terms from one source to another. Therefore, it is crucial that the engineer appreciates that estimates based on the data presented herewith may well be in error by half an order of magnitude. For most serious engineering problems involving cohesive sediment it would be essential to undertake a detailed study. This would involve some of the following techniques: field measurements, laboratory testing of sediment, numerical modelling of hydrodynamics and sediment transport and physical modelling of hydrodynamics.

This paper presents key outcomes of a 3-year Joint Industry Project funded by 6 Operators on the impact of submarine slides on pipelines. This JIP developed new techniques to simulate slide runout, and assess the resulting loading and deformation of seabed pipelines. The work was distilled into guidance for practical application, which has found adoption on projects. The JIP spanned (i) characterization of soils at the solid-fluid transition, (ii) computational modelling of slide runout – via depth-averaged and continuum finite element methods, (iii) physical and numerical modelling of slide runout and pipeline impact, and (iv) analytical studies of pipeline response during slide loading. These elements combine to provide an improved practical basis for quantifying the risk associated with slide-pipeline interaction. To characterize very soft seabed soils, a new geotechnically-based framework was devised based on extensive measurements of different soils. This framework spans the solid-fluid boundary that is crossed as slides evolve into a debris flow and turbidity current. It is shown that the geotechnical link between water content and shear strength extends continuously – with no phase transformation – far into the fluid domain, allowing a single rheology to be applied throughout. Computational modelling of slide runout used a hierarchy of methods, from large deformation finite element analysis (LDFE) (with rate effects and softening at soil element level), through depth-averaged runout, to energy-based analytical solutions. In some regimes of behavior the simpler methods suffice, allowing efficient use of Monte Carlo methods to tackle uncertainty. More complex runout modes can be replicated by newly-developed LDFE techniques. From a runout analysis results, pipeline impact loads can be assessed using new solutions for the bearing capacity and drag forces on pipelines developed from numerical and physical modelling, which again unify concepts from fluid dynamics and geotechnics. Finally, simple analytical methods for assessing the structural response of a pipeline to a known slide loading are provided. These solutions allow rapid assessment of the response of a pipeline to a specified slide loading. These advances improve the methods available for quantitative assessment of slide runout and slidepipeline interaction, allowing better determination of the resulting geohazard risk.

This paper concerns the rheological characterisation of liquefied sands as non-Newtonian Bingham fluids. For this purpose, dam-breaking laboratory tests are often executed and interpreted, offering a viable option to identify the properties of fluidised water-soil mixtures. However, limited attention has been devoted so far to clarify what variables and measurements would allow unambiguous calibration of Bingham parameters, namely, the viscosity η and the yield stress τ y . The numerical results of parametric studies based on the particle finite element method (PFEM) are critically inspected to gain deeper insight into the problem. First, it is confirmed that multiple η − τ y pairs may reproduce the same experimental evidence when formed by only one measurement—usually, the post–dam-breaking displacement of the bottom toe (tip) of the liquefied mass. Then, two alternative procedures are proposed for unambiguous identification of both η and τ y : one is based on monitoring the evolving aspect ratio of the fluid mass during free, gravity-driven flow; the other relies on a slightly different dam-breaking test, also including impact against a rigid obstacle. In particular, the latter approach reduces the relevant duration of the test, reducing the possible influence of reconsolidation effects on the calibration of rheological parameters.

The submarine flow slides and their hydraulic effects are studied by numerical means. These types of landslides are assumed to separate into a dense flow close to the bed and a turbulent dispersion above it. A two-dimensional fluid mechanics mixture model based on Navier-Stokes' equations has been developed to study water waves generated by these landslides. The dense part is considered as a viscoplastic fluid, whereas the dispersed part is modeled by an ideal fluid. The rheological parameters of the model comprise a diffusion coefficient, a Bingham yield stress, a viscosity coefficient, and friction on the slope. First, the numerical model is validated with an analytical solution for a viscous and a Bingham flow. Then, it has been tested for a rigid box sliding into water along an inclined plane. The results of this simulation have been compared with experiments conducted in a channel. Finally, laboratory experiments consisting in the slide of a gravel mass have been carried out. The results of these experiments have shown the importance of the sediment rheology and the diffusion. The model parameters are adjusted by trial and error to match the observed landslide flow.

The study of formation and evolution of waves in mud-flows is strongly motivated by their destructive power. The rheology of these flows, which involve massive solid transport, is often described using linear or non-linear viscoplastic models. The paper analyzes the wave dynamics of mud-flows, presenting the linearized response of the 1D uniform flow of a viscoplastic fluid with yield stress, i.e. Herschel & Bulkley fluid, to a pointwise impulsive forcing term. The solution in either linearly stable or unstable conditions is found, for both subcritical and supercritical flows, through the bilateral inverse Laplace transform. The analysis offers a unified description of the behaviour of different non-newtonian fluids, recovering as particular cases both the Bingham and the power-law models. The influence of the dimensionless governing numbers and of the rheological parameters on the shape and peak of waves is investigated.

The rheology of slurries consisting of ≤2-mm sediment from a natural debris flow deposit was measured using a wide-gap concentric-cylinder viscometer. The influence of sediment concentration and size and distribution of grains on the bulk rheological behavior of the slurries was evaluated at concentrations ranging from 0.44 to 0.66. The slurries exhibit diverse rheological behavior. At shear rates above 5 s−1 the behavior approaches that of a Bingham material; below 5 s−1, sand exerts more influence and slurry behavior deviates from the Bingham idealization. Sand grain interactions dominate the mechanical behavior when sand concentration exceeds 0.2; transient fluctuations in measured torque, time-dependent decay of torque, and hysteresis effects are observed. Grain rubbing, interlocking, and collision cause changes in packing density, particle distribution, grain orientation, and formation and destruction of grain clusters, which may explain the observed behavior. Yield strength and plastic viscosity exhibit order-of-magnitude variation when sediment concentration changes as little as 2–4%. Owing to these complexities, it is unlikely that debris flows can be characterized by a single rheological model.

The rheology of marine sediment-laden flows in a submarine environment is examined. The viscosity and yield stress of marine sediments are key parameters controlling landslide mobility: the former is related to the peak velocity and the latter is related to the final deposition of debris flow. One of the simplest rheological models is the ideal plastic Bingham model, which is often applied to diverse materials. For marine sediments, the flow behavior can be approximated by modified Bingham models, such as the bi-viscosity and Papanastasiou models. The flow of viscoplastic materials can then be described by unyielded (local shear stress τ is lower than a yield stress τy), yield surface (τ = τy), and post-yield (τ > τy) characteristics. It is concluded that modified Bingham models with a proper choice of flow parameters can be used to predict the unyielded, yielding and yielded zones for marine sediments. It is also demonstrated that in some cases the yield surface can be effectively obtained using the relationship between viscosity and shear rate.

The resuspension of contaminated cohesive sediments can impact water quality by mobilizing sediment particles and adsorbed contaminants. Changes in physicochemical and electrochemical environments around resuspended sediment particles may cause some contaminants to desorb into the water column. The contribution of contaminated sediments to degradation of water quality depends on an estimate of sediment resuspension rates. In this study, the resuspension of Georgia kaolinite sediments under varying pH conditions was investigated in laboratory flume experiments. Because the edge charge of kaolinite particles is pH dependent, kaolinite particles exhibit different modes of particle associations under varying pH conditions. The paper characterizes these particle associations and relates them to the resuspension of kaolinite sediments for varying pH values. Variations in sediment water content, changes from a stratified to a uniform sediment bed, changes in rheological properties, and variations in the electrophoretic mobility of kaolinite particles were all indicative of the changes in particle associations that resulted from changes in sediment pH. The critical shear stress and the erosion rate coefficient were evaluated for varying pH values and explained by particle associations. A rheometer was used to measure rheological properties of the settled sediment bed; the measured yield stresses had a direct correlation with critical shear stresses.