Displays

Sediment mobilization in small-medium size mountain catchments occurs by different flow types, categorized as debris-flows, hyperconcentrated-flows and water-flows, depending on the physical mechanisms governing flow rheology and particles interaction. During high-magnitude flow events, such transport mechanisms may take place concurrently in the same catchment at different sites of the channel network. One of the most important tasks in investigating dynamics of floods in these mountain catchments is to identify the transport mechanisms, since different flow types induce peculiar geomorphological hazards and dynamics. This work aims to improve criteria to recognize different flow types, with particular regard to hyperconcentrated-flows, and to analyze the transport mechanisms in mountain catchments in response to high-magnitude hydrological events.

Since direct monitoring of sediment mobilization during a flood is extremely difficult, a sound alternative is to consider the characteristics of the deposited material, which depend on the rheological proprieties of related flow. Through an extensive literature review, we identified the diagnostic criteria of the different flow types, grouping them in morphological and sedimentological evidences. A field-survey worksheet has been developed to ease the field-data collection and interpretation. The case-study selected for applying the survey procedure is the Tegnas catchment (Dolomites, Italy), a mountain basin draining an area of 51 km² affected by the Vaia Storm in October 2018, which induced large floods in several catchments of the Eastern Italian Alps. The deposits field-survey has been conducted in 35 sub-reaches of the Tegnas river and its major tributaries. In addition, we carried out detailed grain-size analyses, measured the angle of clasts-imbrication and collected samples for estimating the vegetal organic matter content through Loss-of-ignition procedure.

Field criteria allowed us to classify each sub-reach according to the deposits left after the event. Most of the steep tributaries have been interested by debris-flows, but also hyperconcentrated-flows have been recognized. Along the main stem, water-flow was the dominant process, although debris-flows and hyperconcentrated-flows deposits are documented where channel slope was very high (i.e. from 9 to 21%).   Hyperconcentrated-flow deposits occur also in the lower sub-reaches (i.e. channel slope from 0.3 to 6%), either at the confluence with debris-flow fed tributaries of where severe bank erosion occurred. We statistically analyzed data about clast imbrication angle (δ) and content of vegetal organic matter (OM_LOI) obtaining significant results for both parameters. δ measured in debris-flow (50°-65°) and hyperconcentrated-flow deposits (45°-60°) is considerably higher than in water-flow sediments (30°-40°). Debris-flow and hyperconcentrated-flow deposits have higher OM_LOI (2.5-5.5%) than water-flow deposits (1.5-3%). 

The combination of field diagnostic-criteria and quantitative measure of additional parameters allows a reliable recognition of the flow types based on post-flood survey. Besides, this study allowed to point out that during high-magnitude floods the sediment mobilization in small-medium size catchments occurs through mechanisms that can be different from those expected for ordinary hydrological events using morphometric approaches. Solid material concentration or dilution (e.g. due to lacking of sediment sources or sediment disconnectivity) can explain the “unexpected” flow types during high magnitude events.

How to cite: Brenna, A., Surian, N., Borga, M., Ghinassi, M., and Marchi, L.: Recognition and Occurrence of Different Sediment-Water Flows Triggered by High-Magnitude Hydrological Events in Mountain Catchments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7333, https://doi.org/10.5194/egusphere-egu2020-7333, 2020.

Nonsuspended sediment transport driven by streams of liquid or air is an important driver of the morphodynamics of planetary landscapes, seascapes, and riverscapes. Laboratory and field measurements of the sediment transport rate as a function of the fluid shear stress have enabled us to predict such processes with reasonable accuracy on Earth. However, sediment transport is also ubiquitous in extraterrestrial environments, such as on Venus, Mars, Titan, and occurs possibly even on Pluto. This raises the question of whether we can extrapolate transport rate expressions validated with measurements on Earth to extraterrestrial environments. The answer is probably, yes, but only if the used expressions capture the essential physics. Here, using coupled DEM/RANS numerical sediment transport simulations, we show that nonsuspended sediment transport in a large range of aeolian and fluvial environments has a conceptually simple common physical underpinning that allows treating these different transport regimes in a universal manner. That is, a conceptually simple universal model captures simulated and measured transport thresholds and transport rates. In particular, when the transport layer thickness substantially exceeds the viscous sublayer thickness (true for many environments), this model yields a mathematically simple transport rate expression that agrees, simultaneously, with existing measurements in air and water.

How to cite: Pähtz, T. and Duran, O.: A simple transport rate relation that unifies aeolian and fluvial nonsuspended sediment transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2074, https://doi.org/10.5194/egusphere-egu2020-2074, 2020.

The Shields curve, which compiles measurements of fluvial sediment transport thresholds in terms of the nondimensionalized threshold fluid shear stress and shear Reynolds number, is a standard reference in geophysics and hydraulic and coastal engineering and commonly thought to describe the critical flow conditions that are required for flow-driven entrainment of bed sediment. However, recent findings from several independent research groups have challenged this belief on various grounds: (i) particle-bed impacts predominately trigger sediment entrainment [1]; (ii) such impact-triggered entrainment can sustain continuous transport even when flow-driven entrainment events are (almost) completely absent [2, 3, (4)]; and (iii) extrapolating measurements of the transport rate of such impact-sustained continuous transport to zero yields transport thresholds that still fall on the Shields curve [5]. The question that thus emerges from these findings is, if not flow-driven entrainment, then what is the physics behind the thresholds shown in the Shields curve? We will try to give an answer to this question based on our latest research.

[1] Vowinckel et al., Journal of Hydraulic Research, 2016, doi: 10.1080/00221686.2016.1140683

[2] Pähtz & Duran, Physical Review Fluids, 2017, doi: 10.1103/PhysRevFluids.2.074303

[3] Lee & Jerolmack, Earth Surface Dynamics, doi: 10.5194/esurf-6-1089-2018

[4] Heyman et al., Journal of Geophysical Research: Earth Surface, 2016, doi: 10.1002/2015JF003672

How to cite: Duran Vinent, O. and Pähtz, T.: Have we misunderstood the Shields curve?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2218, https://doi.org/10.5194/egusphere-egu2020-2218, 2020.

Empirical sediment transport models have common characteristics suggestive of the underlying physics, but mechanistic explanations for these characteristics are lacking due to an incomplete understanding of the fundamental physical mechanisms involved. Hydrodynamic interactions at the grain-scale are thought to be key, however, it is a major challenge to either observe or model these processes. In order to improve our understanding of grain-scale dynamics in sediment entrainment and transport we are studying the detailed mechanics of fluid-grain interactions using a combination of laboratory flume experiments, advanced numerical simulations, and granular mechanics theory. 

The flume experiments are conducted with an emphasis on exploring differences and similarities in the behaviour of glass spheres, a common theoretical tool, to naturally sourced river gravel. Using high-speed cameras coupled with computer-vision based particle tracking, we tracked the majority of grains in the grain bed and water column, with 130,000 glass sphere track paths longer than 10 particle diameters. In particular, we introduce a newly developed a machine learning based particle tracking of the natural grains, with 30,000 gravel track paths longer than 10 mean particle diameters. Fluid flow fields are also observed using particle image velocimetry (PIV). We present the comparison of our detailed observations of granular dynamics between spheres and natural gravel, with a focus on how grain shape impacts fluid-grain and grain-grain interactions.

Using a discrete-element plus Lattice-Boltzmann fluid method (LBM-DEM) we simulate a small portion of the laboratory flume with high temporal and spatial resolution. This method tracks discrete particles interacting with each other through contact laws while mechanically coupled to a dynamic interstitial fluid. We discuss the ability of our simulations to emulate our experiments, the benefits of which are twofold. First, where the simulations work well, we use them to observe grain-scale dynamics that would be difficult or impossible to measure in a laboratory setting or in the field. Second, we learn from situations in which the experiments and simulations diverge, leading to improvements in both the simulations and our understanding of how fluid-grain interactions influence sediment transport.

How to cite: Deal, E., Perron, T., Venditti, J., Zhang, Q., Benavides, S., and Kamrin, K.: Observing and modeling bedload sediment transport at the grain-scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21982, https://doi.org/10.5194/egusphere-egu2020-21982, 2020.

Turbulent bedload transport has a major influence for riverbed evolution and is still lacking a general understanding for realistic configurations with arbitrary slopes and sediments shapes. In this contribution, we explore the importance of the granular bed resistance to the fluid flow. Based on the work of Maurin et al (2018), we show that a generalized version of the repose angle of the granular material can be defined, and is able to characterize the slope influence on sediment transport rate for particle scale simulations (Maurin et al, 2015) over a large range of slopes and fluid forcing (i.e. Shields number). Extending the configuration to arbitrary particle shapes, the sediment transport rate is shown to be correlated to the variation of the granular media repose angle (Monthiller 2019), and the relevance of the latter is discussed.

Maurin, R., Chauchat, J., Chareyre B. & Frey, P. (2015). A minimal coupled fluid-discrete element model for bedload transport, Physics of Fluids, 27, 113302
Maurin, R., Chauchat, J., & Frey, P. (2018). Revisiting slope influence in turbulent bedload transport: Consequences for vertical flow structure and transport rate scaling. Journal of Fluid Mechanics, 839, 135-156. doi:10.1017/jfm.2017.903
Monthiller, R. (2019), Particle shape influence on turbulent bedload transport, Master thesis ENSEEIHT/Toulouse Univ.

How to cite: Maurin, R., Monthiller, R., and Lacaze, L.: Taking into account granular bed resistance in turbulent bedload transport with arbitrary slope and particle shape, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21795, https://doi.org/10.5194/egusphere-egu2020-21795, 2020.

Natural granular flows have a widely dispersed grain size distribution. The majority of the numerical models and laboratory investigations of granular flows are developed assuming a single grain size. However, the geophysical massive flows involve several classes of particles and the bulk solid evolves spatially in a non-uniform state [1]. Segregation causes a different spatial distribution of the particles and influences the kinematic of the bulk solid, like the concentration, the run-out, the velocity and the granular temperature. During the flow motion, the largest particles are found at the surface due to the imbalances in the contact forces, and the smallest at the bottom as they percolate due to gravity [2].

To investigate the physical processes responsible of the particles transfer, we conducted a series of laboratory experiments, using two different grain size classes to reproduce the binary mixture. The measured data are required to calibrate the mathematical model and to set the coefficients that describe the percolation and the kinetic sieving mechanism. The experiments to study the free surface flow started considering the dry condition. Two different type of classes of particles flow over a loose deposit in homogenous and steady conditions. We used spherical particles of non-expanded polystyrene with a density of 1035 kg/m³. The small beads are black with a mean diameter of 0.00075 m and the large beads are white with a mean diameter of 0.0014 m. At the end of the flume there is a weir with two openings. The material is manually inserted and flow in the flume, it is then recirculated by an auger and finally conveyed in a hopper, from where it falls down in the chute again. The system works for at least 30 minutes, after reaching the steady condition.

The measurements were taken through a high speed camera in a section lateral to the flume. The flow field was measured with an optical method, that gives the velocity, the concentration and the granular temperature for both the small and the large particles, from the sidewalls.

Analyzing the experimental data, as regards the longitudinal velocity, it is possible to observe that the velocities of the two classes are similar and the large particles flow a bit faster. In contrast, there is a strong segregation in the concentration rates. After the running time, segregation causes the separation of the two classes: the largest classes are in the upper part and the smallest fraction at the bottom.

References

1 Drahun J.A., Bridgwater J. The mechanisms of free surface segregation, Powder Technology, 36, 39-53, 1988.

2 Savage S., Lun K.K. Particle size segregation in inclined chute of dry cohesionless granular solids, Journal of Fluid Mechanics, 189, 311-335, 1988.

How to cite: D'Agostino, S.: Experimental analysis of segregation in granular flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10063, https://doi.org/10.5194/egusphere-egu2020-10063, 2020.

We studied granular flows of glass beads on an inclined conveyor channel. An upward-moving belt conveyed particles that flowed down the channel under the action of gravity, thus creating a stationary flow. To visualize the internal dynamics of the bulk, we relied on the refractive index matching technique. Under fixed slope and belt velocity, we ran mono- and bi-disperse experiments to characterize spatially and temporally the dynamics and concentration fields of these granular flows. Mono-disperse experiments were done using 6 and 8 mm beads on slopes of 10, 12, 15 and 18° and 3 different belt velocities. Beads of 14 mm were added in concentrations of 10, 20, 30 and 40% for the bi-disperse experiments. The rear part of the flow exhibited well-arranged particle layers that moved relatively between them. This particle arrangement ended with a sharp transition to the front of the flow and a dilated convective front. Bi-disperse experiments with low concentrations of large particles conserved the same layered-convective regime with the few added large beads confined to the convective front, a result of size segregation. When the concentration of large beads was increased to 30%, the described regime disappeared. Large grains were frequently dragged back by the belt, thus disrupting the arrangement of particle layers. A quasi-stationary behavior was observed in these experiments, small particles migrated to the front of the flow in pulses that after a while were dragged back, repeating the cycle. We observed that particle concentration fields, on average, were consistent with the structures observed for the  breaking size-segregation wave phenomenon. The effective basal friction, local concentrations and dilation, among other variables, are responsible for these phenomena.

How to cite: Trewhela, T., Gray, N., and Ancey, C.: Flow regimes, grain mobility and size segregation in stationary bi-disperse granular flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18412, https://doi.org/10.5194/egusphere-egu2020-18412, 2020.

Debris flows are powerful natural hazards posing risk to life, infrastructure, and property.  Understanding the particle scale interactions in these flows is a key component in the development of models to predict the mobility, distal reach, and hazard posed by a given event. In this study we focus on the process of segregation in debris flows, using a large-scale landslide flume to explore segregation in mixtures of 25 mm, 12 mm, 6 mm, and 3 mm diameter particle sizes. Sample volumes, consisting of a multicomponent mixture of materials, up to 1 m³ in size are released at the top of a 6.8 m long, 2.1 m wide slope, inclined at 30 degrees to the horizontal to initiate flow. Subsequent analysis is completed to determine the extent of vertical and longitudinal segregation of the post-landslide deposit morphology. A range of experimental strategies are explored to provide quantitative measures of particle segregation. Particle size is identified via image analysis and various techniques are applied for the longitudinal sectioning of the deposit, using measurements of segregation at the sidewall of the transparent flume, contrasted with planes measured from within the centre of the deposit. Further, replicate experiments are shown to quantify the probabilistic variation in segregation for multicomponent mixtures of dry granular flows, as well as initially saturated granular flows, to explore the effect of pore fluid on segregation processes.

How to cite: Kimball, J. and Take, W. A.: Large-scale flume modelling of segregation processes in debris flows , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21072, https://doi.org/10.5194/egusphere-egu2020-21072, 2020.

Soil liquefaction is one of the most impactful secondary hazards of earthquakes. For example, it played a crucial role in driving the devastating landslides following the 2018 Palu earthquake, Indonesia. While traditionally, the initiation of liquefaction is treated as an undrained phenomenon, evidence shows that a well-drained end-member exists.

We develop a theory for the coupled grains - pore fluid system, and conduct numerical discrete element – fluid dynamics simulations and lab experiments under well-drained conditions. Here, a well-drained layer means that the interstitial fluid can flow out of the layer faster than a single earthquake shaking period. Theory, simulations, and experiments, all suggest that a saturated granular layer, although well-drained, can liquefy when subjected to horizontal cyclic shear. The liquefaction event, evident by high pore pressure, loss of shear strength, and dissipation of shear waves is spatially and temporally controlled by a compaction front that swipes upward through the layer. The compaction front separates the grain-fluid system into two sub-layers: The bottom sub-layer, below the front, is fully-compacted, and the pore pressure gradient across it is hydrostatic. The top sub-layer, above the front, is actively subsiding, and its pore pressure gradient reaches the total solid stress gradient. I.e., the fluid fully supports the granular skeleton. The velocity of the compaction front depends on the permeability of the soil layer and the viscosity of the interstitial fluid. Analytic considerations of the propagation rate of the compaction front allows us to evaluate the duration of a liquefaction event, the magnitude of soil subsidence, and the timing of water seepage at the surface level, which are all independent of the time scales related to the earthquake shaking. Our approach, when combined with field stratigraphy and groundwater level data, could explain and predict the occurrence and duration of soil liquefaction when the soil layer is effectively drained.

How to cite: Ben-Zeev, S., Aharonov, E., Goren, L., Toussaint, R., and Parez, S.: Compaction front controls soil liquefaction dynamics of drained saturated grain layers, as evident by theory, numerical simulations and lab experiments , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10688, https://doi.org/10.5194/egusphere-egu2020-10688, 2020.

The flow resistance in granular mass flows can be due to frictional contacts or collisional interactions between particles. For a constant number of particles, the transition from a frictional to a collisional regime is expected to depend on flow velocity and is associated with an increase of volume and a decrease of bulk density, an effect termed dilation. The relation between velocity, dilation and flow resistance is not well understood. Here we present results of steady, non-uniform flows of ceramic beads (d = 4 mm) in a rotating drum, a setup allowing observations and averaging of parameters measured over an extended period of time. We systematically varied flow mass between 12.3 and 49 kg and flow velocity between 0.2 and 1.2 m/s, while continuously measuring basal normal stress and flow depth. Flow resistance was assessed by calculating average bulk shear stress from torque measurements at the axis of the drum as well as from the deviation of the center of mass from the vertical. Additionally, the flows were captured by high-speed video recordings through the transparent side wall. We find bulk densities at the deepest section of the flow decreasing from 1430 kg/m³ at low velocities to 1370 kg/m³ at the highest velocity for the largest flow mass. At the same time flow resistance increased linearly. When the flow mass was reduced, also bulk density decreased, indicating the importance of overburden pressure for dilation. Video recordings revealed that shear is concentrated in depth zones of lower volume fraction. Our results shall contribute to a better understanding of the transition from a frictional to a collisional flow regime and may help to assess the importance of dilation for gravitational mass flows.

How to cite: Kaitna, R., Shuai, L., Taylor-Noonan, A., Take, W. A., McArdell, B., McElwaine, J., and Bowmann, E.: Dilation and flow resistance of granular flows in a rotating drum, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18418, https://doi.org/10.5194/egusphere-egu2020-18418, 2020.

Earh's surface is covered with soil; particulate mixtures subject to cycles of wetting and drying. The role of this transient hydrodynamic forcing in creating and destroying aggregates is virtually unexplored. We examine this process at the grain scale. When a colloidal suspension is dried, capillary pressure may overwhelm repulsive electrostatic forces, assembling aggregates that are out of thermal equilibrium. This poorly understood process confers cohesive strength to many geological and industrial materials. Here we observe evaporation-driven aggregation of natural and synthesized particulates, and then probe their stability under rewetting using a microfluidics channel as a flume to determine the entrainment threshold. We also directly measure bonding strength of aggregates using an atomic force microscope. Cohesion arises at a common length scale (~5 microns), where interparticle attractive forces exceed particle weight. In polydisperse mixtures, smaller particles condense within shrinking capillary bridges to build stabilizing “solid bridges” among larger grains. This dynamic repeats across scales forming remarkably strong, hierarchical clusters, whose cohesion derives from grain size rather than mineralogy. Transient capillary pressures are even sufficiently large to sinter the smallest particles together. These results may help to understand the strength and erodibility of natural soils, and other polydisperse particulates that experience transient hydrodynamic forces.

How to cite: Jerolmack, D. and Seiphoori, A.: Formation of stable aggregates by fluid-assembled solid bridges, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5806, https://doi.org/10.5194/egusphere-egu2020-5806, 2020.

This contribution presents results of field measurements of wind-blown sand streamers and turbulent flow structures in the boundary layer airflow during gale-force winds on a beach. Sand transport and streamers were measured using Large-Scale Particle Image Velocimetry (LSPIV) combined with laser particle sensors (Wenglors), and airflow turbulence was monitored with a co-located sonic anemometer. The data analysis yields insight into the precise spatio-temporal relationships between sand streamers and near-surface airflow turbulence, at a high resolution of 25 Hz and a centimetre scale, including how high-energy sweeps correlate with the passage of fast-moving saltation clusters, and how Turbulent Kinetic Energy (TKE) may be linked to sediment mass flux.

How to cite: Baas, A.: Wind-Blown Sand Streamers and Turbulent Flow Structures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7777, https://doi.org/10.5194/egusphere-egu2020-7777, 2020.

In the state-of-the-art for suspended-load modeling it is commonly assumed that the concentration profile results from a balance between a settling flux, in which the settling velocity is considered as equal to its value for a single settling particle in quiescent water, and an upward turbulent flux modeled using a Fickian gradient diffusion approximation. While this model provides a general framework, comparison with experiments reveals that the concentration diffusivity is not equal to the eddy viscosity and a turbulent Schmidt number needs to be introduced. Based on Coleman (1970,1981) data, van Rijn (1984) proposed an empirical model which suggests that the Schmidt number is a decreasing function of W_s/u_*. This result is intriguing as it suggests that the turbulent dispersion of sediment concentration is enhanced when the particle’s settling velocity increases relative to the bed friction velocity. Van Rijn suggested that this is due to centrifugal forces that tends to throw inertial particles out of the turbulent vortices leading to an enhanced particle dispersion compared to momentum. In the present contribution, we use high-resolution experimental data and turbulence resolving two-phase flow simulations that directly resolve the turbulent momentum and particle fluxes and the flow turbulence to investigate the different terms appearing on the mass balance mentioned above.  Both the experimental and the numerical results show that the actual turbulent Schmidt number based on the resolved sediment flux is higher than unity meaning that turbulent dispersion efficiency of « heavy particles » is reduced. This contradicts van Rijn’s prediction model of the Schmidt number. One plausible explanation is that the settling velocity of particles is reduced in highly turbulent flows. Using the experimental and numerical results, the actual settling velocity in the turbulent flow is retrieved from the mass balance at steady state. It is found that it is significantly retarded compared with the value in quiescent water (10 to 40%). This result is in good agreement with the one obtained in recent experiments performed in a turbulent grid at KIT (Germany) using the same particles (Akutina et al., 2020). The authors found a settling retardation of 16% for the same turbulent intensities as in the present experiments. The results presented herein completely change the paradigm for turbulent suspension load modeling and open new perspectives on the development of new, physical process-based, parametrizations required for large-scale models. This, of course, will require to extend the proposed methodology to a wider range of flow and sediment conditions.

How to cite: chauchat, J., Revil-Baudard, T., Cheng, Z., Hurther, D., and Hsu, T.-J.: On the physical origin of enhanced turbulent-dispersion of 'heavy particles' in suspended-load, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14748, https://doi.org/10.5194/egusphere-egu2020-14748, 2020.

Turbidity currents are a major way to transport sediment along reservoir, lake and sea beds. They are not fully understood yet due to the difficulty of accessibility. Theoretical criteria have been established for the conditions that generate accelerating turbidity currents, which can produce strong erosion of channel beds, transmit over long distances and thus have important significance for reservoir and sea bed morphology. However, the current theoretical criterion only utilizes local factors of hydraulic, morphology and grain size, which do not necessarily depend on the up- and down- stream boundary conditions. Here, we conducted field surveys on turbidity currents and bed morphology of the Xiaolangdi reservoir on the Yellow River, China. The survey results show clear evidence of accelerating turbidity currents. We identify two types of accelerating turbidity currents: one locates closely to the upstream plunging point where fluvial sediment-laden flow collapses to a stratified turbidity current, concentrating momentum and producing acceleration locally, and the other is located downstream and shows dependence on the enhancement of local slope and potentially on downstream boundary (flushing condition at flow outlets of the dam). So both ends of the boundaries may work together to produce long run-out turbidity currents that transmit through the entire reservoir.  Although preliminary, our dataset indicates that the conditions for accelerating turbidity currents are not only controlled by local morphology and grain size, but also by both upstream and downstream conditions. A comprehensive understanding of the boundary conditions so as to determine conditions for the generation of accelerating turbidity currents will help enhance the sustainability of the dam and reservoir system.

How to cite: Ma, H., Parker, G., Nittrouer, J., McElory, B., Wang, Y., Chen, X., and Fu, X.: Controls of boundary conditions on accelerating turbidity currents in a reservoir: Case study of Xiaolangdi Reservoir on the Yellow River, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20944, https://doi.org/10.5194/egusphere-egu2020-20944, 2020.

Abstract

Recent experiments have established that the sediment particle motion, especially for particles near the bed, may not follow the normal (Fickian) diffusion behavior. To modify the diffusion equation where the fluctuation velocity is based on the normal distribution, this investigation hypothesizes that the fluctuation velocity based on bivariate probability distributions and particle-bed collision in open channel can provide some physical insight into the particle diffusion behavior. The distribution of fluctuation velocity is obtained using the Gram-Charlier expansion which considers the first four statistical moments of turbulent fluctuation velocity. The correlation between two-dimensional fluctuation velocities is modeled by performing Monte Carlo simulations. Besides, the uniform momentum zones (UMZ) are further identified and consequently the spatial locations of the edges that demarcate UMZs can be estimated. Once UMZs in the turbulent boundary layers can be characterized, the streamwise momentum deficit, and occurrences of ejection events and sweep events in the vicinity of UMZ edges under different Reynolds numbers can be simulated. The spatial influence of turbulent coherent structures on sediment particle trajectory can be demonstrated.

How to cite: Tsai, C. and Wu, K.-T.: Beyond Normality: Estimation of Near-Bed Sediment Concentrations Accounting for Asymmetric Distribution and Spatial Influence of Turbulence Coherent Structures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21416, https://doi.org/10.5194/egusphere-egu2020-21416, 2020.

Overtopping of fluvial dikes occurs frequently during major floods and may lead to dike failure, with severe consequences in the protected areas. Mechanisms of fluvial dike breaching remain incompletely understood, while predicting the breach hydrograph is of paramount importance for the flood risk management.

Here, we present a new series of laboratory experiments, in which the evolving 3D fluvial dike geometry was monitored in detail using the laser profilometry technique. The experimental setup extends over about 20 m by 7 m and accommodates a 15 m long main channel and a 7 m-long dike section. The facility is located at LNHE of EDF-R&D (France). The present study extends former experiments by Rifai et al. (2017, 2018), which were conducted with uniform coarse sand (d₅₀ = 1.03 mm). In the new tests, various mixtures of coarse (d₅₀ = 1.03 mm) and fine (d₅₀ = 0.24 mm) sands were used as dike material (Rifai et al., 2020). The fraction of fine sand was varied systematically to assess its influence on the breaching process, specifically as regards the apparent cohesion.

The experimental observations reveal that the frequency of breach slope collapse tends to decrease as the fraction of fine sand is increased; but the collapsing volumes become larger. Consequently, in the tested configurations, the addition of fine sand to the dike material has virtually no effect on the overall breaching dynamics, due to compensation between less frequent but larger collapsing material volumes. In the presentation, the relative importance of the effects will be discussed in comparison with other influencing parameters such as the main channel discharge, floodplain backwater effects and the dike geometry.

All experimental data, including high resolution 3D dynamic models of the breach geometry, are publicly available online (Rifai et al., 2019).

References

Rifai, I., Erpicum, S., Archambeau, P., Violeau, D., Pirotton, M., El kadi Abderrezzak, K., & Dewals, B. (2017). Overtopping induced failure of noncohesive, homogeneous fluvial dikes. Water Resources Research, 53(4), 3373-3386.

Rifai, I., El kadi Abderrezzak, K., Erpicum, S., Archambeau, P., Violeau, D., Pirotton, M., & Dewals, B. (2018). Floodplain backwater effect on overtopping induced fluvial dike failure. Water Resources Research, 54(11), 9060-9073.

Rifai, I., El kadi Abderrezzak, K., Erpicum, S., Archambeau, P., Violeau, D., Pirotton, M., & Dewals, B. (2019). Flow and detailed 3D morphodynamic data from laboratory experiments of fluvial dike breaching. Scientific data, 6(1), 53.

Rifai, I., El kadi Abderrezzak, K., Hager, W.H., Erpicum, S., Archambeau, P., Violeau, D., Pirotton, M., & Dewals, B. (2020). Apparent cohesion effects on overtopping-induced fluvial dike breaching. Journal of Hydraulic Research. In press. https://doi.org/10.1080/00221686.2020.1714760.

How to cite: Dewals, B., Rifai, I., El kadi Abderrezzak, K., Schmitz, V., Hager, W., Violeau, D., Archambeau, P., Pirotton, M., and Erpicum, S.: Addition of fine material is expected to strengthen fluvial dikes ... does it really?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11785, https://doi.org/10.5194/egusphere-egu2020-11785, 2020.

Due to their stochastic and fluctuating nature, the entrainment and transport of sediment particles present great challenges to scientists and engineers who endeavor to formulate a universal framework for quantifying sediment dynamics and transport behaviors. The application of impulse theory represents one of such efforts and has received increased attention over the past decade. Practically, the impulse concept helps to remove the inactive periods of transporting process from the entire transport history, such that the underlying driving mechanism of particle movements can be better identified. This approach not only proves to be useful in characterizing the threshold of motion conditions, as tested against experimental data from an increased body of literature, but also provides a new perspective in formulating the constitutive relation of bedload transport. In this study, we employ a similar criterion, yet based on the pertinent amounts of energy imparted upon sediment particles, to reproduce the stress-transport relations. These post-conditioned stress-transport relations are almost devoid of the inactive periods and, thus, better represent the physics of transport of sediment particles. It is noted that a consistent 1.5th power law has been recovered from a wide range of transport flow conditions, which can be deemed as a constitutive relation for bedload transport. Further examination of these data sets indicates that, in essence, the obtained 1.5th power relation accounts for a constant energy transfer efficiency of fluid flow applied upon the sediment particles. These efforts, based on the impulse concept, lead to a unified approach to sediment transport problems.

How to cite: Diplas, P. and Shih, W.: Impulse concept in formulating a unified approach to bedload transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9217, https://doi.org/10.5194/egusphere-egu2020-9217, 2020.

An ongoing research effort is reported on sediment transport multi-scale modeling in the framework of the ANR project SegSed ‘Size SEGregation in SEDiment transport’. Using a coupled fluid-discrete element model, a variety of numerical experiments were carried out in 2D and 3D bedload configurations studying the dynamics of the depth structure of mono- and bidisperse mixtures. Such models allow access to internal processes in the case of grain sorting and to variables very difficult to measure in the laboratory such as particle shear stress and rate. These variables are the key ingredients to derive constitutive relationships. Such a relationship inspired by one used in dry granular flows was successfully implemented in a Eulerian-Eulerian two-phase flow model. Progress is being made with a multi-class model where the momentum balance for each grain size class should be inferred by discrete element modelling. Ultimately, such studies could be useful to improve Exner-shallow water-type models in particular when grain sorting is considered.

How to cite: Frey, P., Rousseau, H., Chassagne, R., Maurin, R., and Chauchat, J.: Discrete element methods and continuum models in bedload transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-78, https://doi.org/10.5194/egusphere-egu2020-78, 2020.

Scour has been recognized as one of the primary reasons for bridge pier destabilization. As extreme weather intensifies and hydraulic infrastructure such as bridge piers and abutments (many constructed since the Victorian era, for the case of the UK) continues to age, the challenge of scour-induced hazards will keep growing impacting the resilience of our society. Thus, there is an increasing value in studying the highly dynamical process of scour around hydraulic infrastructure. Maximum scour depth estimation has been broadly studied by researchers over the past decades, using phenomenological or empirical approaches, linking mean flow properties, bridge pier and riverbed materials characteristics [1, 2].

This study aims to get a better understanding of how the turbulent flow field modified by the bridge pier, interacts with the bed surface towards the generation of the scour hole. This is pursued by following a dynamical approach via assessing the flow structures that are sufficiently energetic [3] to remove bed material from the vicinity of the bridge pier.

A series of scour experiments with different lengthscale of model bridge piers is conducted in a water recirculating research flume. For each of these cases flow velocity profiles are collected downstream the bridge pier using high resolution acoustic Doppler velocimetry (ADV). Using the raw data collected near the bed surface and information for the bed surface material, the criterion of impulse [4] is used as a metric for assessing the extend and maximum scour depth. The results are compared for the different measurement locations are compared to better understand the process of scour downstream different model piers.

[1]. M Valyrakis, P Michalis, H Zhang, (2015). A new system for bridge scour monitoring and prediction, Proceedings of the 36th IAHR World Congress, 1-4.

[2]. Yagci, O., Celik, M. F., Kitsikoudis, V., Ozgur Kirca, V.S., Hodoglu, C., Valyrakis, M. , Duran, Z. and Kaya, S. (2016) Scour patterns around isolated vegetation elements. Advances in Water Resources, 97, pp. 251-265.(doi:10.1016/j.advwatres.2016.10.002)

[3]. Valyrakis, M. , Diplas, P. and Dancey, C.L. (2013) Entrainment of coarse particles in turbulent flows: an energy approach. Journal of Geophysical Research: Earth Surface, 118(1), pp. 42-53. (doi:10.1029/2012JF002354)

[4]. Valyrakis, M. , Diplas, P., Dancey, C.L., Greer, K. and Celik, A.O. (2010) Role of instantaneous force magnitude and duration on particle entrainment. Journal of Geophysical Research: Earth Surface, 115(F02006), 18p. (doi:10.1029/2008JF001247)

How to cite: Xu, Y., Valyrakis, M., and Michalis, P.: Assessing energetic flow structures responsible for bridge pier scour, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1502, https://doi.org/10.5194/egusphere-egu2020-1502, 2020.

In recent years, the impact of landslides on society has increased due to increasing urbanisation and climate change (as much as up to 30%). In about a decade, around 5000 fatal non-seismic landslides have occurred world-wide resulting in almost 56000 deaths, most of which took place in developing countries, such as China and Philippines. The purpose of studying the characteristics of landslides is to develop a better understanding of their features and to reduce any threat posed by them. Out of these characteristics the runout distance directly determines the impact of the landslide and extend of the affected area which are useful in evaluating risk to infrastructure (such as road pavement or railroad or built structures). Therefore, the study of landslide runout distance prediction has great significance for urban planning and risk assessment, specifically in mountainous areas.

This study focuses on conducting a review of previous literature on landslides reported at the region of Wenchuan in Sichuan (China), aiming to identify any trends connecting the cause and effect relationship between landslides in a phenomenological and empirical manner. Specifically, a dataset of landslides (20 due to rainfall and 50 due to earthquake) is used to statistically link, using multiple regression analysis, the travel distance to five main influencing factors, including landslide volume, height of landslide, landslide plane form, landslide average thickness and relative coefficient of friction. Good results are obtained through error minimisation rendering the developed framework as a useful tool for predictive analysis of the potential extend and impact of landslides using historical regional data.

How to cite: Gu, P. and Valyrakis, M.: A regression analysis framework for the prediction of runout distance of landslides: a case study for Sichuan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1505, https://doi.org/10.5194/egusphere-egu2020-1505, 2020.

Large number of bridges in Europe is at the end of their life span, while the frequency of occurrence for extreme climatic events, driven by climate change, is increasing. Floods influence morphodynamic changes in the riverbed, such as scouring of the riverbed next to the bridge substructure, that can undermine the overall stability of the bridge. Placement of riprap protection around bridge piers is an approach that doesn’t solve scouring problem, it rather displaces the scour hole elsewhere in the river channel, where its location is unknown because it is formed in the interaction between the flow and the structure, in site-specific conditions. Traditional approach to scour monitoring is effective only if surveys are conducted during the flood conditions, while the data acquired post-flood can underestimate the full potential of flood hazard. Detailed field surveys of hydraulic parameters during floods are essential in the understanding of morphodynamic evolution of the river channel, but are often scarce because they are time-consuming and require extensive resources (e.g. the survey equipment). Therefore, the majority of research was conducted using hydraulic flumes where both flow and the riverbed conditions are idealized 

The goal of the R3PEAT project (Remote Real-time Riprap Protection Erosion AssessmenT on large rivers) is to bridge the gap between the real-time scour hole development and flow environment through development of real-time scour monitoring system. The research focus of the project is investigation of scouring processes next to the riprap protection around bridge piers - existing structures whose stability and safety are unknown in the hydraulic environment under the influence of climate change. Research methodology combines experimental investigations on scaled physical model (Phase I) with 3D numerical model (Phase II) into hybrid modelling approach, calibrated and validated with field surveys. The research objectives of the project are: (1) develop ScourBuoy prototype (2); calibrate the physical model with field surveys; (3) improve existing empirical equations for equilibrium scour hole development using hybrid modelling approach; (4) investigate the dependence between turbulent flow characteristics and temporal scour hole development and (5) investigate dependence between turbulent conditions and incipient motion of sediment particles. The impact of the proposed project on the bridge management systems is expected through the development of a practical remote real-time system for erosion estimation around the riprap protection on large rivers that can be basis for the real-time decision support system.

Acknowledgment:
This work has been supported in part by Croatian Science Foundation under the project R3PEAT (UIP-2019-04-4046)

How to cite: Gilja, G., Cikojević, A., Potočki, K., Varga, M., and Adžaga, N.: Remote Real-time Riprap Protection Erosion AssessmenT on large rivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1933, https://doi.org/10.5194/egusphere-egu2020-1933, 2020.

Embankments, found in virtually all transportation and river networks, can be subjected to severe scouring and erosion issues due to more intensified climatic change, which may increase their failure risk [1]. Monitoring of embankment conditions with modern means is essential for ensuring the structural stability of nearby infrastructure (eg. roads and rail networks) against any geotechnical and hydraulic hazards [2, 3]. Additive manufacturing (AM), commonly referred to as 3D printing (3DP), is increasingly finding applications in the construction industry and is defined by the American Society for Testing and Materials (ASTM) International Committee as “the process of joining materials to make objects from 3D model data, usually layer upon layer”. This research is demonstrating the application of additive manufacturing technology in producing an electrical resistance strain gauge mechanism [2] to monitor the probability of embankment scouring failure, thus, warning could be given prior any devastating catastrophes, and preventive measures could be implemented accordingly. Electrical resistance strain gauges could be manufactured utilizing a dual-extrusion 3D printer which allows simultaneous depositions of a conductive material and a structural material in one print. Specifically, a range of control parameters are assessed here including different arrangements of the conductive material within the structural material matrix as well as infill percentages. The parameters aforementioned have effects on the gauge factor of the strain gauges produced. Overall, the 3DP sensors could be deployed to monitor embankment slope failure attributed to erosion, flooding and external loading (eg. due to heavy vehicle passage over it, for road embankments), which are important challenges [2, 3].

Acknowledgements

This research project has been funded by Transport Scotland, under the 2019/20 Innovation Fund (Scheme ID18/SE/0401/014) and the Scottish Road Research Board (Student research competition award 2019).

References

[1] Koursari, E., Wallace, S., Valyrakis, M. and Michalis, P. (2019). The need for real time and robust sensing of infrastructure risk due to extreme hydrologic events, 2019 UK/ China Emerging Technologies (UCET), Glasgow, United Kingdom, 2019, pp. 1-3. doi: 10.1109/UCET.2019.8881865

[2] Michalis, P., Saafi, M. and Judd, M. (2012) Wireless sensor networks for surveillance and monitoring of bridge scour. Proceedings of the 11th International Conference of Protection and Restoration of the Environment (KatsifarakisKL, Theodossiou N, Christodoulatos C, Koutsospyros Aand Mallios Z (eds)). Thessaloniki, Greece, pp. 1345–1354

[3] Michalis, P.; Konstantinidis, F.; Valyrakis, M. (2019) The road towards Civil Infrastructure 4.0 for proactive asset management of critical infrastructure systems. Proceedings of the 2nd International Conference on Natural Hazards & Infrastructure (ICONHIC), Chania, Greece, 23–26 June 2019.

How to cite: Chen, H., Michalis, P., and Valyrakis, M.: Additive Manufacturing of electrical strain gauges for the monitoring of embankment failures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5866, https://doi.org/10.5194/egusphere-egu2020-5866, 2020.

Under climate change, shifting  weather conditions, (both in terms of increasing frequency and intensifying magnitude) result in increasing occurrence of catastrophic failures of the constantly exposed and ageing infrastructure, across the world. Energetic flow events, advected past hydraulic infrastructure (such as bridge piers and abutments), may lead to scour [1, 2, 3], which is the primary cause of bridge collapses, resulting in high socio-economical costs, including loss of life.

This research aims to demonstrate the use of a novel monitoring device for the assessment of scour initiated by turbulent flows. This is pursued via the use of a miniaturized instrumented particle, namely “smart-sphere”, to record directly the frequency of entrainment from its downstream placement a model bridge pier at the Water Engineering lab of the University of Glasgow [4, 5, 6]. The change in entrainment frequencies is used as a metric to assess the increasing risk to scour, with increasing flow conditions, recorded acoustic Doppler velocimetry (ADV). The utility of the method as well as the potential use of the acquired data for prediction of bridge pier scour is presented and the tool as well is discussed with the potential for use to an appropriate field site [7, 8, 9].

Acknowledgments

This research project has been supported by Transport Scotland, under the 2019/20 Innovation Fund and the Student research award.

References

[1] Pähtz, Th., Clark, A., Duran, O., Valyrakis, M. 2019. The physics of sediment transport initiation, cessation and entrainment across aeolian and fluvial environments, Reviews of Gephysics, https://doi.org/10.1029/2019RG000679.

[2] Yagci, O., Celik, F., Kitsikoudis, V., Kirca, O., Hodoglu, C., Valyrakis, M., Duran, Z., Kaya S. 2016. Scour patterns around individual vegetation elements, Advances in Water Resources, 97, pp 251-265, doi: 10.1016/j.advwatres.2016.10.002.

[3] Michalis, P., Saafi, M. and M.D. Judd. (2012) Integrated Wireless Sensing Technology for Surveillance and Monitoring of Bridge Scour. Proceedings of the 6th International Conference on Scour and Erosion, France, Paris, pp. 395-402.

[4] Valyrakis, M. & Pavlovskis, E. 2014. "Smart pebble” design for environmental monitoring applications, In Proceedings of the 11th International Conference on Hydroinformatics, Hamburg, Germany.

[5] Valyrakis M., A. Alexakis. 2016. Development of a “smart-pebble” for tracking sediment transport. International Conference on Fluvial Hydraulics River Flow 2016, St. Liouis, MO, 8p.

[6] Valyrakis, M., Farhadi, H. 2017. Investigating coarse sediment particles transport using PTV and “smart-pebbles” instrumented with inertial sensors, EGU General Assembly 2017, Vienna, Austria, 23-28 April 2017, id. 9980.

[7] Valyrakis, M., Diplas, P., Dancey, C.L. 2011. Prediction of coarse particle movement with adaptive neuro-fuzzy inference systems, Hydrological Processes, 25 (22). pp. 3513-3524. ISSN 0885-6087, doi:10.1002/hyp.8228.

[8] Valyrakis, M., Michalis, P., Zhang, H. 2015a. A new system for bridge scour monitoring and prediction. Proceedings of the 36th IAHR World Congress, The Hague, the Netherlands, pp. 1-4.

How to cite: Corrigan, A., Elmubarak, H., Xu, Y., Michalis, P., and Valyrakis, M.: Bridge pier scour hazards assessment using smart-spheres, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8341, https://doi.org/10.5194/egusphere-egu2020-8341, 2020.

Numerical simulations for the transport of coarse sediment particles in turbulent flows are performed, with particular emphasis on the energy and momentum exchange [1, 2, 3] between the two phases at the particle scale.  The solid particles positions and velocities are solved through the Discrete Element Method (DEM), coupled with a Computational Fluid Dynamics (CFD) model which updates the dynamically evolving flow field through the numerical solution of the Reynolds Averaged of Navier-Stokes equations (RANS).

At the core of this work, the coupling of these two models (DEM-CFD) based on the Fictitious Boundary Method, is analysed. The models have a high mesh resolution, by adopting a meshing strategy which aims at sufficiently discretising the flow field surrounding each particle. Smooth and rough bed cases are simulated, under a wide range of Reynolds numbers covering applications from particle entrainment, up to bulk bedload transport through rolling and saltation. The numerical results are benchmarked against experimental data obtained from controlled laboratory experiments [4, 5, 6].

The implementation of coupled CFD-DEM models provides a very powerful tool for improving the understanding of fluid and particle physics in sediment transport. Particularly, the potential to perform a large number of validated numerical that robustly predict geomorphological changes in aquatic environments and fluvial systems.

References

[1] Valyrakis M., P. Diplas, C.L. Dancey, and A.O. Celik. 2008. Investigation of evolution of gravel river bed microforms using a simplified Discrete Particle Model, International Conference on Fluvial Hydraulics River Flow 2008, Ismir, Turkey, 03-05 September 2008, 10p.

[2] Valyrakis M., Diplas P. and Dancey C.L. 2013. Entrainment of coarse particles in turbulent flows: An energy approach. J. Geophys. Res. Earth Surf., Vol. 118, No. 1., pp 42- 53, doi:340210.1029/2012JF002354.

[3] Pähtz, Th., Clark, A., Duran, O., Valyrakis, M. 2019. The physics of sediment transport initiation, cessation and entrainment across aeolian and fluvial environments, Reviews of Gephysics, https://doi.org/10.1029/2019RG000679.

[4] Valyrakis, M. & Pavlovskis, E. 2014. "Smart pebble” design for environmental monitoring applications, In Proceedings of the 11th International Conference on Hydroinformatics, Hamburg, Germany.

[5] Valyrakis M., A. Alexakis. 2016. Development of a “smart-pebble” for tracking sediment transport. International Conference on Fluvial Hydraulics River Flow 2016, St. Liouis, MO, 8p.

[6] Valyrakis, M., Farhadi, H. 2017. Investigating coarse sediment particles transport using PTV and “smart-pebbles” instrumented with inertial sensors, EGU General Assembly 2017, Vienna, Austria, 23-28 April 2017, id. 9980.

How to cite: Latessa, G., Xu, D., Ji, C., and Valyrakis, M.: Numerical modelling of particle-fluid interaction in fluvial sediment transport , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10186, https://doi.org/10.5194/egusphere-egu2020-10186, 2020.

Road infrastructure is expected to face extreme pressure due to ageing and climatic extremes [1] as evident by recent cases of flash floods followed by drought periods. Among the most vulnerable elements of civil infrastructure are considered to be the road embankments that are not expected to withstand the prospective flood extremes. Seepage and internal erosion patterns inside the body of embankments are difficult to be assessed with conventional methods (e.g. visual inspections) and therefore go undetected leading to irreversible effects with major disruption and costs to road asset owners and maintainers. Flood-induced hazards can cause sudden collapse of bridge infrastructure without prior warning, and with significant socio-economic impacts [2]. Various sensor applications have focused on the development of monitoring systems to assess in real-time hydro and geo-hazards [2, 3, 4, 5]

This study focuses on the development and application of a real-time geo-monitoring system at a pilot road embankment in Scotland (UK) to remotely assess the evolving characteristics of hydro-hazards. The system will also provide early warning of such hazards and timely information to asset owner for proactive actions and early maintenance to avoid irreversible and costly major rehabilitation activities.

[1] Michalis, P., Konstantinidis, F. and Valyrakis, M. (2019) The road towards Civil Infrastructure 4.0 for proactive asset management of critical infrastructure systems. Proceedings of the 2nd International Conference on Natural Hazards & Infrastructure (ICONHIC2019), Chania, Greece, 23–26 June 2019, pp.1-9.

[2] Koursari, E., Wallace, S., Valyrakis, M. and Michalis, P. (2019) The need for real time and robust sensing of infrastructure risk due to extreme hydrologic events, 2019 UK/ China Emerging Technologies (UCET), Glasgow, United Kingdom, 2019, pp. 1-3. doi: 10.1109/UCET.2019.8881865

[3] Michalis, P., Saafi, M. and Judd, M. (2012) Wireless sensor networks for surveillance and monitoring of bridge scour. Proceedings of the XI International Conference Protection and Restoration of the Environment - PRE XI. Thessaloniki, Greece, pp. 1345–1354

[4] Valyrakis M. and Alexakis, A. (2016) Development of a “smart-pebble” for tracking sediment transport. International Conference on Fluvial Hydraulics River Flow 2016, St. Liouis, MO, 8p.

[5] Michalis, P., Saafi, M. and M.D. Judd. (2012) Integrated Wireless Sensing Technology for Surveillance and Monitoring of Bridge Scour. Proceedings of the 6th International Conference on Scour and Erosion, France, Paris, pp. 395-402.

Acknowledgements: This research project has been funded by Transport Scotland, under the 2019/20 Innovation Fund (Scheme ID19/SE/0401/032).

How to cite: Michalis, P., Xu, Y., Koursari, E., Wallace, S., and Valyrakis, M.: Innovative geo-monitoring system to assess hydro-hazards at road embankments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10218, https://doi.org/10.5194/egusphere-egu2020-10218, 2020.

Ageing infrastructure alongside with extreme climatic conditions pose a major threat for the sustainability of civil infrastructure systems with significant societal and economic impacts [1]. A main issue also arises from the fact that past and existing methods that incorporate the risk of climatic hazards into infrastructure design and assessment methods are based on historical records [2].

Major flood incidents are the factor of evolving geomorphological processes, which cause a drastic reduction in the safe capacity of structures (e.g. bridges, dams). Many efforts focused on the development and application of monitoring techniques to provide real-time assessment of geomorphological conditions around structural elements [1, 3, 4]. However, the current qualitative visual inspection practice cannot provide reliable assessment of geomorphological effects at bridges and other water infrastructure.

This work presents an analysis of the useful experience and lessons learnt from past monitoring efforts applied to assess geomorphological conditions at bridges and other types of water infrastructure. The main advantages and limitations of each monitoring method is summarized and compared, alongside with the key issues behind the failure of existing instrumentation to provide a solution. Finally, future directions on scour monitoring is presented focusing on latest advances in soil and remote sensing methods to provide modern and reliable alternatives for real-time monitoring and prediction [5, 6] of climatic hazards of infrastructure at risk.

References

[1] Michalis, P., Konstantinidis, F. and Valyrakis, M. (2019) The road towards Civil Infrastructure 4.0 for proactive asset management of critical infrastructure systems. Proceedings of the 2nd International Conference on Natural Hazards & Infrastructure (ICONHIC2019), Chania, Greece, 23–26 June 2019.

[2] Pytharouli, S., Michalis, P. and Raftopoulos, S. (2019) From Theory to Field Evidence: Observations on the Evolution of the Settlements of an Earthfill Dam, over Long Time Scales. Infrastructures 2019, 4, 65.

[3] Koursari, E., Wallace, S., Valyrakis, M. and Michalis, P. (2019). The need for real time and robust sensing of infrastructure risk due to extreme hydrologic events, 2019 UK/ China Emerging Technologies (UCET), Glasgow, United Kingdom, 2019, pp. 1-3. doi: 10.1109/UCET.2019.8881865

[4] Michalis, P., Saafi, M. and M.D. Judd. (2012) Integrated Wireless Sensing Technology for Surveillance and Monitoring of Bridge Scour. Proceedings of the 6th International Conference on Scour and Erosion, France, Paris, pp. 395-402.

[5] Valyrakis, M., Diplas, P., and Dancey, C.L. (2011) Prediction of coarse particle movement with adaptive neuro-fuzzy inference systems, Hydrological Processes, 25 (22). pp. 3513-3524. ISSN 0885-6087, doi:10.1002/hyp.8228.

[6] Valyrakis, M., Michalis, P. and Zhang, H. (2015) A new system for bridge scour monitoring and prediction. Proceedings of the 36th IAHR World Congress, The Hague, the Netherlands, pp. 1-4.

How to cite: Valyrakis, M., Michalis, P., Xu, Y., and Latessa, P. G.: Monitoring systems for the assessment of water infrastructure hazards due to extreme climatic conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10526, https://doi.org/10.5194/egusphere-egu2020-10526, 2020.

Infrastructure damage due to riverbed and bank destabilisation or localised scour may result in considerable financial costs and even loss of life. As the risk to infrastructure keeps increasing due to climate change, the need to directly monitor it becomes crucial. Typically, hazards assessments for infrastructure near water are performed using relatively expensive and indirect methods that require field visits to remote and harsh environments to obtain mean flow measurements, using acoustic Doppler velocimetry [1], laser Doppler velocimetry [2] or water level stations along with discharge hydrographs [3]. In this work, a miniaturized instrumented particle that can provide a direct, non-intrusive and accessible method for the assessment of coarse sediment particles entrainment is developed, calibrated and tested. The particle has a diameter of only 3cm and is fitted with inertial microelectromechanical sensors (MEMS) that enable recording its three-dimensional displacement [4, 5]. The sensor is capable of recording acceleration, angular velocity and orientation at a rate of up to 1000Hz and has deployment time of at least one hour. The data can be transferred and downloaded to a PC or an SD card at a fast transfer rate and in easy format for further analysis. The calibration process of the sensor consisted of simple physical motions and the results of the calibration show that the uncertainties in the calibration experiments and in the accelerometer’s and gyroscope’s readings are deemed acceptable. The uncertainty quantification and noise estimation for the sensors, provide the input of the appropriate fusion filter that is applied to the raw data to achieve uncertainty reduction. The testing process consisted of moving the particle on a micro-bed topography and using a camera to record the distance it moved. The orientation of the instrumented particle during testing is determined by inertial sensor fusion of the raw readings of the 3 sensors. The results show that the instrumented particle’s motion could be detected accurately and therefore it could provide a method for direct assessment of the sediment entrainment due to hydrodynamic forces at low cost and in a non-intrusive and direct manner. The instrumented particle presented has a potential of use in a wide range of future applications around the fields of geosciences and environmental and infrastructures monitoring where sediment entrainment [5] and transport [6] is considered to be the governing process.

1. Liu, D., Valyrakis, M., Williams, R. 2017. Flow Hydrodynamics across Open Channel Flows with Riparian Zones: Implications for Riverbank Stability.
2. Diplas, P., Celik, A.O., Valyrakis, M., Dancey C.L. 2010. Some Thoughts on Measurements of Marginal Bedload Transport Rates Based on Experience from Laboratory Flume Experiments.
3. Koursari, E., Wallace, S., Valyrakis, M., Michalis, P. 2019. Remote Monitoring of Infrastructure at Risk due to Hydrologic Hazards and Scour.
4. Valyrakis, M. & Pavlovskis, E. 2014. "Smart pebble” design for environmental monitoring applications.
5. Valyrakis M., A. Alexakis. 2016. Development of a “smart-pebble” for tracking sediment transport.
6. Valyrakis, M., Farhadi, H. 2017. Investigating coarse sediment particles transport using PTV and “smart-pebbles” instrumented with inertial sensors.

How to cite: AlObaidi, K. and Valyrakis, M.: Development, calibration and testing of a miniaturized instrumented particle for the study of entrainment of solids in turbulent flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10535, https://doi.org/10.5194/egusphere-egu2020-10535, 2020.

Turbulent flows may destabilise riverbeds and banks, transporting sediment or underscouring hydraulic infrastructure built near water bodies. For example, scour is a significant challenge that can affect the stability of bridge foundations as the transport of sediment around a bridge pier may cause structural instabilities and catastrophic failures. The aim of this study is to use machine learning techniques & data driven algorithms to predict how energetic turbulent flow events can result in the removal of individual sediment grains, resting on the bed surface or on the protective armour layer around built infrastructure. 

The flume experiments involve flow and particle motion data gathering campaigns [1]. Turbulent flow data are collected upstream the exposed target particle using acoustic Doppler velocimetry. Particle's motion data are gathered using novel micro-electro-mechanical sensors embedded within its waterproof casing, for a range of flow conditions. The obtained data are fed into neural networks having distinct algorithmic complexity (inputs, levels and neutrons). A comparison of the performance of the various model architectures, as well as with past ones [2], is conducted to identify the optimal predictive algorithm for the configuration tested. Sensor data fusion combined with artificial intelligence techniques are shown to provide a unique tool for live and robust data-driven predictions to help tackle significant engineering problems, such as geomorphological activity and scouring of infrastructure (eg bridge piers and embankments) due to turbulent flows, which become increasingly more challenging, under the scope of climate change and intensifying extreme weather hazards.

References

[1] Valyrakis, M., Farhadi, H. 2017. Investigating coarse sediment particles transport using PTV and “smart-pebbles” instrumented with inertial sensors, EGU General Assembly 2017, Vienna, Austria, 23-28 April 2017, id. 9980.

[2] Valyrakis, M., Diplas, P., Dancey, C.L. 2011b. Prediction of coarse particle movement with adaptive neuro-fuzzy inference systems, Hydrological Processes, 25 (22). pp. 3513-3524. ISSN 0885-6087, doi:10.1002/hyp.8228.

How to cite: Ahmed, M., Farhadi, H., Michalis, P., and Valyrakis, M.: Geomorphological hazards assessment using machine learning and data fusion , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20770, https://doi.org/10.5194/egusphere-egu2020-20770, 2020.

In 1995, divers noticed a strange circular pattern on the seabed off Japan. The geometric formations mysteriously appeared and dissolved, and no-one knew what made them. Finally, the creator of these remarkable formations was found: a new species of pufferfish from the genus Torquigener. The male puffer fish executes a design of mathematical perfection in the form of ornate circles. As he swims along the seabed, he laboriously flaps his fins and rearranges the sand, creating the geomorphic feature dubbed crop circle by the pioneers who first noticed them. The significance to understand the puffer fish design is magnified when we consider that the nest is able to maintain its morphological features for long periods even though it is built entirely of mobile particles in an area where the flow does not stop.

As a relatively new discovery, the exact reasons behind why the pufferfish spends such a long time constructing and cultivating the nest it still a question that is shrouded in a substantial amount of mystery. Male puffer fish spend many days caring for the eggs, the only puffer fish genus to be overserved doing so; suggesting that Torquigener place an unusually large emphasis on ensuring the survival of their eggs. It is hypothesised that the nest is created as a mating display, as female puffer fish will visit the site, presumably assessing various characteristics of the nest. It is not known exactly what parameters the females judge the nest on; whether it be size, symmetric properties or decorative choice. However, due to some basic hydrodynamic experiments performed by Hiroshi Kawase, there is some evidence to suggest that there may be more to building the nest than solely attracting a mate.

Several questions therefore arise regarding the nest. Is there an evolutionary reason that male puffer fish build these nests? Which characteristics of a nest make it attractive to female puffer fish? Are the eggs safer in a nest, perhaps from incoming currents? How exactly does fluid flow through nest, and can it be replicated and simulated? This project begins to tackle these questions through a numerical investigation (CFD) of the fluid flow through the nest in order to identify key fluid dynamic features, which may play a significant role in egg incubation and spawning using Star CCM+.

How to cite: Clement, L., Campbell, C., Hasibi, A., Zare-Behtash, H., and Valyrakis, M.: Scour-resilient bio-inspired geomorphic designs: The Male Japanese Puffer Fish Nest, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21592, https://doi.org/10.5194/egusphere-egu2020-21592, 2020.

Fine particles represent an important fraction of the mass of sediment transported by rivers (Syvitski et Saito, 2007). Suspended load is therefore a significant contributor to the erosion of landscapes. Fine particles are often considered to travel through streams and rivers with minimal interaction. Yet, recent field campaigns demonstrate that fine particles interact with the bed through erosion and deposition (Misset et al., 2019). Based on this observation, we develop a simplified model of suspended transport that accounts explicitly for the exchange of small particles between the river bed and the water column. This model involves three parameters: (1) a threshold water level above which the flow starts eroding fine particles from the bed, (2) an erosion rate that characterizes the intensity of sediment entrainment, and (3) a characteristic settling time accounting for sediment deposition.

We then test the validity of the model against data collected in the Capesterre catchment, a small catchment (16.6 km2) monitored by the Observatory of Water and Erosion in the Antilles (ObsErA). Located in Basse-Terre Island (Guadeloupe archipelago, lesser Antilles arc), this catchment is regularly exposed to floods induced by hurricanes and tropical storms (Allemand et al., 2014; Gaillardet et al., 2011). The discharge and the turbidity of the river are measured with a time step of 5 minutes. Using in-situ calibrations, we convert the turbidity signal into a suspended load concentration. The resulting data reveal that the transport of fine sediment is highly intermittent: the concentration of suspended particles rises abruptly when the river height exceeds a threshold of the order of 25cm, corresponding to a discharge of 5 m³/s. The concentration decrease following the flood peak is more gentle. The resulting concentration-discharge curve takes the form of a counter-clockwise hysteretic loop, as commonly observed in many streams (Williams, 1989).

How to cite: Roque-Bernard, A., Lucas, A., Gayer, E., Allemand, P., and Lajeunesse, E.: Suspended load transport in a small tropical catchment: data analysis and modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5289, https://doi.org/10.5194/egusphere-egu2020-5289, 2020.

Despite the fact sediment transport has been studied for decades, there is still a need to gain a further insight on the nature and driving mechanisms of bed particle motions induced by turbulent flows, for the low transport stages where the particle transport is relatively intermittent. A custom designed and prototyped instrumented particle, embedded with inertial sensors is used herein to study its transport over hydraulically rough bed surfaces. The calibration and error estimation for its sensors is also undertaken before starting the experiments, to ensure optimal operation and estimate any uncertainties.

The observations and results of this research are obtained from experiments carried out at the University of Glasgow 12 meters long and 0.9 meters wide, tilting and water recirculating flume. The flume walls comprise of smooth transparent glass that enables observing particle transport from the side (also with underwater video cameras) and the bed surface generally is layered with coarse gravel.

The particle is initially located at the upstream end of the test configuration, fully exposed to the uniform and fully developed turbulent channel flow. The top and side cameras are set in their suitable positions to monitor and study the behaviour of particle motion by capturing the dynamical features of sediment motion and to not interfere with flow field that pushes particle downstream. 

Using the sensor data to calculate the kinetic energy for a range of sets of sediment transport experiments with varying flow rates and particle densities, the probability distribution functions (PDFs) of particle transport features, such as particle’s total energy, are generated which give information about particle interaction with the surface bed during its motion. In addition, the effects of different flow rates, particle densities on particle energy are assessed.

How to cite: Al-Husban, Z. and Valyrakis, M.: Assessing sediment transport energetics with instrumented particles for above threshold of motion turbulent flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21727, https://doi.org/10.5194/egusphere-egu2020-21727, 2020.

References:

• Abramian, A., Devauchelle, O., and Lajeunesse, E., “Streamwise streaks induced by bedload diffusion,” Journal of Fluid Mechanics 863, 601–619 (2019).
• Abramian, A., Devauchelle, O., Seizilles, G., and Lajeunesse, E., “Boltzmann distribution of sediment transport,” Physical review letters 123, 014501 (2019).
• Seizilles, G., Devauchelle, O., Lajeunesse, E., and M ́etivier, F., “Width of laminar laboratory rivers,” Phys. Rev. E. 87, 052204 (2013).

• Seizilles, G., Lajeunesse, E., Devauchelle, O., and Bak, M., “Cross-stream diffusion in bedload transport,” Phys. of Fluids 26, 013302 (2014).

How to cite: Lajeunesse, E., Abramian, A., and Devauchelle, O.: Self-organisation of morphology and sediment transport in alluvial rivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2758, https://doi.org/10.5194/egusphere-egu2020-2758, 2020.

Classically, for free surface flows of binary granular mixture, large particles migrate at the top of the flow while small ones percolate to the bottom. The key mechanisms at the origin of this segregation behavior have been identified as a combination of squeeze expulsion and kinetic sieving (Savage & Lun J. Fluid Mech. 1988). In this case, the segregation process is governed by the gravity. We discovered here by means of numerical simulations a new segregation pattern in high speed granular flows where size segregation is driven mostly by granular temperature gradients rather than gravity, which highlight the complexity of providing a complete description of segregation processes.

High speed granular flows are obtained by means of discrete numerical simulations (DEM) in a confined geometry with lateral frictional side-walls. Recently, Brodu et al. (Phys. Rev. E 2013, J. Fluid Mech. 2015) highlighted that this confined geometry allows to produce steady and fully-developed flows at relatively high angles of inclination, including a rich and broad variety of new regimes. In particular, they showed the existence of supported regimes, characterized by a dense and cold (in terms of granular temperature) core floating over a dilute and highly agitated layer of grains, accompanied with longitudinal convection rolls.

We performed extensive numerical simulations within this geometry with binary mixture of spheres with a given size ratio of 2. We analyzed segregation patterns of steady and fully-developed flows for inclination angles ranging from 18° to 50° and various mixture proportions of large particles ranging from 0 to 100%. We evidenced a new segregation pattern that emerge in the supported flow regimes: large particles no longer accumulate in the upper layers of the flow but are trapped in the dense core and localized at the center of the convection rolls. The strong temperature gradients that develop between the dense core and the surrounding dilute layer seem to govern the segregation mechanism. The accumulation of large particles in the dense core, which is the fastest region of the flow, also tends to enhance the total mass flux in comparison with similar mono-disperse flows.

How to cite: Valance, A., Delannay, R., and Neveu, A.: Unexpected segregation patterns in high speed granular flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21839, https://doi.org/10.5194/egusphere-egu2020-21839, 2020.

We investigate numerically high speed granular flows down inclines. Recent numerical works have
highlighted that the presence of lateral frictional walls allows to produce novel Steady and Fully
Developed (SFD) flow regimes at high angle of inclination where accelerated regimes are usually
expected (Brodu et al., 2015). These SFD regimes present non-trivial features, including secondary flows
with longitudinal vortices and “supported“ flows characterized by a central and dense core supported by
a very agitated dilute layer.
We present a review of these new regimes and provide their domain of existence in the parameter space
including the mass hold-up M, the inclination angle θ and the gap width between the lateral walls. We
also investigate the sensitivity of these states to the mechanical parameters of particles such as the
restitution coefficient e for binary collisions.
We emphasize two salient outcomes. (I) First, our simulations reveal that the emergence of the
supported flows is favored by low restitution coefficient (i.e., high dissipation). Surprisingly, increasing
the dissipation leads to faster flows. This is explained by a contraction of the flow, resulting in a lower
contribution of the side-wall friction. (ii) Second, despite the diversity of the supported flow regimes, the
simulations bring to the light that the mass flow rate Q obeys a simple scaling law with the mass hold-up
and the gap width: Q~M3/4W1/4.
Bibliography
Brodu et al., 2015, Journal of Fluid Mechanics, 2015, 769, 218-228

How to cite: Yajuan, Z., Delannay, R., and Valance, A.: High speed granular flows down inclines, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22050, https://doi.org/10.5194/egusphere-egu2020-22050, 2020.

Rivers carry sediments having a wide grain size distribution, ranging from a few hundreds microns to meters. This leads to grain size segregation mechanism that can have huge consequences on morphological evolution.  Accurate comprehension and modeling of this mechanism with continuous equations is a key step to upscale segregation in sediment transport models.
Thornton et al. (2006) developed continuous equations for bidisperse segregation in the context of the mixture theory. Based on the momentum balance of small particles, a simple advection-diffusion equation for the volumetric concentration of small particles was derived. This equation enables to explicit the advection term, that tends to segregate the different particle sizes and the diffusive term, that tends to remix the particles. However, this approach does not immediately provide the physical characteristics of the granular flow in the advection and diffusion terms.

Recently, Guillard et al. (2016) showed, using a Discrete Element Method (DEM), that the segregation force on a large intruder in a bath of small particles, can be seen as a buoyancy force proportional to the pressure. In addition, Tripathi and Khakhar (2011) showed that a large particle rising in a pool of small grains experiences a Stokesian drag force proportional to the granular viscosity.
These new results enable to infer a force balance for a single coarse particle in bedload transport. Solving this force balance showed that the large particle rises with the accurate dynamics, meaning that this force balance is relevant to model grain-size segregation.

Based on these new forces, a continuous multi-class model has been developed to generalize to the segregation of a collection of large particles. The concentration and the segregation velocity of the small particles have been compared with coupled-fluid DEM bedload transport simulations from Chassagne et al. (2020) and show that the accurate dynamics of segregation can be modeled using this continuous model.
Based on this continuum multi-class model, a similar advection-diffusion equation as Thornton et al. (2006)  has been obtained. The latter appears to provide the physical origin of the advection and diffusion terms by linking them to the parameters of the flow.

Chassagne R., Maurin R., Chauchat J., and Frey P. Discrete and continuum modeling of grain-size segregation during bedload transport. J. Fluid Mech. 2020 (in revision).

Gray J. M. N. T., and  Chugunov V. A. Particle-size segregation and diffusive remixing in shallow granular avalanches. J. Fluid Mech. 569: 365-398, 2006.

Guillard F. Forterre Y., and Pouliquen O. Scaling laws for segregation forces in dense sheared granular flows. J. Fluid Mech. 807, R1, 2016.

Thornton A. R., Gray J. M. N. T., and Hogg A. J. A three-phase mixture theory for particle size segregation in shallow granular free-surface flows. J. Fluid Mech. 550: 125, 2006.

Tripathi A., and Khakhar D. V. Numerical simulation of the sedimentation of a sphere in a sheared granular fluid: a granular stokes experiment. Phys. Rev. Lett. 107, 108,001, 2011.

How to cite: Rousseau, H., Chassagne, R., Chauchat, J., and Frey, P.: Continuous modeling of grain size segregation in bedload transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9539, https://doi.org/10.5194/egusphere-egu2020-9539, 2020.

Bedload transport (transport of particles by a flowing fluid along the bed by rolling, sliding and/or saltating) has major consequences for public safety, water resources and environmental sustainabilty. In mountains, steep slopes drive an intense transport of a wide range of grain sizes implying size sorting or segregation largely responsible for our limited ability to predict sediment flux and river morphology. Size segregation can lead to very complex and varied morphologies of bed surface and subsurface, including armouring, and can drastically modify the fluvial morphology equilibrium. In this work, the transport rate of an armoured bed, made of large particles on top of a small particles bed, is studied.

In order to gain understanding of this process, bedload transport numerical experiments of two-size particle mixtures were carried out, using a coupled Eulerian-Lagrangian fluid-discrete element model validated with experiments (Maurin et al. 2015, 2016). It is composed of a 3D discrete element model (based on the open source code Yade), describing each individual particle, coupled with a one dimensional Reynolds Average Navier Stokes model (Chauchat 2017). A 3D 10% steep domain (angle of 5.71°) is considered. Three different configurations are compared: 2 layers or 4 layers of 6mm particles deposited on top of a bed composed of 3mm particles, and a monodisperse case with only 6mm large particles. The bed is then submitted to a turbulent, hydraulically rough and supercritical water flow until steady transport rate. Shields numbers ranging from 0.1 to 0.5 are considered.

The numerical experiments show that in all three configurations, the transport law, relating the dimensionless transport rate to the shields number, is a power law. In addition, it is observed that for the same Shields number, the transport rate is higher in the bidisperse cases than in the monodisperse case. This result can be explained by the rheological properties of bidisperse granular media. Finally, we show that the particles at the interface between large and small particles should be in motion in order to have an increase of particle mobility.

Chauchat J. 2017. A comprehensive two-phase flow model for unidirectional sheet-flows. Journal of Hydraulic Research: 10.1080/00221686.2017.1289260.

Maurin R, Chauchat J, Chareyre B, Frey P. 2015. A minimal coupled fluid-discrete element model for bedload transport. Physics of Fluids 27(11): 113302.

Maurin R, Chauchat J, Frey P. 2016. Dense granular flow rheology in turbulent bedload transport. Journal of Fluid Mechanics 804: 490-512.

How to cite: Chassagne, R., Maurin, R., Chauchat, J., and Frey, P.: Discrete simulations of an armoured sediment bed during bedload transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9367, https://doi.org/10.5194/egusphere-egu2020-9367, 2020.

Gravitational mass flows like debris flows are often physically modelled as an assembly of particles flowing in simplified flume configurations. There is indication that natural flows exhibit a combined movement of sliding and internal deformation, which is not well understood and underrepresented in scaled laboratory experiments. In this study we investigate the effect of the surface roughness on the velocity profile and the runout of small-scale, dry granular avalanches. The experimental set-up is a 0.17 m wide flume with an inclination of 34° for the first 1.5 m, following an 0.8 m curved transition zone with a radius of 1.7 m, and ending in a runout zone with an angle of 4°. The tested material consisted of non-perfect spherical ceramic beads with a diameter of 2.8 to 4.3 mm. We tested four different types of surface roughness ranging from 0 to 6 mm height and additionally one macro roughness, which was higher than the maximum flow height. To also get information about the influence of the relative roughness experiments with three different starting volumes were undertaken. All fourteen experimental variations were repeated three times. Flow heights were measured with laser sensors at four different positions along the channel. Three of them were used to calculate the mean front velocity of the flowing mass in two cross sections. Furthermore, the experiments were recorded with a high-speed camera through one sidewall out of acrylic glass. The recordings were analysed using a PIV (Particle Image Velocimetry) software to derive velocity profiles in 1/1500 second time steps. Results show that the velocity profiles changed from the head to the tail of the flow and that the profiles of the two roughest surfaces are more alike than the smooth roughness configurations. The fraction of sliding on the total movement varied between 0 and close to unity. The runout length decreased the higher the roughness was and increased with higher starting volume. The shape of the velocity profiles at the deepest sections of the flows changed with surface roughness and with starting volumes. Only the velocity profiles for the two roughest surfaces show an inflection point. Our experiments highlight the importance of surface roughness as well as relative roughness for granular mass flows and provide data for model testing.

How to cite: Reider, L. and Kaitna, R.: On the effect of surface roughness on velocity profiles and runout lengths of dry granular flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7631, https://doi.org/10.5194/egusphere-egu2020-7631, 2020.

Scour is the leading cause of bridge collapse worldwide, being responsible for compromising the stability of structures’ foundations. Scour and erosion can take place without prior warning and cause sudden failure. This study describes engineering measures and complications encountered during construction for a case study in the Scottish Borders (A68 Galadean Bridge). The bridge studied carries the A68 road across the Leader Water.

Transport Scotland’s structures crossing or near a watercourse are subject to a two-stage scour assessment following the Design Manual for Roads and Bridges (DMRB) BD97/12 Standard, ‘The Assessment of Scour and Other Hydraulic Actions at Highway Structures’. Structures identified at risk are monitored through Reactive Structures Safety Inspections following events likely to increase water levels. The most common form of monitoring includes visual inspections, however, monitoring sensors are being currently implemented and trialled at locations at high risk of scour.

Scour in the area was identified during a Reactive Structures Safety Inspection, following which a weekly scour monitoring regime was established, alongside further Reactive Structures Safety Inspections, until remediation measures were put in place.

Despite the bridge being constructed perpendicular to the Leader Water, meandering of the watercourse was detected upstream. Sediment transport was the cause of an island formation immediately upstream of the structure. Non-uniform flow and secondary, spiral currents, resulting from the formation of the bend were exacerbating scour and erosion in the area. The design of the remediation measures included the implementation of rock rolls alongside the affected riverbank. However, during construction, increased water levels resulting from thawing snow resulted in the collapse of a significant portion of the embankment supporting the structure’s abutment and the A68 road, prior to the realisation of the remediation measures. An emergency design revision was required and emergency measures had to be enforced.

The urgency of the works led to a two-phase approach being followed for the design and construction of the scour measures in the affected area. The first phase included the construction of a platform in front of the affected road embankment and the implementation of rock rolls to provide scour protection. The two-phase approach ensured the infrastructure at risk was protected from further deterioration while the reconstruction of the embankment was being designed.

The second phase of works included the reconstruction of the affected road embankment, for which the anticipated total scour depth was taken into account.

References:

Koursari E and Wallace S. 2019. Infrastructure scour management: a case study for A68 Galadean Bridge, UK. Proceedings of the Institution of Civil Engineers – Bridge Engineering, https://doi.org/10.1680/jbren.18.00062

Acknowledgements:

The authors would like to acknowledge Transport Scotland for funding this project.

How to cite: Koursari, E., Wallace, S., Michalis, P., Xu, Y., and Valyrakis, M.: A Case Study for Infrastructure Scour Management, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22209, https://doi.org/10.5194/egusphere-egu2020-22209, 2020.

Open channel flows over granular mobile beds are affected by the nature and intensity of hyporheic/surface mass and momentum exchanges. Near-bed surface mean flow and turbulence find an equilibrium with the flow in the hyporheic region and with the type and amount of granular material transported in equilibrium conditions. The processes involved in these adaptive process are not well known. This work addresses this knowledge gap and it is aimed at describing the effect of the hydraulic conductivity on the friction factor and on the parameters of the log-law that is thought to constitute a valid model for the turbulent flow in the overlapping region of fully developed hydraulically rough boundary layers over mobile cohesionless beds. To fulfil the objectives, experimental tests performed in high conductivity beds (mono-sized glass sphere beads) are compared with the existing database of low conductivity beds of Ferreira et al. (2012), keeping constant the range of values of porosity, Shields parameters and roughness Reynolds numbers. The hydraulic conductivity is varied by changing the tortuosity (and the dimensions of the pore paths) and not the porosity.

A new database of instantaneous velocities was acquired with Particle Image Velocimetry (PIV) and processed to gather time-averaged velocities and space-time (double-averaged) quantities, namely velocities, Reynolds stresses and form-induced stresses. The hydraulic conductivity was measured for both types of bed.

The parameters of log-law obtained from high conductivity are compared with low conductivity of existing database, for mobile and immobile bed conditions. The main finding can be summarized as follows.

i. Hydraulic conductivity does not affect the location of the zero plane of the log-law, the thickness of the region above the crests where the flow is determined by roughness.

ii. Increasing the hydraulic conductivity does not appear to decrease the value of bed roughness parameters such as the roughness heigh.

iii. Higher hydraulic conductivity is associated to a structural change: the same near-bed velocity can be achieved with lower shear stress in the inner region. A lower friction factor, (u_*/U)², is thus registered.

iv. Flows over high conductivity beds appear drag-reducing even if roughness parameters do not change appreciably.

This work was partially funded by FEDER, program COMPETE, and national funds through the Portuguese Foundation for Science and Technology (FCT) project ASHES - PTDC/ECI-EGC/29835/2017

How to cite: L Ferreira, R. M., Y Tenzin, R., and Ricardo, A. M.: Turbulent open-channel flows over mobile granular low-tortuosity beds: velocity distribution and friction factor, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11570, https://doi.org/10.5194/egusphere-egu2020-11570, 2020.

Blowout is a common landform on sandy grassland in semi-arid area and part of semi-humid area, and it is the symbol of the activation of fixed dune and the primary manifestation of desertification. This paper selected the south edge of Otingdag Sandy Land as the research area, and used WindSonic and high-precision RTK GPS to measure the airflow and topography of three blowouts with different morphology on the fixed dune. Meanwhile, combining with the image data and meteorological data,we analyzed the morphology evolution process of the three different blowouts and discussed the relationship between airflow and morphology of blowouts. The results showed that the northwest, west and southwest winds were dominant in the study area, and the west wind among them was the most frequent; The average annual wind speed tends to decrease, and the wind direction gradually tends to be stable and unidirectional, which is consistent with the direction of movement of the blowout in this area; From the air inlet to the top of the sand accumulation area, each blowout experienced the process of diffusion deceleration, convergence acceleration, separation deceleration and gradual acceleration along the long axis of the blowout, but the location that highest or lowest wind speed occurred were not the same in different blowout; The relationship between the wind direction and the long axis of blowout determines the airflow pattern inside blowouts. When the airflow diagonally enters the blowout, the airflow pattern tends to be complicated, and the deceleration and acceleration zone in blowout are obviously deviated. After the airflow enters the blowout, the wind speed and direction change obviously, which affects the spatial pattern of erosion or accumulation and further alters the morphology of the blowout. The morphology also in turn reacts on the near-surface airflow, which results in the response and feedback of the morphology and dynamics of the blowout.

How to cite: Zhou, Y. and Hasi, E.: Morphodynamic processes of blowout on the fixed dune, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13595, https://doi.org/10.5194/egusphere-egu2020-13595, 2020.

Hyporheic/surface mass and momentum exchanges are mostly governed by bed tortuosity, porosity and grain size distribution. These determine near-bed mean flow and turbulence. Not many studies explicitly address the differences in turbulent variables between high conductivity and low conductivity bed, which justifies the objective of this paper: to investigate the effects of tortuosity and hydraulic conductivity on the integral length scale, on the parameters of the power spectral density functions (longitudinal and transverse) and 2nd order structure functions and on Reynolds stress anisotropy. Experimental tests were performed in low tortuosity beds (mono-sized glass sphere beads) are compared with the existing database of low conductivity beds of Ferreira et al. (2012), keeping constant the range of values of porosity, Shields parameters and roughness Reynolds numbers. A new database of instantaneous velocities was acquired with Particle Image Velocimetry (PIV) and processed to gather time-averaged velocities and space-time (double-averaged) quantities, namely velocities, Reynolds stresses and form-induced stresses. Turbulence variables obtained from low tortuosity bed are compared with those of the high tortuosity of the existing database, for mobile and immobile bed conditions. The main differences are identified and discussed. They include: increased anisotropy in the flow over high tortuosity beds; smaller integral scale in the near-bed region in the in the flow over high tortuosity beds; different parameters of the 2nd order structure functions. The observations may be used to explain the drag reducing behaviour of the flow over low-tortuosity beds.

This research was partially supported by Portuguese and European funds, within the COMPETE 2020 and PORL-FEDER programs, through project PTDC/CTA-OHR/29360/2017 RiverCure

How to cite: Santos, D., Aleixo, R., Ricardo, A. M., and L. Ferreira, R. M.: Turbulence in open-channel flows over granular low-tortuosity beds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11441, https://doi.org/10.5194/egusphere-egu2020-11441, 2020.

Climate change has resulted in increased storminess and sea-level change that affects the morphodynamics of bays worldwide, impacting both the ecosystem and local infrastructure. This study explores the impact of differing storm winds on the sediment budget of the Bay of Cadiz. The Bay of Cadiz is a highly altered coastal lagoon located in Southwest Spain surrounded by ports, navigation channels, and urban developments and is of high socioeconomic and environmental importance. The human interactions with the bay have already caused morphological impacts, which could be exacerbated by increased storminess. Potential impacts on the sediment budget of the Bay of Cadiz will be modeled using a Delft3D model previously calibrated and tested using field data from December 2011 to January 2012. The model will consider a variety of storm wind scenarios and observe their impacts on sediment transport within the bay, identifying sources and sinks. This will help to estimate the potential impacts of climate change and increased storminess on the bay and the surrounding areas.

How to cite: LeBlanc, M., Plomaritis, T., and Lopez-Ruiz, A.: Response of a coastal lagoon sediment budget to extreme events and climate change implications: The case of the Bay of Cadiz, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21783, https://doi.org/10.5194/egusphere-egu2020-21783, 2020.