GM5.4

Abstract: Evaluation of a large number of rainstorm disasters shows that the coupling effect of sediment supply and floodwaters is one predominant cause for the occurrence of flash flood disasters. Rainfall-induced shallow landslides often provide an adequate source of solid materials to recharge moving sediment during flash floods. In this study, we used the TRIGRS model to analyze the rainfall-related landslide stability in a mountainous basin and gain potential landslide volumes as potential sources for sediment loads. Then, with the calculated results of landslides as input, the Massflow model was used to evaluate how the landslides as sediment loads evolved with flows. The results showed that there was a large amount of sediment deposited in the channel, which can be initiated and transported by heavy rainfalls, leading to the destruction of villages at the mouth of gullies. In general, this study offers a strategy of evaluating sediment-coupled flash flood disasters that the TRIGRS can provides the estimate of landslide distribution and volume first and the Massflow provides the estimate of subsequent movement of the solids caused by flash floods.

Funded by: The National Key R&D Program of China (2019YFC1510702)

How to cite: Sun, T., Wang, X., and Yan, X.: Combining TRIGRS and Massflow Models for the Assessment of Flash Flood Disasters Related to Sediment Supply in Mountainous Watersheds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7089, https://doi.org/10.5194/egusphere-egu21-7089, 2021.

This study used the LES-DEM (Large-Eddy Simulation and Discrete Element Method) model to simulate the lock-exchange particle-laden gravity flow over a flat slope and studied its fluid-particle interactions. The following understandings are obtained. According to the longitudinal particle-fluid interaction force, the flat-slope lock-exchange PGF process can be divided into two stages: fluid conveying particles (Stage I) and particles pushing fluid (Stage II). In the early Stage I, due to the positive vorticity and the positive slip velocity, the lift force plays a leading role in the interaction force. And in the later Stage I, the drag force causes the fluid to push the particles when the lift force decreases and becomes negative due to the negative vorticity caused by the bottom resistance. In Stage II, the lift force hinders the particles’ advancement, which exceeds the drag force that transports the particles forward. The vertical suspension of particles mainly benefits from drag force and contact force, and the former is more prominent. In addition, the longitudinal transport of head particles is mainly controlled by the lift force caused by positive vorticity which is cause by the resistance from the ambient fluid at the current profile. Based on the interaction force, the study distinguishes two energy conversion modes. The final destination of the energy in the two modes is longitudinal particle kinetic energy and longitudinal fluid kinetic energy, respectively.

How to cite: Xie, J. and Hu, P.: Particle-scale fluid-particle interactions in particle-laden gravity flows over the flat slope, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7091, https://doi.org/10.5194/egusphere-egu21-7091, 2021.

Landslide dam breaching is the one of focus topics in the geophysical flows. The frequency of occurrence of landslide dam increases due to earthquake, climate change and mans activities in recent years. Once the dam breaks, it would trigger extreme flood downstream. A field experiment on landslide dam breach has been carried out on a small mountain river in Mianzhu, Sichuan Province, China from 23 November to 29 December, which aims to reveal impact of different diversion channel types on the dam breaching process as well as the resulting flood. The dam is of 4m high, 10~15m wide. the length of the dam crest is 5m, upstream downstream slopes of the dam are 1:2 and 1:5. Results show division channel can reduce the peak flood discharge obviously. The pilot vertical fall can trigger earlier back erosion and thus peak discharge appears earlier with smaller magnitude.

How to cite: Huang, W. and Duan, W.: Large scale filed experiment on the landslide dam breaching, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8169, https://doi.org/10.5194/egusphere-egu21-8169, 2021.

We investigate the removal of a dense bottom layer by a gravity current, via Navier-Stokes Boussinesq simulations. The problem is governed by a dimensionless thickness parameter for the bottom layer, and by the ratio of two density differences. A quasisteady gravity current propagates along the interface and displaces some of the dense bottom fluid, which accumulates ahead of the gravity current and forms an undular bore or a series of internal gravity waves. Depending on the ratio of the gravity current front velocity to the linear shallow-water wave velocity, we observe small-amplitude waves or a train of steep, nonlinear internal waves. We develop a self-contained model based on the conservation principles for mass and vorticity that does not require empirical closure assumptions. This model is able to predict the gravity current height and the internal wave or bore velocity, generally to within about 10% accuracy. An energy budget analysis provides information on the rates at which potential energy is converted into kinetic energy and then dissipated, and on the processes by which energy is transferred from the gravity current fluid to the dense and ambient fluids. We observe that the energy content of thicker and denser bottom layers grows more rapidly.

How to cite: Zhu, R., He, Z., and Meiburg, E.: Removal of a dense bottom layer by a gravity current, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9273, https://doi.org/10.5194/egusphere-egu21-9273, 2021.

Instream vegetation may alter the mean and turbukent flow fields leading to destabilizing riverbed surface, under certain flow conditions. In particular, recent research on instream vegetation hydrodynamics and ecohydrogeomorphology has focused on how energetic flow structures and bulk flow parameters downstream a vegetation may result in riverbed destabilization. This study, demonstrated the application of a 20mm novel instrumented particle in recording entrainment rates downstream simulated vegetation patches of distinct densities, at various distances downstream these. A patch of 6mm acrilic cylinders is used to simulate the emergent vegetation having the same diameter (12cm) and different porosities or densities (void volume equal to 1.25%, 3.15%, 6.25%, 11.25%, and 17.25%). The flow velocity near the instrumented particle is recorded using acoustic Doppler velocimetry (ADV) with appropriate seeding, under clear water conditions. Preliminary results are presented with focus on the effect of vegetation patch density on the flow field and subsequent effects on particle entrainment rates and implications for bed surface destabilisation.

How to cite: Xu, Y., Manousos, V., and Michalis, P.: An initial assessment of bed destabilization risk past vegetation patches using instrumented particles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1808, https://doi.org/10.5194/egusphere-egu21-1808, 2021.

Yellow River has long been suffered from floods and sedimentation in the history, and has brought great catastrophes to the Chinese nation. Therefore, the Yellow River is also called the “China’s sorrow”. From July 25 to 26 of 2017, most of the northern part of the Shanxi and Shannxi Province in the middle Yellow River basin encountered high intensity rainfall with the maximum rainfall of 223.6 mm. In the abstract below, we term this rainfall event as the “7.26 storm”. After the extreme rainfall, hyper-concentrated floods occurred in the Dali River and Wuding River, which are tributaries of the Yellow River. The objective of this research is to study the hyper-concentrated floods of the Wuding River (with a drainage area of 28460 km²) at hourly time-step with a numerical model. The model that we utilized is the Digital Yellow River Model (DYRIM), which a physically based spatially distributed model of watershed sediment dynamics. Due to lack of sub-daily observation data, we first calibrate and verify the model at daily time-step. Then we apply the model to simulate the 7.26 storm at hourly time-step. Results show the DYRIM could well reproduce the peak discharge, peak sediment concentration, flood timing and volume, when compared with the measured data. Furthermore, the DYRIM is able to (1) delineate spatial distribution of hillslope erosion intensity, maximum erosion intensity could reach 10000 t/km²; (2) provide information about proportion of different sources of sediment, channel erosion is the main source of the sediment to the outlet and (3) analysis the influence of check-dams on flow and sediment, the dam trapped about 40 millions tons sediment, their effect on water and sediment reduction under extreme rainfall events is limited though.

How to cite: Zhang, G., An, C., and Fu, X.: Devastating hyper-concentrated flood on the Loess Plateau, China: simulations and implications, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10462, https://doi.org/10.5194/egusphere-egu21-10462, 2021.

While fluvial flows carrying relatively coarse sediments involve strong two-phase interactions, existing numerical modeling in the field-scale is mostly based on quasi-single phase flow model. Here a computationally efficient two-phase hydro-sediment-morphodynamic model is developed with a special focus on field applications. The hybrid LTS/GMaTS method originally developed for quasi-single flow model is extended to the present two-phase flow model, of which the achieved reduction in the computational cost facilitates the present field applications in the Taipingkou Waterway, Middle Yangtze River. To overcome numerical instabilities arising from the relatively large spatial and time steps in field case that lead to an issue of stiff source term, the following numerical treatments are proposed: implementation of theoretically-derived lower and upper limits for the inter-phase interactive forces. Moreover, to improve the numerical accuracy, the HLLC approximate Riemann solver is used for the water phase, whereas the FORCE solver is used for the sediment phase. Both the present two-phase flow model and the existing quasi-single-phase flow model are applied to a series of typical cases, including refilling of a dredged trench, a full dam-break flow in an abruptly widening channel and reproduction of the Taipingkou waterway, Middle Yangtze River. Compared with the quasi-single-phase flow model, the two-phase flow model has better performance as compared to the measure data and has more profound physical significance.

How to cite: Lyu, B., Hu, P., Li, J., Cao, Z., Li, W., and He, Z.: A computationally efficient two-phase hydro-sediment-morphdynamic model and its preliminary field applications , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4258, https://doi.org/10.5194/egusphere-egu21-4258, 2021.

Bedload particle hops are defined as successive motions of a particle from start to stop, characterizing one of the most fundamental processes describing bedload sediment transport in rivers. Although two transport regimes have been recently identified for short- and long-hops, respectively (Wu et al., Water Resour Res, 2020), there still lacks a theory explaining how the mean hop distance-travel time scaling may extend to cover the phenomenology of bedload particle motions. Here we propose a velocity-variation based formulation, and for the first time, we obtain analytical solution for the mean hop distance-travel time relation valid for the entire range of travel times, which agrees well with the measured data (Wu et al., J Fluid Mech, 2021). Regarding travel times, we identify three distinct regimes in terms of different scaling exponents: respectively as ~1.5 for an initial regime and ~5/3 for a transition regime, which define the short-hops; and 1 for the so-called Taylor dispersion regime defining long-hops. The corresponding probability density function of the hop distance is also analytically obtained and experimentally verified. 

How to cite: Wu, Z., Singh, A., Foufoula-Georgiou, E., Guala, M., Fu, X., and Wang, G.: Analytical approach unifying the two-regime scaling for bedload particle motions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4364, https://doi.org/10.5194/egusphere-egu21-4364, 2021.

Recently, there has been an increasing attention on the environmental flow management for the maintenance of habitat diversity and ecosystem health of mountain gravel-bed rivers. More specifically, much interest has been paid to how inter-flood low flow can affect gravel-bed river morphodynamics during subsequent flood events. Such an effect is often termed as “stress history” effect. Previous research has found that antecedent conditioning flow can lead to an increase in the critical shear stress and a reduction in sediment transport rate during a subsequent flood. But how long this effect can last during the flood event has not been fully discussed. In this study, a series of flume experiments with various durations of conditioning flow are presented to study this problem. Results show that channel morphology adjusts significantly within the first 15 minutes of the conditioning flow, but becomes rather stable during the remainder of the conditioning flow. The implementation of conditioning flow can indeed lead to a reduction of sediment transport rate during the subsequent hydrograph, but such effect is limited only within a relatively short time at the beginning of the hydrograph. This indicates that bed reorganization during the conditioning phase, which induce the stress history effect, is likely to be erased with increasing intensity of flow and sediment transport during the subsequent flood event.

How to cite: An, C., Hassan, M. A., Ferrer-Boix, C., and Fu, X.: Effect of stress history on sediment transport and channel adjustment in graded gravel-bed rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6889, https://doi.org/10.5194/egusphere-egu21-6889, 2021.

Sediment transport and associated morphological changes in alluvial rivers occur primarily under unsteady flow conditions that are manifested as well-defined flood hydrograph events. At present, typical bed forms generated by such unsteady flows is far less studied and, thus, more poorly understood, than equivalent bed forms generated under steady flow conditions. In view of this, the objective of this work is to investigate the development of morphological bed features, and specifically variability in the length, height and steepness of bed forms that develop in a mobile coarse-sand bed layer under unsteady flow hydrographs under zero sediment feed conditions. A series of laboratory flume experiments is conducted within which different flow hydrograph events are simulated physically by controlling their shape, unsteadiness and magnitude. Experimental results indicate that different categories of bed forms such as dunes, alternate bars or transitional dune-bar structures develop within the erodible bed layer when subject to varying hydrograph flow conditions. Examination of relative importance of three parameters used to describe the hydrograph characteristics (i.e. asymmetry, unsteadiness and total water work) on bed form dimensional descriptors (i.e. wavelength, height and steepness) reveals that hydrograph unsteadiness and total water work are the primary and second-order controls on bed deformations or corresponding bed form dimensions. By contrast, hydrograph asymmetry appears to have minimal or negligible influence on bed form development in terms of their type and magnitude. Based on these findings, a physical model was developed and tested to describe the effect of unsteady flow hydrographs with varying unsteadiness and total water work on the nature and size of resulting bed forms that are generated in sand-bed layers. 

How to cite: Wang, L., Cuthbertson, A., Pender, G., and Cao, Z.: Effect of unsteady flows on the development and magnitude of bed forms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5854, https://doi.org/10.5194/egusphere-egu21-5854, 2021.

Overtopping dam-break flow has great harm to the earthen embankments due to the hydraulic erosion. Some researchers have carried out relevant model experiments, but it is difficult to achieve the experimental conditions for the actual situation. The common numerical simulation is to express the scouring process through the empirical relationship, which obviously could not reflect the real scouring process. In this paper, a new overtopping erosion model using Smoothed Particle Hydrodynamics (SPH) is proposed. When the shear stress on the sediment SPH particle exceeds the critical stress, the erosion process begins. Then, when a sediment SPH particle is completely eroded, it will begin to move and is described as a non-Newtonian fluid. The un-incipient sediment particles are treated as boundary. This model is well validated with plane dike-breach experiment, and has also achieved a good agreement with erodible bed dam-break experiment.

How to cite: Huang, C., Wang, X., and Liu, Q.: Numerical Simulation of Scouring in Overtopping Dam-break Flow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6798, https://doi.org/10.5194/egusphere-egu21-6798, 2021.

The objective of this paper is to present the ScourBuoy – concept for scour monitoring system. The ScourBuoy prototype is currently under development within the R3PEAT project (Remote Real-time Riprap Protection Erosion AssessmenT on large rivers), which aims to investigate scouring processes next to the riprap protection around bridge piers. ScourBuoy integrates commercially available technical devices into a functional system for scour monitoring during flood conditions. Sensors used are single beam echo sounder that collects depth and temperature data, multi-GNSS device for 3D positioning, compass for orientation respective to the True North and motion sensor for pitch and roll data. Combined output from the sensors allows user to calculate river depth and monitoring of scour development during floods. Advantage of ScourBuoy is adaptability to the field conditions, such as placement over the scour hole, as well as simpler deployment and reallocation in comparison to fix-mount solutions. ScourBuoy prototype was built using a common small-scale pipe float with an 80 mm inner diameter hole, which was used as a holder for an aluminium pipe. Aluminium pipe is used as a casing for echo sounder, positioned as downward-looking, so it stays submerged during deployment. The rest of the sensors are enclosed in the waterproof housing placed atop of the buoy, permanently above the waterline. The ScourBuoy will be a practical and affordable system which will allow researchers and engineers to collect measurements for scouring estimation. It will be used as a support system for rapid and timely decision making. Finally, developed Scour Buoy will present an alternative for real-time scour monitoring which allows responsive adapting to the specific conditions at the locations affected by scour.

How to cite: Harasti, A., Gilja, G., Varga, M., and Fliszar, R.: ScourBuoy – concept for scour monitoring system, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-131, https://doi.org/10.5194/egusphere-egu21-131, 2021.

Physical modelling of local scour around bridge piers is challenging due to the three different similarity criteria that need to be satisfied for the complete similitude of the model and the prototype. To achieve the complete similitude, geometry, flow and sediment material in the model scale need to be scaled simultaneously. Aim of this paper is to calculate the scale parameters of the physical model used for scour assessment next to piers protected with riprap. The scale parameters are calculated individually for each prototype – two bridges located on the Drava River and one on the Sava River. Hydrological conditions are determined by analysis of flood wave amplitude and duration from the nearby hydrological station adjacent to the pilot bridge. Experiments will be conducted with different pier shapes and sizes, as well as with two different materials representing the riverbed – same density as prototype and lower density material. Range of discharges used for simulating various flow conditions are selected to be compatible with the flume pump capacity. To calculate required thickness of sediment trap layer in the hydraulic flume, expected scour depth is estimated. Sensitivity analysis of 13 empirical equations was conducted due to differences in hydraulic and geometric conditions of each prototype [1]. Bathymetric survey was the basis for establishing a numerical model, and its results were input parameters for empirical equations. Finally, scour depths were estimated as the average of all equations results except those results that turned out to be inadequate. Scaling affects the time parameter of the scouring process, and therefore an adequate time scale is calculated to achieve full development of the scour hole with respect to flow conditions. Finally, as a result of the flume restrictions, advantages and disadvantages of the physical model distortion are discussed.

[1] Cikojević, A., Gilja, G., Kuspilić, N.: Sensitivity analysis of empirical equations applicable on bridge piers in sand-bed rivers, Proceedings of International Symposium on Water Management and Hydraulic Engineering, Skopje, Republic of North Macedonia, pp. 100-108, (2019)

Acknowledgments

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

How to cite: Fliszar, R., Gilja, G., Harasti, A., and Potočki, K.: Scaling approach for physical modelling of pier scour, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-171, https://doi.org/10.5194/egusphere-egu21-171, 2021.

Applying an instrumented particle [1-3], the probability density functions of kinetic energy of a coarse particle (at different solid densities) mobilised over a range of above threshold flow conditions conditions corresponding to the intermittent transport regime, were explored. The experiments were conducted in the Water Engineering Lab at the University of Glasgow on a tilting recirculating flume with 800 (length) × 90 (width) cm dimension. Twelve different flow conditions corresponding to intermittent transport regime for the range of particle densities examined herein, have been implemented in this research. Ensuring fully developed flow conditions, the start of the test section was located at 3.2 meters upstream of the flume outlet. The bed surface of the flume is flat and made up of well-packed glass beads of 16.2 mm diameter, offering a uniform roughness over which the instrumented particle is transported. MEMS sensors are embedded within the instrumented particle with 3-axis gyroscope and 3-axis accelerometer. At the beginning of each experimental run, instrumented particle is placed at the upstream of the test section, fully exposed to the free stream flow. Its motion is recorded with top and side cameras to enable a deeper understanding of particle transport processes. Using results from sets of instrumented particle transport experiments with varying flow rates and particle densities, the probability distribution functions (PDFs) of the instrumented particles kinetic energy, were generated. The best-fitted PDFs were selected by applying the Kolmogorov-Smirnov test and the results were discussed considering the light of the recent literature of the particle velocity distributions.

[1] Valyrakis, M.; Alexakis, A. Development of a “smart-pebble” for tracking sediment transport. In Proceedings of the International Conference on Fluvial Hydraulics (River Flow 2016), St. Louis, MO, USA, 12–15 July 2016.

[2] Al-Obaidi, K., Xu, Y. & Valyrakis, M. 2020, The Design and Calibration of Instrumented Particles for Assessing Water Infrastructure Hazards, Journal of Sensors and Actuator Networks, vol. 9, no. 3, 36.

[3] Al-Obaidi, K. & Valyrakis, M. 2020, Asensory instrumented particle for environmental monitoring applications: development and calibration, IEEE sensors journal (accepted).

How to cite: Farhadi, H. and Valyrakis, M.: Exploring probability distribution functions best-fitting the kinetic energy of coarse particles at above threshold flow conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1820, https://doi.org/10.5194/egusphere-egu21-1820, 2021.

Nonsuspended sediment transport (NST) refers to the sediment transport regime in which the flow turbulence is unable to support the weight of transported grains. It occurs in fluvial environments (i.e., driven by a stream of liquid) and in aeolian environments (i.e., wind-blown) and plays a key role in shaping sedimentary landscapes of planetary bodies. NST is a highly fluctuating physical process because of turbulence, surface inhomogeneities, and variations of grain size and shape and packing geometry. Furthermore, the energy of transported grains varies strongly due to variations of their flow exposure duration since their entrainment from the bed. In spite of such variability, we here propose a deterministic model that represents the entire grain motion, including grains that roll and/or slide along the bed, by a periodic saltation motion with rebound laws that describe an average rebound of a grain after colliding with the bed. The model simultaneously captures laboratory and field measurements and discrete element method (DEM)-based numerical simulations of the threshold and rate of equilibrium NST within a factor of about 2, unifying weak and intense transport conditions in oil, water, and air (oil only for threshold). The model parameters have not been adjusted to these measurements but determined from independent data sets. Recent DEM-based numerical simulations (Comola, Gaume, et al., 2019, https://doi.org/10.1029/2019GL082195) suggest that equilibrium aeolian NST on Earth is insensitive to the strength of cohesive bonds between bed grains. Consistently, the model captures cohesive windblown sand and windblown snow conditions despite not explicitly accounting for cohesion.

How to cite: Pähtz, T., Liu, Y., Xia, Y., Hu, P., He, Z., and Tholen, K.: Unified model of sediment transport threshold and rate across subaqueous bedload, windblown sand, and windblown snow, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7022, https://doi.org/10.5194/egusphere-egu21-7022, 2021.

Bed load experiments reveal a range of possibilities for the downstream velocity distributions of moving particles, including normal, exponential, and gamma distributions. Although bed load velocities are key for understanding fluctuations in transport rates, existing models have not accounted for the full range of observations. Here, we present a generalized Langevin model of particle transport that includes turbulent drag and episodic particle-bed collisions. By means of analytical calculations, we demonstrate that momentum dissipation by particle-bed collisions controls the form of the bed load velocity distribution. As collisions vary between elastic and inelastic, the velocity distribution interpolates between normal and exponential. These results add context to conflicting experiments on bed load velocities and suggest that granular interactions regulate sediment dynamics and transport rate fluctuations.

How to cite: Pierce, K. and Hassan, M.: Collisional Langevin approach to bed load sediment velocity distributions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8148, https://doi.org/10.5194/egusphere-egu21-8148, 2021.

Sediment transfer in mountain streams occurs by processes classified as debris flows, hyperconcentrated flows, debris floods, and water flows. One of the most important tasks in investigating floods in mountain catchments is to identify the transport mechanisms since different sediment-water flows induce peculiar geomorphological dynamics and hazards. This study aims at testing how the energy of water and the amount of sediment involved during a high-magnitude hydrological event can modify the mechanisms of sediment transfer with respect to those occurring during ordinary floods.

The selected case study is the Tegnas catchment (Dolomites, Italy), which, in October 2018, was affected by a severe hydrological event (Vaia Storm) with a recurrence interval of about 200 years. The studied catchment drains an area of 51 km², with a range in elevation between 2872 and 620 m a.s.l.. The classification of flows that occurred during the Vaia storm was addressed at the sub-reach scale applying a field survey protocol developed to classify the flood deposits based on their sedimentological and morphological features. Following the same approach, we also determined the flow types typifying the stream network during ordinary floods. Additionally, we considered flows predicted by three morphometric approaches for high-magnitude events, and took into account the geomorphological dynamics (e.g., channel changes) and the hydraulic constraints (i.e., unit stream power) that occurred during the Vaia storm.

Water flow was the dominant process during Vaia storm in the Tegnas main steam (12 sub-reaches), although debris flow and debris flood deposits were documented at 3 and 7 sub-reaches, respectively. Water flow was observed in response to ordinary events along the entire Tegnas Torrent. Most of the steep tributaries were affected by debris flows (6 tributaries), but also debris floods were recognized at 3 steep channels. The morphometric approaches had a satisfactory performance in predicting the two end-member flows, but often failed in recognizing sub-reaches affected by debris floods.

The comparison between the occurred high-magnitude flows, and the ordinary flows allowed us to infer the existence of relationships between the transport mechanisms, the hydraulic forcing, and channel dynamics. The upheaval of the ordinary flow types did not occur along the entire stream network. The transition from water flows to debris floods occurred for unit stream powers exceeding the threshold of 5000-6000 Wm^-2 or downstream of a channel delivering a large amount of sediment mobilized by debris flow to the receiving stream. The occurrence of debris floods, causing higher channel widening than water flows, appears to be facilitated by the injection of fine material into the flow, which can occur as consequence of channel-bank erosion and overbank floodwater re-entering the channel. Finally, morphometric approaches turned out to be adequate to provide a first-order discrimination of expectable high-magnitude flow types. However, the complex relationships found between flow types and a range of hydraulic, morphological, and geological controlling factors, reveal that a more detailed characterization is necessary for understanding the transport mechanisms and predicting geomorphic hazard that can affect specific channel sites during high-magnitude to extreme hydrological events.

How to cite: Brenna, A., Borga, M., Ghinassi, M., Marchi, L., Zaramella, M., and Surian, N.: Sediment-water flows in mountain catchments: Variability in response to high-magnitude hydrological events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4605, https://doi.org/10.5194/egusphere-egu21-4605, 2021.

Sediment transport is considered to be the governing process in many applications around the fields of geosciences and engineering as well as infrastructure and environment monitoring. Of a special interest to which scientists and engineers have dedicated a lot of time and experimental studies in the last century is the conditions for initiation of sediment entrainment, or incipient motion. In the literature, there are different criteria for determining the conditions that can result in initiation of sediment entrainment. Among these criteria, the impulse (or energy) criterion [1-2] captures the actual physics of sediment entrainment since it accounts for both the magnitude and the duration of the turbulent flow events that can result in initiation of a particle’s motion. The experimental and field studies of incipient motion use relatively expensive tools, like Particle image velocimetry (PIV) or Acoustic Doppler velocimetry (ADV), with indirect methods to determine flow parameters that could be related to predicting sediment entrainment. However, technological developments in recent decades has made it possible to assess sediment entrainment directly. Recently, a number of research studies [3-4] have suggested linking micro-electromechanical system (MEMS) recordings that consist of accelerometers, gyroscopes and magnetometer as well as an internal digital motion processor that are interconnected forming inertial measurement units (IMUs) to the probability of entrainment of individual particles. The particles have been presented provide a direct, non-intrusive, low-cost and accessible method for assessing the probability of entrainment of individual sediment particles rather than inferred using near bed flow diagnostics. In this work, an instrumented particle of 3cm in diameter [5] is used to investigate experimentally the conditions that can result in initiation of sediment entrainment for a range of flowrates that represent the near threshold conditions. The data is used to derive metrics like frequency of entrainment that could be linked to the probability of entrainment of individual sediment particles which could be used as an indicator of the risk of riverbed destabilization based on well-established theories in hydraulic engineering. Additionally, the novelty of this work is explicitly linking the probability of entrainment to the flow hydrodynamics. In addition to that, a stochastic analysis is performed to identify the relevance of certain flow structures (sweeps) to the incipient entrainment of the instrumented particle.

[1] Valyrakis M., Diplas P., Dancey C.L., Greer K., Celik A.O. (2010). Role of Instantaneous Force Magnitude and Duration on Particle Entrainment. JGR, 115, 1-18.

[2] Valyrakis M., Diplas P., Dancey C.L. (2013). Entrainment of Coarse Particles in Turbulent Flows: An Energy Approach. JGR- Earth Surf., 118, 42-53.

[3] Valyrakis, M., Alexakis, A. (2016). Development of a “smart-pebble” for tracking sediment transport. River Flow 2016, MO, USA.

[4] Al-Obaidi, K., Xu, Y., Valyrakis, M. (2020). The Design and Calibration of Instrumented Particles for Assessing Water Infrastructure Hazards. JSAN, 9, 3, 36.

[5] Al-Obaidi, K., Valyrakis, M. (2020). A sensory instrumented particle for environmental monitoring applications: development and calibration. IEEE sensors journal (accepted).

How to cite: AlObaidi, K. and Valyrakis, M.: Assessing thresholds for fluvial entrainment with instrumented particles, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3325, https://doi.org/10.5194/egusphere-egu21-3325, 2021.

Aeolian sediment transport in desert and sandy coastal environments affects civil structures and infrastructures, such as pipelines, industrial facilities, towns, single buildings, farms, roads, and railways [1]. The wind flow interacts with surface-mounted obstacles of any kind inducing sand erosion, transport, and sedimentation around them. This can lead to detrimental effects such as the loss of functionality of the endangered structure or infrastructure, or even danger for users when structural failure is involved [2]. In order to cope with the effects above, the demand for the characterization of aeolian sand transport and the design of Sand Mitigation Measures (SMMs) has grown in the last decade and is expected to further increase in the next years [1]. The multiphase and multiscale nature of the aeolian flow ranging from the sand grain diameters to the obstacle characteristic lengths make the problem only tractable by means of physical experiments and computational simulations. On the one hand, in-situ full scale field tests are expensive, time-consuming, and subject to environmental setup conditions difficult to control. On the other hand, numerical models shall be carefully validated against physical experiments. Hence, experimental Wind-Sand Tunnel Tests (WSTTs) are often carried out.

In this study, windblown sand transport on flat ground is reproduced by means of WSTTs carried out in the wind tunnel L-1B of von Karman Institute for Fluid Dynamics. The aim of WSTTs is twofold. On one hand, they are intended to characterize the incoming sand flux in open field conditions. On the other hand, they allow to properly tune cheaper Wind-Sand Computational Simulations [3], so as to assess the performance of SMMs in full-scale. The wind tunnel setup implements a uniform 5-meter-long sand fetch as sand source. The wind speed boundary layer and sand flux saltation layer are characterized through 2D Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) techniques, respectively. Wind flow and sand transport state variables are assessed along the sand fetch by setting the wind speed equal to 1.3, 1.5, 2 times the threshold one, and by assessing the influence of a monoplane grid installed at the inlet of the wind tunnel testing sections. Results from WSTTs are critically discussed by investigating the effects induced by the sand fetch length, wind speed, and turbulence intensity on the sand transport. Finally, a Eulerian multiphase computational fluid dynamics model is tuned in order to reproduce the obtained results.

References

[1] Bruno L, Horvat M, Raffaele L. Windblown sand along railway infrastructures: a review of challenges and mitigation measures. J Wind Eng Ind Aerodynam 2018;177:340–65.
[2] Raffaele L, Bruno L. Windblown sand action on civil structures: Definition and probabilistic modelling. Eng Struct 2019;178:88-101.
[3] Lo Giudice A, Preziosi L. A fully Eulerian multiphase model of windblown sand coupled with morphodynamic evolution: Erosion, transport, deposition, and avalanching. Appl Math Model 2020;79:68-84.

How to cite: Raffaele, L., Coste, N., Lo Giudice, A., Glabeke, G., and van Beeck, J.: Characterization of windblown sand transport in open field conditions through wind-sand tunnel testing and computational simulation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9574, https://doi.org/10.5194/egusphere-egu21-9574, 2021.

The ongoing fourth industrial revolution has accelerated the transformation of management and maintenance of assets into the digital era. This involves the application and interoperability of management systems in an upper system like the one described as Civil Infrastructure 4.0 [1]. CI4.0 involves the collection and process of data from the surrounding infrastructure over a wide range of assets and systems, incorporating a multi-integrated decision support system for efficient asset management. This is particular important for ageing water infrastructure as it is threatened by the occurrence of flood-related hazards, which have significant degradation impact and consequences to transport systems, e.g. bridges, embankments, waterways etc.

Despite the recent advances in the development and application of immersive technologies, transport and water infrastructure are still considered to be managed in a traditional way. This process involves on-site engineers making decisions based on their skills and experience, while in the majority of the times using paper-based analytics.

This study presents the development of intelligent tools to efficiently advance decision making about the maintenance procedure of water infrastructure, aiming to reduce costs and assessment times. One of the technological pillars, which can upgrade the traditional procedures is Augmented Reality (AR) technology, which is already used in other industries like Manufacturing and Automotive [2]. AR creates a combined environment in which the views of real and virtual worlds co-exist. AR technology provides valuable key information to inspectors, through AR glasses or mobile devices, pointing out areas of interest. Such an AR solution can register the coordination of location of the defects, analysing the possible maintenance solutions, and communicating effectively between in-house operators and inspectors on-site.

[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 (ICONHIC), 23–26 June Chania, Greece, pp. 1-9.

[2] Konstantinidis, F.K., Kansizoglou, I., Santavas, N., Mouroutsos, S.G. and Gasteratos, A., 2020. MARMA: A Mobile Augmented Reality Maintenance Assistant for Fast-Track Repair Procedures in the Context of Industry 4.0. Machines, 8(4), p.88.

How to cite: Konstantinidis, F., Michalis, P., and Valyrakis, M.: Digital Transformation of Critical Water Infrastructure, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1589, https://doi.org/10.5194/egusphere-egu21-1589, 2021.

An abrupt transition in river bed grain size occurs from gravel to sand over a short downstream distance, often only a few channel widths, and is termed the gravel-sand transition (GST). At this point, the bed structure also changes from framework- to matrix-supported. Whether the GST is externally imposed, a result of internal dynamics (sediment sorting, abrasion, suspension deposition) or due to some other emergent property is unclear. There is also a general absence of rivers beds with median surface grain sizes between ~1 and 5 mm, often referred to as the grain size gap. Here we present two sets of new laboratory experiments, examining changes in fluid and sediment dynamics across the GST. In the first set, we created stable GSTs with a 10 mm gravel and 0.5 mm sand that show GST formation is consistent with  previous theory suggesting that at shear velocities of ~0.1 m/s, sand particles rapidly fall out of suspension as a result of a particle Reynolds number dependency (i.e. a viscous effect). In a second set of experiments, we explored the fate of grain size gap material. We formed a gravel wedge composed of ~2 to 5 mm sediment, then fed 0.5 mm sand.  Our observations indicate that where sand rapidly starts to fall out of suspension, the gravel bed becomes inherently unstable. Gravel is transported downstream until the grain size gap material is largely exhausted from the system (e.g. buried under sand or rafted out of the flume). This occurs because sand sized particles fill or bridge interstitial pockets in the fine gravel bed surface, generating fluid acceleration in the near-bed region (i.e. a geometric effect specific to these grain sizes). As such, particles in the grain size gap do not form the dominant mode in river bed sediments. 

How to cite: Dingle, E. and Venditti, J.: Experiments on the grain size gap across gravel-sand transitions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13093, https://doi.org/10.5194/egusphere-egu21-13093, 2021.

The prediction of particle motion in a fluid flow environment presents several challenges from the quantification of the forces exerted by the fluid onto the solids -normally with fluctuating behaviour due to turbulence- and the definition of the potential particle entrainment from these actions. An accurate description of these phenomena has many practical applications in local scour definition and to the design of protection measures.

In the present work, the actions of different flow conditions on sediment particles is investigated with the aim to translate these effects into particle entrainment identification through analytical solid dynamic equations.

Large Eddy Simulations (LES) are an increasingly practical tool that provide an accurate representation of both the mean flow field and the large-scale turbulent fluctuations. For the present case, the forces exerted by the flow are integrated over the surface of a stationary particle in the streamwise (drag) and vertical (lift) directions, together with the torques around the particle’s centre of mass. These forces are validated against experimental data under the same bed and flow conditions.

The forces are then compared against threshold values, obtained through theoretical equations of simple motions such as rolling without sliding. Thus, the frequency of entrainment is related to the different flow conditions in good agreement with results from experimental sediment entrainment research.

A thorough monitoring of the velocity flow field on several locations is carried out to determine the relationships between velocity time series at several locations around the particle and the forces acting on its surface. These results a relevant to determine ideal locations for flow investigation both in numerical and physical experiments.

Through numerical experiments, a large number of flow conditions were simulated obtaining a full set of actions over a fixed particle sitting on a smooth bed. These actions were translated into potential particle entrainment events and validated against experimental data. Future work will present the coupling of these LES models with Discrete Element Method (DEM) models to verify the entrainment phenomena entirely from a numerical perspective.

How to cite: Latessa, G., Busse, A., and Valyrakis, M.: Investigation on hydrodynamic forces leading to coarse particle entrainment using Large-Eddy Simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13221, https://doi.org/10.5194/egusphere-egu21-13221, 2021.

Scour action still remains the leading cause of numerous bridge failures each year and is considered one of the most destructive flood related hazards occurring around underwater foundation elements [1]. Undetected erosion related processes are therefore the cause of major disruptions to the transportation network with significant socio-economic losses and disruption to users, maintainers and asset owners. Recent cases of bridge failures due to extreme climatic events have highlighted the need for a reliable scour monitoring and early warning system to assess flood and geo-related hazards in real-time, providing advanced key info for repair and maintenance actions. Despite the past efforts to provide such a system for scour assessment, most of these instruments have not managed to realise a solution for scour monitoring due to technical and cost issues. The existing practices to assess, manage and maintain transportation assets are mainly based on visual inspection procedure which is also considered to be insufficient [2]. As a result there currently exists a gap in the knowledge and understanding of scour mechanism during flood incidents.

This study presents the architecture of ‘Climatic Hazard Monitoring and Bridge Scour Early Warning System’ (CliHaMoS) project, which is expected to significantly assist towards the optimisation of bridge performance against scour issues with a real-time data driven approach. CliHaMoS platform comprises of a new structural health monitoring system based on a novel bio-inspired sensing system aiming to deliver key information under different hydrodynamic events for real-time and forecasted assessment of flood hazards at bridges. The sensing solution is coupled by an early warning system, with advanced interoperability characteristics, to provide a holistic interactive platform and ensure that risks associated with flood hazards are properly and timely communicated to end-users. The obtained information is expected to enable stakeholders to plan adaptation strategies and proactively manage and maintain transportation infrastructure.

[1] Michalis, P., Saafi, M., and Judd. M. (2013) Capacitive sensors for offshore scour monitoring. Proceedings of the ICE – Energy, 166 (4), pp. 189-197

[2] 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.

ACKNOWLEDGMENT:

This research is co-financed by Greece and the European Union (European Social FundESF) through the Operational Programme «Human Resources Development, Education and 4 Lifelong Learning» in the context of the project “Reinforcement of Postdoctoral Researchers - 2nd Cycle” (MIS-5033021), implemented by the State Scholarships Foundation (ΙΚΥ).

How to cite: Michalis, P., Valyrakis, M., and Vintzilaiou, E.: A New Structural Health Monitoring System to Assess Bridge Scour, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1652, https://doi.org/10.5194/egusphere-egu21-1652, 2021.

Gravity currents propagating over and within porous layers occurs in natural environments and in industrial processes. The particular modes by which the dense fluid flows into the porous layer is a subject that is not sufficiently understood. To overcome this research gap, we conducted laboratory experiments aimed at describing experimentally the dynamics of the drainage flow.

The experiments were conducted in a horizontal channel with a rectangular cross-section. The channel is 3.0 m long, 0.05 m wide. The porous bottom was composed of 5 cm and 10 cm layers of 3 mm borosilicate spheres – unimodal bed – and of a mixture of 3 mm (50% in weight) and 5 mm spheres (50%) – bi-modal bed. The porosity of the unimodal bed ranged between 0.60 and 0.64 (compatible with loose packing). The porosity of the bi-modal bed ranged between 0.61 and 0.65. All gravity currents were generated by releasing suddenly denser fluid locked by a thin vertical barrier placed at 0.2 m from the channel end. The dense fluid consists in a mixture of freshwater and salt (coloured with Rhodamine) while the ambient fluid is a solution of freshwater and ethanol. The density difference between the ambient fluid and the current, and the need to maintain the same refractive index, determine the amount of salt and alcohol added in each mixture. Here we report the findings of currents with a reduced gravity of 0.06 ms^-2.

Each experiment was recorded by an high-speed camera with a frame-rate of 386 Hz and a resolution of 2320 x 1726 pxxpx. Measurements were based on light absorption techniques: a LED light panel 0.3 m high and 0.61 m long was used as back illumination. All images were calibrated to ascribe, pixel by pixel, a concentration value from a 8 bit gray level. Different calibrations were performed for the porous layer and for the surface current.

Results show that, in the slumping phase, the gravity current flows with velocities compatible with those over rough beds. As the current progresses further attenuation of momentum is noticed owing to mass loss to the porous bed.

The flow in the porous bed reveals plume instability akin to a Saffman-Taylor instability. The growth of the plumes seems independent from the initial fluid height in both types of porous beds. The wavelength and the growth rate of the plumes depends on the bed material. Plumes grow faster in the case of the bi-modal bed and the wavelength of the bi-modal bed is about 1.5 as that of the unimodal bed. It is hypothesised that the gravity-induced porous flow is best parameterized by a Péclet number defined as a ratio of dispersive (mechanical diffusion) and advective modes of transport. Smaller wavelengths and slower growths are attained for stronger dispersion, characterisitic of the unimodal bed. For bimodal beds, permeability is larger, and thus also advection. This causes the flow to concentrate in faster growing but farther apart plumes.

This research was funded by national funds through Portuguese Foundation for Science and Technology (FCT) project PTDC/CTA-OHR/30561/2017 (WinTherface).

How to cite: L Ferreira, R. M., Solis, G., Adduce, C., and Ricardo, A. M.: Gravity induced vertical motion of dense fluids into saturated granular beds, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16313, https://doi.org/10.5194/egusphere-egu21-16313, 2021.