Dam breach is a large-scale, highly unsteady, and complex water-sediment flow. Dam breach events span scenarios involving natural dams (created following valley blocking landslides) to scenarios involving anthropogenic structures such as water-retaining dams and dams designed specifically to store mine waste (e.g. tailings). Management of the risk posed by a potential breach of a dam structure requires a careful analysis of the consequences of failure. These analyses, often called dam breach studies, aim to improve safety and risk management through the prediction of travel time, flow velocity, and spatial extent of the hazard (e.g. map of depth of inundation) in the event of a breach. These factors in turn define the consequence classification of a dam and guide the development of emergency preparedness plans. The key boundary condition required for flood routing numerical simulations often conducted for dam breach studies is the outflow hydrograph which describes the relationship between outflow of the retained volume with time. In this study we explore the effect of failure mechanism on the characteristics of the outflow hydrograph of otherwise identical physical model dams. Physical model dams of 1 m in height were constructed of fine sand near the midspan of the 36 m long, 2.1 m wide, and 1.2 m high large landslide flume facility at Queen’s University Coastal Engineering Laboratory. Three failure mechanisms are explored; a) notch overtopping, initiated by incising a v-notch into the dam crest and allowing the impounded reservoir to intersect this local low point; b) wide-width overtopping, where a retaining wall placed along the crest of the dam allows for an additional reservoir capacity above the height of the crest that when rapidly removed a full width sheet of water cascades over the dam; and c) geotechnical seepage failure, where closing the toe drain allows a seepage face to develop that causes a failure in the downstream face of the dam. The reservoir surface elevation during breach was monitored with a series of five Akamina AWP-24 wave capacitance height gauges distributed centerline in the upstream reservoir. The evolution of breach shape is captured every 3 s using five Canon EOS Rebel T5 Digital Single-lens Reflex (DSLR) that capture a plan view area of the entire 2.1 m width of the dam and a combined upstream and downstream length of 4.3 m. To further capture the evolution of failure a Blickfeld LiDAR sensor was positioned oblique to the downstream slope to capture a point cloud scan ever 1.4 s. These data sets are then used to compare the physical characteristics of each breach process and the resulting implications for the observed outflow hydrographs for each failure mechanism.

How to cite: McKellar, M., McDougall, S., Evans, S., and Take, A.: Comparison of outflow hydrographs following dam breach arising from overtopping, wide-width overtopping, and geotechnical seepage failure mechanisms., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10650, https://doi.org/10.5194/egusphere-egu23-10650, 2023.

Debris flows are the common gravity-driven mass flows in mountain areas that have high sediment concentration, wide grain sizes, and strong impact force.  Erosion or entrainment is the main mechanism by which the flows significantly increase their volume and destructive potential when they progressively move down over colluvial or alluvial beds. However, the scale and mechanism of erosion are poorly understood due to scarcity of field data. We present on-site data of a rare event in which three consolidated landslide dams were incised deeply by debris flows in a small catchment, southwestern China. The highest erosion rate was up to 1.3 m/min or 568 m³ per unit channel length. The channel topographical condition controls transition from erosion to deposition and the locations of local erosion maxima. An outburst-flood erosion model incorporating flow discharge, channel slope and erodibility is adopted to simulate the progressive erosion of these dams. The infrequent case confirms the key role of debris flows in alpine landscape evolution and provides field data (case data) for developing advanced erosion models. This work provides new insights into the role and scale of debris-flow erosion in catchment evolution.

How to cite: Hu, K., Ning, L., Wei, L., and Zhang, Q.: Progressive channel erosional processes of the 2020 Heixiluo debris flow in Dadu River, southwestern China, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2278, https://doi.org/10.5194/egusphere-egu23-2278, 2023.

Floods cause severe natural disasters over the world generating property and infrastructures damages, poverty and loss of human life, among others. Mediterranean watersheds are especially sensible to floods due to their typical drainage basin features (steep slopes, short concentration times, complex orography, etc.) and the high rainfall intensity typical of convective systems. Knowing channel dimensions and other fluvial morphological features is key to (i) understanding the morphological evolution of the fluvial system in response to changes in land use and climate, and (ii) as an input for hydrological and erosion modelling. The objective of this study is to develop a simple method to obtain reliable estimates of channel dimensions and granulometry of bed material, based on statistical relations with catchment characteristics (e.g. topography, land use, soil properties, lithology, precipitation, connectivity indicators).

First, channel dimensions were estimated based on GIS analysis using a high resolution digital elevation model (2x2m) and ortophotos (50 cm resolution) for the Upper Segura River catchment, a Mediterranean catchment of 2,592 km² located in the southeast of Spain. These estimates were validated with field measurements of depth and width of bankfull channels in the catchment headwaters. This comparison revealed that there was generally good agreement between channel dimensions obtained with the GIS method and those observed in the field for all evaluated channels (depth: RMSE=0.06; R²=0.93; width: RMSE=0.59, R²=0.45), although with better results for dry channels than for channels with continuous water flow. At each field observation, we also took sediment samples and characterised the granulometry of the channel bed material in the laboratory through dry sieving. Preliminary results show that bed material is composed mainly by gravels (67.8%), followed by sands (31.1%) and clays and silts (1.1%).    

Next, channel dimensions, obtained using GIS analysis across the entire catchment, and granulometry of bed material were used as dependent variables in advanced statistical analyses (such as machine learning algorithms) at sub-basin scale, with catchment characteristics as independent variables. The outcome of this analysis is now being used to make spatially continuous predictions of channel dimensions and granulometry of bed material at the catchment scale. This information will then ultimately serve as input for a coupled model that simulates channel hydraulics and morphodynamics at the catchment level.

Keywords: geomorphology; sediment yield; depth and width of bankfull channels; Mediterranean environment; GIS based tools; southeast of Spain.

We acknowledge funding from the Spanish Ministry of Science and Innovation and ‘Agencia Estatal de Investigación’ (PID2019-109381RB-I00/AEI/10.13039/501100011033).

How to cite: Jodar-Abellan, A., Carrillo-López, E., Eekhout, J., Boix-Fayos, C., Pérez-Cutillas, P., and de Vente, J.: Predicting channel dimensions and bed materials in intermittent Mediterranean rivers., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7170, https://doi.org/10.5194/egusphere-egu23-7170, 2023.

Abstract: A very common engineering task we encounter in practice is calculating the annual balance of sediments on some watercourse. This is particularly challenging when assessing the backfilling of river reservoirs that have a multifunctional function.

Trakošćan Lake was built in the period from 1850 to 1862 as a pond and landscape addition to the park and Trakošćan castle. The topographic catchment area of the lake is 10.7 km², it is about 1.5 km long, and its area is about 17 hectares, with an average depth of about 2.5 meters. The lake's total volume was originally 400,000 cubic meters, and downstream from the dam, the water of the Čemernica stream flows into the Bednja River.

After 61 years, the lake was drained, and in 2022, work began on sediment excavation to improve the lake's ecological condition due to about 200,000 cubic meters of deposited silt in the lake. The projected depth of the lake after cleaning would be about 6 meters.

In the example of this artificial lake, an estimate of the annual sediment spread by empirical parametric methods was carried out. Furthermore, the results were compared with the results of previous analysis obtained based on geotechnical sediment investigation at Lake Trakošćan.

How to cite: Oskoruš, D., Leskovar, K., and Pavlić, K.: Parametric methods for assessing the production of suspended sediment and its deposition in artificial lakes - an example of Lake Trakošćan, Croatia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13546, https://doi.org/10.5194/egusphere-egu23-13546, 2023.

Bedload transport has major consequences for public safety, water resources and environmental sustainability. 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 mobility of bidisperse beds is studied with coupled fluid-Discrete Element Method (DEM) simulations of bedload transport. Initially, a large particle layer is deposited over a 10% slope bed made of small particles. A gravity-driven water free surface flow induces a downslope shear-driven granular flow of the erodible bed. It is observed that, for the same water flow conditions, the bedload transport rate is higher in the bidisperse configuration than in the monodisperse one. Depending on the Shields number and on the depth of the interface between small and large particles, different transport phenomenologies are observed, ranging from no influence of the small particles to small particles reaching the bed surface due to diffusive remixing. In cases where the small particles hardly mix with the overlying large particles and for the range of studied size ratios (r < 4), it is shown that the increase of mobility of the sediment bed is a granular effect, which can be explained within the mu(I) rheology framework. The buried small particles are more mobile than larger particles and play the role of a “conveyor belt” for the large particles at the surface. Based on rheological arguments, a simple predictive model is proposed for the additional transport in the bidisperse case. It reproduces quantitatively the DEM results for a large range of Shields numbers and for size ratios smaller than 4.

Finally, a phenomenological map is proposed. It presents the different transport regimes of bidisperse mixtures, depending on the mechanism responsible for the mobility of the small particles.

How to cite: Chassagne, R., Maurin, R., Chauchat, J., and Frey, P.: Mobility of bi-disperse sediment beds in bedload transport, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11802, https://doi.org/10.5194/egusphere-egu23-11802, 2023.

Particle size segregation is a phenomenon that generates preferential sorting of particles, based on size, in material flows of non-uniform size distribution. Landslide hazards, such as debris flows, involve materials of non-uniform particle sizes and therefore generate flow structures which arise from particle size segregation. The mobility, distal reach and impact forces associated with these natural hazards are influenced by these processes. Understanding the mechanisms of this phenomenon is essential for acquiring accurate input parameters that are needed to model these flows and properly design debris flow barriers and retaining structures. While the dynamics of particle size segregation in flow and deposition have been furthered through studying granular flows, studies to date have had several limitations. They primarily examine flows of bidispersed material, are small in scale, and rely on observations from flume sidewalls, precluding the study of dynamics along the centreline of flows. In this study, a large scale 6.8 m long and 2.1 m wide slope inclined at 30 degrees was used to generate dry tridispersed granular flows with 0.6 m³ of material. The tridisperse mixture consisted of even proportions by mass of 3 mm, 6 mm and 12 mm diameter spherical particles. Replicate tests were conducted to observe flow dynamics and assess methods for sampling along the internal plane of the test deposit. Image analysis techniques were developed to quantify particle size distributions within the deposit. Flume sidewall and internal observations were found to differ significantly from each other, in that side wall observations contained significantly higher proportions of the largest particle size. Additional replicate tests were conducted with saturated material to further examine the impact of pore fluid on segregation. This work will allow for future calibration of both numerical and theoretical models of particle size segregation and ultimately enable better debris flow modelling and mitigation practices.

How to cite: Kimball, J., Bowman, E., Gray, N., and Take, A.: Assessing Experimental Methods for the Quantification of Particle Size Segregation in Large Scale Flume Tests using Image Analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10517, https://doi.org/10.5194/egusphere-egu23-10517, 2023.

How to cite: Wang, X., Campmans, G., Weinhart, T., Thornton, A., Luding, S., and Wijnberg, K.: Discrete element modelling of grain-scale aeolian sediment transport on moist beach surface, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-385, https://doi.org/10.5194/egusphere-egu23-385, 2023.

ABSTRACT: Gravity currents are flows generated by density differences within two contacting fluids. In this work the interaction between lock-release gravity currents propagating over a horizontal rectangular channel and an emergent cylinder is analyzed through velocity measurements obtained through PIV. Two-dimensional instantaneous velocity fields are measured in a plane perpendicular to the bottom along the center axis of the channel upstream of the obstacle. The experiments were also conducted without the cylinder for comparison purposes and ten repetitions were carried out for each configuration. The analyses focus on the effects that the presence of an adverse pressure gradient has on both the mean velocity field and the turbulence of the leading part of the current, the head, before the impact. The mean velocity field is not affected by the presence of the obstacle and since no differences were found in the spatial distribution of the mean velocity components, the necessary cylinder-induced deceleration occurs uniformly. Turbulence is studied through the components of the Reynolds stress tensor and their fluxes within the head. In the configuration with the cylinder, there are no fluxes of Reynolds stresses in the inner part of the section. Consequently, the Reynolds stress intensity decreases inside the head compared to the configuration without the obstacle. In conclusion, the presence of an adverse pressure gradient stops the mechanism of Reynolds stress distribution from the main source of production, i.e. the front region, to the inner region of the flow. This leads to a decrease in Reynolds stresses in the inner part of the head and an increase in the frontal region.

Acknowledgements: This work was partially supported by Foundation for Science and Technology's through funding UIDB/04625/2020 (CERIS research unit).

Keywords: Gravity currents, lock release, Particle Image Velocimetry, adverse pressure gradient, Reynolds stress.

How to cite: Di Lollo, G., Adduce, C., Brito, M., Ferreira, R. M. L., and Ricardo, A.: Reynolds stresses in gravity currents approaching an obstacle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15236, https://doi.org/10.5194/egusphere-egu23-15236, 2023.

The risk of scour of hydraulic infrastructure, such as bridge piers and abutments, may considerably rise during extreme weather events. The removal of coarse bed material and consequent critical failure of the riverbed surface layer's protective layer against scour can result from sufficiently energetic flow structures, which normally scale with their cross-sectional lengthscale [1] (typically a naturally formed armour layer or implemented rip-rap protection, comprising of cobbles and rocks). This study intends to directly monitor the likelihood of entrainment of an instrumented particle [2] that is properly positioned on the riverbed surface near a bridge pier, in order to evaluate the probability of critical failure of the scour protection layer. By directly observing (visually with an underwater camera) as well as monitoring with inertial sensors within the instrumented particle its likelihood of being entrained, this study seeks to evaluate and validate the risk of a critical failure of the scour protection layer close to build hydraulic infrastructure (here, a model bridge pier). As the physical model of a bridge pier is laid downstream, a series of flume experiments (four flow rates) are conducted under carefully controlled flow conditions to evaluate the change in entrainment frequencies of the instrumented particle. The experimentally obtained highly resolved (at 200Hz) time series of the instrumented particle's entrainment, are validated with the camera placed underwater, for the various flow conditions. The instances of instrumented particle entrainment - from which the rate of entrainment is found (matching the probability of bed surface destabilization [3]) - are derived from the analysis of fused raw data from the calibrated embedded sensors (accelerometer, magnetometer, and gyroscope) to identify entrainment events. Acoustic Doppler velocimetry (ADV) under proper configurations [4], is used to collect flow profiles at various distances downstream of the model pier in an initial effort to connect the local and dynamic driving processes for particle entrainment to the phenomenologically significant bulk flow and pier characteristics (such as pier lengthscale, average flow velocity and depth, the median size of armour layer particles). In order to examine the incipient destabilization of riverbed material, typically leading to the disruption of the bed surface protective layer and catastrophic scour, this research effectively demonstrates the employment of purpose designed instrumented particles, showcasing it as a method that is affordable, non-intrusive, long-lasting, and with readily accessible results.

References

• 1. Xu, Y., Valyrakis, M., Gilja, G., Michalis, P., Yagci, O., Przyborowski, L. (2022). Assessing riverbed surface destabilization risk downstream isolated vegetation elements, Water, 14(18):2880. DOI: 10.3390/w14182880.
• 2. AlObaidi, K., Valyrakis, M. (2021). A sensory instrumented particle for environmental monitoring applications: development and calibration, IEEE Sensors, 21(8), pp.10153-10166, DOI: 10.1109/JSEN.2021.3056041.
• 3. AlObaidi, K., Valyrakis, M. (2021). Linking the explicit probability of entrainment of instrumented particles to flow hydrodynamics, Earth Surface Processes and Landforms, 46(12), pp. 2448-2465 DOI: 10.1002/esp.5188.
• 4. Liu, D., AlObaidi, K., Valyrakis, M. (2022). The assessment of an Acoustic Doppler Velocimetry profiler from a user’s perspective, Acta Geophysica, 70, pp. 2297-2310. DOI: 10.1007/s11600-022-00896-3.

How to cite: Valyrakis, M. and Xu, Y.: Using instrumented particles for monitoring the likelihood of bridge scour protection destabilization, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14619, https://doi.org/10.5194/egusphere-egu23-14619, 2023.