GM5.4

EDI
Multi-scale Investigation of sediment transport processes in geophysical flows

Transport of sediments in geophysical flows occurs in mountainous, fluvial, estuarine, coastal, aeolian and other natural or man-made environments on Earth and has been shown to play important formative roles in planets and satellites such as Mars, Titan, and Venus. Understanding the motion of sediments is still one of the most fundamental problems in hydrological and geophysical sciences. Such processes can vary across a wide range of scales - from the particle to the landscape - which can directly impact both the form (geomorphology) and, on Earth, the function (ecology and biology) of natural systems and the built infrastructure surrounding them. In particular, feedback between flow and sediment transport as well as interparticle interactions including size sorting are a key processes in surface dynamics, finding a range of important applications, from hydraulic engineering and natural hazard mitigation to landscape evolution and river ecology.

Specific topics of interest include (but are not restricted to):

A) particle-scale interactions and transport processes:
-mechanics of entrainment and disentrainment (for fluvial and aeolian flows)
-momentum (turbulent impulses) and energy transfer between turbulent flows and particles
-upscaling and averaging techniques for stochastic transport processes
-interaction among grain sizes in poorly sorted mixtures, including particle segregation

B) reach-scale sediment transport and geomorphic processes
-bedform generation, evolution and disintegration dynamics (e.g. for dunes and other formations)
-discrete element modelling of transport processes and upscaling into continuum frameworks
-derivation and solution of equations for multiphase flows (including fluvial and aeolian flows)
-shallow water hydro-sediment-morphodynamic processes

C) large-scale, highly unsteady and complex water-sediment flows:
-flash floods, debris flows and landslides due to extreme rainfall
-natural and build dam failures and compound disasters (due to landslides, debris flow intrusion and downstream flooding)
-reservoir operation schemes and corresponding fluvial processes
-design of hydraulic structures such as fish passages, dam spillways, also considering the impact of sediment
-dredging, maintenance and regulation for large rivers and navigational waterways

Co-organized by NH1
Convener: Manousos Valyrakis | Co-conveners: Zhixian Cao, Rui Miguel Ferreira, Eric Lajeunesse, Anita Moldenhauer-RothECSECS
vPICO presentations
| Fri, 30 Apr, 13:30–17:00 (CEST)

vPICO presentations: Fri, 30 Apr

13:30–13:32
|
EGU21-7089
Tong Sun, Xiekang Wang, and Xufeng Yan

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.

13:32–13:34
|
EGU21-7091
|
ECS
Jiafeng Xie and Peng Hu

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.

13:34–13:36
|
EGU21-8169
|
ECS
Wei Huang and Wengang Duan

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.

13:36–13:38
|
EGU21-9273
|
ECS
Rui Zhu, Zhiguo He, and Eckart Meiburg

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.

13:38–13:40
|
EGU21-1808
|
ECS
Yi Xu, Valyrakis Manousos, and Panagiotis Michalis

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.

13:40–13:42
|
EGU21-10462
Ga Zhang, Chenge An, and Xudong Fu

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.

13:42–13:44
|
EGU21-10565
|
ECS
Jingyao Chen, Yanan Chen, Zhiguo He, and Benjamin Kneller

The density currents’ velocity structure, which can be divided into a jet region (JR) and a wall region (WR, thickness hr) according to their distinct dynamics, may be significantly modified as the current crosses an obstacle, thus leading to variations in the flow propagation process. However, there is a lack of direct observation of the response of different parts of the velocity structure to a three-dimensional obstacle due to the challenges in 3-D flow field measurement. To address this knowledge gap, a series of laboratory experiments have been devised to examine the separate influence of the WR and JR on mixing and propagation processes of density currents. A particle image velocimetry system and a high-speed camera are used to obtain the detailed velocity and vorticity fields with high temporal resolution. Compared with the no-obstacle counterpart that is uniform in the spanwise direction, the time-averaged current height (hc) in obstacle cases gradually thickens in that direction, and both the WR and JR thicken accordingly. The ratio of the obstacle height (ho) to hc influences the velocity structure. Specifically, hr/hc upstream is larger than that downstream when ho>hc, and vice versa. It is noteworthy that the variation of hr/hc in the spanwise direction is nonmonotonic with ho. Furthermore, the obstacle also influences the velocity profile upstream. The flow is obstructed on the center line when ho>hc. When ho<hr, the obstacle divides the wall region upstream into two parts above and below ho, and the gradient of the velocity profiles of the upper one is larger than the lower one. The results suggest that the obstacle plays an important role in determining the dissipation on the interface between the JR and the environment, and changing the current’s capacity on carrying the sediment since both the settling and resuspension of particles and sediment mostly happen in the WR. Our findings can improve understanding of the influence of submarine topography and provide a reference for underwater engineering.

How to cite: Chen, J., Chen, Y., He, Z., and Kneller, B.: Experimental Study of Three-Dimensional Obstacle Effect on Velocity Structure of Density Currents, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10565, https://doi.org/10.5194/egusphere-egu21-10565, 2021.

13:44–13:46
|
EGU21-3589
|
ECS
|
Highlight
Peng Gu and Manousos Valyrakis

Rockfalls can be detrimental to the safety of people and exposed infrastructure or property. Especially in the mountainous areas, rockfall disasters are common and unpredictable. In many countries and regions around the world, rockfalls have directly and indirectly caused great economic losses and even loss of life. In Scotland, an average of 1.4 million pounds a year is lost due to rockfalls. In China, a direct economic loss of 170 million pounds was caused and 858 people were killed by rockfalls In 2018. Most of the current prevention methods are costly and time-consuming. The objective of this study is to develop a new sensor that could monitor the dynamics of rockfall process. This paper discusses the development and calibration of a spherical instrumented rock with low-cost sensors, simulating spherical rocks and stones, which can be used to record the triaxial accelerations and angular velocities. The instrumented rock is tested on an appropriately designed dry flume for a range of slopes and fixed bed roughness. The preliminary experimental setup and results will be presented and discussed. By utilizing the impact forces obtained from the instrumented rock for the assessment for rock-fall dynamics with the designed dry flume physical experiments, we demonstrate how rockfall hazards can be monitored directly or indirectly using a low-cost tool.

Key words: rockfalls, instrumented rock, sensors, dynamics of process, earth surface hazards

How to cite: Gu, P. and Valyrakis, M.: Development and calibration of instrumented rock for monitoring rockfalls, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3589, https://doi.org/10.5194/egusphere-egu21-3589, 2021.

13:46–13:48
|
EGU21-3819
Yufang Ni, Zhixian Cao, Wenjun Qi, Xiangbin Chai, and Aili Zhao

Hydraulic lifting dams become increasingly popular in China for water storage, river landscaping and environmental restoration. Inevitably, dams influence riverine morphology. Unfortunately, current understanding of this topic has remained rather limited. Here, the morphological effects of a hydraulic lifting dam on the middle Fenhe River, China are investigated. This reach features a compound channel and floodplains, and the riverbed is mainly composed of silt that can be easily eroded, indicating potential significant bed deformation. A computationally efficient depth-averaged two-dimensional shallow water hydro-sediment-morphodynamic model is employed. Unstructured meshes are refined around dam structures to accurately present topography. The numerical predictions show discrepancies of morphological responses of the main channel and floodplains to different operation schemes of the hydraulic lifting dam. This work helps to support decisions on the management of hydraulic lifting dams on the middle Fenhe River and reveals a general pattern for the morphological impact of hydraulic lifting dam.

How to cite: Ni, Y., Cao, Z., Qi, W., Chai, X., and Zhao, A.: Morphological effects of a hydraulic lifting dam on the middle Fenhe River, China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3819, https://doi.org/10.5194/egusphere-egu21-3819, 2021.

13:48–13:50
|
EGU21-4146
Yunhui Sun, Xiaoliang Wang, and Qingquan Liu

Natural disasters normally involve the flow of polydispersed granular materials with interstitial fluid which may change the flow dramatically. Here we focus on a typical small-scale case of fluid–particle mixture flows, i.e., the immersed granular collapse using computational fluid dynamics coupled with discrete element method (CFD-DEM). The simulation parameters are calibrated with laboratory experiments and the immersed granular collapse process is reproduced in terms of different aspect ratios. We present a deeper investigation of the collapse based on simulation results. The granular front evolves in three stages, i.e., acceleration, steady propagation, and deceleration. We found that the constant propagation stage is maintained by the transition of particles’ motion from vertical to horizontal and the drag of the fluid. The constant propagation velocity is proportional to the free-fall velocity with a Stokes-number-dependent coefficient and the normalized final runout is linearly correlated with the densimetric Froude number. These conclusions may find its significance in geophysical applications.

How to cite: Sun, Y., Wang, X., and Liu, Q.: A Coupled CFD-DEM Simulation of Immersed Granular Collapse, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4146, https://doi.org/10.5194/egusphere-egu21-4146, 2021.

13:50–13:52
|
EGU21-4258
|
ECS
Binghan Lyu, Peng Hu, Ji Li, Zhixian Cao, Wei Li, and Zhiguo He

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.

13:52–13:54
|
EGU21-4364
|
ECS
Zi Wu, Arvind Singh, Efi Foufoula-Georgiou, Michele Guala, Xudong Fu, and Guangqian Wang

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.

13:54–13:56
|
EGU21-4656
Yining Sun, Ji Li, Zhixian Cao, and Alistair G.L. Borthwick

For reservoirs built on a hyper-concentrated river, tributary inflow and sediment input may affect the formation and evolution of reservoir turbidity current, and accordingly bed morphology. However, the understanding of tributary effects on reservoir turbidity currents has remained poor. Here a series of laboratory-scale reservoir turbidity currents are investigated using a coupled 2D double layer-averaged shallow water hydro-sediment-morphodynamic model. It is shown that the tributary location may lead to distinctive effects on reservoir turbidity current. Clear-water flow from the tributary may cause the stable plunge point to migrate upstream, and reduce its front speed. Sediment-laden inflow from the tributary may increase the discharge, sediment concentration, and front speed of the turbidity current, and also cause the plunge point to migrate downstream when the tributary is located upstream of the plunge point. In contrast, if the tributary is located downstream of the plunge point, sediment-laden flow from the tributary causes the stable plunge point to migrate upstream, and while the tributary effects on discharge, sediment concentration, and front speed of the turbidity current are minor. A case study is presented as of the Guxian Reservoir (under planning) on the middle Yellow River, China. The present finding highlights the significance of tributary inflow and sediment input in the formation and propagation of reservoir turbidity current and also riverbed deformation. Appropriate account of tributary effects is warranted for long-term maintenance of reservoir capacity and maximum utilization of the reservoir.

How to cite: Sun, Y., Li, J., Cao, Z., and Borthwick, A. G. L.: Effects of tributary inflow and sediment input on reservoir turbidity current formation and evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4656, https://doi.org/10.5194/egusphere-egu21-4656, 2021.

13:56–13:58
|
EGU21-6889
|
ECS
Chenge An, Marwan A. Hassan, Carles Ferrer-Boix, and Xudong Fu

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.

13:58–14:00
|
EGU21-5490
Kaiheng Hu, Xiaopeng Zhang, and Li Wei

Large-magnitude debris flows up to a volume of 1.0 million m3 happen frequently in the southeastern margin of Tibetan plateau due to rapid rock uplift, high relief and abundant rainfall. These flows with high bulk density can easily block main rivers. Such debris-flow barrier dams fail very quickly, resulting in outburst floods and intensive sed-iment transport. We collect data of four recent large-scale debris-flow damming events at Peilong, Yigong, Tianmo and Sedongpu catch-ments, and examine the process of riverbank erosion and sediment transportation under dam narrowing and outburst flooding. More than 10% of debris mass was delivered downstream when the dams breached. It is concluded that debris flow is main erosion way in this area, and the very high erosion rate play a key role on river morpholo-gy in southeast Tibet.

How to cite: Hu, K., Zhang, X., and Wei, L.: Typical debris-flow barrier dams and associated outburst floods in the southeastern Tibet, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5490, https://doi.org/10.5194/egusphere-egu21-5490, 2021.

14:00–14:02
|
EGU21-5854
|
ECS
Le Wang, Alan Cuthbertson, Gareth Pender, and Zhixian Cao

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.

14:02–14:04
|
EGU21-6789
|
ECS
Wei Li, Lehong Zhu, and Peng Hu

In history, channel avulsion occurred frequently in the Yellow River Delta featuring by the combination of large-scale north-south shifts and small-scale evolution of “wandering-merging-meandering-diverting” patterns. However, these evolution processes are lack of quantitative investigations due to the complex interactions between riverine and tidal flows, and between sediment-laden flow and river bed as well. Since public observations are scarce, we numerically study this problem focusing on the controlling factors for reproducing the “wandering-merging-meandering” evolution patterns and the characteristics of relative morphological equilibrium under constant discharge and sediment conditions. Using a 2-D depth-averaged fully coupled morphological model, numerical experiments are carried out for a schematic Yellow River Delta. The results show that random disturbance on initial topography is the key factor to initiate wandering patterns. Moreover, the development of river patterns and the associated morphological time scales are strongly related to initial bed slopes and upstream discharge and sediment conditions. Generally, a small bed slope and a low discharge favor the formation of wandering patterns in the early stage, while a large bed slope and a high discharge may accelerate the merging and routing processes. In the case of upstream clear flow, channel formation is dominated by erosion processes. Yet with increasing sediment, it results from the combination of levee lip sedimentation and channel erosion. In addition, the flow routing may be facilitated by enhanced tidal ranges whereas decelerated when subaqueous sedimentation extends to the sea. Regarding the equilibrium state, the morphological time scales are 4~8 years in most cases and the width-depth ratio increases longitudinally following a power-law function.

How to cite: Li, W., Zhu, L., and Hu, P.: Modelling Morphological Evolution of Deltaic Lobes in the Yellow River Mouth, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6789, https://doi.org/10.5194/egusphere-egu21-6789, 2021.

14:04–14:06
|
EGU21-6798
|
ECS
Can Huang, Xiaoliang Wang, and Qingquan Liu

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.

14:06–14:08
|
EGU21-6884
|
ECS
Aofei Ji, Peng Hu, Zhiguo He, and Fengfeng Gu

Abstract: In the Yangtze River Estuary deep-water channel regulation project, soft mattresses have been widely used to reduce bed erosion and thus improve stability of bridges/piers/levees/dikes. However, soft mattresses are also subject to failure due to the continuous and gradual scour in their edges, which have been a major risk for their stability. Here we report a preliminary numerical study on this issue. Firstly, a depth-averaged two-dimensional hydro-sediment-morphodynamic model is applied to simulate edge scour process for the submerged dike of the Jiangyanansha in the Yangtze estuary. For this purpose, physically-based sediment erosion parameterization is proposed to take account of the effect of the soft mattresses. Compared with the inner area of the soft mattress, only the edge area has stronger erodibility. Numerical comparative studies indicate that a scouring pit may develop to the vicinity of the submerged dike without the protection of the soft mattress, whereas under the protection of the soft mattress, the scouring pit can be largely controlled. Nevertheless, as the scouring process continues, the pit region and depth increase, which may finally lead to failure of the soft mattress. Finally, full 3D high-resolution simulations of the near-bed flow structure with/without edge scour are conducted using flow3D to shed light on the failure mechanisms of the soft mattresses.

Keywords: submerged dikes, soft mattress, erodibility, Yangtze estuary, edge scour, flow structure

How to cite: Ji, A., Hu, P., He, Z., and Gu, F.: Numerical study on the effect of edge scouring on stability of soft mattress in the Yangtze estuary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6884, https://doi.org/10.5194/egusphere-egu21-6884, 2021.

14:08–15:00
Break
15:30–15:32
|
EGU21-131
|
ECS
Antonija Harasti, Gordon Gilja, Matej Varga, and Robert Fliszar

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.

15:32–15:34
|
EGU21-171
Robert Fliszar, Gordon Gilja, Antonija Harasti, and Kristina Potočki

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.

15:34–15:36
|
EGU21-1820
|
ECS
Hamed Farhadi and Manousos Valyrakis

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.