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.