Scouring at bridge piers is troublesome and inevitable at the same time. Numerous empirical studies have been conducted in the last century to predict scour depth, but they completely ignore the physics of the problem. The physics behind scouring at bridge piers can be best understood in terms of the effect of the flow field around the pier at different stages of scour. This study comprises experimental and numerical parts. Experiments are conducted in the laboratory in which the flow field data at equilibrium is collected using Acoustic Doppler Velocimeter (ADV) and the equilibrium scoured bed is measured around isolated and In-Line Piers. Additionally, the commercial CFD code “FLOW-3D HYDRO 2022 R1” is utilized to simulate the flow field and scour around bridge piers. The FLOW-3D model solves the three–dimensional momentum and continuity equations coupled with the sediment transport equations to calculate and predict the flow field and the equilibrium scoured bed. While the maximum scour depth at equilibrium has been used to validate various CFD codes in the past, point-wise comparison of scour depth is scanty in previous research works. Moreover, the flow field at the equilibrium scour stage obtained using FLOW-3D has also been compared with experimental data available in the literature and experiment conducted in the laboratory. The performance of the CFD model is evaluated, the flow field and scoured bed geometry at equilibrium are analyzed and results are presented.

How to cite: Harshvardhan, H. and Kaushal, D. R.: CFD Modelling of Local Scour and Flow Field around Isolated and In-Line Bridge Piers using FLOW-3D, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3820, https://doi.org/10.5194/egusphere-egu23-3820, 2023.

The formulation of the reservoir desiltitation strategy has been addressed as an essential issue worldwide because the water resources crisis has become serious in recent years. The sediment-releasing operation using the current sluice gates or the bypass tunnel during flooding events can slow down the reservoir deposition and keep the storage capacity. However, the variation of downstream river morphology inevitably affected the channel stability due to the reservoir sediment-releasing operation. The sediment transportation downstream of the reservoir needs to be further investigated to determine the potential risk. This study adopted a calibrated two-dimensional numerical model, SRH-2D, to investigate the river morphology in the Tamshui River in northern Taiwan, East Asia. Three different typhoon events, Typhoon Hinnamnor, Soudelor, and Aere were considered slight, moderate, and severe scenarios. In addition, the original operation, sediment releasing, and bypass joint operation could be represented as the lowest, medium, and highest sediment released rate situation. The simulation shows the erosion and deposition location of different typhoon events and reservoir operations to highlight the potential disaster hotspot. As a result, this research can be a reference to reservoir management to adjust the operation principle to obtain the balance between reservoir storage capacity and downstream river stability.

How to cite: Huang, C.-C.: Simulation of River Morphology Change due to Reservoir Desiltitation Operation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-109, https://doi.org/10.5194/egusphere-egu23-109, 2023.

Habitat structures such as the Interception Rearing Complexes (IRC) are intended to aid endangered species such as the larval pallid sturgeon by increasing the movement of larval sturgeon out of the main river channel and into the channel margin through the construction or modification of river training structures. The studied IRC are designed to create fish pathways that access the lower velocities and generally shallower depths in the channel margin where juvenile sturgeon have a chance to mature in areas with critical food resources. A Two-Dimensional (2D) Adaptive Hydraulics/Sediment Library (AdH/SEDLIB) depth-averaged, shallow-water, finite element model has been created to study the potential effects that the construction of an IRC may have on other factors, such as long-term bed morphology, navigation, velocity distribution, water surface elevations, and potential flood risk. The model simulates a stretch of river under consideration for the construction of IRC structures. First, the model was set up with pre-existing riverbed and structure geometry. The model was validated for a period between 2017-2020. After calibration, the model was run with the implementation of the proposed IRC structure geometry and compared to system behavior without IRC construction. The validated model was used to evaluate the effects of the proposed IRC on other river processes, by performing simulations both with and without the implementation of the IRC and analyzing inter-model comparisons. The presentation will focus on the details of the modeling system including numerics, application and analysis of results.

How to cite: Savant, G., Denney, C., and Brown, G.: Numerical Model Based Analysis of River Training and Habitat Structures on River Processes: Straubs Bend of the Missouri River, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1717, https://doi.org/10.5194/egusphere-egu23-1717, 2023.

Intense sediment transport situations such as debris flows and mudslides consisting of fine-grained particles can pose serious threats to human infrastructures and lives. A thorough understanding of the rheology of such cohesive granular flows is crucial to predict the behavior of these types of flow and to mitigate their damaging effects. Whereas the rheology of non-cohesive granular flows has been studied extensively in the literature, the effect of cohesive inter-particle forces on the rheological behavior is still obscure and has not been sufficiently addressed. In this study, employing particle-resolved Direct Numerical Simulations, we simulate non-cohesive and cohesive dense suspensions sheared by moving walls. We perform several high-resolution simulations and compare the rheological parameters of the suspensions for different values of a dimensionless cohesive number, Co. Direct Numerical Simulations enable us to delve into the stress profiles in the vertical and streamwise directions and explore the contribution of different particle and fluid stresses to the total stress in each direction. We will also investigate the microstructure of the suspension and relate the microscopic interactions to macromechanical, rheological behavior of the dense cohesive suspension. 

How to cite: Khodabakhshi, A., Konidena, S., and Vowinckel, B.: Rheological behavior of dense granular suspensions of cohesive particles, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2270, https://doi.org/10.5194/egusphere-egu23-2270, 2023.

Floods including sediment transport are a significant threat to  communities in Austria. With climate change impacting the Alps, it is crucial to enhance and reassess protective measures. Predictions for the future of the Alpine climate indicate that there will be more winter precipitation and stronger, convective rain in the summer, even if the temperature goals set by the  Paris Agreement are met.

However, analyzing the impact of increased extreme precipitation on flood events in small Alpine catchments remains a challenge, and knowledge of the subsequent impacts on sediment transport is still insufficient. The catchment area of the Schöttlbach and thus the town of Oberwölz (Murtal, Styria) were affected by extreme flood events with extreme sediment transport in both 2011 and 2017, which caused heavy damage. The region was therefore selected to work with local stakeholders (especially the Austrian Torrent and Avalanche Control authorities) to improve the process understanding of flood events and sediment transport in a torrent catchment area and to draw possible climate change scenarios for the future.

It was the aim of the project „RunSed-CC“ (i) to estimate future runoff and sediment transport in an Alpine catchment using the latest climate projections (ÖKS15), (ii) to consider them in the light of the associated model uncertainties, and (iii) the potential for extrapolating the results to other Alpine catchments to test.

One focus of the project was the collection of natural data using a wide variety of measurements in the catchment area of an alpine torrent.The project RunSed-CC developed further a model that connects rainfall and runoff to sediment transport. The  hydrological model WaSiM was used, in combination with data on the  evolution of sediment source areas, to drive the 2D numerical models Telemac-2D (hydrodynamics) & Sisyphe (sediment transport). The main focus of the project was to  understand the potential impacts of future climate change on the  hydrologic regime, changes in sediment dynamics and sediment yield, and  associated uncertainties in the model.

This article is intended to provide a brief outline of the results of the extensive project, with a focus on the final findings of the sediment balances at the outlet of the catchment area.

How to cite: Schneider, J., Gegenleithner, S., Pessenteiner, S., Sass, O., and Schöner, W.: Determination of runoff and sediment transport in an Alpine torrent in Austria, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13076, https://doi.org/10.5194/egusphere-egu23-13076, 2023.

Morphodynamic modelling relies on different types of riverbed surveys. Surveys are essential as the basis of the evaluation of temporal river bed development, mesh creation, and model calibration. Spatial data, for example, obtained by topo-bathymetric airborne laser scanning (ALB) or sonar surveys results in a dense point cloud, providing detailed information on the river bathymetry. However, data gaps can occur due to restrictions in data acquisition (e.g. high water turbidity or water depth for ALB, low water for boat-mounted sonar). In contrast, cross-profiles contain only limited information on the bathymetry strongly dependent on the cross-profile and point spacing.

To assess the effect of the two survey data types on river bed development and morphodynamic predictions, the temporal evolution of a river stretch in the upper Danube at Donauwörth was analysed. The study area contains homogeneous river sections and sections with complex river geometry due to scours, bridge foundations, and river mouths.  Spatial sonar and ALB surveys were conducted from 2013 to 2020 and give detailed documentation of the river bed development. Cross-profiles with a cross-profile spacing of 200 m were derived from the spatial data. The spatial and cross-profile datasets show continuous river bed erosion. However, in this case, cross-profile data overestimate the overall erosion compared to spatial data. The geometry of homogeneous river stretches is depicted very similarly in the two datasets. For cross-profile data two cases exist for reaches with more complex river bed geometry: (i) The geometry lies in between two cross-profiles and it is missed entirely. (ii) The geometry is covered by a cross-profile and the resulting geometry is smeared in between the cross-profiles due to the interpolation process. Both possibilities result in an unsatisfactory depiction of the riverbed geometry.

To analyse the effect of morphological developments two morphodynamic models based either on the spatial or cross-profile datasets were set up. The models were calibrated against the datasets from 2013 to 2020 by adjusting the Strickler value for river sections with a length of 200 m. The Strickler values differ over the entire river stretch and not only in sections where complex river bed geometry occurs, meaning that the calibration errors propagate through the entire study area. Consequently, the deviations in calibration outcomes affect the model predictions, which simulate 7 years. In this case, the general shape of the predicted riverbed is similar, but due to the overestimation of riverbed erosion by cross-profile data, the morphodynamic model overestimates the erosion compared to the spatial data. However, the obtained error is for river reaches with low local variability within an acceptable range. If a project demands a highly accurate depiction of the river bed and the river geometry is known for having complex features, the use of spatial data is strongly advised.

How to cite: Siedersleben, J., Jocham, S., Klar, R., and Aufleger, M.: The Effect of Spatial and Cross-Profile Data on Morphodynamic Modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13691, https://doi.org/10.5194/egusphere-egu23-13691, 2023.

Estuarine sand dunes are primary bedforms existing in many sandy estuaries. They generally have lengths between those of river dunes (tens of meters) and marine sand waves (on the order of hundred meters). Estuaries are known for their complex flow patterns, arising from a mix of riverine and tidal flow, bringing in freshwater from land and salt water from the sea. Two particular flow patterns arising from the interaction between salt- and freshwater are the gravitational circulation and the strain-induced circulation, induced by a longitudinal and a vertical salinity gradient, respectively. Recent research was directed to understanding the influence of these flow patterns on estuarine sand dunes through a linear morphodynamic model ([1], [2]). Linear stability models are capable of capturing initial growth from a flat bed, and yield the system’s preferred bedform length and migration rate.

Here, we build upon the linear modeling approach with a nonlinear morphodynamic model capturing both the subsequent bedform development towards equilibrium, and the effect of dunes on the flow as they develop. The model domain has spatially periodic boundary conditions and a rigid lid at the surface; the hydrodynamic module is non-hydrostatic and is solved with a k-omega turbulence closure. Furthermore, we include bed-load sediment transport with a formulation that contains a slope term. Results show the height, length, shape and migration rate of dunes in an estuarine environment, and reveal the flow- and turbulence patterns over these bedforms. Furthermore, we show the deceleration of the flow with development of dunes, and thus quantify the effect of dunes on flow resistance.

[1]          Van der Sande, W. M., Roos, P. C., Gerkema, T., & Hulscher, S. J. M. H. (2021). Gravitational circulation as driver of upstream migration of estuarine sand dunes. Geophysical Research Letters, 48(14). DOI: 10.1029/2021GL093337

[2]          Van der Sande, W. M., Roos, P. C., Gerkema, T., & Hulscher, S. J. M. H. (in press).  Shorter estuarine dunes and upstream migration due to intratidal variations in stratification. Estuarine, Coastal and Shelf Science. DOI: 10.1016/j.ecss.2023.108216

How to cite: van der Sande, W., Roos, P., Gerkema, T., and Hulscher, S.: Finite-amplitude modeling of estuarine sand dunes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6198, https://doi.org/10.5194/egusphere-egu23-6198, 2023.

Shifting runoff dynamics and highly intensified geomorphic processes are immediate consequences of the evident glacier mass loss in high-alpine headwater catchments. Rapidly retreating glaciers expose unconsolidated sediments to erosion in the proximity of meltwater-fed mountain streams impacting the catchment-scale sediment dynamics. Altering sediment fluxes can have considerable implications for the operation and management of water infrastructure, especially hydro-electric power facilities in otherwise non-regulated glaciated catchments. Bedload-rich outwash plains with typical braided channel networks serve as a deposition area for glacier debris under average runoff conditions. During flood flow conditions, the proglacial areas connect with the downstream catchment, delivering subglacial sediments to lower stream sections.

As such, they represent key elements in high-alpine river systems when considering future discharge and sediment yield from deglaciating catchments. Establishing a numerical model of this important component of the headwater catchment illuminates a data scarce fluvial process domain. Yet, model parametrization and setting boundary conditions for a glacier forefield are challenging. Direct measurements in the paraglacial transition zone of retreating glaciers are usually complicated to achieve, especially since outwash plains are frequently subject to intensive geomorphic processes. Therefore, innovative methods, minimizing labour-intensive and time-consuming manual surveying, are needed to overcome data scarcity in paraglacial environments.

A combined methodological approach to parameterize key boundary conditions of an Alpine proglacial outwash plain (Jamtal valley, Austria) with an area of 0.035 km² and an average channel inclination of 4.8 % is presented. Measuring discharge in situ is difficult since the braided riverbed is not stable due to frequent relocation of sediment. Therefore, close range sensing techniques based on RGB imagery from hand-held and fixed time-lapse cameras used in combination with maximum water level gauges are used directly in the outwash plain to monitor flood runoff events. A conventional discharge gauge (non-contact flow velocity and water level sensor) was realized 3 km further downstream covering the recent hydrologic summers (2019-2022). UAV-borne RGB imagery was used to detect changes in topography, sediment budget and composition.

We present results on key parameters, essential for numerical modelling of hydraulic flood flow conditions, including: (i) multi-annual high-resolution topographic 3-D models of the frequently changing channel geometry, (ii) hydraulic roughness of surface sediments derived from areal grain size distribution maps (i.e., D50, D84) and (iii) spatio-temporal flood flow maps indicating the annual variability in the surveyed proglacial outwash plain. These interrelated survey results are then used to parameterize and calibrate a 2-D numerical model (TELEMAC 2-D) to simulate hydraulic base and flood flow conditions, demonstrating the applicability and robustness of the presented multi-method approach.

How to cite: Hiller, C., Siedersleben, J., Leistner, S., Wibmer, T., Helfricht, K., and Achleitner, S.: From multi-method monitoring towards numerical simulations of flood flow events in a proglacial outwash plain, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16035, https://doi.org/10.5194/egusphere-egu23-16035, 2023.

In hydraulic engineering, river discharge estimation is an important requirement for managing and planning the channel flow. Discharge measurements are of utmost importance for the purposes such as water availability analysis, reservoir operation, flood forecasting, designing of hydraulic structures, etc. In the discharge estimation process, the spatial velocity distribution in the transverse cross-section at the desired location of measurement is required. In traditional approaches such as Prandtl-Von Karman logarithmic law and power law, the velocity is employed deterministically, making its utility easier and providing accurate results for the wide channels only. In contrast, Shannon’s information entropy concept, which evaluates random variables probabilistically, is used in hydrology to determine entropy-based velocity distributions. In this approach, the velocity distributions obtained depends on a parameter called as entropy parameter, which is considered to be a fundamental measure of information about the channel characteristics such as channel bed slope and roughness. It provided better results for both the clear water and sediment-laden flow as compared to the former. In the present study, experiments for discharge estimation were performed on the experimental flume to collect the velocity data at different channel bed slope conditions to demonstrate the accuracy of the entropy-based concept. To prove the truthfulness of the entropy-based concept, the results were compared with the ones obtained from the classical method (velocity area method). Both the approaches have their respective advantages and limitations. Therefore, error analysis was necessary to check the efficiency and accuracy of the entropy-based model, which was performed by comparing the percentage error between the observed and computed discharge values. The final results revealed that the entropy model was a quick and accurate technique for discharge estimation as absolute percentage errors were less than 5% and the 95^th percentile was 3%.

How to cite: Singh, G. and Khosa, R.: Discharge Estimation for Different Bed Slope Conditions Using Entropy-Based Concept: An Experimental Investigation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-868, https://doi.org/10.5194/egusphere-egu23-868, 2023.

Small mountainous rivers provide an important part of the sedimentary inputs to the oceans. The particularity of theses rivers comes from the fact that their inputs take place mostly during brief and intense flood events. While the quantification of fine sediment flux is fairly well known, sandy inputs are very poorly known and field measurements are scarce. River mouth sand discharge is a key variable in the coastal sediment budget as it participates to the coastline evolution and its protection against marine events. Coarse sediments are mainly transported as bedload, making it difficult to estimate with traditional methods such as traps. In this study, a fixed Acoustic Doppler Current Profiler (ADCP Nortek Aquapro 1 MHz) was deployed on a bottom frame, near the mouth of the Têt river (SE France) to estimate near-bottom sediment transport during flood events. In addition, cross sections have been undertaken with a Sontek Hydroboard equipped with a Sontek M9 ADP at different water discharge during several flood events. A calibration of the backscatter index was carried out using gravimetric measurements and granulometric analysis of water samples to estimate the sediment flux and the sand proportion. Sediment fluxes were then compared with the altimetric variations observed from bathymetric surveys. Results allowed to characterize the variability of the boundary layer thickness and the sediment concentrations during flood events. Those results give useful information to estimate sand fluxes from mountainous rivers to the coastal area in a context where these are considered to be low or non-existent compared to large coastal rivers.

How to cite: Meslard, F., Bourrin, F., Balouin, Y., and Robin, N.: Near-bottom sediment transport during flood events on a small mountainous river, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8007, https://doi.org/10.5194/egusphere-egu23-8007, 2023.

Many water bodies are, as a result of anthropogenic influences, such as river straightening, river bank fixation, or damming in accordance with the EU Water Framework Directive (2000/60/EC) not in a good ecological state anymore. With the aim to return to a good ecological status for surface waters, restoration measures are implemented in many rivers. However, the success and sustainability of such measures are often site-dependent and require hence an objective assessment.

In this study, the Multi-Parameter Approach to assess Clogging (MultiPAC) was used to assess the suitability and sustainability of different riverbed restoration strategies. MultiPAC is based on several measured physico-chemical parameters, which enable a detailed investigation of in-situ conditions of gravel-bed rivers. The approach includes measurements of the sediment composition for identifying surface and subsurface grain size distributions and fine sediment fractions. In addition, measurements of the porosity are obtained by using Structure-from-Motion and the Water Replacement Method of freeze-core samples. Finally, measurements of the interstitial oxygen concentration and so-called slurping rates, which are converted into hydraulic conductivity, were performed with a double-packer system called VertiCo.

The residual river stretch between Jettenbach and Töging at the Inn River in Germany provided a means to evaluate riverbed restoration measures, implemented in February and March 2020. Investigations were performed for several gravel bars, where sediment was replenished and a mechanical break-up of the bed armour layer was conducted. The MultiPAC investigations were performed before measure implementation, shortly afterwards (March 2020) and in November 2020 to investigate the impact of a flood event with a 10-year return period, which occurred in August 2020, and thus may have influenced the sustainability of the restoration measures.

From the measurements, it can be seen that sediment replenishment and the mechanical break-up of the armour layer significantly improved the ecological functioning of the riverbed. However, it became evident that the increase in the quality of the riverbed was only temporary. Hence, these measures will need to be repeated regularly with the aim of maintaining ecologically-valuable riverbed habitat conditions. The results of this study also showed that MultiPAC provides detailed insights into the riverbed sediments, their composition, and the permeability of the riverbed.

How to cite: Haun, S., Negreiros, B., Schwindt, S., Aybar Galdos, A., Noack, M., and Wieprecht, S.: MultiPAC as a tool to monitor the sustainability of riverbed restoration measures, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7152, https://doi.org/10.5194/egusphere-egu23-7152, 2023.

Northern rivers are responding to global warming by changes in seasonal discharges, sediment transport rates and morphology. Very limited amount of studies about bedload transport rates have been carried out in the winter-season. Traditional methods of measuring bedload transport are limited by their proneness to user error, small spatial scales and uncertainties related to the equipment itself. Computer vision-based particle image velocimetry (PIV) and particle tracking velocimetry (PTV) methods have been successfully applied to measurements of water surface velocities and preliminary results show that they can be applied to underwater sediment transport velocity measurements. 

The aims of this study are to 1) to enhance the bedload calculations, by comparing traditional mechanical methods and computer vision-based particle image velocimetry methods applied to underwater video data sets. Additionally, topography created from underwater imagery is used to scale and  georeference the results with very good precision, and 2) understand the seasonal variation in bedload transport amounts based on both mechanical and image velocimetry methods.

The study is based on field data, measured at sub-arctic Pulmanki river, located in northern Finland. The data has been gathered in 2021 autumn and winter, 2022 spring, 2022 autumn, to cover different possible sediment transport conditions, from low flow ice-covered to high flow open channel periods. The preliminary results are presented. They show that the method is promising in enhancing the understanding of sediment transport processes and the seasonal transported amounts.

How to cite: Välimäki, J.-M., Lotsari, E., Takala, T., Wolff, F., Pajunen, V., and Eltner, A.: Subwater particle image velocimetry and  photogrammetry  as solution for enhanced seasonal measurements of river dynamics (more precisely: bedload transport), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8386, https://doi.org/10.5194/egusphere-egu23-8386, 2023.

Sediment transport is a fundamental process to understand river morphodynamics. Bedload sediment transport during high flow energy governs channel geometry, though with variable response to flood events. Therefore, the sensitivity of bedload sediment transport to flood events is an important geomorphic query. We analysed the bedload transport process during the extreme flood event in the Purna River, a partial-bedrock river in peninsular India. The Purna River originates from an elevation of ~900 m and drains ~18,450 km² area. This major tributary of the Tapi River flows ~360 km. The flood events were characterised through flood frequency analysis using the Gumble distribution on peak discharge data from 1980-2016. The bedload sediment transport was assessed for the highest flood event using the Mayer-Peter Muller equation. Daily data of discharge, wetted area, wetted perimeter, grain size (D₅₀) data of pre- and post-monsoon, and the cross-section was obtained from the Central Water Commission (CWC), India. The bed slope was analysed using Manning’s equation. Our analysis shows that the return period for the highest flood at upstream station (Gopalkheda) is 35 years, while it is 136 years for downstream station (Yerly). The average bed shear stress was ~1.04 and ~1.55 times more than the critical shear stress using D₅₀ during the flood event for upstream and downstream reaches, respectively. The average bed shear stress exceeded the equal mobility condition (τ_eq≈1.45τ_c) in the downstream reach leading to full mobilisation. Therefore, it causes high bedload transport and scouring of bed level by more than 1 m in the downstream reach. However, at upstream reach, there was low bedload sediment transport and insignificant change in the bed level due to partial mobilisation. Also, the Maximum Flow Efficiency (MFE) at the downstream station is 6-7 times more than the upstream reach, representing high erosion in the downstream reach. Therefore, Purna River is characterised by reach-scale variability in the channel process to the same flood event. The downstream reach is more sensitive than the upstream reach and hence more prone to morphological change. These alterations have implications for designing hydraulic structures, water management, and river ecology.

How to cite: Bind, V. K. and Jain, V.: Bed level variation and channel sensitivity analysis of a river channel to the extreme flood event, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11614, https://doi.org/10.5194/egusphere-egu23-11614, 2023.

Low water depth can be a problem for navigation on large rivers. Since the last century, a frequently-used method of river regulation has been the installation of wing dams (wing dikes, spur dikes, groins). With their help, the riverbed narrowed during low water, which created a greater water depth and a higher water level. However, the narrowed flow also generated a higher bed shear stress, which, due to bed erosion, simultaneously increased the water depth and lowered the water level.

From a flood protection point of view, questions arise as to how wing dam fields change flood levels as a result of the bed change caused by such intervention. The complexity of the answer increases if we examine the problem not only in a cross-section (or short reach scale), but at long scale, as in the case of the Mississippi River, USA, where a system of wing dams hundreds of kilometers long was installed.

We analyzed the problem in two steps: we apply a 3D sediment transport model on a local scale, and the results are then upscaled and implemented in a 1D model to enable study of the problem at large spatio-temporal scale (hundreds of km, several centuries).

How to cite: Török, G. T. and Parker, G.: Study of morphodynamic change caused by wing dams at large spatio-temporal scale, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10815, https://doi.org/10.5194/egusphere-egu23-10815, 2023.

In the 19^th and 20^th centuries, numerous alpine rivers were modified into straightened and monotonous channels by river regulations, which had significant negative effects on the ecology of the river systems and their floodplains. River revitalisations aim to restore river sections in order to create stepping stones in anthropogenic altered rivers. A large-scale example are the measures along the alpine Inn River between Stams and Rietz in Tyrol (Austria) over a length of about 3 river kilometres, which are implemented within the extension project of the hydropower plant Sellrain-Silz. During two low-water periods (October - April), starting in 2021, the existing bank protections were removed to a large extent, the river bed was widened up to 75 meters and a back water zone as well as a branch were created. As a result, the river stretch can develop by its own dynamics and ecologically valuable areas such as shallow water zones as well as gravel and sand banks are supposed to be formed.

In order to assess the morphodynamics and ecological functionality of these measures, while also maintaining flood protection, a comprehensive monitoring program is being conducted. This includes nine photogrammetric surveys using an unmanned aerial vehicle (UAV) between March and October 2022. The UAV data were used to generate ortho-images and digital elevation models. The accuracy was assessed by comparison with airborne LiDAR data from one flight in March 2022. The objective is to quantify the erosion and deposition processes in the area of the measures and to determine the sedimentological processes that occurred during the survey period using the UAV results and hydro-morphological data (e.g., hydrograph, suspended sediment concentration) from nearby gauging stations on the Inn River. In addition, the suitability of high-resolution data from UAV surveys for monitoring sediment and bedload dynamics in alpine rivers will be evaluated. First results show that deposition processes dominated in the area of the measures, while relatively low discharge values were recorded throughout the study period. Based on the data analysis, we elaborate a suggestion for further monitoring in addition to the cross-section surveys already conducted since 1980, i.e., one UAV flight per year at low-flow conditions in order to establish a long term monitoring of morphodynamics.

How to cite: Zöschg, H., Bacher, T., Boschi, M., Fey, C., Schöber, J., Reindl, R., Schletterer, M., and Aufleger, M.: UAV-based monitoring of sedimentary processes at a large-scale revitalisation of an alpine river – concept and outlook, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13887, https://doi.org/10.5194/egusphere-egu23-13887, 2023.

Suspended sediment transport is an integral part of river systems and provides important services for ecological functioning and human use of river channels. Thus, measuring and monitoring suspended sediment is of great importance to understand the implications of suspended sediment transport and to improve sustainable river management. Suspended sediment in the German waterways is monitored at 62 monitoring stations by the German Water and Shipping Authorities staring in the 1960ties. The dataset provides valuable information on the long-term developments of suspended sediment transport in Germany. After the analysis of suspended sediment rating (Hoffmann et al. 2020) and the long-term trend (Hoffmann et al. 2022) based on the extensive suspended sediment dataset, we present first results on the seasonal variation and the seasonal shifts of suspended sediment transport in Germany. The results indicate that river systems in Germany are characterized by strongly differential seasonal behavior despite modest spatial variations of climatic conditions in Germany. These results will be discussed in terms of seasonal shifts during the last 50 years and the expected changes of the sediment regimes in German river systems due to future climate changes.

References:

Hoffmann, T.O., Baulig, Y., Fischer, H., Blöthe, J., 2020. Scale breaks of suspended sediment rating in large rivers in Germany induced by organic matter. Earth Surface Dynamics, 8(3), 661-678.

Hoffmann, T.O., Baulig, Y., Vollmer, S., Blöthe, J., Fiener, P., 2022. Back to pristine levels: a meta-analysis of suspended sediment transport in large German river channels. Earth Surf. Dynam. Discuss., 2022, 1-28.

How to cite: Hoffmann, T.: Seasonal variability and seasonal shifts of suspended sediment transport in large German river system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14612, https://doi.org/10.5194/egusphere-egu23-14612, 2023.

Although the importance of studying channel bifurcations is widely recognised, their hydraulic behaviour in shallow, rough mountain rivers has so far received little attention from researchers. Understanding the specific hydraulics of such units is essential for predicting and interpreting their morphodynamic evolution. Water discharge measurements in the incoming channel and distributaries are often difficult to perform in steep streams characterised by high relative roughness (grain size to depth ratio d/h > 0.1), aerated flow, and marked free-surface waves. Nonetheless, recent advances in Acoustic Doppler Current Profiler (ADCP) technologies open new possibilities for studying the flow configuration at stream bifurcations.

We monitored the flow repartition in a bifurcation of a mountain gravel-bed river by deploying an ADCP specifically designed for shallow flow conditions. This field campaign was combined with photogrammetric surveys for documenting the geomorphological evolution of the river bed, its surface grain size distribution and structure. Integrating data from these different sources provided useful information on the bifurcation evolution and hydrodynamics. During a period in which the river bed did not undergo noticeable elevation changes, we observed that the water discharge ratio of the distributaries was approximately constant for sensibly different total discharge values. Such result was compared with the outcomes of numerical simulations.

How to cite: Pascal, I., Miazza, R., de Graffenried, B., and Ancey, C.: Bifurcations in mountain rivers: insights on their hydraulics from field measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8056, https://doi.org/10.5194/egusphere-egu23-8056, 2023.

Where fine-grained marine sediments are being brought in suspension and become subject to transport by currents, aggregation of cohesive primary particles (flocculation) can occur. By changing the inherent particle properties, flocculation processes play an important role in speeding up settling and redeposition of sediment particles. However, while numerous laboratory experiments have been conducted to understand properties and behavior of flocs, so far, there is not yet an appropriate method to monitor flocculation in-situ. Gaining a better understanding of the flocculation process and how it will affect the dispersion of man-made sediment plumes is important to assess the impact of human activities on the environment, for example in the context of deep-sea mining or offshore dredging.
In this study, the inherent acoustical and optical properties of sediment particles are studied using in-situ plume monitoring data collected by the MiningImpact2 project consortium during the first deep-sea mining trial of a pre-prototype polymetallic nodule collector vehicle. The trial was conducted in April 2021 at 4500 m water depth in the Clarion-Clipperton Zone (eastern equatorial Pacific Ocean) by the Belgian contractor DEME-GSR. During this trial, one of the main goals was to monitor the spatiotemporal evolution of the sediment plume generated by the mining vehicle. For this purpose, numerous sensors were deployed around the test area including ADCPs of different frequencies, OBSs and deep-sea particle camera. In this study, the main interest is to use this dataset to gain knowledge on the variability of particle properties and to monitor flocculation in the generated plume.
The monitoring array of sensors proved successful in measuring the dispersion of the plume around the mining site. In the data recorded in the plume, a gradient in optical and acoustic response was found, suggesting a change in inherent particle properties such as their size and shape induced by flocculation. The evolution of particle size as inferred by the particle camera recordings (PartiCam) corroborated this finding. In combination with currents and environmental measurements, this dataset provided valuable information to better understand the flocculation process.

How to cite: Diaz, M., Stigter, H., Gillard, B., Gazis, I.-Z., Mohrmann, J., Heger, K., Baeye, M., Thomsen, L., and Greinert, J.: Monitoring flocculation during a deep-sea mining test in the Clarion-Clipperton Zone, eastern equatorial Pacific Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17507, https://doi.org/10.5194/egusphere-egu23-17507, 2023.