Hydro-morphological processes in open water environments - measurement and monitoring techniques

Sedimentary processes in aquatic environments, including erosion, transport, and deposition of sediment by hydrodynamic mechanisms, are key features for various research disciplines, e.g., geomorphology and paleoclimatology or hydraulics, river engineering and water resources management and hydrology. Accurate quantification of erosion, transport, and deposition rates, conditioning river channel morphology, and bed composition, is fundamental for adequate development of conceptual sediment budget models and for the calibration and validation of the numerical tools.
The main goal of this session is to bring together the community of scientists, scholars, and engineers, investigating, teaching, and applying novel measurement techniques and monitoring concepts, which are crucial in determining sedimentary and hydro-morphological processes in rivers, lakes, and reservoirs, estuaries as well as in coastal and maritime environments. It focuses on the quantification of bedload and suspended load, bedforms migration, channel horizontal migration, bed armoring and colmation, but also the transport mode, flocculation, settling, and re-suspension of the sediment particles.
Contributions are welcome with a particular focus on single and combined measurement techniques, post-processing methods as well as on innovative and advanced monitoring concepts for field and laboratory applications. We welcome contributions containing recent results in a temporal and spatial scale on sediment budgets as well as on sedimentary and morphodynamic processes in open water environments.
Contributions may refer but are not restricted to:
• Measurements of suspended sediment and/or bedload transport in open water environments, e.g., with classical or novel methods;
• Determination of sediment characteristics, e.g., with mechanical bed material samplers or freeze core technique;
• Innovative measurement approach or techniques aimed for validation and calibration of numerical models;
• Measurements of critical bed shear stress of cohesive sediments, e.g., with benthic flumes or miscellaneous devices;
• Monitoring of morphological changes like lake and reservoir sedimentation, bank erosion or bed armoring, meandering
migration, river bends evolution;
• Measuring networks / multiple point datasets;
• Large- or small-scale monitoring concepts including case studies;
• In-situ or laboratory calibration of measurement data using classical or novel (e.g., machine learning) approaches;

Co-organized by GM2
Convener: Slaven ConevskiECSECS | Co-conveners: Stefan Achleitner, Kordula Schwarzwälder, Axel Winterscheid
| Mon, 23 May, 08:30–10:00 (CEST)
Room 2.17

Presentations: Mon, 23 May | Room 2.17

Chairpersons: Slaven Conevski, Kordula Schwarzwälder, Axel Winterscheid
Presentation form not yet defined
Giacomo Pellegrini, Riccardo Rainato, Luca Mao, and Lorenzo Picco

In many environments, climate change causes an increase in the frequency and magnitude of Large Infrequent Disturbances (LIDs). LIDs make fragile areas, as mountain basins, even more vulnerable. Among all LIDs, windthrows are one of the most relevant disturbances affecting the Alpine region. Windthrows can affect the forest cover and morphological settings at the basin scale (e.g., due to associated landslides), changing the supply of sediments to river networks and affecting the cascading processes. This work aims to measure the sediment contribution of a managed windthrow-affected area during the snowmelt (1st April - 15th June 2021) in the Rio Cordon basin (5 km2, eastern Italian Alps). The study reach crosses the area affected by windthrow and receives sediments from six sediment sources. Two multiparameter sondes measuring the turbidity and the water level were installed upstream and downstream the windthrow-affected area. Moreover, water samples and salt dilution discharge measurements were collected to obtain the rating curves and calibrate the turbidity meters in order to derive suspended sediment loads (SSL). The cumulative precipitation registered 231.2 mm during the entire 2021 snowmelt period. The total runoff recorded was 3,054,239 m3 and the total SSL at the outlet was 109 t. Two relevant events peaking at 1.13 and 1.86 m3 s-1 were recorded in the study period, and in both cases the SSL was higher at the downstream end of the reach (+4.4% and +4.0% respectively). However, clockwise hysteresis loops were identified in both sections and events. Although these preliminary results suggest that the managed windthrow-affected area can be a potential source of sediment, the greatest contribution of sediments seems to have been provided by other sediment sources, either or both located on the slopes and in the channel bed upstream the monitoring area. This study represents a suitable way of understanding the cascading processes in a mountain basin, to improve both risk-and conservation-related management strategies. Further analysis to comprehend the all-seasons basin responses are undergoing.


How to cite: Pellegrini, G., Rainato, R., Mao, L., and Picco, L.: Contribution of a windthrow-affected area to the suspended sediment transport in an Alpine Mountain catchment: a focus on the snowmelt period., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5456, https://doi.org/10.5194/egusphere-egu22-5456, 2022.

Presentation form not yet defined
Marlene Haimann, Mario Klösch, Patrick Holzapfel, Franz Steiner, Kevin Merl, Erich Busch, Michael Krapesch, Christopher Tomaschitz, Peter Baumann, and Helmut Habersack

Downstream of Bernhardsthal to its confluence with the Morava River, the Thaya River forms the border between Austria and the Czech Republic. Here, in the about 15 km long river section, the river was channelized in the 1970ies and 1980ies mainly by meander cut-offs and bank protection. In addition, this section is impacted by sediment retention in upstream reservoirs and by the effects of climate change, which has led to adverse morphological response of the river and a decline in habitat diversity. Since original habitats in the form of oxbows and riparian forests still exist in the immediate vicinity of the Thaya River, the potential for restoration is particularly high. This was exploited in two projects (Dyje 2020/Thaya 2020 and Thaya Wellendynamik/Dyje, rovnovážnádynamika odtokových poměrů, funded by the EU through INTERREG AT-CZ), where restoration measures such as the removal of bank protection and enhanced bank erosion by large woody debris structures, as well as the reconnection of meanders were implemented. Without monitoring of the morphodynamics before and after restoration, the effects of these efforts would remain unclear.

In the EU-funded project SEDECO (INTERREG AT-CZ), morphological changes and the current morphodynamics of the Thaya River in this section were investigated. The analysis is mainly based on cross-sectional measurements from 1996, which were resurveyed within the project. Furthermore, the current morphodynamics, occurring after the implementation of the restoration measures, are surveyed in detail. By comparing the different data sets, the development of the river was assessed and a sediment budget was calculated applying the newly developed sediment budgeting tool BudSed. Additionally, the suspended sediment transport is measured at a monitoring station in the upstream part of this section. These data were supplemented by orthophotos to determine the evolution of the active channel. Meso- and micro-scale habitat modeling, including climate change scenarios, will be conducted to evaluate habitat enhancement resulting from the meander reconnections. Besides numerical simulations, physical modeling of morphodynamics will be performed in the new hydraulic engineering laboratory, built as part of the project, allowing the performance of large-scale tests.

The project results show that the entire section is affected by erosion. This is most likely the result of the straightening and slope increase of the river, as well as the sediment deficit caused by the upstream reservoir. Sections without bank protection exhibited less incision and more lateral dynamics such as widening and migration of the river axis. The smaller width/depth ratio in reaches with protected banks indicates that impeded bank erosion is compensated by more bed incision. The sediment budget shows an imbalance as more sediment is transported out of the section. This imbalance will persist until the river has changed sufficiently to compensate for human impacts or new measures are taken to reduce their effect. In this respect, the recent reconnections of meanders seem promising by reducing the slope and thus sediment transport capacity. Furthermore, preliminary results of the habitat modeling indicate that the depth and width variations increased and habitat availability became larger even at low flow conditions.

How to cite: Haimann, M., Klösch, M., Holzapfel, P., Steiner, F., Merl, K., Busch, E., Krapesch, M., Tomaschitz, C., Baumann, P., and Habersack, H.: Investigation of the morphological and ecohydraulic evolution in the course of river restoration works in a transboundary section of the Thaya River, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9319, https://doi.org/10.5194/egusphere-egu22-9319, 2022.

On-site presentation
Jessica Droujko, Srividya Hariharan Sudha, Gabriel Singer, and Peter Molnar

Estimates of suspended sediment concentration (SSC) at high spatial resolution can be used to identify sediment sources, track the natural erosion gradients over entire mountain ranges, and quantify anthropogenic effects on catchment-scale sediment production, e.g. by dam construction or erosion control. Measurements of SSC at a basin outlet yields a basin-integrated picture of possible hydroclimatically-driven sources of sediment. However, a statistical analysis of one-dimensional input-output relations does not give us a full spatial perspective on sediment pathways of production and, potentially transient, storage within the catchment. These sediment pathways within catchments are difficult to identify and quantify due to the lack of affordable monitoring options that can create both spatially and temporally highly resolved datasets. Here, we propose a methodology to quantify these pathways using Sentinel-2 Level-1C imagery and in-situ measurements from a small network of sensors. The study is carried out on the Vjosa river, which represents one of the last intact large river systems in Europe. Geological diversity in the catchment and its widely unobstructed fluvial morphology over the entire river length makes it extremely interesting to monitor natural sediment dynamics. The remote sensing signal from the river’s water column, extracted from satellite imagery, contains an optical measure of turbidity. Furthermore, in-situ turbidity measurements between May 2019 and July 2020 from seven turbidity sensors located across the Vjosa provide ground-truthing. A significant multiple linear regression model between turbidity and reflectance was fitted to these data. The regression model has a low adjusted R2 value of 0.30 but a highly significant p-value (< 2.2e-16). The satellite data together with the regression model were used to generate longitudinal profiles of predicted turbidity over the catchment from August 2020 to August 2021. Validation of these predictions for two different Sentinel-2 acquisition dates was done with in-situ turbidity measurements taken from a kayak during descents of the entire river. This validation showed accurate prediction of trends on a catchment scale but poor accuracy in the prediction of pointwise turbidity quantification. The model also showed accurate estimation of trends during different climatic seasons, suggesting that our approach captures the temporal variability in suspended sediment concentrations driven by long-term hydrological processes. Gridded rainfall from E-OBS was used to identify short-term hydrological forcing such as storm-driven activation of sediment sources. In order to monitor the many physical connections between hydrology, river processes, and sediment fluxes, future work will include extension of the in-situ turbidity sensor network with new sensors developed by our group. We plan to place these low-cost sensors at the outlet of every major tributary, on the main stem both above and below a confluence with a tributary, and within morphodynamically unique reaches.

How to cite: Droujko, J., Hariharan Sudha, S., Singer, G., and Molnar, P.: Sediment source and pathway identification using Sentinel-2 imagery and (kayak-based) lagrangian river profiles on the Vjosa river, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5665, https://doi.org/10.5194/egusphere-egu22-5665, 2022.

Virtual presentation
Fine Scale Turbulent and Granular Processes in Intense Sediment Transport Regimes
julien chauchat, helder Guta, remi chassagne, cyrille bonamy, david hurther, and tian-jian hsu
Virtual presentation
Aron Slabon and Thomas Hoffmann

Monitoring suspended sediment transported in fluvial systems is of major importance regarding natural hazards, water quality, and sustainable river management. However, monitoring is challenged by the spatial and temporal variability of suspended sediment transport and thus time consuming and costly. Here we analyze the spatial variability of suspended sediment in the German waterways using data from suspended monitoring networks of the German water and shipping authority (WSV). The data consists of four stations with cross-sectional measurements (isokinetic sampling with 20-25 samples/sampling campaign and 3-4 campaigns per cross-section/year) along with three stations with frequent (daily) point measurements. As the distribution of SSC with water depth is well established through the Rouse profile, uncertainties are induced through the determination of the settling velocity and the assumption of suspended sediment being transported as primary particles.

The lateral and vertical variability are quantified through the mean standard deviation for each vertical profile and sampling depth for each sampling campaign respectively. First, we investigate general patterns (including the vertical and lateral variability) of suspended sediment concentration (SSC) in the four different cross-sections. Second, we link the lateral and vertical variability with discharge, the magnitude of SSC, and flow velocity. Third, we estimate differences between the cross-sectional sampling and single point sampling.

Our preliminary results indicate an increase of vertical and lateral variability with average SSC in the cross-section. This involves a strong vertical gradient at high average SSC and increased variability at the bottom compared to near-surface SSC. As the flow velocity is smaller at the bottom, we detect a decrease in variability with higher flow velocity. These general patterns are present at each cross-section. However, site specific variations are abundant; caused by site specific properties, such as local morphology, lithology, and the impact of tributaries. Mean standard deviation of laterals and verticals shows the strongest connection to SSC, rather than discharge and flow velocity. Comparing cross-sectional average SSC with surface-sampling from the middle of the river ranges from strong underestimations (> 70 %) to strong overestimations (> 100 %) for single years with an average underestimation of approx. 11 % for all three stations over the 30-year sampling period used in this study. Thereby, incorporating cross-sectional measurements reduce uncertainties induced by point-sampling. Further, site specific adaptations regarding the sample location and an optimization of the sampling process utilizing simultaneous sampling could improve cross-sectional sampling.

How to cite: Slabon, A. and Hoffmann, T.: Vertical and lateral variability of suspended sediment in cross-sections at the river Rhine, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11793, https://doi.org/10.5194/egusphere-egu22-11793, 2022.

On-site presentation
Dhruv Sehgal, Núria Martinez-Carreras, Christophe Hissler, Victor Bense, and Ton (A.J.F.) Hoitink

In recent decades, optical backscatter techniques have increasingly become used to measure turbidity for the quantification of suspended sediment (SS) concentrations. One of the limitations of this method is that site-specific calibrations between SS concentration and turbidity (NTU) are needed. This is because turbidity (NTU) readings respond to factors other than SS concentrations, such as organic and mineral fractions, SS density, particle size distributions, and particle shape. Organic matter introduces irregularity in the shape of suspended particles that may aggregate to form flocs, which are not spherical, and different SS particle fractions (clay, silt, and sand) show different optical responses. Even though organic content is known to influence particle size and density and, as a result, turbidity, an explicit formulation of turbidity accounting for organic content is still missing. We conjecture that a better understanding of the relations between turbidity, SS carbon content (proxy for SS organic content under specific conditions) and particle size can help us to move from local calibrations towards ‘global’ dependencies. In this study, we investigate this by means of (i) a laboratory experiment, and (ii) in-situ high frequency SS characterization of carbon content and particle size. We collected sediments from 6 sites in Luxembourg representing different land use types and geological settings. The sampled sediments were wet sieved into 3 size classes and one part of the sieved samples were oxidized with hydrogen peroxide to investigate the effect of carbon content on turbidity and particle size. To this end, we first conducted laboratory experiments using a tailor-made setup consisting of a cylindrical tank (40-L) with an open top. A stirrer facilitated the homogeneous mixing of SS and prevented settling of heavy particles. Here, a submerged UV-VIS spectrolyser was used to estimate SS carbon content, a LISST-200X sensor to measure particle size distribution and a YSI EXO2 multi-parameter sensor to measure turbidity (NTU). Carbon content was measured in the laboratory with a CHNS Elemental analyser to calibrate the spectrometer readings, and a Mastersizer 3000 to measure particle size distribution. Laboratory results were then validated using field data from two instrumented sites in Luxembourg (Alzette River at Huncherange and Attert River at Useldange). Ongoing analysis will be discussed, and a global calibration equation between turbidity and SSC based on particle size, density and carbon content will be presented.

How to cite: Sehgal, D., Martinez-Carreras, N., Hissler, C., Bense, V., and Hoitink, T. (A. J. F. ).: Influence of riverine suspended sediment carbon content and particle size on turbidity, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9307, https://doi.org/10.5194/egusphere-egu22-9307, 2022.

Virtual presentation
Daniele Gasparato, Lisdey Veronica Herrera Gomez, Giovanni Ravazzani, and Marco Mancini

Bedload discharge measurement in riverine environment is crucial to monitor and understand the morphological evolution of the riverbed and its interaction with existing and new infrastructures.  Despite their relevance, bed load solid transport measures are most of time not available due to the difficulties in their acquisition.

The paper investigates the potentiality of Acoustic Doppler Current Profiler (ADCP ) technique, well established for measuring discharge and flow velocity in a river,  for the measure of bedload discharge in a more manageable way than the traditional ones.

A specific field campaign was organized at Boretto (Italy) cross section on the Po river where the riverbed sediment consists of uniform sand with a mean diameter of 0.4 mm. ADCP measures of bedload discharges were done at the same time as the ones acquired by traditional Helley Smith sampler.

The ADCP data are used in two different ways to obtain the value of the bedload solid discharge. The first approach computes the bedload discharge using the literature formulas where the shear velocities are computed by the logarithmic fit of the velocity profile given by the ADCP. The second approach uses the instrument Bottom Tracking function to obtain a measure of the sediment velocity on the river bed. The sediment velocity computed with this latter method is then used to calculate the bedload discharge with a kinematic model, whose parameters of active layer thickness and concentration are estimated using the Van Rijn model.

The comparison of traditional measures with the one based on the ADCP show comparable values of bed load discharges of the same order of magnitude.

How to cite: Gasparato, D., Herrera Gomez, L. V., Ravazzani, G., and Mancini, M.: Potentiality of bedload measures using Acoustic Doppler Current  profiler technique, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5978, https://doi.org/10.5194/egusphere-egu22-5978, 2022.

Virtual presentation
Mina Tabesh, Julius Reich, and Axel Winterscheid

Assessment of bedload transport rates is of great importance for river morphology. Over the years, many efforts have been made to get a realistic estimate of the bedload transport rate in a river. Several researchers suggested that a reliable estimate of the bedload transport rate can be computed from migration of dunes based on the so-called dune tracking method. To apply this method, bed elevation profiles have to be measured using echosounding of the river bed to determine dune geometries (length (λ) and height (H)) and dune migration rate (C).
The migration rate of dunes (C = ∆X/∆T) is calculated by cross-correlation based on dune migration distance (∆X) and time interval (∆T) between echosounded profiles of two successive measurements. Based on literature, the ∆T between successive echosoundings has to be small enough for the same dunes to be clearly detectable in both measurements. However, the parameter ∆T has not been yet quantified for different river conditions. The objective of the present study is to get an appropriate estimation of the ∆T in order for the cross-correlation to work properly and thus to get a reliable magnitude of bedload transport rate which is at the same time also a specification for the execution of the measurements.
To provide an accurate estimate of the ∆T, both the dune migration distance (can be related to the dune geometries) and the dune migration rate need to be known. Since both parameters (∆X and C) are not available at the beginning of measurements in the field, they need to be estimated based on the existing predictors (e.g. Allen (1968), Tsuchiya & Ishizaki (1967), Van Rijn (1984), Wilbers (2004)) in the literature. The predictors’ verification has been carried out by using the dune geometries and the dune migration rate obtained based on the echosounded profiles. The analysis has been conducted by using the echosounding data of the LiLaR campaign (November 2021) from the Rhine River around the German-Dutch border between km 858-859. For the verification, the dune tracking method has been used. The applied dune tracking method is based on a combination of the software RhenoBT (Frings et al. 2012) and Bedforms ATM (Gutierrez et al. 2018), which determine the dune geometries. Besides, the dune migration rate has been calculated by the cross-correlation using an R script.
This study shows that the dune migration distance can be related to the dune length (∆X = p.λ). The p parameter depends on echosounding measurement uncertainties and dune geometries changes as they migrate downstream. Furthermore, the migration rate would be probably predicted best with Wilbers (2004) predictor in which dune migration rate is related to dune length. While large dunes migrations show high correlation (> 0.7) for time interval of more than 20 hours, small superimposed dunes only show high correlation for time interval lower than 2 hours. Knowing the required time interval can be a helpful factor during echosounding measurements which results in finding the dunes that are most active in transporting bedload material as they migrate downstream.

How to cite: Tabesh, M., Reich, J., and Winterscheid, A.: Temporal resolution of echosounding measurements for assessing bedload transport rates via dune tracking, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6369, https://doi.org/10.5194/egusphere-egu22-6369, 2022.

Presentation form not yet defined
Sándor Baranya, Hojun You, Marian Muste, Dongsu Kim, Tate McAlpin, and David Abraham

Non-intrusive technologies for the in-situ measurement of river hydro-morphological features are increasingly popular in the scientific and practice communities due to their efficient and productive data acquisition. Our research team have successfully demonstrated through laboratory experiments and field measurements that, by combining acoustic mapping with image velocimetry concepts, we can characterize the planar dynamics of the bedform migration and eventually rates of bedload transport. The technique, labeled Acoustic Mapping Velocimetry (AMV), is currently transferred to field conditions using multiple-beam echo-sounders (MBES) and Acoustic-Doppler Current Profilers (ADCP) for producing acoustic maps and tracking the bedform dynamics.

A constant preoccupation of the research team during this transfer has been the validation of the AMV in field conditions. Such validation requires the use of identical input data and the availability of a similar capability measurement system in terms of measurement output, spatial and temporal coverage for the measurement. Fortunately, there is a similar system for estimation of bedload transport labeled Integrated Section Surface Difference Over Time (ISSDOT).  The latter method has been developed and extensively tested by a research group of US Corps of Engineers. While the data inputs (acoustic maps) and the underlying principle (i.e., dune tracking) are the same as for AMV, ISSDOT is based on purely geometrical estimation of the bedload transports rates. The present paper described a comparison between AMV and ISSDOT applied to a set of repeated maps acquired in the Mississippi River. In the absence of a third measurement alternative to be used as benchmark, the paper draws inferences from the comparisons of the two instruments.

How to cite: Baranya, S., You, H., Muste, M., Kim, D., McAlpin, T., and Abraham, D.: Inferences from the comparison of two non-intrusive methods for estimation of bedload transport, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9427, https://doi.org/10.5194/egusphere-egu22-9427, 2022.

On-site presentation
Stefan Haun, Beatriz Negreiros, Maximilian Kunz, Sebastian Schwindt, Alcides Aybar Galdos, Markus Noack, and Silke Wieprecht

Riverbed clogging, also referred to as colmation, describes the infiltration of fine sediment in gravel bed rivers. The infiltrated fine sediment leads to a reduction of the pore space and, in the worst case, to a sealing of the riverbed. As a result of severe colmation, negative effects on the environment may occur, such as a limited oxygen supply for fish eggs or for macrozoobenthos.

The quantification of the degree of colmation and its impact on the ecological status of a river is often based on an expert assessment or only on a single parameter, such as the amount of fine sediment. However, depending on the sediment matrix of the riverbed, the packing arrangement of particles, or the organic material in the riverbed, a single parameter may not be sufficient to evaluate the degree of colmation. In addition, most expert-based assessments, such as mapping of inner and outer colmation, are on the one hand biased due to subjectiveness and on the other hand, only investigate the surface layer of the riverbed. Knowledge on possible occurring colmation layers in deeper regions of the interstitial will not be gained by using these methods.

In this study, a novel MultiParameter Approach to assess Colmation (MultiPAC), is presented, which measures several physical parameters, and provides insights into the status of colmation conditions in the interstitial. These are:

  • measurements of the sediment composition for identifying surface and subsurface grain size distributions and for assessing fine sediment fractions,
  • measurements of porosity by using Structure-from-Motion in combination with freeze-core sampling, and
  • measurements of oxygen concentration and hydraulic conductivity by using a newly developed double-packer system, called VertiCo.

The VertiCo (Vertical profiles of hydraulic Conductivity and dissolved Oxygen) enables measurements with a high spatial resolution over the vertical axis of the riverbed to enable the quantification of possible colmation layers or changes of the conditions in the interstitial over depth.

With the MultiPAC it is feasible for the first time to holistically assess the influence of oxygen and hydraulic conductivity in the interstitial. By taking also into account the properties of the sediment matrix and the porosity, the degree of colmation of a riverbed can be identified. In addition, these findings may provide important information to support the classification of the ecological state of river sections.

How to cite: Haun, S., Negreiros, B., Kunz, M., Schwindt, S., Aybar Galdos, A., Noack, M., and Wieprecht, S.: MultiPAC: A novel approach to quantify the clogging degree of a riverbed, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1318, https://doi.org/10.5194/egusphere-egu22-1318, 2022.

On-site presentation
Georg Stauch, Alexander Esch, Lukas Dörwald, Verena Esser, Simone Lechthaler, Frank Lehmkuhl, Philipp Schulte, and Janek Walk

The sediments of the artificial Urft reservoir preserve a century of environmental information due to undisturbed sedimentation conditions. The Urft reservoir is located in the Eifel Mountains in western Germany and was built between 1900 and 1905. At the time of its construction, the Urft reservoir was the largest reservoir (45.51 million m³) and drove, with 12 MW, the most powerful water storage power plant in Europe. During construction works in November 2020, the reservoir was nearly completely drained. This offered the unique possibility to analyse the sediment volume and the composition deposited during the last 115 years.

We used high resolution maps with a scale of 1:1,000 from 1898 which were compiled to calculate the original storage volume of the reservoir. To assess the present-day surface, the entire lake area was photogrammetrically surveyed using an Unmanned Aerial Vehicle (UAV). Additionally, 10 drill cores were retrieved in 2020 to quantify the anthropogenic influence on the sediments in the form of mining-induced sediment-bound pollutants (e.g., heavy metals) and to relate this to the history of use in the catchment area. Furthermore, microplastics were studied in the sediments. To derive the sediment ages, a detailed Cs-137 chronology was created for one of the cores.

In summer 2021, the northern Eifel Mountains were impacted by a catastrophic flooding event, resulting in massive destructions in the catchment of the Urft and strong relocation of sediments in the floodplain. To assess these geomorphologic changes in the Urft reservoir, the water level was lowered again in December 2021. Consequently, an additional digital elevation model was produced by UAV surveying. Furthermore, additional sediment cores were taken to get information on changes in the sediment composition due to the flood event. In the upper part of the reservoir, up to 30 cm of sediments were deposited in summer 2021 while channels below the water surface experienced strong modifications.

How to cite: Stauch, G., Esch, A., Dörwald, L., Esser, V., Lechthaler, S., Lehmkuhl, F., Schulte, P., and Walk, J.: A century of sedimentation in a reservoir in central Europe – sedimentation rate and characteristics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5480, https://doi.org/10.5194/egusphere-egu22-5480, 2022.

On-site presentation
Alexandre Hauet, Hélène Scheepers, Dominique Lepa, and Bruno Capon

EDF is the largest producer of hydroelectricity in Europe and France, with about 640 dams and an installed capacity of about 20GW. Sedimentation in dam reservoirs is a paramount issue for EDF, including impacts on electricity generation, on dam stability, on spillway discharge capabilities and operation of bottom gates, and on the sediment starvation downstream.

This study focusses on EDF’s dam of Chambon, in the French Alps. In order to guarantee the proper operation of the Chambon dam's outlet bottom gate (the main safety device) between 45 and 55,000   m3 of fine sediments upstream of this gate have to be cleaned out and thus create a stable release zone.  The dredging was conducted by a dilution-pumping method that consists in pumping the sediment deposited in the area upstream the bottom gate, and released them upstream the power-plant intake so that they can transit through the turbines and return to the river downstream to be diluted. The outlet of the pipe dredge was stalled several meters in front of the water intake in order to entrain the fine sediment plume while allowing sand and gravel (which can create serious damage to the turbine) to settle before the intake.

To verify the efficiency of this method, and to ensure that the fine sediments were well entrained in the power-plant intake, adcp measurements were conducted to map the acoustic backscatter intensity that reflects the sediment concentration. A TRDI RioGrande 600 kHz was used, tilted by 20° in order to point the Beam 1 to the Nadir and avoiding side-lobe perturbation close to the bottom. The acoustic backscatter from Beam 1 is used as a proxy of sediment concentration, in a qualitative approach (without estimating the sediment concentration in g/L), in order to map areas of no-, low- or high- sediment concentration.

The measurements show that the release of sediment from the dredging nozzle is highly variable over time, and causes sediment puffs which are diffused towards the free surface and laterally downstream. The sediment plume is homogenized in the body of water, then plunges towards the power-plant intake where it is entrained. 

How to cite: Hauet, A., Scheepers, H., Lepa, D., and Capon, B.: Qualification of the efficiency of a dam dredging by mapping the sediment plume with adcp acoustic backscatter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8903, https://doi.org/10.5194/egusphere-egu22-8903, 2022.

Virtual presentation
Ignacio Pereyra Yraola, Slaven Conevski, Massimo Guerrero, Hanne Novik, Siri Stokseth, and Nils Ruther

The hydropower industry is facing serious challenges handling sediment hazards. In fact, the world’s total storage capacity is continuously decreasing 1-2% per year due to the sedimentation of reservoirs. The necessity to act accordingly, implies choosing the proper sediment management strategy, as early as in the design phase, and to adapt the system on the sediment and water discharge inflows during the operation of the hydropower plant (HPP). To achieve this goal, a detailed monitoring strategy must be implemented.

Periodic bathymetric surveys are crucial for obtaining reliable information about the sediment deposition. The Banje reservoir is located in Albania, in the Devoll river valley, which is a catchment with approximately 2000 t/ (km2 year) sediment yield. The reservoir was commissioned in 2016 and until now two bathymetric studies were conducted. The measurements were performed using a single beam echosounder with dual frequency (80/200 kHz) and the RiverPro RDI, a five-beam acoustic doppler current profiler (ADCP): one vertical beam working at 600 kHz and four 1200 kHz slanted beam. In addition, 22 sediment samples were taken from the reservoir bottom with an Ekman grab sampler.

Both the echosounder and the ADCP are ultrasound instruments; besides the registering of water depth, they also give information about the strength of the returned acoustic signal (i.e., the backscatter). It is well known that the backscatter is highly sensitive to different roughness and river or reservoir bed composition of the reflecting material. In addition to the regular depth measurement, this study aims to correlate the density and particle size distribution of the bed sediment samples to the corrected backscatter signal. Furthermore, combining the observed changes of bed position and the investigated sediment characteristics, details about the total sediment deposition are inferred. The signal intensity from both instruments was corrected by applying an updated ultrasound equation, which yield the corrected backscatter signal. The first and the second returns (i.e., echoes) to the echosounder were used as an input data, whereas the ADCP bottom track signal strength indicator (RSSI) was included in the equation. The recorded raw data was previously processed and smoothed, carefully filtering errors and outliers.

A good correlation was obtained between the sediment samples density and the backscatter signal from the second echo. The ADCP backscatter is reasonably correlated to the particle size distribution of the bed material, but only for reflecting flat regions. The corrected first echo showed abrupt changes which are most likely produced by roughness variability of the reflecting region.

The combining of ADCP and single beam echosounder enabled a detailed analysis of the sediment characteristics and depositions in the reservoir. However further research is necessary to efficiently discard the false data reflected from submerged vegetation, buildings and debris. In addition, frequency dependent returns may be exploited to investigate the sediment layer consolidation.

How to cite: Pereyra Yraola, I., Conevski, S., Guerrero, M., Novik, H., Stokseth, S., and Ruther, N.: Ultrasound investigation of sediment depositions in hydropower reservoir  - case study Banje, Albania, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10587, https://doi.org/10.5194/egusphere-egu22-10587, 2022.