Displays

Measuring sediment fluxes in rivers and laboratory flumes has long been a challenge. Different definitions of sediment transport rates have been proposed over the past decades. Most measurement techniques involve collecting a volume of sediment in a sampler or counting the number of particles crossing a reference section within a given time interval. In laboratory experiments, scientists routinely use high-speed cameras and particle tracking techniques for monitoring bedload transport, but measuring the relevant transport parameters (i.e. the number of moving particles, their velocities and size, etc.) remains a demanding task. Moreover, no clear consensus has emerged on how to define mean bedload transport rates. To address this controversy, we ran an experiment in which we measured the particle flux in different places along a flume using high-speed cameras. Furthermore, we also determined the number of particles moving in a fixed control volume, their trajectories, and their velocities. Even under steady-state conditions, particle transport rates exhibited significant non-Gaussian fluctuations, which caused the time-averaged transport rate to fluctuate widely. In such a situation, determining the mean transport rate becomes a non-trivial operation. To solve this issue, we developed a procedure for estimating the uncertainties associated with the time-averaged transport rates. The theoretical underpinnings are provided by a Markovian model of bedload transport. We demonstrated its versatility by applying it to other laboratory and field cases with different monitoring systems.

How to cite: Pascal, I. and Ancey, C.: Measuring bedload transport rates in a laboratory flume: fluctuations and uncertainties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18452, https://doi.org/10.5194/egusphere-egu2020-18452, 2020.

Throughout the world’s oceans, water layers with increased suspended particulate matter concentrations, so called nepheloid layers, play an important role in the lateral transport of sediment, organic matter and pollutants. Nepheloid layers are persistent features in submarine canyons, where they are formed under influence of energetic hydrodynamics. To evaluate their importance it is crucial to properly quantify the amount and type of material that is transported. However, interpretation of turbidity data is not straightforward, since the detected signal is not only dependent on the concentration of particles, but also on the physical characteristics. Therefore we investigated how turbidity fluctuations induced by internal tides in the Whittard Canyon (northern Bay of Biscay, NE Atlantic Ocean) are reflected in time series data, recorded by different types of commonly used optical and acoustic sensors. Results show that in the surface water the transmitted light signal is strongly affected by the chlorophyll-bearing phytoplankton, whilst only a modest response is found in backscattered light. If left unaccounted for, this would result in an overestimation of the suspended particulate matter concentration in this layer. At the bottom of the canyon optical and acoustic sensors responded differently during one tidal cycle, interpreted as cyclic resuspension, whereby different phases of disaggregation, reaggregation and settling of particulate matter were observed. The differences in the records have important implications on the estimation of mass fluxes of suspended particulate matter, which are vital for understanding for instance carbon transport processes in the bottom boundary layer.

How to cite: Haalboom, S., de Stigter, H., Duineveld, G., Reichart, G.-J., and Mienis, F.: Suspended particulate matter in a submarine canyon: What are we looking at?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1638, https://doi.org/10.5194/egusphere-egu2020-1638, 2020.

Quantification of bedload flux along with its boundary conditions is essential to advance our understanding of rivers and to reduce human and economic threats. However, gaining continuous high resolution empirical data is challenging. Seismic sensors can provide time-resolved quantitative bedload flux data, given adequate data processing. The mechanistic model by Tsai et al. (2012) predicts the power spectral density (PSD) of Rayleigh waves caused by impacts of saltating particles on the river bed, allowing to invert seismic signals to obtain bedload flux. Here we test the robustness of bedload flux inversions of seismic observations against in-situ continuous monitoring of bedload flux and select bedload grain sizes made in the Nahal Eshtemoa, Israel. Proper testing is further ensured by wave propagation (the Green’s function) being fully constrained from active seismic survey experiments. We find that there is a discrepancy of approximately one order of magnitude between the measured and reconstructed bedload flux. We support that this discrepancy can be due to the largest grains, though constituting an infinitely small fraction, generating considerable seismic signals but not being caught by the bedload samplers. It is also possible that this discrepancy is due to model simplification regarding grains in motion. Based on these findings we support that seismic observations may be complementary rather than redundant to in-situ measured bedload flux, because they may give constraints on the fraction of large grains, which is challenging to monitor otherwise.

How to cite: Lagarde, S., Laronne, J., Gimbert, F., Turowski, J., Dietze, M., and Halfi, E.: Comparison between bedload flux from inverse modelling of seismic ground motion data and direct monitoring by Reid bedload samplers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3522, https://doi.org/10.5194/egusphere-egu2020-3522, 2020.

Recent mapping of sediment sources and erosion processes in the Congo basin show that sediment loads may be higher than previously estimated. Stark temporal changes in water turbidity in some of the tributaries observed by satellite images over the past 25 years indicate a need for closer monitoring of sediment load transported in the River. Turbidity sensors present an attractive option for sediment monitoring due to their ability to provide automated continuous time series data for estimation of suspended sediment concentration and suspended sediment fluxes in rivers; an attribute that is particularly important for remote rivers like the Congo. Continuous in-situ turbidity measurements were made using an OBS-501 turbidity sensor at the Kutu Moke monitoring site on the Kasai River, a major tributary of the Congo River between July 2018 and August 2019. The sensor infers turbidity by detecting the intensity of light scattered from suspended particles in water. We explore a field calibration of turbidity measurements with over 120 simultaneous suspended sediment concentration (SSC) measurements for the same period. Sediment loads estimated using high frequency turbidity data measurements (hourly) are then compared to loads estimated using classical sediment rating curves to establish if the turbidity provides a better representation of the suspended sediment load.

How to cite: Mushi, C., Ndomba, P. M., Neal, J., Beya, J., and Trigg, M.: Use of turbidity measurements to monitor suspended sediment loads on the Congo River, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-518, https://doi.org/10.5194/egusphere-egu2020-518, 2020.

For future strategies in water depth maintenance in the Port of Hamburg, determining the navigability limit (i.e. the nautical safe depth) is of major importance. For this purpose, a project "Nautical Depth" was set up at the Hamburg Port Authority (HPA), which is dedicated to dealing with this issue. The aim is to measure a nautical safe depth under various boundary conditions and to identify limits for a safe passage of high concentrated soil suspensions. Among other things, the project cooperates with the Antwerp Port Authority, the Port of Rotterdam and the TU Delft. The project is also embedded in a research platform or network called MUDNET (www.tudelft.nl/mudnet).

In order to achieve the required acceptance for a reassessment of the nautical depth, it is necessary to determine the rheological properties of soil suspensions in-situ. The rheological parameters - which will be used to describe the nautical depth - have still to be determined. For a permanent identification of nautical relevant rheological properties of the soil suspensions, existing in-situ measuring devices have been tested and, under certain circumstances, new equipment has to be developed. However, these devices cannot be used for the spatial determination of the rheological properties but will reproduce these on cross-sections and depth profiles. Therefore, new evaluation algorithms should be developed in echo-sounding technology - which have to be correlated with the in-situ rheological properties - in order to ensure spatial representations of a safe nautical depth.

In a first step, measurements of nature conditions in the water column and at the riverbed were carried out in 9 areas and in 12 measuring campaigns in 2018 and 2019 in the Hamburg Port. Therefore, different sediment profiler devices (Rheotune, Graviprobe, Admodus USP) have been tested. Sediment samples were taken with a modified Frahm-Lot. All investigations were combined with hydro-acoustic measurements which includes multibeam echo-sounders and sub-bottom profilers with Silas processing software.

The presentation will give a closer look to the sampling strategies and results of the different soil properties within the Hamburg port and the river Elbe, which serves as fairway to the port. The investigations show that the soil properties are dependent from local and regional boundary conditions, as flow velocity, grain size distribution and especially in Hamburg from the organic matters and nutrients within the suspended and the soil material. Moreover, the laboratory data will be compared with hydro-acoustical and in-situ monitoring devices. Advantages and disadvantages of the different systems will be discussed.

Kamphuis et al. (2013) Fluid Mud and Determining, Nautical Dept Hydro International, 22-25;

Malcherek, A. et al. (2011) Zur Rheologie von Flüssigschlicken: Experimentelle Untersuchungen und theoretische Ansätze, Mitteilungen des Instituts für Wasserwessen der Universität der Bundeswehr, München 111:1-191;

Metha et al. (2013) Fluid Mud Properties in Nautical Depth Estimation, Journal of Waterway, Port, Coastal & Ocean Engineering, 140:210-222;

Ohle, N. et al (2019) Introduction and first results within the project “Nautical Depth” in Hamburg, 11^th International SedNet conference, 3-5 April 2019, Dubrovnik;

How to cite: Ohle, N., Thies, T., Lüschow, R., and Schmekel, U.: Sediment sampling and soil properties of sediments in the Hamburg port and the river Elbe in comparison with hydro-acoustic measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16468, https://doi.org/10.5194/egusphere-egu2020-16468, 2020.

Decisions on water resources infrastructures’ planning and management are, at the moment, only rarely taking into consideration their impacts on sediment transport dynamics and on river geomorphological changes at basin scale. The reasons for this are the fact that these changes in most cases happen over time scales longer than the so called “engineering scale” (around 70/100 years) and the inherent complexity in accurately modelling these processes at the scales of interest. Recently, simplified schematizations have been proposed to assess different scenarios for river basin development strategies (e.g. dams development and operations). The possibility to calibrate and validate these models still is hindered by the scarcity of measurements of sediment fluxes in most catchments worldwide. If one could accurately infer transport processes (fluxes) (e.g. travel distances, source areas, relative production etc.) from sediment properties (shape, size, lithology, etc.), this would allow the use of already available data and would simplify the collection of future datasets.

Recent studies on sediment attrition claim the existence of a “universal” relation between particles relative mass loss and their circularity (Novák-Szabó et al., 2018). The relationship between relative mass-loss and travel distance is, however, still unclear and thought to depend on the transport conditions and on particles mechanical properties.

In order to start assessing the importance of the latter, we identified a case study, the Sarzana River basin, in North-East Italy, characterized by the presence of localized sources of arenites and metabasalts, which are expected to have different abrasion rates. We measured sediment size and shape properties with a photogrammetric method and compared their longitudinal evolution. We also performed the same analysis on mixed samples, collected using the Wolman method. The inclusion of particles lithology among the control variables of the study allowed the identification of a series of transport dynamics that would otherwise be completely overlooked by mixed sampling. This example stresses the importance for a thorough basin analysis when designing a sediment monitoring campaign in order to maximise the amount of information minable from data.

How to cite: Cattapan, A., Paron, P., McClain, M., Piégay, H., and Franca, M.: Using sediment morphometry to infer transport dynamics in Alpine catchments: which variables matter?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17575, https://doi.org/10.5194/egusphere-egu2020-17575, 2020.

Lake Baikal, located in eastern Russia, is the oldest (25 million years) and the deepest (~1800 meters) lake in the world. There are many rivers flowing into the Lake Baikal (~ 365 rivers), of which the Selenga River is the most important one being responsible for almost 55% of the runoff water into the system and also 60% of the transported sediments. As the hydrological changes of the river and its delta enormously alter the neighbouring area, it is of utmost importance to explore the dynamics of change in terms of flow magnitude, paths and fluvial geomorphology, and the related tipping points defining different states. The questions this study aims to answer are: What are the fluvial geomorphological and hydrological changes? What fluvial geomorphological tipping points can be identified during the last 34 years and what are the discharge and climatic conditions that induce them? In this study, we use the Global Surface Water Dataset (GSWD) to analyze the changes in the river’s stream network. With these products, we assess changes in several fluvial geomorphological proxies (e.g., sinuosity, fractal dimension, meandering characteristics, planform information) and identify possible tipping points. We relate these changes to different hydrological and climatic conditions such as precipitation, river discharge and Lake Baikal water level. We find evident changes in the meandering behaviour and flow path of the Selenga River tributaries in the Delta. The number of oxbow lakes based and corresponding size distribution has varied in time, and evident flow path changes occur that seem to be related to flooding periods, and there appears to be a consistent relationship between meandering and the river discharge variability. These results enable policymakers to understand different contributing factors altering the Selenga River Delta and ultimately leading to better decisions to manage the effects of these changes in the area.

How to cite: Aminjafari, S., Brown, I., Jarsjö, J., R. Chalov, S., and Jaramillo, F.: Studying Fluvial Tipping Points with Remotely Sensed Observations and Hydroclimatic Data in the Selenga River Delta, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20542, https://doi.org/10.5194/egusphere-egu2020-20542, 2020.

Changes in fluvial morphology, such as the migration of channels and sandbars, are driven by many factors e.g. water, woody debris and sediment discharges, vegetation and management practice. Nowadays, increased anthropic pressure and climate change are accelerating the natural morphologic dynamics. Therefore, the monitoring of river changes and the assessment of future trends are necessary for the identification of the optimal management practices, aiming at the improvement of river ecological status and the mitigation of hydraulic risk. Satellite data can provide an effective and cost-effective tool for the monitoring of river morphology and its temporal evolution.

The main idea of this work is to understand which remote sensed data, and particularly which space and time resolutions, are more adapt for the observation of sandbars evolution in relatively large rivers. To this purpose, multispectral and Synthetic Aperture Radar (SAR) archive data, with different spatial resolution, were used. Preference was given to satellite data freely available. Moreover, the observations extracted by the satellite data were compared with ground data recorded by a fixed camera.

The study case is a sandy bar (area about 0.4 km² and maximum width about 350 m) in a lowland reach of the Po River (Italy), characterized by frequent and relevant morphological changes. The bar shoreline changes were captured by a fixed video camera, installed on a bridge and operating for almost two years (July 2017 - November 2018). To this purpose, we used: Sentinel-2 multispectral images with a spatial resolution of 10 m, Sentinel-1 SAR images with a resolution of 5 x 20 m and CosmoSkyMed SAR images with a resolution of 5 m. It is worth noting that the Sentinel data of the Copernicus Programme are freely available while the CosmoSkyMed data of the Italian Space Agency (ASI) are freely distributed for scientific purpose after the successful participation to an open call. In order to validate the results provided by Sentinel and CosmoSkyMed data, we used very high resolution multispectral images (about 50 cm).

Multispectral images are easily interpreted, but are affected by the presence of cloud cover. For instance, in this analysis, the expendable multispectral images were equal to about 50% of the total archive. On the other hand, the SAR images provide information also in the presence of clouds and at night-time, but they have the drawback of more complex processing and interpretation. The shorelines extracted from the satellite images were compared with those extracted from photographic images, taken on the same day of the satellite acquisition. Other comparisons were made between different satellite images acquired with a temporal mismatch of maximum two days.

The results of the comparisons showed that the Sentinel-1 and Sentinel-2 data were both adequate for the shoreline changes observation. Due to the higher resolution, the CosmoSkyMed data provided better results. SAR data and multispectral data allowed for automatic extraction of the bar shoreline, with different degree of processing burden. The fusion of data from different satellites gave the opportunity of highly increase the sampling rate.

How to cite: Papa, M. N., Nones, M., Cavallo, C., Gargiulo, M., and Ruello, G.: Data-fusion of satellite and ground sensors for river hydro-morphodynamics monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20381, https://doi.org/10.5194/egusphere-egu2020-20381, 2020.

There is a strong interaction between the appearance and dimensions of bedforms in rivers and the prevailing hydraulic and morphological conditions. The availability and mobility of sediments, in response to hydraulic variables like flow depth and velocity, determine the bedform characteristics. Vice versa, bedforms have a strong impact on the hydraulic conditions by exerting a flow resistance. Further on, with a thorough knowledge of the dimensions and migration velocities, predictions about sediment transport rates can be made.

Bedform geometries can be derived from multibeam echo sounding data. There are methods to discriminate several layers of superimposed bedforms and to identify the individual geometric attributes (length, height and shape). The calculated results, however, strongly depend on the setting of various input parameters. For choosing the values for these parameters there are mostly no theoretically sound criteria and the process itself is also strongly influenced by the individual experience of the researcher. If repeated several times by several researchers the analysis of the same data set would ultimately lead to different results. Only by means of a structured and traceable approach the level of inherent subjectivity and uncertainties can be reduced.

For the processing of multibeam echo sounding data we combined the existing software tools Bedforms ATM (Gutierrez et al., 2018) and RHENO BT (Frings et al., 2012) using an R-script. The concept of Bedforms ATM is based on a wavelet analysis in order to detect predominant bedform lengths. Applying this tool provides the rationale for deciding on the respective window sizes, which is a required input parameter for RHENO BT. The latter one is used to identify individual bedform geometries from longitudinal bedform profiles.

For estimating the sensitivity of all relevant input parameters to Bedforms ATM and Rheno BT an algorithm was developed in which a Monte Carlo-like simulation is performed. Assuming an individually chosen distribution function, random values are generated for each parameter. Multiple repetitions of the calculation with varying input parameters reveal the possible range of results. The algorithm has been tested on longitudinal profiles of Parana River in Argentina (Parsons et al., 2005) and an own data set of River Oder in Germany. The two case studies cover different ranges of bedform geometries, long and high bedforms characterize the morphology of the Parana River in contrast to much smaller and lower bedforms in the River Oder.

In the simulations carried out several input parameters turned out to be very sensitive. In some cases it can be shown that even slight variations lead to an increase in calculated mean bedform height of about 30 %. Further on, the type of statistical evaluation determines the robustness of the results. These uncertainties underline the need for comprehensive analyses before further processing in order to choose a reliable setting of input parameters and a suitable evaluation method.

How to cite: Reich, J.: A Monte Carlo approach to determine the sensitivity of bedform analysis methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5200, https://doi.org/10.5194/egusphere-egu2020-5200, 2020.

Confluences are common components of all riverine systems, and are characterized by converging flow streamlines and mixing of separate flows, which can take some significant distance to be complete. Whilst turbulent diffusion and Taylor dispersion are expected to affect mixing in any open channel flow, the analysis of mixing at river confluences should also consider some peculiar processes, which could be divided between near-field processes and far-field processes. The former, which have been well studied, are those operating at the junction itself and lead to rapid mixing only if some form of asymmetry (geometry, discordance, momentum, density difference) between the tributaries exists. The latter are advective processes, such as secondary circulation, that can enhance mixing to degrees greater than those associated with turbulent diffusion or Taylor dispersion combined. These processes, which have received less attention, were investigated using a three-dimensional computation of the Reynolds averaged Navier-Stokes equations combined with a Reynolds stress turbulence model for the confluence of the Kama river and Vishera rivers in the Russian Urals. To test the hypothesis that far-field mixing can be both enhanced and reduced by the type of secondary circulation that develops, numerical simulations on an idealized configuration (rectangular channel with no curvature) and on the real configuration with the natural planform and/or bathymetry were carried out to isolate the relative impacts of real planform and bathymetry on secondary circulation and mixing for different combinations of momentum/discharge ratio. Results show that if the rivers are represented as an idealized junction, the initial vortices that form due to channel-scale pressure gradients decline rapidly with distance downstream. Mixing is slow and incomplete at more than 10 multiples of channel width downstream from the junction corner. On the other side, if the real configuration is introduced, rates of mixing increase dramatically. This is related to both increase intensity of secondary circulation at the junction and the formation of a single channel-scale vortex downstream of the junction. The latter appears to be aided by curvature of the post-junction channel. This effect is strongest when the discharge of the tributary that has the same direction of curvature as the post junction channel is greatest.

The study was performed under financial support of the Government of Perm Krai (grant C 26/788) and Russian Foundation for Basic Research (grant 19-41-590013).

How to cite: Lyubimova, T., Lepikhin, A., Parshakova, Y., Gualtieri, C., Roux, B., and Lane, S.: A numerical study about the influence of channel-scale secondary circulation on mixing processes at Kama/Vishera confluence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12953, https://doi.org/10.5194/egusphere-egu2020-12953, 2020.

The development of morphodynamic models to simulate sediment transport accurately is a challenging and highly complex process given the non-linear and coupled nature of the sediment transport problem. We implement a new depth-averaged coupled hydrodynamic and sediment transport model within the coastal ocean model Thetis, built using the code generating framework Firedrake which facilitates code flexibility and optimisation benefits. To the best of our knowledge, this represents the first full morphodynamic model using a discontinuous Galerkin based finite element discretisation, to include both bedload and suspended sediment transport. We apply our model to problems with non-cohesive sediment and account for effects of gravity and helical flow by adding slope gradient terms and parametrising secondary currents. For validation purposes and to demonstrate model capability, we present results from the common test cases of a migrating trench and a meandering channel comparing against experimental data and the widely used model Telemac-Mascaret.

There is a high degree of uncertainty associated with morphodynamic models, in part due to incomplete knowledge of various physical, empirical and numerical closure related parameters in both the hydrodynamic and morphodynamic solvers. We therefore also present examples of how an adjoint model can be used to calibrate or invert for the values of these parameters from either experimental results or real-world erosion profiles.

How to cite: Clare, M., Percival, J., Kramer, S., Angeloudis, A., Cotter, C., and Piggott, M.: Hydro-morphodynamics 2D modelling using a discontinuous Galerkin discretisation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4990, https://doi.org/10.5194/egusphere-egu2020-4990, 2020.

Riverine environments are amongst the most complex ecosystems on the planet. As several anthropogenic factors have increasingly disrupted the natural dynamics of rivers, namely through stream regulation, the need for re-establishing the ecological role of these systems has gained relevance.

Of particular interest are floodplains in compound channels, primarily regarded for safety against floods, but which also comprise an extensive realm for ecological functions and establishment of various species. Floodplain vegetation affects flow resistance and dispersion, playing a fundamental role in erosion and deposition of suspended sediment.

The present work aims at quantifying the interaction between vegetation and suspended sediment transport on floodplains in compound channels by numerical simulations. The employed numerical tool is BASEMENT v3, a GPU-accelerated hydro-morphodynamic 2D model developed at the Laboratory of Hydraulics, Hydrology and Glaciology of ETH Zurich. In the context of the present study, the model is extended with turbulence and suspended sediment transport capabilities. The implemented closure models for turbulence pertain to three major groups, namely (i) mixing-length, (ii) production-dissipation and (iii) algebraic stress models. For suspended sediment transport, the main classical formulations from fluvial hydraulics were implemented in the numerical model.

Laboratory data from flume experiments featuring suspended sediment load and vegetation-like proxies are used for model validation. The numerical results are compared with the observed water depths, velocities and sediment concentrations for different sets of experiments with varying properties, such as density and submergence. The implemented closure models for flow resistance, turbulence and suspended sediment are then combined, calibrated and classified in terms of numerical output quality.

The obtained results from this modelling effort mainly contribute to understanding the applicability of 2D (depth-averaged) models to complex eco-morphodynamics scenarios. The calibration and rating of well-known closure models for turbulence and sediment transport provides relevant guidelines for both future research and practice in fluvial modelling.

How to cite: Conde, D. A. S., Boes, R. M., and Vetsch, D. F.: Hydro-morphodynamic modelling of floodplains: the role of vegetation in suspended sediment transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22003, https://doi.org/10.5194/egusphere-egu2020-22003, 2020.

Sediment flushing has been reported as one of the most efficient techniques for reservoir desiltation. This technique consists in opening the bottom outlets of a dam to induce an accelerated flow that mobilizes part of the sediments deposited in the reservoir. The efficacy of flushing depends much on conditions such as the hydraulic head in the reservoir, the discharge capacity of the outlets, the sediment characteristics, and the topography of the reservoir, among others. In this context, numerical models become an extraordinarily useful tool for reservoir operators, as the efficacy of flushing can be previously evaluated by means of numerical modeling. However, though there are several studies that have simulated flushing numerically, most of them are based on specific case studies whose conditions cannot be generalized. This study aims to analyze the capacity of three hydrodynamic models (HEC-RAS-1D, IBER-2D and FLOW-3D) to simulate flushing events. For that purpose, those conditions tested in laboratory for two experimental setups were implemented and simulated in these hydrodynamic models. The first experimental setup was based on a one-dimensional approach in which the width of the outlet coincided with the width of the reservoir. This experimental setup was carried out in a 12.5 m long and 0.30 m wide horizontal rectangular flume at Universidad Politécnica de Cartagena, Spain. Here, 10 pairs h_s – h_w were tested, where h_s and h_w stand for the initial sediment and water elevations, respectively. Sediments consisted of a uniform sand with d₅₀ = 0.7 mm, bulk density ρ_b = 1650 kg/m³, and grain density ρ_s = 2650 kg/m³. In these experiments, the evolution of the water surface and bed surface, as well as the liquid and solid hydrographs, were characterized by means of videos recorded from a side of the flume. The second experimental setup consisted of 3 of the experiments documented in the PhD thesis by Lai (1994), which were conducted in a 50 m long, 2.4 m wide and 1.5 m high rectangular concrete flume at University of California at Berkeley. In this experimental setup, the reservoir was emptied through a 0.15 m wide and 0.25 m high sluice gate., which allows analyzing the influence of the width ratio between outlet and reservoir. Sediments consisted of walnut shell grit with d₅₀ = 1.25 mm and ρ_s = 1390 kg/m³. In these experiments, liquid and solid hydrographs were characterized by means of discrete measurements of the water surface and sediment concentration at the outlet. To assess the capacity of the hydrodynamic models to simulate flushing, the hydrographs obtained from laboratory experiments are compared to those obtained numerically. Preliminary results show that the model FLOW-3D obtained the best approach to the results obtained in laboratory. The results obtained with HEC-RAS also show a good approach to the experimental results, but with comparatively high differences in magnitudes for the peaks of the liquid and solid hydrographs. The results obtained with IBER show the greatest differences with respect to the results obtained in laboratory.

How to cite: Guillén Ludeña, S., Carrillo, J. M., Toapaxi, J. A., and Castillo, L. G.: Modeling of reservoir flushing by means of existing hydrodynamic models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22243, https://doi.org/10.5194/egusphere-egu2020-22243, 2020.

Density-driven gravity flows frequently occur in nature, due to density difference between inflowing and ambient water. When a sediment-laden flow reaches the backwater zone of a reservoir, with a greater density than the ambient waters, an underflow can occur along steep bottom slopes. The formation and evolution of an underflow depend on various natural conditions. It is necessary and crucial for reservoir management to understand the dynamics and prediction of the turbidity currents. In addition to field investigation and laboratory experiments, numerical models are gaining popularity for solving open-channel flows and sediment transport processes such as turbidity currents in reservoirs.

SCHISM (Semi-implicit Cross-scale Hydroscience Integrated System Model) is a 3D seamless cross-scale model grounded on unstructured grids for hydrodynamics and ecosystem dynamics. A general set of governing equations are used for the flow and tracer transport, and a new higher-order implicit advection scheme for transport (TVD²) is proposed. A mixed triangular-quadrangular horizontal grid and a highly flexible vertical grid system are developed in the model to faithfully represent complex geometry and topography of environmental flows in open channel cases. SCHISM has found a wide range of cross-scale applications worldwide including general circulation, storm surges, sediment transport and so on. However, the feasibility of simulating turbidity currents caused by sediment-laden flows in a reservoir is rarely validated. In this study, SCHISM is applied to a lab experiment to simulate the turbidity currents on a flume slope to examine how the model predicts the hydraulic characteristics of turbidity currents in a reservoir.

Model results can describe the process of the turbidity current plunging beneath the free surface with the time step of 0.1s. It is relatively uncommon in previous studies to clearly show the evolution of the velocity and sediment concentration profiles in such a short time step. The simulated velocity and sediment concentration profiles of the turbidity currents match well with the measured profiles at the cross section downstream of the plunge point. The calculated depth-averaged velocity, thickness, and depth-averaged concentration of the turbidity current all agree well with the measured values. The correlation coefficient between the measured and calculated values is 0.92, 0.95, and 0.94, respectively. Also, the densimetric Froude number of the stable plunge point is found to be approximately 0.54 in this study, which is between 0.5 and 0.8 based on previous research. The plunge depth is smaller with higher sediment concentration and smaller discharge of the inflow. Besides, the ratio of plunge depth to inlet depth is proportional to the densimetric Froude number of inflow conditions. This finding can be used to predict the depth and location of the plunge point based on the inflow conditions in a reservoir, which has great practical implications in reservoir management. Our results demonstrated that SCHISM is generally applicable to simulate the turbidity currents in small-scale water environments, and has the potential to be adopted in large-scale open water environments.

How to cite: Zhang, R., Wu, B., and Zhang, Y. J.: Three-dimensional numerical simulation of the turbidity current on a flume slope, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4346, https://doi.org/10.5194/egusphere-egu2020-4346, 2020.

Drava River confluence is characterized by specific morphodynamic processes under which significant sediment deposition is occurring at the Drava River mouth, impeding fairway conditions. Morphodynamic analysis requires long-term hydraulic and sediment transport regime data as input for estimation of equilibrium conditions, taking into account baseline conditions of both rivers. This paper presents results of detail investigations of morphodynamic changes at the Drava River confluence during the 2-year period. Quantification of morphodynamic processes is conducted indirectly through interpretation of ADCP transects surveyed over wider confluence zone, estimation of sediment transport intensity and bathymetric surveys. Purpose of the conducted analysis was to estimate morphodynamic development of the riverbed based on the 1D numerical model results. Numerical model is calibrated using flow velocity field and sediment transport pattern for range of hydrological events. Validation of sediment transport method is done through comparison of morphological changes on characteristic profiles between two consecutive surveys.

How to cite: Cikojević, A., Gilja, G., Baranya, S., Kuspilić, N., and Pomázi, F.: Sediment transport modelling of the Drava River confluence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3873, https://doi.org/10.5194/egusphere-egu2020-3873, 2020.

Regularities of the river channel processes are closely related to the magnitude and variability of the river load, and the sediment yield, and the sediment yield is one of the principal factors of the riverbed formation. Cycling and dynamics of the sediment yield need to be taken into account when making hydrotechnical calculation, project work and investigating of the riverbed processes. The amount of the river loads, which find their way into the sub-basin of the Pripyat River each year, depends more on the meteorological conditions of the year. That’s why sediment yield and water turbidity are non-permanent from year to year. Since the rivers of the investigated region belong to different hydrological zones and regions, characterized by the uniqueness of water regime, which is caused by the peculiarities of hydrographic and orographic territory indexes, the content of the sediment yield and the amount are also not the same.

The water turbidity at the right-bank tributaries of the Pripyat River, which flow within the Ukrainian Polesie, is not large. The water turbidity of the Turia, Ubort River and Vyzhivka is especially small. In certain years the concentration of sediment yield can be bigger. Much more suspended particles are observed in the Styr, Sluch, and Horyn, the upper catchments of which are strongly cut and partially covered with easily washable sediments. The biggest annual average water turbidity was recorded on the Ikva River. Within the accumulative lowland in the downstream river sections, the right-bank tributaries of the Pripyat River carry cleaner water than in the upstream section since the part of sediment load of the river is build up on the riverbeds and creeks. It occurs as a result of the slope and speed reducing. However, on some right-bank rivers of Pripyat, which flow down from small local terrain uplands with cover of loamy forest and sandy sediments, water turbidity can be quite large.

The estimation of the spatio-temporal dynamics of the sediment load is accomplished by difference integral curves taking into account an average annual water discharge, maximum annual water discharge and also average annual sediment discharge and the biggest annual sediment discharge during the whole period of investigation of current hydrological posts for the right-bank tributaries of the Pripyat River within Ukraine. The analysis of synchronicity and equiphase condition of these oscillations had made it possible to identify general noticeable opposite orientation of the set of curves to the oscillation curve of general sediment load. Constructed graphs show interdependence of maximum annual water discharge, average annual sediment discharge, maximum annual sediment discharge because the sediment discharge has to react to hydraulic fluctuation in the flow.

How to cite: Filippova, Y.: Spatio-temporal analysis of the sediment yield and water turbidity at the right-bank tributaries of the Pripyat River within Ukraine, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4913, https://doi.org/10.5194/egusphere-egu2020-4913, 2020.

Runoff attenuation features such as bunds and leaky barriers are increasingly incorporated into catchment flood management schemes. However, with any structure resulting in a barrier to flow, sediment dynamics are also affected, which will in turn affect the feature’s hydraulic effectiveness over time. The geomorphological impact of these features is often overlooked. This work looks at using the CAESAR-Lisflood landscape evolution model to assess how to implement runoff attenuation features into a catchment and evaluate their corresponding impact on sediment dynamics and subsequent change to water storage efficacy. The simulations were based on a small catchment, situated south of the Yorkshire Dales, UK, where the land is primarily used for grazing livestock. Features were implemented through the editing of the underlying topography allowing features to be fully erodible and scenarios were created based on feature shape, size and quantity. Of the features implemented, there was no unified response to the flood event simulated. Generally, many of the features themselves were affected by erosion, reducing their ability to hold water over time. Fewer features experienced deposition upstream compared to those experiencing erosion, which may suggest scour as opposed to sedimentation as a management issue that would need to be monitored. Nonetheless, the model scenarios run permitted an optimal design and layout of runoff attenuation features within the catchment to be established.

How to cite: Pearson, E., Carrivick, J., and Lamb, R.: Implementation of runoff attenuation features into a landscape evolution model for the assessment of the impact on catchment sediment dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5008, https://doi.org/10.5194/egusphere-egu2020-5008, 2020.

After the application of the large reservoirs, the conditions of discharge and sediment are changed. Based on a large number of measured hydrological and topographic data, this paper studies the deformation characteristics of sandbars micro-geomorphology in sandy reach of jingjiang river after the impounding of the three gorges reservoir(TGD), and discusses the adjustment mechanism of sandbars. The result shows that the sandy bars showed the head scoured and the shrink of area, and the sandbars in the reach with revetment project was relatively stable.The evolution of the sandbars was mainly influenced by riverbed composition, discharge and sediment process and revetment project. The composition of the riverbed determined the scour resistance of the sandbars, while the change of flow process determined the location and property of scour and silting, and the amount of incoming sediment determined the extent of scour and silting, the implementation of revetment project was beneficial to sandbars stability. After the TGD operation, the erosion of the bars in the Jingjiang reach ranked the strongest when the discharges fall in 15000 m³/s~25000 m³/s. The duration of this flow range increased after the TGD operation in 2003, and the bars presented an erosion state. Due to the impacts of river bed armoring and the significantly reduced sediment, there existed certain interactive relationships between the adjustment in the erosion and deposition of bars and the changes in the percentage of the grain size belonging to 0.125<d<0.25mm. The reduction of the fine sand had a negative impact on the sedimentation of bars after erosion. The layout of the revetment project had a certain control effect on the sandy reach, but the unguarded sandy bars presented scour and deposition with the fluctuation of discharge and sediment process between years.

How to cite: Yang, X.: Adjustment mechanism of the sand bars in the jingjiang reach after the impounding the three gorges, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5464, https://doi.org/10.5194/egusphere-egu2020-5464, 2020.

Groundsill is one of the hydraulic structures used to stabilize the riverbed and prevent the erosion of riverbank. Therefore, groundsill may have the negative effects on the ecological environment. Comparing with a traditional groundsill, a Cross-Vane concentrates the water flow and create a downstream pool. This may improve the diversity of the aquatic habitats. The aim of this research is to analyze the scour phenomena and morphologies downstream of an arched Cross-Vane with different geometrical dimensions in a straight channel by using the numerical model. The riverbed slopes of 0%, 2%, 4% and 6% were tested. Among them, the ratio (L/B) between the arc length of the structure (L) and the channel width (B) represents the camber of structures, including 13 kinds of arches. For each arch structure, Densimetric Froude numbers (F_d) , approach flow depths (h₀) and drop heights (Δy) were tested in different flow rate, and the flow rate was between 0.01cms and 0.04cms.The results showed the downstream scour pattern of the arched Cross-Vane had a significant correlation with F_d and Δy, and could be classified according to the scour length (l_m) and the ridge length (l_n). Scour typology included five types of scour. Type 1 : l_m/B > 2.5 and l_n /B < 1. Type 2: l_m/B was located about 2.0 to 2.5 and l_n/B > 1. Type 3: l_m/B=2.0 and l_n/B < 1. Type 4: l_m/B was located about 1.5 to 2.0 and l_n/B > 1. Type 5 : l_m/B < 1.5 and l_n /B > 1. L/B was one of the most important parameters affecting the maximum scour depth and its position. When L/B was less than 1.4, the scour holes were similar to the traditional groundsill. When the L/B ranged between 1.4 and 2.3, the maximum scour depth was located at about 0.5 to 0.65 times of scour length downstream the Cross-Vane. When L/B was greater than 2.3, the maximum scour depth was located adjacent to the Cross-Vane.

Keyword : Cross-vane, Scour morphology, Numerical model

How to cite: Lai, Y.-W., Chen, P.-A., and Chan, H.-C.: Local Scouring Characteristics Downstream of Arched Cross-Vane Structures, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6564, https://doi.org/10.5194/egusphere-egu2020-6564, 2020.

We propose a one-way coupled model that tracks individual primary particles in a conceptually simple cellular flow setup to predict flocculation in turbulence. This computationally efficient model accounts for Stokes drag, lubrication, cohesive and direct contact forces on the primary spherical particles and allows for a systematic simulation campaign that yields the transient mean floc size as a function of the governing dimensionless parameters. The simulations reproduce the growth of the cohesive flocs with time and the emergence of a log-normal equilibrium distribution governed by the balance of aggregation and breakage. Flocculation proceeds most rapidly when the Stokes number of the primary particles is O(1). Results from this simple computational model are consistent with experimental observations, thus allowing us to propose a new analytical flocculation model that yields improved agreement with experimental data, especially during the transient stages.

How to cite: Zhao, K., Vowinckel, B., Hsu, T.-J., Köllner, T., Bai, B., and Meiburg, E.: An efficient cellular flow model for cohesive particle flocculation in turbulence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7463, https://doi.org/10.5194/egusphere-egu2020-7463, 2020.

Due to the effects of extreme climate, the frequency of drought has increased around the southern Taiwan in recent years. The development and utilization of water resources, especially in dry season, becomes more significant in the hillsides of southern Taiwan. Farm ponds are useful facilities for agriculture, especially in the hilly areas. In addition, farm ponds can provide the functions of water retention and detention, ground water recharge, land subsidence mitigation, water purification, ecological conservation and habitat environment. According to the results of fieldwork, about ten percentage of ponds are facing more serious sedimentation problems. Among these farm ponds, deposition in dam farm ponds may be more severe. Because the farm ponds are located on hilly areas where agriculture is active, the problem of sediment deposition is always issued. This study used an experiment model to study the feasibility of hydraulic desilting of dam farm pond. The experimental study was conducted in a tank of cube which has a volume of 1.0 m³, a stand pipe area of 0.01 m², an inclined pipe area of 0.01 m² and desilting pipe diameter of 0.03 m. The experimental arrangements included three positions of desilting pipes (setting0, setting1, setting3), two bottom orifice sizes (D_o = 1.0 cm, 2.0 cm), three sediment deposition depths (EL = 30 cm, 40 cm, 50 cm) and three water levels (WL = 60 cm, 70 cm, 80 cm). Our study aims to figure out the most effective sediment removal ratio in the different arrangement of experiment. The inflow discharge varied from 450 to 4,000 cm³/s. The median diameter (d₅₀) of the sediment used in experiments was 0.12 mm. According to the results of experiment, sediment removal ratio with bottom orifice of 2.0 cm is higher than bottom orifice of 1.0 cm under conditions of three different sediment deposition depths and three different water levels. The experimental results indicate that desilting pipes installed in the dam farm pond is helpful for removing sediment deposition. This study will also conduce to the promotion of conservation measures and water resources that related to farm ponds.

How to cite: Hsu, Y.-C., Wang, J.-S., Su, Y.-W., Hung, T.-C., Jan, C.-D., Fu, G.-L., Lin, J.-J., and Fang, Y.-C.: Experimental Study on the Hydraulic Desilting System of Dam Farm Pond, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8768, https://doi.org/10.5194/egusphere-egu2020-8768, 2020.

The study of resonant shallow flows past a lateral cavity is of great relevance due to their interest in civil and environmental engineering [1]. Such flows exhibit the presence of a standing gravity wave, called seiche, which is coupled with the shedding of vortices at the opening of the cavity. A complete understanding of such phenomenon is necessary as it may determine the mass exchange between the main channel and the cavity [2]. A better insight into this phenomenon helps to improve the design and implementation of innovative river bank restoration techniques. An experimental study of the resonant flow in a laboratory flume with a single lateral cavity is herein presented. Five different flow configurations at a fixed Froude number (Fr=0.8) are considered. The main novelty of the present work is the use of a pioneering non-intrusive experimental technique [3] to measure the water surface at the channel-cavity region. This optical technique offers high resolution 2D data in time and space of the water surface evolution, allowing to determine the relevant features of the seiche oscillation, i.e. spatial distribution of oscillation nodes and anti-nodes, oscillation modes and amplitude of the oscillation. Such data are supplemented with Particle Image Velocimetry measurements to perform a more detailed study of the resonance phenomenon. High-resolution two-dimensional amplitude oscillation maps of the seiche phenomenon are presented for the experimental water depth. Experimental velocity fields inside the cavity are presented and confirm the inherent coupling between the unstable shear layer at the opening of the cavity and the gravity standing wave. The high quality of the experimental data reported in this work makes this data set a suitable benchmark for numerical simulation models in order to evaluate their performance in the resolution of turbulent resonant shallow flows.

[1] C. Juez, M. Thalmann, A. J. Schleiss & M. J. Franca, Morphological resilience to flow fluctuations of fine sediment deposits in bank lateral cavities, Advances in Water Resources, 115 (2018) 44-59.

[2] I. Kimura & T. Hosoda, Fundamental properties of flows in open channels with dead zone, Journal of Hydraulic Engineering 123 (1997) 98-107.

[3] S. Martínez-Aranda, J. Fernández-Pato, D. Caviedes-Voullième, I. García-Palacín & P. García-Navarro, Towards transient experimental water surfaces: a new benchmark dataset for 2D shallow water solvers, Advances in water resources, 121 (2018) 130-149.

How to cite: Martínez Aranda, S., Navas-Montilla, A., Lozano, A., and García-Navarro, P.: Experimental study of resonant shallow flows past a lateral cavity: a benchmark test for high-resolution numerical models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9288, https://doi.org/10.5194/egusphere-egu2020-9288, 2020.

Steady shallow flows past an open channel lateral cavity have been widely studied in the last years due to their engineering and environmental relevance, e.g. for river restoration purposes [1]. Such flows can induce the excitation of an eigenmode of a gravity standing wave inside the cavity, called seiche, which may be coupled with the shedding of vortices at the opening of the cavity. A complete understanding of such phenomenon is necessary as it may determine the mass exchange between the main channel and the cavity [2]. A numerical study of the resonant flow in a channel with a single lateral cavity is herein presented. Five different flow configurations at a fixed Froude number (Fr=0.8), measured in the laboratory [3], are used as a benchmark. Such experiments are reproduced using a high-order 2D depth-averaged URANS model based on the shallow water equations, assuming that shallow water turbulence is mainly horizontal [4]. The large-scale horizontal vortices are resolved by the model, whereas the effect of the small-scale turbulence is accounted for by means of a turbulence model. Water surface elevation and velocity measurements are used for comparison with the numerical results. A detailed comparison of the seiche amplitude distribution in the cavity-channel area is presented, showing a good agreement between the numerical results and the observations. Frequency analysis techniques are used to extract the relevant features of the flow. It is evidenced that the proposed model is able to reproduce the observed spatial distribution of oscillation nodes and anti-nodes, as well as the time-averaged flow field. The coupling mechanism between the gravity wave inside the cavity and the unstable shear layer at the opening of the cavity is also accurately captured.

[1] C. Juez, M. Thalmann, A. J. Schleiss & M. J.  Franca, Morphological resilience to flow fluctuations of fine sediment deposits in bank lateral cavities, Advances in Water Resources,  115 (2018) 44-59.

[2] I. Kimura & T. Hosoda, Fundamental properties of flows in open channels with dead zone, Journal of Hydraulic Engineering 123 (1997) 98-107.

[3] S. Martínez-Aranda, J. Fernández-Pato, D. Caviedes-Voullième, I. García-Palacín & P. García-Navarro, Towards transient experimental water surfaces: a new benchmark dataset for 2D shallow water solvers, Advances in water resources, 121 (2018) 130-149.

[4] A. Navas-Montilla, C. Juez, M.J. Franca & J. Murillo, Depth-averaged unsteady RANS simulation of resonant shallow flows in lateral cavities using augmented WENO-ADER schemes, Journal of Computational Physics, 24 (2019) 203-217.

How to cite: Navas-Montilla, A., Martínez-Aranda, S., Lozano, A., and García-Navarro, P.: Numerical study of resonant shallow flows past a lateral cavity: benchmarking the model with a new experimental data set , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9289, https://doi.org/10.5194/egusphere-egu2020-9289, 2020.

Force measurements using load cells equipped with strain gauges are widely applied in hydraulic experimentation and field surveys. In our case, drag on 5 horizontal cylinders in cross-flows had to be measured directly in a hydraulic water flume. We tested different setups and came up with a two-load cell solution per cylinder as mechanically best way to fix the cylinders in the flume.
The costs for an amplifier system of industrial standard is in the order of magnitude of 10.000 Euro and for load cells around 500 Euro. For our multiple cylinder application, the costs of an industrial standard solution exceeded the budget and forced us to find alternatives. Chinese-made load cells cost only a few Euros each. We designed our own measuring system, consisting of an external analogue-digital converter and a microcontroller. A Python script was programmed to operate the microcontroller and analyse the data.
In the session, we will give an overview of the flume setup and the measuring system – including live operation. We will discuss the required calibration procedure for the load cells and data quality and give recommendations for further improvements.

How to cite: Wolff, M. and Schnauder, I.: Strain gauge measurements in hydraulic experiments: Chinese firecrackers versus industrial solutions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10360, https://doi.org/10.5194/egusphere-egu2020-10360, 2020.

Reservoir sedimentation reduces not only the available storage volume of reservoirs, but may also create other serious problems, such as an increase of bed levels or accumulations of nutrients and contaminants, which affect the environment. An increase in bed levels at the head of the reservoir can reduce flood safety and increase the risk for the surrounding areas. Deposited sediments close to the dam may block hydraulic structures, such as the bottom outlets, or, in case they enter the intake, lead to possible abrasion of plant components (e.g. wear of turbines and pipes).

Prior to reservoir construction, a pre-evaluation of the sediment yield from the catchment is usually performed by using soil erosion and sediment delivery models. However, the trapping efficiency is often only obtained by empirical approaches, such as Brune’s or Churchill’s curve, which are based on the capacity of the reservoir and the mean annual inflow. This is still common practice, although 3D hydro-morphodynamic models became powerful tools to numerically study sediment transport and reservoir sedimentation prior to the construction of reservoirs as well as during its operation.

Within this study, a fully 3D hydro-morphodynamic numerical model, based on the Reynolds-averaged Navier-Stokes equations, is applied to a case study to simulate, on the one hand suspended sediment transport within a hydropower reservoir and on the other hand a reservoir flushing operation as potential management scenario, with the goal to remobilize already deposited sediments and to release these sediments from the reservoir. The modeled reservoir has a total storage capacity of around 14 million m³, whereby the water level can fluctuate due to pumped-storage operation by 40.5 m (difference between the maximum operation level and the operational outlet). At the head is the natural inflow of two creeks into the reservoir and a lateral transition tunnel is located on the orographic right side, which collects several headwater streams from adjacent catchments.

Simulations are performed for different operation modes of the reservoir. The results clearly show that through active reservoir management (variation of water levels as well as using the momentum of the discharge from the transition tunnel) the sediment motion in the reservoir can be affected to a certain extent. It is for instance possible to almost completely avoid reservoir sedimentation in front of the dam and the hydraulic structures (water intake and bottom outlets) during sediment-laden flows when simultaneously high discharges are provided from the laterally located transition tunnel. The conducted simulation results of reservoir flushing also show that the success of the flushing operation is strongly dependent on the water level. As expected, flushing with full drawdown of the water level is the most efficient method to release sediments.

Through the detailed results of the 3D hydro-morphodynamic model, it is feasible to receive a deeper knowledge of the ongoing sediment transport processes within the studied reservoir. The gained knowledge can further be used to derive sustainable and efficient management strategies for the sediment management of the reservoir.

How to cite: Mouris, K., Saam, L., Beckers, F., Wieprecht, S., and Haun, S.: 3D hydro-morphodynamic models as support tools for obtaining sustainable sediment management strategies of reservoirs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10548, https://doi.org/10.5194/egusphere-egu2020-10548, 2020.

Observation of bedload is quite important for understanding temporal and spatial variation of sediment transport in mountainous regions. In government-owned mountain watersheds in Japan, Japanese pipe-type hydrophones (Hydrotech Co., Ltd.) have been installed as a surrogate monitoring tool since about 2009 and continuous observations have been conducted. According to positive correlation between sound pressure and bedload transport rate, observed sound pressure is used to be integrated with respect to time and its value is converted into bedload transport rate using proportionality constant. However, it remains challenging to obtain precise bedload transport rate with high accuracy, because we should consider the difference of the way to fix the hydrophone on river bed among installed sites, deformation of steel pipe due to collision of sediment particles, and the difference of initial performance and aging of microphone. Hence, we have to calibrate frequently the proportionality constant. In this study, we investigate a calibration method which is easily conducted by engineers. Because it takes time and effort to obtain time integral of sound pressure, we try to calibrate the proportionality constant with the maximum sound pressure, which can be obtained easily.

How to cite: Izumiyama, H., Uchida, T., Iuchi, T., Yoshimura, N., and Yamakoshi, T.: A calibration method for monitoring bedload transport rate using Japanese pipe-type hydrophone considering installation condition and aging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12205, https://doi.org/10.5194/egusphere-egu2020-12205, 2020.

There is a wide range of geophysical flows, such as flow in open channels and rivers, tsunami and flood modeling, that can be mathematically represented by the non-linear shallow water 1D equations involving hydrostatic pressure assumptions as an approximation of the Navier Stokes equations. In this context, special attention must be paid to bottom source terms integration and numerical corrections when dealing with wet/dry fronts or strong slopes in order to obtain physically-based solutions (Murillo and García-Navarro, 2010) in complex and realistic cases with irregular topography. However, although these numerical corrections have been developed in recent years achieving not only more robust models but also more accurate results, they still might find a limit when dealing with specific scenarios where vertical information or disspersive effects become crucial. This work presents a 1D shallow water model that introduces vertical information by means of a non-hydrostatic pressure correction when necessary. In particular, the pressure correction method (Hirsch, 2007) is applied to a 1D finite volume scheme for a rectification of the velocity field in free surface scenarios. It is solved by means of an implicit scheme, whereas the depth-integrated shallow water equations are solved using an explicit scheme. It is worth highlighting that it preserves all the advantages and numerical fixes aforementioned for the pure shallow water system. Computations with and without non-hydrostatic corrections are compared for the same cases to test the validity of the conventional hydrostatic pressure assumption at some scenarios involving complex topography.

[1] J. Murillo and P. Garcia-Navarro, Weak solutions for partial differential equations with source terms: application to the shallow water equations, Journal of Computational Physics, vol. 229, iss. 11, pp. 4327-4368, 2010.

[2] C. Hirsch, Numerical Computation of Internal and External flows: The fundamentals of Computational Fluid Dynamics, Butterworth-Heinemann, 2007.

How to cite: Echeverribar, I., Brufau, P., and García-Navarro, P.: On the necessity of non-hydrostatic pressure models for free surface flow over complex topography , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19946, https://doi.org/10.5194/egusphere-egu2020-19946, 2020.

Nowadays, the aquatic biodiversity is highly under pressure due to anthropogenic changes
of rivers such hydraulic structures changing the diversity of flow and aquatic fauna as well
as sediment continuity. This can have severe consequences on the fish population in the
river reach. Fish are strongly depending on a certain substrate composition throughout all
their life stages. Juveniles e.g. are depending on a certain availability of shelter in the
substrate in order to survive this stage. Therefore we investigate the effects of changes in
the sediment composition at a Hydropower plant in Switzerland on the availability of
potential shelter for juvenile fish. The investigation is conducted as part of the EU Horizon
2020 funded project FIThydro (funded under 727830).
To reach this goal, we measured the sediment compositions at several locations in the
bypass reach with different measurement techniques such as sieving, photogrammetry
(Basegrain) and the pebble count method. Further we measured the shelter availability in
the corresponding locations, using the so called Finstad method. As the method was
developed purely for Atlantic salmon, we modified it by expanded the variability of
available sizes. The resulting correlation of the grain size distribution with the potential
shelter availability at different locations showed a fairly high correlation coefficient. This
equation can then be used in hydro-morphological models to estimate the spatial
distribution of potential shelter availability for any given flow regime and grain size
distribution. Further investigation at other sites will over time enlarge the database and
therefore improve the correlation.

How to cite: Pritsis, S., Schwarzwälder, K., Szentkereszty, W., and Rüther, N.: Linking changing grain size distributions with the development of shelter availability for fish in the bypass reach of a hydro power plant, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22184, https://doi.org/10.5194/egusphere-egu2020-22184, 2020.

Fine sediment transport processes and the thermodynamics in reservoirs are key processes governing the water quality of reservoirs. With regard to a sustainable sediment management of reservoirs, the prediction of sediment transport and deposition is becoming increasingly important.

The subject of the present work was the 3D numerical simulation of fine sediment transport in a reservoir taking into account stratification and mixing effects which in turn are caused by temperature gradients and wind effects. In order to understand and investigate the driving factors for stratification processes and their impact on fine sediment distribution, the great pre-dam of the Dhünn reservoir in Germany served as case study. The investigations were conducted in sensitivity analyses adopting a 3D sediment transport model with Delft 3D. The impact of various physical and numerical parameters on temperature and fine sediment transport modeling was examined: the number of vertical layers, the input data for the heat model (e.g. relative humidity, air temperature, cloud coverage, solar radiation), the vertical diffusivity and wind effects. The sensitivity studies showed that the input data for the heat model have a minor impact on the temperature and sediment transport modeling within the tested range of parameters. However, the vertical diffusivity and especially the inclusion of wind showed a greater influence on the simulated temperature and suspended sediment concentration gradients. The temperature modeling results by inclusion/exclusion of wind were qualitatively compared with temperature data from literature and with measurement data over a period of one month. Hereby, the simulations showed a good agreement with measurement data by exclusion of wind effects.

The results of the studies provide a solid basis for the development of further models in fields where fine sediment transport is affected by stratification processes and can also be very useful in terms of a better understanding of the interactions between temperature, wind and fine sediment transport.

How to cite: Gonzalez, W., Klassen, I., Jakobs, A., and Seidel, F.: 3D numerical studies on stratification and mixing processes affecting fine sediment transport in the pre-dam of the Dhünn reservoir in Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20418, https://doi.org/10.5194/egusphere-egu2020-20418, 2020.

The longitudinal dispersion of bed load particles as they move downstream in a river is relevant both to cases of polluted sediment and pulses of sediment released during reservoir flushing events or dam removals.  To quantify the rate of bed-load dispersion, researchers with the U.S. Army Corps of Engineers conducted a series of flume experiments using successive additions of different-colored sediment in a 22m x 0.9m, upstream-fed, tilting flume at the U.S. Engineer Research and Development Center's (ERDC) Coastal and Hydraulics Laboratory.  Here we show that longitudinal bed-load dispersion can be accurately modeled in a one-dimensional sediment transport model (HEC-RAS) that does not explicitly simulate dispersion.  We accomplished this by adjusting the active layer thickness and the bed-load depositional exchange increment.  The bed-load depositional exchange increment sets the ratio of active layer vs. bed-load material that are mixed into the bed during deposition.  The optimal parameters varied between the flume experiments, but smaller active layer thicknesses generally performed better. 

How to cite: Dahl, T., Gibson, S., Floyd, I., and Sanchez, A.: Numerical Dispersion of Bed Load in a 1D Model Mimics Physical Flume Results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20357, https://doi.org/10.5194/egusphere-egu2020-20357, 2020.

Sedimentation processes are in a “dynamic balance” in most natural rivers, but the construction of dams and reservoirs influences these natural conditions. The flow velocities, turbulences and bed shear stresses in reservoirs are reduced compared to free flow conditions, which lead to the deposition of the transported sediment particles. As a further consequence the sediment depositions reduce the storage volume by “filling up” the reservoir. This “reservoir sedimentation” is a problem in several Alpine reservoirs.

In the case of Alpine reservoirs with a small storage volume compared to the annual inflow, such as reservoirs of run-off river power plants, the water depth are usually lower than in reservoirs of storage and pump-storage hydro power plants. A larger part of the suspended sediments is thus transported through the reservoir and deposition of bed load fractions is the main problem. The deposition of coarse sediments at the head of the reservoir may cause problems regarding flood protection by raising the bed level and thus, raising the water level too.

This contribution focus on the bed load transport processes during a flushing event in an Alpine reservoir. The reservoir is approximately 1 km long with an initial storage volume of about 250.000 m³. The annual bed load input is rather high, thus the remobilization of the sediment in the reservoir in case of flood events was investigated.

An open source three-dimensional numerical model with an internal coupled hydrodynamic and morphological part was used to simulate the flushing process. The calibration of the hydrodynamic model was done using ACDP measurements performed at the prototype to calibrate the roughness at the river bed. Additionally an extensive sensitivity analysis was carried out and several sediment transport formulae were tested.

How to cite: Harb, G. and Schneider, J.: Numerical Modelling of Gravel Transport during Flushing Processes in an Alpine Reservoir, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7806, https://doi.org/10.5194/egusphere-egu2020-7806, 2020.

A flood risk assessment has implemented with an inundation map or with other simulated results; e.g., a rainfall-runoff simulation. In order to conduct the flood risk assessment, it is usual that the case with maximum floods are subject for discussion. At the same time, it is usual that observed data of the maximum floods are not available, since the maximum floods has not experienced, or observation have not conducted. Estimation of the discharge values are not simple, since the river flow at the targeted cross section are affected by river shape, or roughness changes. Both of them are sensitive with different flow stage.

The present study discusses about constructing the stage discharge relationship with numerical simulation. For this purpose, the author implements the 2-D depth integrated flow simulation including the flow resistance. The flow resistance is one of the traditional studies of the sediment hydraulics. It deals with the changing of resistance with different micro-scale bed forms as the bed shear stress changes. Similar with the one by Engelund (1966), the relationship with grain shear stress and total shear stress are constructed in qualitative manner by Kishi and Kuroki (1973). It is useful to obtain the bed roughness with different flow stage. The author implements the changes of the roughness in the 2-D depth integrated flow simulation and obtains the flow field in actual river flow in order to obtain the discharge values.

The authors conducted the numerical simulation in steady flow condition. In order to construct the stage-discharge relationship based on the results, 10 different cases with appropriate ranges of stage were conducted. The domain of the simulation is 5 times longer than the width of the targeted section. In order to construct the initial condition, bathymetry data in the one point in 5 m with the laser technique, and sediment size distribution at the different location; e.g., at center of flow, top of the dune and etc., were obtained. The calculated results were compared with observed flow field by float measurements and other non-contact current meter. The results indicate that the numerical stage-discharge relationship shows some good agreements and few disagreements with the one created based on observation. For example, at the water stage which represents the dune I, the simulated results are similar with observed. However, at the stage of dune II, simulated velocity shows smaller velocity than observed. As Hirai (2015) suggested, shape of micro-bed form classified as Dune II is unstably changes between Dune and flat bed. Therefore, velocity at the stage is sensitively changes as well. From this aspect, the authors concluded that not only the numerical simulation but also field measurement are necessary in order to construct good stage-discharge relationships, in particular if the shear stress at the targeted discharge involves the Dune II.

How to cite: yorozuya, A.: Constructing stage discharge relationship with numerical simulation including hydraulic resistance, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14004, https://doi.org/10.5194/egusphere-egu2020-14004, 2020.

The ever-increasing demand for fluvial navigation and the more and more efforts made for ecologically sustainable water usage (facilitated by e.g. the Water Framework Directive of the EU) have highlighted potential conflicts of interests in river management. Riverine traffic has notable hydrodynamic effects, i.e. the local hydraulic regime of river reaches may get significantly altered by wave events generated by passing vessels. As ship waves reach the shallower areas, the related hydrodynamic stresses affect the near-bed boundary layer increasingly, bed shear stress increases gradually, leading to the resuspension of fine sediments. In order to find out more about the nature of this phenomenon, simultaneous ABS (acoustic backscatter sensor) and ADV (acoustic Doppler velocimeter) measurement were performed in the Hungarian Danube. Such measurement not only offer the opportunity to reveal the likely interconnections between hydrodynamic variables (e.g. flow velocity, turbulent kinetic energy) and suspended sediment concentrations (SSC), but the found correlation between ABS data and the backscatter strength of the ADV also suggests the applicability of the latter for the estimation of instantaneous SSC in a high temporal resolution.

How to cite: Fleit, G. and Baranya, S.: Estimation of wave induced sediment resuspension using an ADV, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21659, https://doi.org/10.5194/egusphere-egu2020-21659, 2020.

Sedimentation of small as well as large water storage reservoir has become a major issue. Due to the fact that we observe a 1% decrease of reservoir volume every year due to sedimentation and that the largest part of the reservoirs have been built between 70 and 40 years ago, many HPPs are confronted with the threatening scenario that soon the active storage and therefore their lifetime is dramatically diminished. Due to the above mentioned combination, active and sustainable sediment management has become the last option to retain or preferable enlarge the left-over reservoir volume. There are several options for a sustainable sediment handling, each for a different boundary condition, which must be evaluated carefully in order to be successful. For a successful choice, design and conduction of a sediment handling technique, usually a physical scale model will be conducted. Physical scale model have the advantage that there is a lot of experience in conducting these models and that they are illustrative. The disadvantage of scale models is that there are restrictions in the use of certain sizes of sediments due to scaling issues and that they are rather expensive.

This study attempt to use a 3D numerical model to overcome the above mentioned disadvantages and to serve as an additional source of alternatives in finding the right sediment handling techniques in reservoirs with high discharges of suspended and bed load. The goal is to simulate several flood events in order to gain insights in the current situation as well as to have a better understanding of the physical processes in the reservoir. This will support and positive influence the sustainable design of sediment handling techniques. The numerical model will be verified with flow measurements a physical model study and with bathymetry measurements from field observations. Based on the actual deposition pattern and the given input data, different sediment handling techniques are planned and conducted by means of the numerical model. The results show that the 3D numerical model is able to simulate sediment transport deposition pattern, bed load guide vane structures, as well as bed load diversion structures.

How to cite: Lal Maskey, D., Nepal, D., Herman, D., Gaiti, G., and Rüther, N.: 3D numerical modeling of sediment handling techniques in a hydro power reservoir, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19713, https://doi.org/10.5194/egusphere-egu2020-19713, 2020.

Change detection analysis for monitoring and modeling riverine systems requires detailed spatiotemporal surveying of river morphology dynamics. An accurate high-resolution surface model of the river channel and floodplain enables a more comprehensive view of the riverbed evolution and allows monitoring the morphodynamics of the entire river channel more precisely compared to the traditional methods. Unmanned Aerial Vehicle (UAV) based Structure from Motion (SfM) techniques have renovated 3D topographic monitoring of earth surface, offering low-cost, rapid and reliable data acquisition and processing. Herein, the acquisition of repeated topographic surveys helps us to characterize the flow regime and to monitor the sediment dynamics. Multitemporal models of the river environment can be produced by autonomous operation to determine erosion, subsidence, landslide, soil transport and surface deformation in the riverbeds. The ‘meandering’ phenomenon takes its denomination from the Büyük Menderes River (BMR), which flows in a winding course in western Turkey, known as the Maiandros River in ancient times. Meandering rivers generally consist of a single, highly sinuous channel responding to erosion and sedimentation processes. This study presents the hydromorphological changes of the meandering structures by using multitemporal UAV surveys between 2017 and 2019 in the BMR. In the study, multitemporal topographic data were produced and morphodynamic processes in the lower course of the BMR were modelled by Digital Shoreline Analysis System (DSAS) and Digital Elevation Model of Difference (DoD) methods. These methods were employed to examine the changes in the shoreline and to analyse the size of geomorphological changes and spatial patterns. The results showed that the change in the shoreline of the meanders varied from 3 to 27 meters, and the water levels varied between approximately 0.3 and 3 meters. Although there was both sediment erosion and deposition along the shoreline, the predominant process was identified as deposition in the shoreline. Besides, major changes on the deposition rate were found to occur mostly after the summer season. Ultimately, a significant correlation was found between the deposited sediments and the sinuosity index values (r=0.88) according to the changes in water level over the months. This research showed that UAVs could provide a suitable measurement model for determining areal and volumetric eroded/deposited sediment quantities along the meandering fields.

How to cite: Özcan, O., Akay, S. S., and Şen, Ö. L.: Multitemporal Monitoring of the Morphodynamics of a Meandering River by UAV-Based Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-125, https://doi.org/10.5194/egusphere-egu2020-125, 2020.