HS9.3

Within the joint project Integrated Water Governance Support System (iWaGSS) funded by the German Federal Ministry for Education and Research (BMBF, reference numer: 02WGR1424C) the Institute of Water and River Basin Management (IWG) of the Karlsruhe Institute of Technology (KIT) developed a benthic flume. The benthic flume HIPPO (Hydro-morphological Investigation of riverbed Particle Performance On-site) is an adjustable in situ device to reliably determine the start of erosion of fine sediments.

In advance 3D-CFD simulations have been carried out to optimize the components and the setup of the measurement system. The final product is primarily a benthic flume, which has a downwardly opened sampling area at the bottom and is placed on the river or reservoir bed. This underwater flow channel can be adapted to the local conditions with further components and is connected via a tube system to a measurement boat or raft. On the boat a pump creates a steady flow velocity in the system. The velocity in the benthic flume is gradually increased at fixed time intervals and is monitored using a built-in flow velocity meter (Acoustic Doppler Velocimeter). In addition the entire erosion process is recorded visually with video cameras. Also the turbidity of the water flowing through the system is continuously measured by a turbidity probe installed behind the pump. The amount of flow induced by the pump is controlled by a valve close to the end of the system. With the pump currently installed flow velocities of up to v = 0.8 m/s at the sampling area can be achieved, which is sufficient for the determination of the critical flow rate for erosion of most types of clay, silty and fine sandy sediments. During the process of erosion also the remobilization of fluid mud can be monitored. The critical flow velocity for the start of sediment transport is determined on the basis of the turbidity of the pumped water and data from the flow velocity probe and is verified using the camera system.

In addition to the critical threshold flow velocities, the critical bed shear stress is often required as input or evaluation variables for morhpodynamic numerical models. The conversion can be made, for example, using the quadratic velocity approach originally used in pipe hydraulics. The determination of the required resistance coefficient λ is based on the Moody Chart. However, it should be considered that this procedure entails some uncertainties with regard to the measurement system presented here. Still for cohesive sediments, the natural values measured in this way represent a significant added value compared to common estimates based on only partially known bed parameters, since factors such as vegetative cover, consolidation or even a developed biofilm can influence the timing of erosion. Especially against this background, possible effects of the change of hydraulics by the measuring system (geometry, velocity profile) seem to be small compared to the uncertainties of contemporary morphodynamic analyses.

How to cite: Kerlin, T., Musall, M., Oberle, P., and Nestmann, F.: HIPPO – In situ device to monitor the remobilization process of fine sediments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-256, https://doi.org/10.5194/egusphere-egu21-256, 2021.

The Swiss plate geophone (SPG) system is an indirect bedload transport monitoring device that records the acoustic signals generated by bedload particle impacts, with the goal to derive the bedload flux and grain size distribution. Particle drop experiments with quartz spheres in quiescent water in a flume setting were performed to investigate the dynamic signal response of the SPG system impacted by particle-like objects varying in size and impact location. Systematic flume experiments with natural bedload particles in flowing water were conducted to study the effects of impact angle and transport mode (saltating, rolling and sliding) on the SPG signals. For each impact caused by a single particle, the number of signal impulses, the amplitude, the positive area surrounded by the signal envelope, and the centroid frequency were extracted from the raw geophone monitoring data. The finite element method (FEM) was used to construct a virtual model of the SPG system and to determine the propagation characteristics of the numerical stress wave in the material structure. The experimental and numerical results showed a qualitative and partially quantitative agreement in the changes of the signal impulses, the amplitude, and the envelope area with increasing colliding sphere size. The centroid frequencies of the SPG vibrations showed qualitatively similar dependencies with increasing particle size as some field measurements for the coarser part of the investigated range of impact sizes. The effects of variable particle impact velocities and impact locations on the geophone plate were also investigated by drop experiments and compared to FEM simulations. In addition, the signal response for different bedload transport modes and varying impact angles were explored. In summary, the FEM simulations contribute to the understanding of the signal response of the SPG system and the findings in this study may eventually result in improving the bedload grain size classification and transport mode recognition.

How to cite: Chen, Z., He, S., Nicollier, T., Ammann, L., Badoux, A., and Rickenmann, D.: Controlled experiments and finite element simulations with the Swiss plate geophone bedload monitoring system: particle size identification and transport mode, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2078, https://doi.org/10.5194/egusphere-egu21-2078, 2021.

River ecosystems are diverse and dynamic habitats which are strongly influenced by direct and indirect consequences of human interventions. Several initiatives have been started all over Europe to fulfill the European guidelines for the protection of the local water bodies, but a standardized procedure fulfilling all relevant aspects and parameters of the Water Framework Directive (WFD) does not exists. To evaluate water quality, the WFD predefines biotic and abiotic parameters, such as morphology, hydrology, water chemistry as well as biological quality components, including fish fauna. In this context, we propose a new methodological approach based on salmonid fish populations to assess river quality. Our approach is based on European standardization of the Austrian and Italian methods and it has been tested in the context of an international fish project in 81 stream sections in the European Alps, having homogeneous morphological characteristics. The assessment procedure is composed of a set of 11 indicators, which were selected to evaluate longitudinal and lateral morphological and hydrological conditions: stream passability, reproduction sites, riverine dynamic, shoreline, shoreline vegetation, structure, substrate and degree of hydrological disturbance, a descent speed indicator as well as discharge conditions of hydropeaking. The indicators were then combined to 3 indices, namely: morphology index (I_M), hydrology index (I_H) and hydromorphology index (I_HM), to create a holistic picture of the total stream conditions. The indicator and index definition, the compilation and practical testing of the data entry form in the field, as well as the calculation of the values, were carried out jointly by a team of experts. The combination of that created a new hydromorphology index (I_HM) for Alpine streams. The application of the proposed method was shown in 31 river streams in South Tyrol (Italy) and Tyrol (Austria) covering a wide range of different anthropogenic changes and pressure degree, which enabled the trial of the methodology and the refinement of the indicators and indices. The outcomes of our study lead to interesting insights regarding applicability, strengths and weaknesses of the proposed approach.

How to cite: Schmölz, K., Felber, A., Mark, W., Thaler, M., Wieser, J., Persiano, S., Bertoldi, G., and Tasser, E.: A methodical approach to analyze the effect of river morphology and hydrology on fish fauna in the inner-Alpine space, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4425, https://doi.org/10.5194/egusphere-egu21-4425, 2021.

As part of AQUIMAR project (MAR2020 nº MAR-02.01.01-FEAMP-017 – AQUIMAR – Caraterização geral das áreas aquícolas para estabelecimento de culturas marinhas), intensive CTD surveys and turbidity/concentration data were collected in four cruises along the Portuguese continental shelf (30-200m depth), in 5 aquaculture areas from 2018 to 2020. In-situ calibration of the turbidity sensor (Seapoint Turbidity Meter) was done using the traditional gravimetric method of suspended sediments concentration (SSC) determination with water sampling and filtering. The obtained FTU/SSC relations resulted in correlations in the order of R²=70-80% for all considered surveys.

Measured turbidity and concentration values, were generally very low (<2 FTU and <2 mg/l) for all measuring periods, however variations of the FTU/SSC sensitivity between the different areas indicate that significant variations of suspended matter composition exist throughout the Portuguese continental shelf.

This study aims to understand the seasonal and spatial variations of the turbidity signal sensitivity to SSC. To this end, a closer look will be given to samples collected during two contrasting seasonal periods (spring and late autumn 2019), as well as to the general water column structure at the time of the sample collection. Additionally, results from X-Ray diffraction analysis performed in some of the filtered samples will be used to better understand the variations of the suspended sediment composition in open clear waters. The mineralogical signal shows a dominance of clay minerals in suspension (mean 83%) and calcite (mean 10%), reflecting the detritic and organic fraction of SSC, respectively.

How to cite: Oliveira, A., Santos, A. I., Santos, R., and Zacarias, N.: Turbidity sensor response to seasonal and spatial variability of suspended particle composition in open clear waters –(Portuguese continental shelf), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7610, https://doi.org/10.5194/egusphere-egu21-7610, 2021.

The aim of the present study was to characterize the size and shape of sediments along a reach of a mountain river in Maquiné municipality, southern Brazil, to establish an efficient methodology in river sediments analysis. In Brazil, this might be a pioneering study of mountain rivers characterized by the presence of gravel, cobble, and boulders sediments. The study catchment, covered by Dense and Mixed Rain Forest and high-altitude grasslands (Campos de Cima da Serra), has an altitude difference of 900 m. Its geology is characterized by the Serra Geral Formation (basaltic rocks) and pedology by Cambisols and Neossols. The mean annual rainfall is 1200 mm. According to the Köppen classification, the regional climate is humid subtropical with hot summers (Cfa) in lower areas and humid subtropical with mild summers and cold winters (Cfb) in higher areas. The catchment outlet has a fluviometric station, and at its headwater, there is a rainfall gauge, both of which perform automatic measurements every 10 min. For the bed sediments diameter analysis, 500 grains were sampled, following the Wolman Pebble Count methodology. The measurements were carried out along the same reach (100 m) in five stages (December 2019; February, May, August, and November 2020) to observe sediment dynamics over time. During these measurements, the mean values of water depth and discharge were 0.4 m and 0.8 m³/s, respectively. To determine the size and shape, the three axes A (longest), B (intermediate), and C (shortest) were measured by using the tree caliper. With the axes’ values, the sediment shape was classified into four types: sphere, rod, disc, and blade. Linear correlation and multiple regression analyses were performed to evaluate the influence of each sediment axis on determining the nominal diameter (D_n). The mean values of D_max, D₉₀, D₈₄, D₅₀, D_16, and D₁₀ of all the sampled sediments were 290.61, 114.40, 103.52, 56.27, 35.89, 28.0, and 18.40, respectively. Preliminary results indicate that 38% of the sampled sediments corresponded to the disc format and did not vary over the year. The characteristic diameters remained constant throughout the monitoring period, even though strong rainfall-runoff events sometimes occurred (~ maximum runoff was 33 m²/s in July 2020). The D_n values calculated with the multiple regression model based on the analysis of the axes (D_n = f (A, B, f (A, B))) were very close (R² = 0.95) to those calculated through an original definition of D_n, i.e., D_n = (A·B·C)^1/3. During the monitoring period, notable changes in the size and shape of the sediments were not observed. The axes analysis confirms that the D_n value can be estimated only with the measurement of axes A and B, without axis C. Therefore, this methodology (without the axis C) may be recommended to characterize the size and shape of bed sediments in mountain rivers. Finally, the present study highlights the importance of fieldwork to advance basic river sciences in Brazil.

How to cite: Menezes, D. and Kobiyama, M.: Sediment size and shape observation in a mountain river, southern Brazil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9884, https://doi.org/10.5194/egusphere-egu21-9884, 2021.

Flooding of rivers is one of the major causes of soil erosion leading to significant changes in the geomorphological environment. Particularly, in countries such as Afghanistan, where the transboundary are designated according to the Amu River shorelines, are significantly affected by riverbank erosions. Amu River is driven by streamflow from the Pir Pranjal ranges of Afghanistan and Tajikistan. Numerical analysis of the river flow dynamics in such regions is subject to the scarce data availability on ground stations. Thus, ERA5 Reanalysis data provides a significant means for the temporal analysis of the geomorphological changes in such multi-national watersheds.

In this study, we propose a framework to quantify the Amu riverbank erosion in the Kaldar District of the Balkh Province of Afghanistan. The proposed framework is based on establishing an empirical relationship between the riverbank erosion area based on the discharge intensity and the specific stream power. To determine these two parameters, the river discharge is modeled using the ERA5 Reanalysis hydrological parameters based on multivariate regression. The river width is determined using the Normalized Difference Water Index-based (NDWI) derived from the Landsat-7 and Landsat-8 datasets. The riverbank erosion area is determined using shoreline analysis carried out using these datasets. The shoreline analysis indicates that Afghanistan is losing precious land due to the riverbank erosion over the past two decades (2004-20) amounting to as much as 86 sq. km and on average 5.4 sq. km every year. According to the ERA5 Reanalysis data, the water contribution from snowmelt in the spring and the summer was significantly dominant compared to the precipitation, which is consistent with several other watersheds in the north-western Himalayas. The river width and the discharge are observed to follow a power-law relation with an r² of 0.7. Additionally, the discharge intensity and the specific stream power showed significant relation (r² of 0.84 both) corresponding to the riverbank erosion area, where the peak flood events were observed to be outliers.

How to cite: Mahmoodzada, A. B., Varade, D., Shimada, S., Okazawa, H., and Vinay, C.: Average Quantifying the snowmelt dominant river erosion in Afghanistan between 2004-2020, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10888, https://doi.org/10.5194/egusphere-egu21-10888, 2021.

Manual and unattended sampling in the field and laboratory analysis are common practices to measure suspended sediment (SS) carbon content and particle size. However, one of the major drawbacks of these ex-situ methods is that they make high frequency measurements challenging. This includes restricted data collection due to limited access to the sampling locations during turbulent conditions or high flows, when the largest amount of sediments is transported downstream, introducing uncertainty in quantification of SS properties (particle size and carbon content) and sediment loads. Knowledge on SS carbon content and particle size is also important to better understand the multi-component form of suspended sediments (i.e. flocs) that directly affect sediment transport and other sediment properties (e.g. settling velocity and density). Moreover, SS carbon content and particle size exert an impact on the optical sensor readings that are traditionally used to measure turbidity. In that respect, high frequency measurements of SS carbon content and particle size could eventually help us to move from ‘local’ calibrations towards ‘global’ dependencies based on in-situ SS characterization.

In this study, we propose to use a submerged UV-VIS spectrometer to infer SS carbon content and particle size. The sensor measures the entire light absorption spectrum of water between 200 nm and 750 nm at sampling intervals as short as 2-minutes. To this end, we first test our approach under controlled conditions with an experimental laboratory setup consisting of a cylindrical tank (40-L) with an open top. An UV-VIS spectrometer and a LISST-200X sensor (to measure particle size distribution) are installed horizontally. A stirrer facilitates the homogeneous mixing of SS and prevents the settling of heavy particles at the bottom. We use the sediments sampled from 6 sites in Luxembourg with contrasting composition and representing different land use types and geological settings. The sampled sediments were wet sieved into 3 size classes to clearly recognize the effect of particle size on absorption. In our investigation, we use specific wavelengths, chemometric techniques and carbon content specific absorbance indices to infer SS composition and particle size from the absorption spectrum. Results are then validated using in-situ field data from two instrumented field sites in Luxembourg. Amid the challenge of associating laboratory and field results, the preliminary results indicate that the absorption spectrum measured with a submerged UV-VIS spectrometer can be used to estimate SS particle size and carbon content.

How to cite: Sehgal, D., Martínez-Carreras, N., Hissler, C., Bense, V., and Hoitink, A. (.: Inferring suspended sediment carbon content and particle size at high frequency from the optical response of a submerged spectrometer, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12317, https://doi.org/10.5194/egusphere-egu21-12317, 2021.

Long term and high-frequency monitoring of water quality, particularly the suspended particulate matter (SPM) concentration are crucial to decipher the health and sustainable development of marine ecosystems. However, in-situ measurements based on indirect optical or acoustic techniques are often associated with large uncertainties due to the dynamics of natural SPM, especially throughout the land-sea continuum. Therefore, this study aims to improve the accuracy of long term in-situ measurements by quantitatively elucidating the physical mechanisms by which sand and fine sediment respond to multi-wavelength optical and multi-frequency acoustic signals. We hypothesize that whilst fine sediment is very sensitive to optical signals, the coarser particles are more sensitive to acoustic signals, and vice versa. We further hypothesize that the SPM compositions and variability can be differentiated and derived based on such sensitivities and differences in behaviors of sand and fine sediment under different types of signals, i.e., optical and acoustic. 

Before testing the hypotheses, a novel laboratory device that is capable of 1) generating homogeneous suspended concentration and 2) providing sufficient space for multiple sensors to operate simultaneously must be developed. The new device, DEXMES (dispositive experimental de quantification des matières en suspension), primarily consists of two main components. The upper part is a cylindrical tank with an inner diameter of 0.96 m and 1.4 m high. To break up the large vortexes and mitigate the vortex-induced bubbles (e.g., generated by the impeller), four baffles with dimensions of 0.09 x 1.31 m are evenly attached to the inner side of the tank. The bottom part of the DEXMES device is a convex, elliptical Plexiglas bed. Turbulent flow is generated by an impeller with a diameter of 0.36 m placed approximately 1 m below the water surface. The speed of the impeller, ranging from 0 to 235 rpm, is regulated by a controller box.

To test the hypotheses, 30 experiments, consisting of 6 concentrations and 5 mixture ratios (by mass) of Bentonite and fine sand (d₅₀ = 100 µm), i.e., 100/0, 75/25, 50/50, 25/75, and 0/100, were thoroughly investigated using three acoustic sensors (ADV, AQUAscat, LISST-ABS) and three optical sensors (Wetlabs, HydroScat, LISST-100X). On average, each data point is the averaged value of 10 min of recording at 1 or 32 Hz. First, results show logarithmic/linear relationships between concentration and acoustic/optical signals respectively for a given bentonite/sand. Second, the slope of this relation is a function of the Bentonite/sand ratio. Third, the results confirm the hypotheses that coarser particles are more sensitive to acoustic signals and fine sediment is more sensitive to optical signals. Simple regression models were developed for different pairs of acoustic and optical sensors based on their relative sensitivity to SPM characteristics. The correlation coefficient, bias, and RMSE between observed and predicted concentrations then were examined. The results also show that it is possible to use a combination of one acoustic and one optical sensor to infer the concentration and the ratio of fine/coarse sediment in suspension with minimum use of water samples calibration.

How to cite: Tran, D., Jacquet, M., Pearson, S., and Verney, R.: Investigating suspended particulate matters from multi-wavelength optical and multi-frequency acoustic measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13022, https://doi.org/10.5194/egusphere-egu21-13022, 2021.

The capability of ADVs (Acoustic Doppler Velocimeters) to estimate suspended sediment concentration (SSC) has been widely investigated using commercial glass microspheres of the same size or well-sorted fractions in experimental studies. In the natural environment, sediment samples may be composed of different types of sediments having various types of grain size distribution.

This study aims to analyze experimentally the effect of clay ratio in sediment content on acoustic response. Modification of scattering and attenuation characteristics for different clay ratios is evaluated theoretically. In laboratory experiments, four different sediment mixtures constituting non-cohesive sand and cohesive clay materials were prepared with clay ratios of 0, 5, 10 and 15% by dry mass. A-10 MHz acoustic Doppler velocity profiler (ADVP, The Nortek Vectrino Profiler) was used in controlled laboratory environments under a wide range of concentration conditions up to 10 g/L. Acoustic backscatter measurements were made by immersing the ADVP in a well-mixed circulation tank filled with mixtures with known concentration and sediment composition. The backscattered signals were recorded at 100 Hz, from which 1.5-min ensemble averages were obtained. For each sediment mixture, calibration curves representing the relationship between SSC and acoustic backscatter were obtained based on the sonar equation. Acoustic estimates of suspended sediment parameters obtained for mixtures with different clay contents are compared to identify the effect of increasing clay content on the acoustic signal.

The experimental results showed that the slope of the calibration curve decreases with increasing validity range as the clay ratio of the mixture increases. Under the fixed SSC condition, the backscatter strength is greater for the mixture with a lower clay ratio. The theoretical analysis indicated that changing clay content modifies the scattering and attenuation properties compared to the mono-size suspension with the same mean size. Introducing clay material in a mixture affects the scattering properties more significantly than the attenuation properties. Therefore, information on the form of the sediment distribution and the sorting of sediments in suspension is crucial for acoustic estimates of suspended sediment parameters.

Acknowledgments

This research is supported by the Scientific and Technological Research Council of Turkey (TUBITAK) with project number 218M428.

How to cite: Aras, A. and Sahin, C.: Effect of clay content on Acoustic Doppler Velocimeter backscatter for suspended sediment concentration measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14330, https://doi.org/10.5194/egusphere-egu21-14330, 2021.

Turbidity currents are in the range of highly sediment concentrated flows, challenging traditional (i.e. optical and acoustic) techniques that aim to measure concentration and velocity quantities. In typical laboratory conditions, difficulties increase in the presence of highly non-uniform and unsteady flows. However, the measurement of those quantities along with a longitudinal profile is necessary to quantify and depict key mechanisms of mass and momentum transport, related to the mean and turbulent flow fields. The possible solutions often require prohibitive costs or resources. In this work, visual, acoustic, electrical, and statistical tools are tested. The aim of these tests is to find appropriate techniques and strategies for measuring concentration and velocity quantities in the broader research scope involving turbidity currents triggered by a 2D water jet. The outcomes will be applied in the quantification of turbidity currents with various boundary and initial conditions in a flume 4 m long, 2 m deep, and 22 cm wide. Additionally, the findings can potentially be transferred to other laboratory applications involving turbidity currents or other types of sediment-laden flows.

Acknowledgements: CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, a Foundation within the Ministry of Education in Brazil), grant number 88881.174820/2018-01.

How to cite: Buffon, P., Valero, D., Uijttewaal, W., and Franca, M.: Concentration and velocity measurements in experimental turbidity currents, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14713, https://doi.org/10.5194/egusphere-egu21-14713, 2021.