Sedimentary processes in aquatic environments, including entrainment, transport and deposition of sediment by hydrodynamic mechanisms, are key features for various research disciplines, e.g. geomorphology and paleoclimatology or hydraulics and river engineering. An accurate evaluation of entrainment, transport and deposition rates, conditioning river channel morphology and bed composition, is fundamental for an adequate development of conceptual sediment budget models and for the calibration and validation of numerical tools.
The main goal of this session is to bring together the community of scientists, scholars and engineers, investigating, teaching and applying novel measurement techniques and monitoring concepts, which are crucial to determine sedimentary and hydro-morphological processes in rivers, lakes and reservoirs, estuaries as well as in coastal and maritime environments. Within the focus of this session are the evaluation and quantification of bed load and suspended load, flocculation, settling, and re-suspension of such processes relevant to morphological channel changes as bed form development, horizontal channel migration, bed armouring and colmation.
Contributions are welcome with a particular focus on single and combined measurement techniques, on post-processing methods as well as on innovative and advanced monitoring concepts for field applications. Furthermore, we welcome contributions containing recent results in a temporal and spatial scale on sediment budgets as well as on sedimentary and morphodynamics processes in open water environments.
Contributions may refer, but are not restricted, to:
- measurements of suspended sediment transport in open water environments, e.g. with optical, acoustical, traditional sampling methods or others;
- measurements of bed load transport, e.g. with bed load samplers, sediment traps, tracers or acoustic and optical methods;
- determination of sediment characteristics, e.g. with mechanical bed material samplers or freeze core technique;
- measurements of critical bed shear stress of cohesive sediments, e.g. with benthic flumes or miscellaneous devices;
- monitoring of morphological changes like lake and reservoir sedimentation, bank erosion or bed armouring, meandering migration, river bends evolution;
- measuring networks / multiple point datasets;
- monitoring concepts including case studies;
- in-situ as well as laboratory calibration of measurement data;
vPICO presentations: Tue, 27 Apr
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 estimation of Suspended Sediment Concentration (SSC) on the surface of the Pearl River is of great significance to the management of water and soil erosion and water quality in the Pearl River. Previous studies lack of measured reflectance data and enough field samples and the distribution of sediment-concentration field samples were uneven. In response to the above problems, we combined the sediment concentration data (proceed by filtered weighing method) collected on the spot, high-precision ground measured spectral data (obtained by ASD) with multi-source remote sensing satellite images (MODIS and Sentinel-2), employing simple linear regression model (single logarithmic transformation) and neural network learning algorithm to fit the relationship model between SSC and surface reflectance (Surface Reflectance, SR). The preliminary results showed that SSC and the surface SR based on the red band (wavelength=665 nm) had a stable correlation (R2>0.83), and the red band of Sentinel 2 was appropriate for the estimation of SSC. Compared with previous studies, this study synthesized higher-precision spectrum measured data and higher-resolution remote sensing satellite data to improve the estimation accuracy of SSC. In addition, based on the SSC model under study, we will couple long-time series of satellite data to explore the spatiotemporal variation characteristics of SSC in the Pearl River, so as to provide a reference for soil erosion monitoring and water resources management in the Pearl River Basin.
How to cite: Cao, B., Yang, X., Qiu, J., Xie, X., and Li, H.: Estimation of Sediment Concentration in the Pearl River Estuary Based on Remote Sensing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-331, https://doi.org/10.5194/egusphere-egu21-331, 2021.
How to cite: Haun, S., Mayar, A., Noack, M., and Wieprecht, S.: Dynamic development of sediment infiltration in an artificial river bed, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1603, https://doi.org/10.5194/egusphere-egu21-1603, 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.
Suspended sediment contributes to the vast majority of the annual sediment load transported by rivers to the global oceans. At the same time, this large fraction is transported just in a fraction of time. Towards achieving sustainable sediment management and healthy fluvial systems, identifying the impact of the temporal variability on annual load estimates becomes indispensable in order to reduce uncertainties.
We aim to estimate the temporal variability of suspended sediment transport and the uncertainty of annual suspended sediment loads. Our approach is based on high-resolution time series (15 min sampling interval) of discharge and suspended sediment concentration (SSC) at four monitoring stations with different degrees of discharge variability. The quantification of the variability of discharge and sediment yield is achieved through the exceedance time. The uncertainty of the annual sediment load is estimated using a bootstrap approach. We assess the impact of the sampling interval and link the optimal sampling interval to different SSC-variability. Further, the impact of rating parameters on the uncertainty of annual loads is investigated.
Our results indicate an increase in SSC-variability with decreasing discharge, leading to a negative relationship with the contributing catchment area. The 80 % exceedance times for the annual sediment load range from less than 10 % for the river Ammer (catchment area 608 km²) between 10 – 20 % for the rivers Ilz (765 km²) and Moselle (27 088 km²) to more than 40 % for the river Rhine (109 806 km²). Simultaneously, the variability increases with a decrease in sampling frequency. Our preliminary results indicate a negative exponential relationship between exceedance time and uncertainties in annual load estimates. This relationship can be used to estimate the uncertainty of annual loads estimated based on low frequency sediment sampling at the continental to global scale.
How to cite: Slabon, A. and Hoffmann, T.: Temporal variability of annual suspended sediment yield estimates and their uncertainties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2989, https://doi.org/10.5194/egusphere-egu21-2989, 2021.
Differences in deposit geometry and texture with depth along ephemeral gravel-bed streams strongly reflect fluctuations in bedload, which occur due to environmental changes at the basin scale and morphological channel adjustments. In this study, non-destructive methods, 2D and 3D electrical resistivity tomography(ERT), have been combined with datasets from borehole logs to identify, quantify and analyze the internal geometry of cross-sections of the gravel-bed ephemeral channel, known as Azohía Rambla (southeastern Spain). The electrical survey was performed through longitudinal and transverse profiles in two channel reaches, upper and middle stretches. Both profiles utilized 28 stainless steel electrodes reaching 14-30 m in length and an investigation depth of 3-5 m, approximately. Electrical resistivity values were correlated with data obtained from the samples collected from borehole logs (e.g. sediment strength, grain size distribution, compaction, porosity (ϕ), and hydraulic conductivity (k)). To determine ϕ and K granulometric and morphometric variables, such as shape-sphericity indices, particle sorting, effective grain-sizes and void ratios, were used.
The alluvial channel-fills showed the superposition of four layers with uneven thickness and arrangement: 1) a lower sandy-gravel body, scarcely thick, characterized by moderate resistivity (150-500 Ω · m); 2) a thicker intermediate layer, with moderate to high resistivity values (500 to 1600 Ω · m); and 3) an upper set composed of coarse gravel and supported matrix, ranging from 1600 to 3000 Ω · m, and a narrow subsurface layer, the most resistive (> 3000 Ω · m), corresponding to the most recent armored deposits (gravel and pebbles). Consequently, the ERT results coupled with borehole data suggest that since the channel entrenchment in the Miocene marl substrate, different pulses of vertical sedimentary accretion were produced, denoting a general trend to increase in grain-size (coarsening-upwards) and hydraulic conductivity towards the top of the sedimentary sequence. This research was funded by ERDF/Spanish Ministry of Science, Innovation and Universities—State Research Agency/Project CGL2017-84625-C2-1-R; State Program for Research, Development and Innovation Focused on the Challenges of Society.
How to cite: Martínez-Segura, M. A., Conesa-García, C., Pérez-Cutillas, P., and Vásconez-Maza, M. D.: Using Electric Resistivity Tomography and Borehole Logs to Detect Sedimentation Changes in a Gravel-Bed Ephemeral Channel, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3860, https://doi.org/10.5194/egusphere-egu21-3860, 2021.
Knowledge about bedload transport in rivers is of high importance for many hydraulic engineering applications, in particular related to flood protection measures. Passive acoustic surrogate measurement techniques provide useful continuous estimates of bedload transport in terms of total mass, as well as for different grain-size classes.
We compare different surrogate measurement systems regarding their performance in quantifying total and fractional bedload transport in three alpine streams. The investigated measurement systems are the well-established Swiss plate geophone (SPG), an equivalent system in which the geophone sensor is replaced by an accelerometer sensor, and the miniplate accelerometer (MPA) system. The latter is a more recent device and consists of four small square metal plates embedded in elastomere elements. While the signal recorded with the SPG is known to be proportional to the transported bedload mass, we find that the MPA-signal shows a non-linear dependency. In addition, the MPA reacts more sensitively to small grain size classes than the other two systems, indicating a possible alternative to improve the quantification of bedload transport consisting of those classes.
Based on the raw signal recorded with the SPG and the MPA in a flume experiment, we test the ability of different empirical models to predict the known weight of the impacting particle. We show that it is possible to identify the particle weight with high accuracy with relatively simple models using data of either of the two measurement systems. One remaining challenge is to account for the site-to-site variability in the (amount of) signal caused by the combination of differing numbers of plates in the measurement setup and the lateral transmission of the signal across multiple plates, especially for the SPG system.
How to cite: Ammann, L., Nicollier, T., Badoux, A., and Rickenmann, D.: Total and fractional bedload transport in alpine streams approximated by different surrogate measurement systems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3983, https://doi.org/10.5194/egusphere-egu21-3983, 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 (IM), hydrology index (IH) and hydromorphology index (IHM), 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 (IHM) 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.
Over many decades it has become evident, that sediment accumulation threatens the fundamental operation of reservoirs by reducing the storage volume, hindering technical functions and deteriorating water quality over time. Most scientists, operators and authorities are aware of this, often “silent” but enduring process. However, not often mitigation measures are applied with foresight and in an appropriate manner according to this global problem. One fundamental reason for the often hesitant implementation of measures is the lack of precise and applicable assessment techniques. The type of reservoir, available historic data and especially the composition of the sediment may allow only for one available method to be applied successfully. In this study we present a workflow to select the best available method to detect the sediment thickness correctly. We compare topographic differencing, dual-frequency echo sounding, sub-bottom echo sounding, free-fall penetrometer measurements and sediment coring. Next to the general applicability, the precision (vertical resolution) and the time requirement for the measurement are relevant factors. A special point of discussion is the presence of free gas inside the sediment, often creating measurement errors, leading to underestimation of the sediment thickness.
How to cite: Hilgert, S., Sotiri, K., and Fuchs, S.: Comparative overview of reservoir siltation assessment techniques depending on the type of sediment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5893, https://doi.org/10.5194/egusphere-egu21-5893, 2021.
Fine sediments exhibit various stages of deposition and erosion during their transport from hillslopes to the ocean. In mountainous environments, high fine sediment load during runoff or dam flushing events can lead to important amounts of deposits in gravel bed rivers. Massive deposits may lead to bar elevation, riparian vegetation growth and consequently to bar stabilization, which can increase flood risks. High amount of fine sediment deposits alters also aquatic life and habitat.
In order to better understand the dynamics of re-suspension of these deposits, and to accurately predict it with numerical modelling, field monitoring campaigns were performed to assess both the spatial variability and the controlling factors of the erodibility of fine deposits. The cohesive strength-meter (CSM) device, a pocket penetrometer and a pocket shear vane were used to evaluate the erodibility of fine sediments deposited in two rivers in the French Alps: The Isère and Galabre.
The results highlight the specificity of gravel bed rivers with an abundance of areas of deposition of fine sediments, which are discontinuous compared to estuaries and lowland rivers. A high spatial variability of the erodibility was observed and related to the spatial organization of the deposits. The location of the deposit and its elevation, the moisture and the grain sizes are inter-related and have important correlations with the erodibility. Measurements show that high altitude dry deposits and low altitude humid deposits are more easily eroded than intermediate deposits with medium moisture. The measured variables explain part of the variability of the erodibility but other processes such as the history or the origin of the deposit might also be important factors to consider.
How to cite: Haddad, H., Jodeau, M., Antoine, G., and Legoût, C.: Erodibility of fine sediment deposits in gravel bed rivers: investigation of the spatial variability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6103, https://doi.org/10.5194/egusphere-egu21-6103, 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.
Pipelines that traverse a river are often buried beneath the river bed. However, the pipeline may be exposed due to scoured riverbed during floods. The exposed pipeline vibrates in a frequency band depending upon the flow velocity, size, and shape of the pipe. These vibrations are detrimental to the pipeline safety and stability due to their cyclic nature. In fact, these vibrations are induced by the turbulence around the cylinder which is a function of the flow velocity apart from the diameter of the cylinder and the bed roughness. The main objective of this paper is to investigate the structure of turbulent flow in the recirculation, reattachment and recovery regions behind a horizontal circular cylinder placed on the rough bed. In this direction, different experiments were conducted in a wide flume for various flow Reynolds numbers and cylinder Reynolds numbers. The Acoustic Doppler Velocimetry (ADV) was used for measuring the instantaneous point velocities. The raw velocity data were properly processed before the analysis. The approach flow was found to be a canonical near wall turbulent flow. In the immediate downstream of the cylinder, flow is characterized by recirculation, boundary layer reattachment and recovery. The reattachment length was determined using the established forward fraction method and reattachment length is independent of the flow Reynolds number. In addition, enhanced turbulence intensities, Reynolds shear stress, and turbulent kinetic energy were observed in the separated shear layer and they rapidly decreased in the recovery region. The present investigation will boost the understanding of hydraulics of flow around the horizontal bed-mounted cylindrical objects in rough bed natural streams under different flow conditions.
Keywords: Wall mounted horizontal cylinder; Boundary layer; Separated and reattached turbulent flows; Wall Wake flows; ADV; Open channel flow.
How to cite: hanmaiahgari, P. and devi, K.: Turbulent Flow Structure around a Horizontal Circular Cylinder Placed on a Rough Bed, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8112, https://doi.org/10.5194/egusphere-egu21-8112, 2021.
The entrainment and transport of sediment by hydrodynamic mechanisms is strongly related to bed shear stress exerted by flow. Therefore, to quantify sediment transport and to determine sediment incipient motion conditions, accurate estimations of bed shear stress are required. Most of the existing methods used in hydraulics and river engineering to determine bed shear stress are indirect, and are mostly restricted to limited flow conditions or contain a large degree of uncertainty. Although devices to perform direct measurements of boundary shear stress exist, they are normally based on expensive technology. We developed a shear plate for direct shear stress measurements, using relatively low cost components. In this work we present preliminary results of measurements performed with the new shear plate, to characterize the bottom shear stress generated by a ship propeller. The data result in the expected quadratic relation between bed shear stress and jet velocities, and also give evidence of a good reproducibility. We show that the new shear plate appears to be a promising device for reliable measurements of submerged boundary shear stress under a wide range of environments and flow conditions.
How to cite: Niewerth, S., Núñez-González, F., and Llull, T.: An affordable and reliable device for direct bed shear stress measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9721, https://doi.org/10.5194/egusphere-egu21-9721, 2021.
Quantifying bedload transport is important for many applications such as river management and hydraulic structures protection. Bedload flux measurements can be achieved using physical sampler methods. However, these methods are expensive, time-consuming, and difficult to operate during high discharge events. Besides, these methods do not permit to capture the spatial and temporal variability of bedload transport flux. Recently, alternative measuring technologies have been developed to continuously monitor bedload flux and grain size distribution using passive or active sensors. Among them, the hydrophone was used to monitor bedload transport by recording the sounds generated by bedload particles colliding on the river bed (referred as self-generated noise SGN). The acoustic power of SGN was correlated with bedload flux in field experiments. To better understand these experimental results and to estimate measurement uncertainties, we developed a theoretical model to simulate the SGN. The model computes an estimation of the power spectral density (PSD)by considering the contribution of all signals generated by impacts between bedload particles and the riverbed, and accounting for the attenuation of the acoustic signal between the source and the hydrophone position due to river propagation effects,. In this model, we
The energy of acoustic noise generated from the collision between two particles is mainly dependent on the transported particles' diameter and the impact velocity. We tested different empirical formulas for the estimation of the number of impact (impact rate) and the impact velocity depending on particle size and hydraulic conditions. To characterize the acoustic power losses as a function of distance and frequency, we used an attenuation function which was experimentally calibrated for different French rivers.
We tested the model on a field dataset comprising acoustic and bedload flux measurements. The results indicate that the PSD model allows estimating acoustic power (in between a range of one order of magnitude) for most of the rivers considered. The model sensitivity was evaluated. In particular, we observed that it is very sensitive to the empirical formulas used to determine the impact rate and impact speed. In addition, special attention should be kept in mind on the assumption of the grain size distribution of riverbed which can generate large variability in some rivers particularly in rivers with a significant sand fraction.
How to cite: Nasr, M., Geay, T., Zanker, S., and Alain, R.: Development of Model for Acoustic Noise Generated from Bedload in Rivers , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9822, https://doi.org/10.5194/egusphere-egu21-9822, 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 (Dn). The mean values of Dmax, D90, D84, D50, D16, 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 Dn values calculated with the multiple regression model based on the analysis of the axes (Dn = f (A, B, f (A, B))) were very close (R² = 0.95) to those calculated through an original definition of Dn, i.e., Dn = (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 Dn 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.
Paleoreconstruction is a group of methods for evaluating geophysical parameters based on indirect indicators, such as the chemical composition of foraminiferal shells, the concentration of various gases in sedimentary grains on the ocean floor, etc. For example, with the distribution of types and sizes of foraminifera or sizes and shapes of crystals of various minerals in sediment deposits, one may reconstruct the temperature and salinity of water and even the direction and speed of currents. A number of methods are used to explore sediment particle composition nowadays: examining color characteristics, physical properties of grains, their isotopic composition, etc. However, the visual examination of sedimentary particles under a microscope remains one of the most exploited methods. In our study, we consider sediments' fraction of size from 100 microns under an 80x microscope. Our study aims to infer the distribution of sizes and shapes of the grains in a sediments sample. However, the problem we face is that it takes a great time and effort of an expert to classify the particles. In our study, we propose to employ artificial intelligence methods for automating this task. We collect optical imagery of prepared sediment samples. Using the images, we collect visual representations of different grains of sediments. We propose using a version of convolutional variational autoencoder for reducing the dimensionality of the visual representations. Then we apply a clustering algorithm for splitting the grains into groups of similar ones. An expert examines the groups for further classification.
In this work, we will demonstrate the preliminary results of the clustering of sedimentary particles and outline further development of the presented method for their automated classification.
The work is supported by the Russian State assignment no. 0149-2019-0007
How to cite: Golikov, V., Krinitskiy, M., and Borisov, D.: Artificial neural networks for clustering sediment grains in microphotographs., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14262, https://doi.org/10.5194/egusphere-egu21-14262, 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.
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.
The continuous need for beach nourishment requires a detailed understanding of the sediment transport characteristics at the shelf borrow sites, to assess their recovery rate and to evaluate the long-term sustainability of these operations.
The main objective of this work is to assess sediment transport conditions at an inner shelf borrow site exploited to nourish a beach located at the updrift boundary of the same sedimentary cell (Belharucas, Albufeira, south coast of Portugal).
The work is supported by a sand tracer experiment, where 600 kg of coated sand with fluorescent ink was deposited (August 2020) by divers at 11 m depth (referred to the mean sea level). Periodic sediment sampling using a Van Veen grab was performed using an adaptative sampling grid that accounted for tracer’s dispersion trough time. The samples were washed and dried in laboratory and tagged particles were automatically identified using an automated image analysis procedure based on ultraviolet lighting.
Preliminary results show that sediment transport is dominated by a eastward component,probably related with the energetic events from the SW. Ongoing work relates the tracer’s displacement with ADCP (wave and current) data measured nearby the borrow site during the experiment.
The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL and ECOEXA project (MAR-01.04.02-FEAMP-0016).
How to cite: Cascalho, J., Taborda, R., Rosa, M., Garel, E., Teixeira, S., Alberto, A., and Drago, T.: Monitoring inner shelf sediment transport using fluorescent sand tracers: an example from the south coast of Portugal , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15834, https://doi.org/10.5194/egusphere-egu21-15834, 2021.
Measuring and assessing the bedload data is a crucial for successful and efficient river management. Hence, the information about the bedload transport and characteristics helps to describe the dynamics of the river morphology and to evaluate the impacts on boat navigation, hydropower production, ecological systems and aquatic habitat.
Although the acoustic Doppler current profilers are designed to measure water velocities and discharges, they have been successfully used to measure some bedload characteristics, such as the apparent bedload velocity. The correlation between the apparent bedload velocity and the bedload transport rates measured by physical bedload samplers (e.g. pressure difference) has been examined and relatively high correlations have been reported. Moreover, laboratory experiments have proven that there is a strong correlation between the bedload concentration and particle size distribution and corrected backscattering strength obtained from the ADCPs.
The bedload transport rates yielded from the ADCPs outputs are usually derived as regression model-fitting of the measured apparent velocity and the physically collected bedload samples at the same time and position. Alternatively, a semi-empirical kinematical approach is used, where the apparent bedload velocity is the main component and the bedload concentration is empirically estimated. However, the heterogeneous and sporadic motion of the bedload particles is often followed by high uncertainty and weak performance of these approaches.
Machine learning offers a relatively simple and robust method that has the potential to describe the nonlinearity of the complex bedload motion and so far, it has not been previously exploited for predicting transport rates. This study implements artificial neural network techniques to develop a model for predicting bedload transport rates by using only ADCP data outputs as training data. Data processing techniques are used to extract relevant features from the corrected backscattering strength and apparent velocity obtained from the ADCPs. More than 60 features were derived in the ADCPs dataset, and the most relevant features are selected through neighborhood component analysis. These features are used as inputs in conventional supervised neural network architecture which consists of two hidden layers and 35 neurons. This model is used to capture the distribution of the ADCP features for each output (e.g., physically measured transport rates and grain size from bedload samples) in the training sample. The back-propagation algorithm (BPA) is still one of the most widely used learning algorithms in the training process and thus herein applied. The learning rate, number of neurons and hidden layers were optimized by using Bayesian optimization techniques. The network was trained with more than 60 bedload samples and corresponding 5 - 10 min time series of ADCP preprocessed data. The rest of the samples were used for validation of the model. The validation resulted in correlation coefficients higher than 0.9 and the, which is significantly higher value than the corresponding values for the methodologies developed before. Aiming to develop a more robust and stable ANN model, further testing of different training algorithms must be performed, different ANN architecture should be tested, and more data shall be included.
How to cite: Conevski, S., Guerrero, M., Winterscheid, A., and Ruther, N.: Application of artificial neural network to estimate bedload transport rates and bedload granulometry using outputs of stationary ADCP measurements , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16412, https://doi.org/10.5194/egusphere-egu21-16412, 2021.
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