Global change alters hydro-climatic conditions, affects land use, and contributes to more frequent droughts and floods. Large artificial reservoirs may effectively alleviate hydro-climatic extremes, ...but their storage capacities are threatened by sedimentation processes, which in turn are exacerbated by land use change. Envisioning strategies for sustainable reservoir management requires interdisciplinary model chains to emulate key processes driving sedimentation under global change scenarios. Therefore, we introduce a model chain for the long-term prediction of complex three-dimensional (3d) reservoir sedimentation considering concurrent catchment, hydro-climatic, and land-use conditions. Applied to a mountainous Mediterranean catchment, the model chain predicts increased sediment production and decreased discharge for high and medium emission pathways. Increased winter precipitation, accompanied by a transition from snowfall to rainfall, is projected to aggravate reduced summer precipitation, emphasizing a growing need for reservoirs. Additionally, higher winter precipitation proliferates sediment production and reservoir sedimentation. Land use change can outweigh the increased reservoir sedimentation originating from hydro-climatic change, which highlights the significance of localized actions to reduce sediment production. Finally, a 3d hydro-morphodynamic model provides insights into interactions between global change and reservoir sedimentation with spatially explicit information on future sedimentation patterns facilitating the implementation of management strategies.
•Riverbed clogging blocks vertical connectivity and its assessment is challenging.•Multiple parameters may indicate different or opposing degrees of clogging.•New fuzzy-logic indicators evaluate ...clogging and declogging for stream restoration.•The fuzzy-logic method provides quantitative assessment of (de)clogging.•Placement of large wood can alleviate clogging, as demonstrated by the new method.
Rivers provide dynamic habitats with ecological niches, particularly in their mobile sand, gravel, and cobble riverbed patches that create an active hyporheic zone. Natural or artificial deposition of fine sediment may clog the porous matrix of the hyporheic zone, impairing exchange processes between the subsurface and surface water. Clogging reduces the permeability of the sediment matrix, thus degrading the ecological functionality of the hyporheic zone. Once clogged, the ecological functions may be recovered through active stream restoration, which requires considerate site assessment. To this end, clogging is typically assessed by expert opinion of substrate characteristics including grain size characteristics, porosity, hydraulic conductivity, and interstitial oxygen content. To overcome limitations of expert assessment, such as subjectivity expressed in noisy decision-making, this study introduces a novel fuzzy-logic method based on physically sound rules. The method provides quantitative indicators for clogging and declogging to evaluate the effectiveness of stream restoration. We applied the fuzzy-logic method to test whether the placement of large wood, a common restoration practice, can locally prevent or reduce clogging. Two measurement series from before and after a morphologically effective flood suggested that large wood placements perpendicular to the flow generate elevated amounts of declogging. The tested logs caused a greater amount of declogging within their region of influence than observed at a reference point. The effect was stronger for a log emergent at baseflow. The declogging assessment showed that the novel fuzzy-logic indicators can reasonably overcome subjective judgment by accounting for multi-variate quantitative changes rather than individual parameter trends.
•A combined use of the LISST-SL optical sampler and a down-looking 1200kHz ADCP profiler, gave insights in the performances of optical and acoustic methods when investigating suspended sediment in ...the Danube River.•A combined deployment of those measuring techniques may aid to detect ideal conditions for LISST-SL measurement across a large river channel. For example, a moving ADCP may be used to track regions characterized with acoustic parameters values where LISST-SL could subsequently be deployed for a more accurate and quantitative assessment.•Obtained vertical profiles of sediment concentration, mean size and acoustic parameters from repeated samples by LISST-SL and ADCP recording, respectively, pointed out density anomalies at shallow depths rather than actual concentration values of suspended sediment. These density anomalies were related to the cold water inflow from Hron River.•The LISST-SL required 2min measuring in fixed positions, eventually limiting the standard deviation to 10% of the expected mean value, except for low concentrations (i.e., in the range of 5–10mg/l) of sand that resulted in the same order of the corresponding standard deviations over 2min.•The acoustic device appeared particularly sensitive to suspended sediment from the riverbed. A moving ADCP located regions of sediment entrainment from riverbed, fine sediment fully suspended in the water column and the extent of Hron River inflow dispersion into Danube main streamflow.
The use of acoustic and optic devices has become more and more common for estimating suspended sediment loads in rivers. The echo intensity levels (EIL) recorded by means of an Acoustic Doppler Current Profiler (ADCP) have been applied in different methods, which provided relationships between scattering particles features derived from samples (i.e., concentration and grain size) and corresponding backscattering strength and sound attenuation. At the same time, the laser diffraction was applied by an in-stream sampler (LISST-SL) to measure suspended sediment concentration and the corresponding particle size distribution (PSD). These two techniques exhibited different limitations in terms of the measured range of concentration, sensitivity to a certain spectrum of particle sizes, and instruments deploy feasibility especially in large rivers, in a way that the use of sampled PSD by LISST-SL to validate ADCP methods may not be trivial. The aim of this study was to combine the vertical profiling of EIL by an ADCP with results from LISST-SL, eventually demonstrating the possibility of using moving ADCP measurements to detect different suspended matters along a Danube River section characterized by a small tributary junction. At the same time, this work elucidates optical to acoustic method deviations that hinders an actual validation of ADCP methods based on LISST-SL rather than with physical samplings.
In curved channels, the flow characteristics, sediment transport mechanisms, and bed evolution are more complex than in straight channels, owing to the interaction between the centrifugal force and ...the pressure gradient, which results in the formation of secondary currents. Therefore, using an appropriate numerical model that considers this fully three-dimensional effect, and subsequently, the model calibration are substantial tasks for achieving reliable simulation results. The calibration of numerical models as a subjective approach can become challenging and highly time-consuming, especially for inexperienced modelers, due to dealing with a large number of input parameters with respect to hydraulics and sediment transport. Using optimization methods can notably facilitate and expedite the calibration procedure by reducing the user intervention, which results in a more objective selection of parameters. This study focuses on the application of four different optimization algorithms for calibration of a 3D morphodynamic numerical model of a curved channel. The performance of a local gradient-based method is compared with three global optimization algorithms in terms of accuracy and computational time (model runs). The outputs of the optimization methods demonstrate similar sets of calibrated parameters and almost the same degree of accuracy according to the achieved minimum of the objective function. Accordingly, the most efficient method concerning the number of model runs (i.e., local optimization method) is selected for further investigation by setting up additional numerical models using different sediment transport formulae and various discharge rates. The comparisons of bed topography changes in several longitudinal and cross-sections between the measured data and the results of the calibrated numerical models are presented. The outcomes show an acceptable degree of accuracy for the automatically calibrated models.
Abstract
Understanding the complexity of the siltation process and sediment resuspension in shallow reservoirs is vital for maintaining the reservoir functionality and implementing sustainable ...sediment management strategies. The geometry of reservoirs plays an indispensable role in the appearance of various flow structures inside the basin and, consequently, the pattern of the morphological evolution. In this study, a three-dimensional numerical model, coupled with optimization algorithms, is used to investigate the morphological bed changes in two symmetric shallow reservoirs having hexagon and lozenge shapes. This work aims to evaluate the applicability, efficiency, and accuracy of the automatic calibration routine, which can be a suitable replacement for the time-consuming and subjective method of manual model calibration. In this regard, two sensitive parameters (i.e., roughness height and sediment active layer thickness) are assessed. The goodness-of-fit between the calculated bed levels and the measured topography from physical models are presented by different statistical metrics. From the results, it can be concluded that the automatically calibrated models are in reasonable agreement with the observations. Employing a suitable optimization algorithm, which finds the best possible combination of investigated parameters, can considerably reduce the model calibration time and user intervention.
A combination of two indirect methods to measure sediment flux is presented in this study to evaluate suspended sediment transport in a hydropower reservoir. The acoustic backscatter signal (ABS) ...from an Acoustic Doppler Current Profiler (ADCP) is therefore applied in pre-defined transects within the reservoir in combination with a Laser In-Situ Scattering Transmissometry – stream lined device (LISST-SL). The stationary LISST-SL derived suspended sediment concentration (SSC) measurements are used to calibrate the ABS. From the LISST-SL measurements a time series of SSC is obtained. This enables, in addition, a comprehensive data analysis to evaluate the influence of natural fluctuations of the SSC on the calculated sediment flux, which should be taken into account when assessing sediment transport. Furthermore SSC measurements are done with the LISST-SL close to the reservoir bed. In areas close to the bed no information regarding the ABS is available from the ADCP measurements due to the side-lobe interference. In various studies the information from the last three valid cells is used for extrapolation. However, as result of a comparison of the LISST-SL measurements with extrapolated SSC values from the ADCP measurements it can be seen that, especially in deep reservoirs, this method has to be adapted to the in-situ conditions.
The use of echo-levels from Acoustic Doppler Current Profiler (ADCP) recordings has become more and more common for estimating suspended bed-material and wash loads in rivers over the last decade. ...Empirical, semi-empirical and physical-based acoustic methods have been applied in different case studies, which provided relationships between scattering particles features derived from samples (i.e., concentration and grain size) and corresponding backscattering strength and sound attenuation. These methods entail different assumptions regarding sediment heterogeneity in the ensonified volume (e.g., particle size distribution (PSD) and spatial concentration gradient). Our work was to compare acoustic backscatter and attenuation properties of suspended sediments, sampled in the rivers Parana and Danube that represented rather different hydro-sedimentological conditions during the surveys. The Parana represents a large sandy river, characterized through a huge watershed and the typical bimodal PSD of sediment in suspension, while the Danube represents in the investigated reach an exposed sand-gravel bed and clay-silt particles transported in the water column in suspension. Sand and clay-silt concentrations clearly dominate the analyzed backscattering strength in the rivers Parana and Danube, respectively, with an effect of PSD level of sorting in the latter case. This comparison clarifies the extent of assumptions made, eventually advising on the actual possibility of applying certain ADCP methods, depending on the expected concentration gradients and PSD of suspended sediment to be investigated.
Soil slides can occur when the water level in a lake or a reservoir is lowered. This may take place in situations when a reservoir is flushed to remove sediments. The current study describes a ...three-dimensional numerical model used for the simulation of reservoir flushing that includes the slide movements. The geotechnical failure algorithms start with modelling the groundwater levels at the banks of the reservoir. A limit equilibrium approach is further used to find the location of the slides. The actual movement of the sediments is computed by assuming the soil to be a viscous liquid and by solving the Navier–Stokes equations. The resulting bed elevation changes from the slides are computed in adaptive grids that change as a function of water level, bed erosion and slide movements. The numerical model is tested on the Bodendorf reservoir in Austria, where field measurements are available of the bank elevations before and after a flushing operation. The results from the numerical simulations are compared with these observations. A parameter test shows that the results are very sensitive to the cohesion and less sensitive to the
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The catchment of the Dashidaira reservoir located on the Kurobe River has high sediment yield. Because of the sufficient available amount of water in the catchment during flood events, the free-flow ...sediment flushing operation with full water-level drawdown is employed every year to preserve the effective storage capacity of the Dashidaira reservoir. This paper focuses first on the numerical simulation of a previously conducted free-flow flushing operation in the Dashidaira reservoir using the available in situ obtained data. Afterwards, to improve the flushing efficiency, the effects of water and discharge manipulation and the construction of an auxiliary channel on the total volume of the flushed sediment were studied. A fully 3D numerical model using the finite volume approach in combination with a wetting/drying algorithm was utilized to reproduce the flow velocity field and simulate the movable bed variations. The outcomes revealed that increasing the average free-flow discharge during the free-flow stage by approximately 56%, in the form of multiple discharge pulses, can enhance the flushing efficiency by up to 13%, and the construction of an auxiliary channel in the wide midstream of the reservoir can locally increase the sediment erosion from this area.
Suspended sediment and particle size measurements were in this study performed with two instruments based on laser diffraction. The measurements were conducted in the river Danube during a 1-year ...flood, which enabled measurements in a large scale and under conditions with high sediment load. The aim of this study was to compare LISST-SL stationary measurements with LISST-SL moving measurements to gain knowledge for extensive field surveys of large areas. The moving measurements were performed in four different depths along a transect and compared with stationary measurements in five verticals in the same depths. The results showed that the measured suspended sediment concentrations are smaller for the moving measurements compared to the stationary measurements. In addition thus the d50 was smaller for the moving measurements, which is an indicator for the phenomena of non-isokinetic sampling. A comparison of the pump speed of the two operation modes proofed that during LISST-SL moving measurements a non-isokinetic sampling effect occurred. Hence, a suction effect happened, resulting in an underestimation of particles >63µm. Beside the LISST-SL simultaneously a LISST-STX was used during the field survey to investigate the behavior and the results of the instrument under conditions where it is originally not designed for. However, the predominant suspended sediment concentrations in the river influenced the measurements and thus multiple scattering occurred. As a consequence the measurements of the suspended sediment concentrations showed lower values for the measured suspended sediment concentrations compared to the LISST-SL stationary measurements. It could also be seen that due to the high amount of fine particles, which were transported during the flood event, a re-scattering of light occurred also in case of measurements where the value for the optical transmission was with 0.38 above the standardized used limit of 0.3.
•The SSC and the PSD were in this study evaluated by two measurement devices based on laser diffraction.•The measurements were performed during an 1‐year flood event in the river Danube in Hungary.•A LISST‐SL measuring device was compared in stationary and moving mode to evaluate nonisokinetic sampling effects.•The predominant SSC partly influenced the measurements and multiple scattering effects occurred.
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