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Strengthening of non-structural measures for flood protection by enforcing natural retention processes on river floodplains not only is required by the EU Floods Directive but also implicates a variety of socio-economic conflicts related to the multipurpose use of floodplains. In order to ensure the achievement of multiple benefits of such non-structural measures in the most effective way, it is crucial to determine which floodplains are highly relevant for preservation and/or restoration not only concerning flood protection (hydrology/hydraulics) but also for ecological and sociological reasons. Therefore, the main objectives of this paper are to identify on different spatial scales those hydrological/hydraulic, ecological and sociological parameters that are important for an integrated evaluation of floodplain effectiveness. The results are finally assembled in the novel multidisciplinary floodplain evaluation matrix (FEM) serving as decision support for the relevant stakeholders and indicating where efforts of floodplain preservation/restoration should be spent first within an integrated flood risk management. At the end of this paper, an integrative evaluation on macro-scale level for the Austrian Kamp River is exemplarily presented. Detailed sectoral investigations of the hydrological and hydraulic parameters are further described in a case study of the Austrian Danube (see paper 'Assessing the role of floodplains along the Austrian Danube for flood hazard reduction using the hydrological and hydraulic parameters of the FEM-method'). [ABSTRACT FROM AUTHOR]

During the last decades, river floods accounted for enormous damages especially in highly developed and/or densely populated regions worldwide. Moreover, due to anthropogenic alterations of hydrology and river morphology (climate change, land use changes in the catchment, channelling and constricting rivers) and due to the ongoing accumulation of values (such as settlements, infrastructure facilities, etc.) in flood prone areas, this amount of damages is likely to rise in future. Integrated flood risk management is legally in force and aims at reducing the negative effects of floods by combining structural and non-structural flood protection measures. Non-structural measures such as the preservation or restoration of floodplains are considered by the EU Floods Directive as an effective tool for reducing flood risks. For most of the rivers, however, very little is known about the effectiveness of floodplains in regard to hydrological and hydraulical flood hazard reduction. This lack in knowledge often obstructs the integration of these natural flood retention processes into the concepts of integrated flood risk management. In the present study, the Austrian Danube was investigated along its entire 350 km length, determining reaches and floodplains with high relevance for flood water retention and thus for reducing flood hazards downstream. A novel analysis based on one-dimensional and two-dimensional hydrodynamic-numerical modelling, using hydrological and hydraulic parameters defined under the so-called floodplain evaluation matrix method (FEM; Habersack et al. in Nat Hazards, in print, ), was carried out to evaluate retention effectiveness on various spatial scales. The results illustrate the magnitude and the variability of flood retention and hydraulic parameters with respect to different hydrological settings (flood wave shape, recurrence probability). [ABSTRACT FROM AUTHOR]

ABSTRACT Most of the large rivers are heavily degraded and lack near-natural conditions due to high human pressure (agricultural use and settlements) especially on former inundation areas. Hence, it is rarely possible to 'restore' predisturbance conditions of rivers and their floodplains. Further, river or floodplain restoration programs are often based on type-specific reference conditions. Those reference conditions are mainly determined on the basis of historical maps not giving any information of, for example, sediment supply, flood frequency and vegetation cover (density). Especially for improving the ecological status of rivers with abandoned channel features, key habitats for target fish species have to be restored by reconnecting floodplains and their secondary channel system. In addition, because of the necessity of improving the ecological status, there is growing interest in interdisciplinary river restoration techniques. Within the presented article, an integrative concept is derived based on Light Detection and Ranging measurements and numerical modelling with respect to river dynamics (hydrologic and morphological). Further habitat modelling, based on unsteady depth-averaged two-dimensional hydrodynamics, is applied with a focus on the mesounit scale. For testing the conceptual model, various river reaches at the Morava River were selected, featuring different morphological characteristics. It was found that the applied management concept allows considering the important issues of river dynamics (morphological/hydrologic) using a flow- and flood-pulse approach for identifying bottlenecks of target species at the Morava River. The reconnection of abandoned channels will result in an increase of hydromorphological heterogeneity and/or woody debris within the study reach. This might be of high relevance for habitat features (e.g. backwater habitats) especially for flow pulses between low flow and mean flow and/or in reaches without abandoned channels between low-flow and the bankfull stage. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Hydropeaking due to operations of storage hydropower plants was highlighted as one of the main stresses on aquatic ecology by the European Water Framework Directive. Nevertheless, hydropower must be seen as a high-value resource in the renewable energy sector. However, very few studies deal with the physical processes behind the artificial unsteady flows, as the variability of local river morphology and roughness in general. Thus, the aim of the present study is to address these issues on the basis of one-dimensional hydrodynamic numerical modelling. In total, four Austrian rivers have been investigated, which were further divided into 13 sub-reaches according to their bed slope. From single and multiple linear regression models, it was possible to relate potential retentions of peak flow for peak duration of <2 h and for bed slopes <0.005 to the spatial extent of wetted width between base and peak flows, independent of the selected ramping rate (discharge relationship between base and peak flows). Moreover, the ramping velocity (rate of decreasing water level between peak and base flows) correlated positively with an increase in depth variance during peak flow, which was related further to an increase in mean flow velocities and a higher specific discharge over the 13 selected reaches. Nevertheless, within sensitivity testing, it could be proven that increases in channel roughness cause an increase in peak flow retention and a decrease of the ramping velocities for the decreasing limb. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Examines the role of river-scaling concept on determining the ecological integrity of biotic river. Derivation of the abiotic components; Assessment of morphodynamics and habitat quality; Analysis of the boundary conditions.

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In the Johnsbach Valley (Austria), a medium size non-glaciated torrent catchment, enormous amounts of sediment have been made available due to the brittle dolomite bedrock. This occurs mainly in the Zwischenmäuerstrecke (ZMS) (English translation: “reach between the walls”) and presents a major challenge to local river management. Within a renaturation project, which followed several decades of disturbance (flood protection and gravel mining) in the ZMS, it is of particular importance to understand where the sediments come from and the transport pathways through the system to prepare future forecasts. In the present study, we investigate the recent sediment cascade in a comprehensive analysis of the ZMS that was achieved by means of airborne laser scanning campaigns in 2010 and 2015. The current bedload yield at the outlet was measured using an integrative bedload monitoring system. Historical data from 1954 was used to illustrate the effects of the mining period on the former sediment routing. Finally, we evaluated the expected sediment transport rates in the near future. The results show that from the hillslopes sediments are mainly transported via the active side trenches to the main channel (~7000 m 3  yr −1 ). The sediment transport in the Johnsbach River consists mainly in relocating the periodically occurring sediment entries of the side trenches. The bedload transport rates at the outlet sum up to annual bedload yields of 2000 m 3  yr −1 to almost 12,000 m 3  yr −1 during the observation period. Especially those areas inside the side trenches that were heavily affected by gravel mining (excavated amount of sediment during the mining period: ~25,000 m 3  yr −1 ) are now accumulating sediment since the end of this period (~8000 m 3  yr −1 ). Future scenarios will depend heavily on the progress in the mining affected side channels. The impacts of this period are continuously being reworked and a natural sediment flow will adjust in the near future. The sediment input into the Johnsbach River will rise significantly and could lead to a doubling in the annual sediment yield at the outlet compared to now. In particular, the reaches along the Johnsbach River following the confluences with the mining affected side trenches are already showing morphological changes due to the recently imported sediments. [ABSTRACT FROM AUTHOR]

This study conducted scaled physical model tests of a run-of-river hydropower plant (RoR). The physical model represented an idealized gravel bed river with a width of 20 m, a slope of 0.005, a mean annual flow of 15 m 3 s − 1 and a 1-year flood flow of 80 m 3 s − 1 . Substrate and bed load had a characteristic grain size d m of 56 mm. The model scale was 1:20. The study was designed to test the effects of weir height and reservoir widening on sediment continuity and flushing efficiency at low-head RoR plants. Three different weir heights and two reservoir widths were investigated to assess and quantify their influence. As long as equilibrium conditions had not been established in the reservoir the flushing efficiency was low and thus the sediment continuity was not met. The experiments revealed that both parameters had a significant effect regarding the time needed to reach equilibrium conditions. The equilibrium slope that established was independent of weir height. Weir height, however, determined the starting point from where the equilibrium slope was built up. Accordingly, an increase in weir height will increase the reservoir bed levels by the same amount. A large weir height leads to huge sedimentation volumes and thus to a lack of sediments and to river bed erosion downstream of the hydropower plant. It could also considerably increase the flood risk at the head of the reservoir. This calls for regular maintenance work to re-establish flood safety. The experimental results suggest new concepts of low weir heights and cross sectional reservoir widths in the design of low-head RoR plants to facilitate frequent and efficient flushings, to enhance sediment continuity, and to reduce maintenance and operational costs. [ABSTRACT FROM AUTHOR]

A Large Eddy Simulation of a fully developed turbulent open-channel flow with a Reynolds number of R e = 71,755 was performed to study dynamics of coherent structures in the vicinity of a stationary sediment particle. The sediment particle was positioned at the bottom on a smooth channel bed. The impulse concept was employed by computing three-dimensional hydrodynamic forces acting on the sediment particle in order to predict the onset of particle motion. Results of quadrant analyses at high impulse events yielded mostly outward and sweep events. Auto- and cross-correlation of the hydrodynamic forces and the flow field were studied and discussed. An identification of coherent structures at high impulse events showed that large-scale, long-lived hairpin-like vortices whose bed-normal lengths are larger than ≈ 100 viscous units were resulting in an increase of lift and drag forces simultaneously, facilitating initiation of particle motion. [ABSTRACT FROM AUTHOR]

The dynamic behavior of a subaqueous cylindrical pendulum and corresponding flow dynamics are investigated. The objectives were twofold: (i) to examine whether the two-dimensional model equations sufficiently capture the three-dimensional dynamics and (ii) to investigate the emerging three-dimensional vortical flow structures. Large eddy simulations with two-way coupling fluid structure interaction were carried out using the immersed boundary method to simulate the motion of the pendulum and its interactions with the initially stagnant water. The resulting pendulum motion is compared against measured data obtained in a series of experimental tests to validate the simulation results and the model equations with and without wake corrections. An analysis of the flow vorticity revealed the development of a vortex ring during the first swing and the formation of tip vortices. The evolution of the vortex rings emerging from the motion of the subaqueous cylindrical pendulum was visualized using Q-criteria showing a reasonable agreement with vortical structures observed in the experiment using particle imaging velocimetry. The hydrodynamic moments acting on the simulated pendulum and the moments calculated from the model equations are analyzed. Using the insights from these numerical simulations, a modification of the wake correction is proposed to enhance the accuracy of the rate of decay and period. The transient effect of coherent flow on pendulum dynamics, especially the added mass effect, is discussed. [ABSTRACT FROM AUTHOR]

Sediment plume development was modelled using an Euler–Lagrangian two-way coupled large eddy simulation. Momentum exchange was calculated based on interaction forces in the Maxey–Riley equation. Validation showed good agreement with experimental data from literature. For analysis of interaction between sediment and fluid 6,859 particles were released into turbulent fluid flow. In order to ensure independence of the results from arbitrary turbulent velocity fluctuations in the initial fluid velocity field, 41 simulations of the test case starting from different initial flow fields were analysed. Results of these simulations are normally distributed. Mean values indicate three phases of the development of the sediment plume: (i) acceleration phase, (ii) transport phase and (iii) deposition phase. Significant slow down of fluid flow and particle sorting turned out to be relevant processes in the initial development of the sediment plume which are not accounted for in one-way coupled models. • Transient two-way coupled large-eddy simulations of a sediment plume. • Results of 41 simulations indicate three phases of the development of the sediment plume. • Two-way coupling particularly important in the initial sediment acceleration phase. • Significant slowdown of fluid flow primarily in the sediment acceleration phase. • Development of the sediment plume is influenced by particle sorting. [ABSTRACT FROM AUTHOR]

River sediment budgets are prepared globally, under different conditions and requirements. This paper proposes a general methodology for sediment budgeting – the River Sediment Budget Approach (RSBA), to demonstrate the challenges and opportunities involved in establishing a sediment budget. The methodological approach focuses on the bedload, suspended sediment load as well as changes in the river bed and morphology. Four approaches are provided for sediment budgeting, which can be used individually or in combination and are selected based on the time scale and data availability. If measurement data is available or can still be undertaken, a (i) field-based approach is selected. Specifically, the morphological changes or data required for formulas can be gathered using a (ii) GIS-based approach. When calculations are carried out without measured data, a (iii) formula-based approach is pursued. Due to the numerous budgets already conducted and sediment data already measured, an approach dependent on (iv) literature data and estimations can also be considered. Due to the complexity of sediment dynamics in rivers, strict rules are not beneficial in sediment budgeting and, to avoid misinterpretation, it is essential to validate the results. • A budget approach (RSBA) with a hierarchical decision matrix. • Sediment budgets as a methodology to budget suspended sediment and bedload at different temporal and spatial scales. • Opportunities for sediment budgeting with weak datasets. [ABSTRACT FROM AUTHOR]

This study investigated the sediment resupply and transport dynamics at the Upper Drau River upstream of Lienz (Eastern Tyrol, Austria). Due to a hydropower plant, a 24 km long river reach of this alpine gravel bed river is under residual flow conditions, although sediment is still resupplied into the reach through many active torrents and tributaries. As a result, sediment deposition in the residual flow reach intensified, hence increasing maintenance efforts to stabilize this river section and ensure flood protection. In combination with a new sediment management program, a continuous bedload monitoring system was installed 2 km downstream of the residual reach in 2001 to support the development of adapted sediment management strategies. The surrogate bedload monitoring system consists of 16 impact plate geophones, installed over a 17 m wide cross section. The unprecedented 15-year dataset of high-resolution bedload intensity revealed a complex process of gravel storage and intermittent resupply from the residual reach, allowing the authors a detailed analysis of frequently occurring bedload pulses. These transport features are triggered by increased discharges during floods in the residual reach and created pronounced anticlockwise bedload hysteresis or, with a temporal shift to the event peak, caused distinct shifts in the bedload activity downstream. Bedload pulses produce very high bedload fluxes while in transit, tend to increase bedload flux in the post-event phase, and can alter and reduce the upstream sediment storage leading to a lowering of bedload availability for future pulses. The observed time lags between main discharge events and the arrival of the macro-pulses are correlated with mean water discharge during pulse propagation, thus enabling a prediction of the pulse arrival at the monitoring station solely based on the hydrograph. In combination with the hydrological setup of the reach, the observed bedload pulse time lags allowed an estimation of pulse velocities in the range 0.002 − 0.05 m s − 1 . [ABSTRACT FROM AUTHOR]

Knowledge about the magnitude of bedload fluxes at given hydraulic conditions in natural streams is essential for improved process understanding, for the application, calibration and validation of bedload transport formulas, and for numerical sediment transport models. Nonetheless, extensive field measurements of bedload transport are challenging and therefore data from such efforts are rare. Bedload transport has been measured comprehensively at the downstream section of the Urslau torrent in Salzburg, Austria, since 2011. We used an integrative monitoring system that combines direct (mobile basket sampler, slot sampler) and indirect measuring devices (geophone plates). Continuous information about the intensity and distribution of bedload transport within the channel cross-section is available in high spatial and temporal resolution. Seven geophone plates at a stream width of 8 m are part of a measurement system that delivers data in 1-min intervals. These geophone data are calibrated using results of direct bedload measurements, providing an opportunity to calculate bedload rates and bedload yields in selected time periods. Continuous data on the bedload transport process over three years enabled assessing several bedload transport events. The investigation of bedload transport rate/discharge relationships reveals order-of-magnitude changes. For individual events, we observed shifts in the data, reflecting different bedload rates at comparable hydraulic conditions. This study reveals that variable sediment supply conditions affect the prevailing bedload transport rates at the Urslau stream. Calculating the bedload transport efficiency enables comparing bedload transport events that exhibit similar process characteristics. Finally, we provide a conceptual model of bedload transport process types as a function of bedload transport efficiency and dimensionless stream power. [ABSTRACT FROM AUTHOR]