River engineering structures, such as bank protection or bed sills, act as constraints on the hydromorphology of rivers and limit morphodynamic processes. Accordingly, the deviations of a river's ...morphology from a natural reference condition have been attributed to the degree of artificiality in the observed river section and river restoration works mainly aimed at reducing artificial constraints within the river reach. Less attention has been drawn to alterations of the sediment continuum between sediment production in the river's catchment and downstream river reaches. However, the sediment supply from upstream is strongly reflected in the morphodynamics such as bar formation or the reworking of the riverbed. Any alteration of sediment supply may affect the morphological appearance of a reach and determine its deviation from an undisturbed condition.
We introduce the Hydromorphological Evaluation Tool (HYMET), which accounts in a hierarchical procedure for sediment supply and sediment transfer as catchment and river network based preconditions for sustainable morphodynamics in river reaches. At the reach scale, artificiality and the sediment budget are assessed. In contrast to existing evaluation methods for assessing hydromorphological state, no reference condition is needed for determining hydro-morphological alterations. Here, with re-established sediment supply and reduced artificiality, a river reach is expected to develop morphodynamics that approach a morphodynamically and ecologically sustainable condition.
Application to the Drau River showed that the alteration of sediment supply strongly affects the hydromorphological condition and thus the evaluation result of a restored reach, indicating the remaining potential for the re-initiation/re-establishment of morphodynamics through catchment-wide restoration plans.
•Evaluation method managing without the definition of a reference condition.•Hierarchical procedure ensures consideration of sediment connectivity and transfer.•Budgeting of sediment transfer links reach's trajectory to upstream alterations.•HYMET indicates remaining potential for morphodynamics in already restored reaches.
Plastic waste as a persistent contaminant of our environment is a matter of increasing concern due to the largely unknown long-term effects on biota. Although freshwater systems are known to be the ...transport paths of plastic debris to the ocean, most research has been focused on marine environments. In recent years, freshwater studies have advanced rapidly, but they rarely address the spatial distribution of plastic debris in the water column. A methodology for measuring microplastic transport at various depths that is applicable to medium and large rivers is needed. We present a new methodology offering the possibility of measuring microplastic transport at different depths of verticals that are distributed within a profile. The net-based device is robust and can be applied at high flow velocities and discharges. Nets with different sizes (41 µm, 250 µm, and 500 µm) are exposed in three different depths of the water column. The methodology was tested in the Austrian Danube River, showing a high heterogeneity of microplastic concentrations within one cross section. Due to turbulent mixing, the different densities of the polymers, aggregation, and the growth of biofilms, plastic transport cannot be limited to the surface layer of a river, and must be examined within the whole water column as for suspended sediments. These results imply that multipoint measurements are required for obtaining the spatial distribution of plastic concentration and are therefore a prerequisite for calculating the passing transport. The analysis of filtration efficiency and side-by-side measurements with different mesh sizes showed that 500 µm nets led to optimal results.
River floodplains allow for higher levels of conveyance and a greater capacity for floodwater, often improving the flood situation locally and regionally in hydrological and hydraulic terms. The ...European Floods Directive aims to protect existing and to restore potential floodplains to make use of their beneficial functions within an integrated plan of flood risk management. However, river floodplains are confronted with many other demands such as development of settlements, industry, and commerce and its supporting infrastructure. With such developments flood protection measures such as dykes and walls often follow restricting the floodplain's natural boundaries. To limit the loss of floodplains it is necessary to evaluate the importance of floodplains for flood risk management and to develop ultimately support tools for legal and spatial planning. This paper presents a method for evaluating and ranking river floodplains according to hydrological and hydraulic parameters and demonstrates its application at two very different Austrian river reaches. The results show which parameters and associated thresholds prove to be of greatest applicability to support future spatial planning strategies.
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,
2013
), 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).
Floodplains play a central role in flood risk management since they function as retention areas which attenuate and decelerate flood waves. However, during the last decades land use has changed ...distinctly on floodplains which has led to a change in topography due to the construction of levees and dykes. Using geographic information system analysis we assessed floodplain developments over 60 years for five Austrian rivers. We used these findings as input for hydrodynamic‐numerical modelling. A comparison of computations of current and historic floodplain topographies demonstrated the complex impacts that changes on floodplains have on catchment level flood risk. Results showed that the losses of floodplains were in general linked to a deterioration in hydrological (flood peaks and travel times) and hydraulic (water level) parameters. In rare cases the unintentional overtopping of dykes resulted in an improved reduction of the peak of the flood wave, but included a worsening of local hydraulic conditions. Hence, this study demonstrates that general conclusions about an alteration of flood risk cannot be easily reached, with a demand for further site‐specific assessment. This novel way of investigating the trends of flooding characteristics by including the historic development within a catchment offers valuable information to planners for a future flood risk management.
Bedload monitoring techniques have been developed and applied for many years in rivers ranging from steep mountain torrents to the large gravel-bed Danube River in Austria. Most monitoring stations ...use a combination of direct (mobile bag samplers, slot samplers) and indirect (geophones, hydrophones) measurement methods. Each individual technique is adequate, yet features particular boundary conditions and limitations related to hydraulic and sampling efficiency, functionality during floods, sampling duration or grain size. We show the capabilities and limitations of the different monitoring devices with respect to technical, operational and economic criteria, evaluating their suitability for determining bedload transport parameters. Bedload monitoring results of a measuring site at the Drau River in Carinthia/Austria are used to illustrate the specific range of the device application. We present an integrated automatic and continuous bedload monitoring system. It complements the specific limitations of single monitoring methods by additional measurement devices, enabling comprehensive monitoring of the bedload transport process. We then derive the Bedload Discharge Integrated Calculation Approach and the Bedload Rating Curve Approach and discuss their application for determining bedload discharge Qb and total bedload mass Vb. Whereas the integrated approach combines data from direct monitoring methods with indirect techniques, the rating curve approach uses only data from direct bedload monitoring devices. We demonstrate that applying an integrated automatic and continuous bedload monitoring system and combining the Bedload Discharge Integrated Calculation Approach and Bedload Rating Curve Approach yields accurate bedload discharge results.
Periodic flushing operations during moderate flood events (≤annual flood flow HQ1) are an approach to counteract problems caused by disturbed sediment continuity in rivers, which is possibly an ...effect of run-of-river hydropower plants (RoR-HPPs). Considering ecology, flood risk, technical, and economical reasons, discharge values of 0.7 × HQ1 are a good reference point for the initiation of gate operations. This work aimed to investigate the role of different gate opening actions on the effectiveness of such flushing measures. Physical model tests were performed, to capture bed load rates, together with 2D velocity measurements in the vicinity of two movable radial gates above a fixed weir. The length scale of the idealized model arrangement was 1:20, and a conveyor-belt sediment feeder was used to supply a heterogeneous sediment mixture. Velocities were acquired using 2D laser doppler velocimetry (LDV). Based on the LDV measurements, mean velocity profiles and Reynolds stresses were derived. The full opening of both radial gates led to the highest bed load mobility. While the flushing efficiency drastically decreased, even for slightly submerged gates, an asymmetrical gate opening initially led to the formation of a flushing cone in the vicinity of the weir, accompanied by temporarily high flushing efficiency. In conclusion, our results stress the importance of full drawdowns in successfully routing incoming bed load downstream of the HPP. However, the combination of an asymmetric gate opening followed by a full drawdown could be a promising approach to further improve the flushing efficiency of RoR-HPPs.
Racetrack flumes are established experimental settings in ecohydraulics and sediment studies. Their experimental results are often coupled with numerical simulations. The two-equation turbulence ...closures of the Reynolds-Averaged Navier-Stokes equations are applied widely in such engineering applications. They are preferred for their ease of use and low computational costs compared with more sophisticated turbulence models involving large-scale fluvial simulations. Here, three variants of two-equation models, i.e. the standard k-ϵ model and two variants of k-ω models, were applied to tackle mean flow and turbulence in a racetrack flume and a river reach. Regarding model performances, we found an overall reasonable agreement between simulated and measured mean velocity values. Nonetheless, the simulated turbulent kinetic energy exhibited discrepancies to the measured values. The computational costs of all investigated model variants are comparable. Therefore, there is no significant preference among models. The findings confirm the application of these model approaches for mean flow-related investigations (e.g. habitat modeling) but suggest exercising caution for applications sensitive to turbulent processes.
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