Volker Weitbrecht

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Weitbrecht

First Name

Volker

ORCID

0000-0003-3008-5167

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03820 - Boes, Robert / Boes, Robert

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Publications 1 - 10 of 31

Morphological response of channelized, sinuous gravel‐bed rivers to sediment replenishment
Rachelly, Cristina; Friedl, Fabian; Boes, Robert; et al. (2021)
Water Resources Research
Anthropogenic alterations of sediment supply and transport processes may impact the ecological state of riverscapes and threaten infrastructure along the river. Sediment replenishment is one restoration method that is employed in channels impacted by sediment deficit. We performed flume experiments to investigate the channel bed response of a channelized, sinuous gravel-bed river to periodic and episodic sediment replenishment. The grain size distribution of the replenished material, flow discharge, and sediment supply level were varied in long-term steady-state experiments. In addition, the channel routing of a single sediment pulse was investigated. The long-term channel response included intensified sediment relocation and transversal bed leveling. Sediment supply level and flow discharge thereby exerted the strongest control over channel response, whereas the influence of the grain size distribution of the replenished material was minor. A simple habitat analysis using grayling as example species revealed that replenished sediment retained within the channel and thus providing episodically renewed clean gravel patches may increase spawning habitat availability. However, the general shortage of shallow habitats for grayling fry and juveniles in channelized rivers persisted regardless of sediment replenishment. Overall, the experiments illustrate that sediment replenishment may provide valuable habitats within a channelized river. Accompanying measures such as channel widening and the careful consideration of other remaining stressors are strongly recommended to increase restoration benefits further.
WoodFlow project - large wood management in rivers
Steeb, Nicolas; Badoux, Alexandre; Boes, Robert; et al. (2021)
14th congress INTERPRAEVENT 2021. Natural Hazards in a Changing World
This contribution summarises the most important practical findings from the WoodFlow research project. The main goal was to improve understanding of the processes governing large wood (LW) dynamics in watercourses and to provide practitioners with suitable tools to help assess LW-related hazards. The results provide a basis for estimating potential LW quantities, modelling wood transport during floods and describing the associated hazards due to wood accumulations. The resulting recommendations can be used by specialists as a basis for silvicultural and river-engineering measures.
Mesures morphologiques ponctuelles dans le cadre de l'assainissement des éclusées: quels bénéfices pour le macrozoobenthos?
Firese, Nathalie; Weber, Christine; Rachelly, Cristina; et al. (2022)
Wasser Energie Luft
Geschiebedurchgängigkeit an Wasserkraftanlagen
Boes, Robert; Albayrak, Ismail; Friedl, Fabian; et al. (2017)
Aqua Viva
Quantification of large wood (LW) impact forces at field-scale using SmartWood
Spreitzer, Gabriel; Schalko, Isabella; Boes, Robert; et al. (2022)
Proceedings of the 39th IAHR World Congress
While wood in rivers constitutes an essential element for the regulation of stream power and habitat creation, large wood (LW) carried during floods poses a high risk for interactions with in-stream structures such as bridges, dams or weirs. Although significant damage or total failure of impacted structures is frequently reported, there are no field-scale data of LW impacts available to date. Thus, acceleration data from innovative inertial measurement units (IMUs), which are deployed in the course of the SmartWood_3D research project at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of ETH Zurich, were used to measure field-scale impact forces of transported LW during floods. The field experiments considered the release of up to four sensor-tagged prototype logs – SmartWood – at a time into flooded channels (approximately HQ₁) in Switzerland. Each SmartWood-log is fully debranched and measures 4.40 m in length at a mean diameter of 0.33 m. During the preparation of SmartWood in the laboratory, wood density decreased from 680 kg/m³ for the freshly cut and wet logs directly from the forest to 450 kg/m³ for the completed SmartWood logs at dry condition. The wetted density of SmartWood during the field experiments was roughly 500 kg/m³, yielding an impacting mass of roughly 188 kg. After SmartWood had been released into the channel, the logs were instantly mobilised by the high flow. On their journey downwards, complex LW dynamics were observed and successfully measured with high temporal resolution for the first time to the authors’ knowledge. Of particular interest were interactions of SmartWood with channel boundaries and in-stream obstacles (e.g., boulders). Deceleration of impacting logs were found to be significant, reaching the maximum measurable acceleration range (± 16 g) of the applied smart-sensors. The gained results contribute to a better understanding of LW dynamics in rivers and will help engineers to assess the vulnerability of existing structures as well as to improve the design of future flood-resilient structures in fluvial environments.
Macro-Roughness Elements for River Restoration: Flow Structures
Speltoni, Simone; Schalko, Isabella; Weitbrecht, Volker; et al. (2023)
Proceedings of the 40th IAHR World Congress
River restoration projects aim at reintroducing habitats and ecological functions that might have been lost due to anthropogenic forcings, such as flood protection measures or hydropower plants. The effort to restore the ecological value of a river can imply different concepts, considering target species, ecological habitats, as well as available space. In cases where space is limited, macro-roughness elements can be built to create morphological and flow heterogeneity to provide fluvial habitats such as pools, riffles, and runs. Their placement in the river reach, as well as their shape and geometry affect local flow and morphological structures. Besides improved ecological conditions these measures may as well have an impact on flood protection issues. In this paper, we present a brief literature review about previous laboratory and field experiments on the physical behavior of wood placements as macro-roughness elements with respect to flow dynamics. To complement previous studies, field experiments will be conducted on the effect of engineered logjams, rootwads, and boulder sills on the local flow conditions. The methods and research questions of the planned experiments using largescale surface particle image velocimetry are herein summarized.
Einfluss des Geschiebeeintrags auf die morphologische Entwicklung in Flussaufweitungen bei 1 Prozent Sohlneigung
Rachelly, Cristina; Demuth, Paul; Vetsch, David F.; et al. (2024)
Wasser Energie Luft
Effect of discharge variations and sediment supply on the stability of artificial step-pool sequences
Maager, Fiona; Hohermuth, Benjamin; Weitbrecht, Volker; et al. (2022)
Proceedings of the 39th IAHR World Congress
Built step-pool systems are promising to stabilize the bed in steep streams. Flume experiments have already been conducted to assess scour development and stability of such step-pool sequences. However, mainly stationary conditions were tested so far, which hardly represent the processes in steep streams. In this study, three distinct flow loading conditions (LC) were applied to the same step-pool sequence to determine potential differences in scour development and step stability. The 1:20 scaled physical experiments were conducted in a flume with a bed slope of 8%. The base material was selected according to typical Swiss mountain streams with dm = 0.005 m and d₉₀ = 0.0126 m (model scale). Six equally spaced steps were artificially placed into the channel bed. The steps consisted of two rows of blocks with a weight between 1.1 and 1.4 kg and a height of ~0.065 m each. First, a stationary hydraulic load was applied to the step-pool sequence for 80 min (LC A). The unit discharge was stepwise increased by Δq = 0.014 m²/s until the steps collapsed. Second, a series of hydrographs was applied to represent the processes occurring in steep streams more realistically (LC B). Similarly to LC A, the peak discharge of each hydrograph was increased by Δq = 0.014 m²/s until the steps collapsed. The total duration of the hydrographs was set to 54 min, the peak arrived after 13.5 min and lasted for 4.5 min. Due to the shorter peak duration and the descending limb of the hydrographs in LC B, differences in scour depth and stability were expected. LC C was identical to LC B, but a small stationary discharge was applied in between each hydrograph to simulate low flow periods occurring between major flood events. An increase in scour depth was expected in LC C, because the jet of low flows impinges more vertically on the channel bed. The step-pool system sustained a discharge of q = 0.110 m²/s for LC A and C, and q = 0.124 m²/s for LC B. Considering the uncertainties, the step-pool system failed at similar hydraulic loads independently of the LC applied. Major changes in channel bed appeared to occur within 5 to 10 min after the peak discharge was reached. Furthermore, all LC led to a similar scour depth. Even for periods of low flow in LC C, scour depths did not further increase at the toe of the steps. Consequently, applying either one of the LC appears to be reasonable to test step stability. However, all three LC were conducted under clear water conditions and the effect of sediment feed on the stability was neglected. Scour development of step-pool systems with sediment feed will be investigated in future experiments.
SmartWood: field-based analysis of large wood movement dynamics using inertial measurement units (IMUs)
Spreitzer, Gabriel; Schalko, Isabella; Boes, Robert; et al. (2024)
Environmental Sciences Europe
Wood plays an important ecological role in rivers. Yet challenges arise when large wood (LW) is mobilised and transported during floods. Due to a lack of quantitative data, movement behaviour of LW during floods is still not well understood to date. A proof-of-concept study was conducted at three Swiss rivers to test state-of-the-art sensor-tagged logs, so-called “SmartWood” and collect quantitative field-scale data about LW movement behaviour. The experiments utilised innovative inertial measurement units (IMUs), which have been developed at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at ETH Zurich and implanted into wood logs (SmartWood) at prototype scale. Each IMU comprised three individual sensors (gyroscope, accelerometer, and magnetometer) and was equipped with an on-board processor, an AA battery (4.35 V), a memory (8 MB), and a Wi-Fi transmitter (100 m) for data transfer. After successful initial verification tests of the sensors, the IMUs were installed into debranched wood logs, measuring 4.35 m in length and 0.33 m in diameter. At the time of the field experiments, each SmartWood-log weighted between 170 and 220 kg, yielding a density of roughly 500 kg∙m⁻³. At the Limmat, Thur, and Grosse Melchaa Rivers in Switzerland, innovative yet discontinuous data were obtained. Results revealed consistent movement dynamics across all field sites. Specifically, we observed positive yaw movement during transport of SmartWood along the left river bank and negative yaw movement along the right river bank. Furthermore, interactions of SmartWood with channel boundaries, riparian vegetation, and objects (e.g., ferry dock) were registered and quantified, even when the SmartWood-log was transported out of sight of traditional sensing methods. The conducted field experiments enabled the initial testing of SmartWood in the field and exposed critical limitations of the IMUs and software algorithms for the reconstruction and analysis of floating LW dynamics. The gained knowledge and introduced sensing method will benefit the quantitative assessment of LW dynamics in rivers to maintain safety and functionality for instream structures (e.g., considering LW movement dynamics for the robust design of LW retention and guiding structures), but also river restoration projects and numerical models that rely on quantitative field-scale data.
Wood Retention at Inclined Bar Screens: Effect of Wood Characteristics on Backwater Rise and Bedload Transport
Schalko, Isabella; Ruiz-Villanueva, Virginia; Maager, Fiona; et al. (2021)
Water
In forested mountain catchment areas, both bedload and large wood (LW) can be transported during ordinary flows. Retention structures such as sediment traps or racks are built to mitigate potential hazards downstream. Up to now, the design of these retention structures focuses on either LW or bedload. In addition, the majority of LW retention racks tend to retain both LW and bedload, while bedload transport continuity during ordinary flows is an important aspect to be considered in the design. Therefore, a series of flume experiments was conducted to study the effect of LW accumulations at an inclined bar screen with a bottom clearance on backwater rise and bedload transport. The main focus was put on testing different LW characteristics such as LW size, density, fine material, and shape (branches and rootwads), as well as a sequenced flood. The results demonstrated that a few logs (wood volume of ≈ 7 m3 prototype scale with a model scale factor of 30) are sufficient to reduce the bedload transport capacity to below 75% compared to the condition without LW. Fine material and smaller wood sizes further reduced bedload transport and increased backwater rise. In contrast, LW density and LW shape had a negligible effect. The test focusing on a sequenced flood highlighted the need for maintenance measures to avoid self-flushing of the bed material. The results of this study further indicate that an inclined bar screen may need to be adapted by considering LW characteristics in the design of the bottom clearance to enable bedload continuity during ordinary flows.

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Volker Weitbrecht

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