Cristina Rachelly

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Rachelly

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Cristina

ORCID

0000-0003-3147-3807

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Publications 1 - 10 of 15

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.
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
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
Sediment Supply Control on River Widening Morphodynamics and Refugia Availability
Rachelly, Cristina (2021)
How does sediment supply influence refugia availability in river widenings?
Rachelly, Cristina; Mathers, Kate L.; Weber, Christine; et al. (2021)
Journal of Ecohydraulics
Habitats that mitigate the effects of a disturbance event (e.g. flood) are referred to as refugia. Their occurrence in heavily impacted river systems is often limited, and their restoration rarely pursued. This paper presents the results of a combined laboratory and numerical modeling study to assess flood refugia availability to mobile aquatic organisms in the context of river restoration and dynamic river widening. We used a calibrated 2D hydrodynamic model based on eight topographies obtained in laboratory experiments to assess refugia availability by analyzing the hydro-morphological conditions under varying sediment supply. Overall, sediment equilibrium sustains more complex hydro-morphological conditions with low bed shear stress zones being maintained during elevated discharges. Furthermore, our results suggest that the floodplain is an important potential refuge that becomes accessible for discharges with a return period of approximately one year. Conversely, sediment deficit results in a homogeneous flow field with steadily increasing hydraulic forces for high flows and impaired lateral connectivity except for very large flood events of a 30- to 100-year return period. Dynamic river widening implemented in a channel with sediment equilibrium conditions as opposed to a sediment deficit is thus more likely to provide flood refugia.
Bed-Load Diversion with a Vortex Tube System
Rachelly, Cristina; Albayrak, Ismail; Boes, Robert; et al. (2019)
Proceedings of the IAHR World Congress
Impaired sediment connectivity in river systems due to transversal hydraulic structures is a primary cause of channel narrowing and down-cutting, as well as reduced morphodynamic variability. Among various restoration measures, the vortex tube is a structural measure to re-establish bed-load conveyance during flood events at low-head hydraulic structures. Bed-load particles are entrained through a slit in the tube soffit and sluiced from an upper channel to a lower parallel channel. This paper presents and compares the results of two laboratory model studies on the optimal design of vortex tubes for run-of-river hydropower plants. Although the two case study hydropower plants are situated within 1.5 km distance along the same river, differences in boundary conditions considerably reduce the suitability of one of the plants for the effective operation of a vortex tube. The test results show that the energy head difference between the two channels is a decisive hydraulic boundary condition and should be larger than approx. 1 m at all times. The dimensions of the vortex tube should be adapted to the grain sizes of the transported bed-load. If the vortex tube does not span the entire channel width, an open tube end is recommended to increase its zone of influence. A diversion efficiency of 94% was determined for the optimal design. The present findings demonstrate that vortex tubes may be an appropriate structural measure to re-establish sediment continuity at hydraulic structures, primarily where the upstream and downstream channels are located in close proximity.
Dynamic river widening under variable bed-load supply
Rachelly, Cristina; Vetsch, David F.; Boes, Robert; et al. (2024)
River Flow 2022
Reach-scale widening of channelized rivers is a common river restoration measure. The goal is to reactivate morphodynamic processes and increase habitat heterogeneity locally. Yet, the influence of bed-load supply on widening morphodynamics is still inadequately understood. Using mobile-bed laboratory experiments, we studied the morphological development of one-sided dynamic river widenings in channelized subalpine gravel-bed rivers in response to different sediment supply levels. The initial experimental setup consisted of a channel with a longitudinal slope of around one percent and a floodplain on one side, which was twenty-five channel widths long and three widths wide. After removing the fixed bank separating the channel from the floodplain, lateral floodplain erosion and channel widening were allowed. Sediment supply was found to be a critical driver of morphodynamic activity in river widenings, with sediment supply rates close to the channel's transport capacity promoting active channel widening, sediment relocation, and lateral channel-floodplain connectivity. In contrast, low sediment supply maintained a stable single-thread channel disconnected from its floodplain.
Typical layouts of river widening projects in Switzerland
Demuth, Paul; Rachelly, Cristina; Vetsch, David F.; et al. (2023)
Proceedings of the 40th IAHR World Congress
Due to anthropogenic activities, many rivers in Switzerland and worldwide are highly degraded. Hence, they are limited in width, channelized, straightened, and the sediment budget is often strongly imbalanced due to sediment mining and damming, leading to sediment retention. Such rivers are characterized by decreased morphological heterogeneity and dynamics and substantial losses in riparian biodiversity. Based on regulations such as the EU Water Framework Directive 2000, many countries nowadays pursue river restoration strategies to mitigate these negative effects. Swiss river restoration efforts include re-establishing sediment continuity at hydropower plants and reach-scale restoration measures along 4'000 km of streams and rivers, among other targets. River widenings, varying in length from a few hundred meters up to several kilometers, are an oftenimplemented restoration measure. In this study, we investigated the layout of 105 existing and planned river widenings in Switzerland. The first widenings realized in Switzerland were fully excavated to immediately establish the design geometry. More recently, dynamic river widening has become popular, where bank protection is removed, and bank erosion can progress during morphologically relevant discharges. Most river widenings in Switzerland are one-sided, i. e., on one orographic bank only. 20% of the projects considered in this study involved widening on both sides of the river. The present paper gives an overview of the restoration progress by river widening in Switzerland and referring to a data set of the above-mentioned 105 widening projects including their most important characteristics.
The effects of sediment traps on instream habitat and macroinvertebrates of mountain streams
Mathers, Kate L.; Kowarik, Carmen; Rachelly, Cristina; et al. (2021)
Journal of Environmental Management
Sediment transport in mountain streams can be a major hazard to downstream infrastructure. Consequently, sediment traps are a common feature in many high gradient streams to retain large volumes of sediment and protect settlements from major sediment transport events. Despite the wide application of these instream structures, there is little knowledge regarding the environmental and ecological effects on streams. Here, we investigated the hydromorphological effects of sediment traps on instream habitats and associated macroinvertebrate communities in four impacted and three non-impacted streams in Switzerland. Sediment traps significantly disrupted the sediment regime homogenising grain size percentiles compared to the natural stepwise downstream fining in control streams. This disruption in the sediment regime resulted in finer grain size distributions upstream of the sediment trap, and reduced substrate diversity in the sediment retention basin and just downstream of the trap. The reductions in substrate diversity resulted in an altered macroinvertebrate community composition. Further, the disconnection in sediment transport led to a lack of longitudinal correlation in macroinvertebrate communities. Refugia provision downstream of the sediment trap, and resource availability within the retention basin, were diminished, potentially reducing resilience of macroinvertebrate assemblages to instream disturbances. The effects of sediment traps were most likely localised in three of the four streams with substrate diversity recovering to comparable control values within 8 wetted widths (ca. 50 m) downstream of the trap associated with natural longitudinal fining. In contrast, ecological and environmental effects propagated downstream in one impacted stream with no recovery being evident. Sediment retention basins in the impacted streams provided a local artificially unique habitat of dynamic-braided channels. Our results indicate that sediment traps can significantly disrupt the sediment regime with important consequences for instream ecology and environmental conditions, although these effects can be system specific. Further work is needed to fully understand the effects of sediment traps in mountain streams to assist resource managers in the mitigation and future construction of these structures.

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Cristina Rachelly

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