GM5.2

EDI
Geomorphic, ecological, hydrological & wood-related river restoration and management in a changing climate

The United Nations has designated the 2020s as the decade of ecosystem restoration; and restoration of streams, rivers and their catchments is particularly important to restore ecosystems and halt biodiversity loss, in addition to achieving several sustainable development goals. Within Europe, river restoration is used to meet the EU Water Framework Directive objectives, and EU LIFE projects provide millions of euro per year to physical restoration. Furthermore, restoration of rivers and their catchments will prove both more important in the coming decades in order to mitigate and adapt to the effects of climate change and more challenging when restoring a moving target with altered flow, sediment, and ice regimes and habitat conditions. Restoration and management of rivers and their catchments will require a holistic view of multiple facets of river systems and will need to be process-based, including geomorphic, hydrological and ecological processes, incorporating an understanding of how these evolve and interact following restoration interventions. In addition, large wood (LW) is a key component of fluvial ecosystems and affects both flow and sediment transport processes. LW jams (i.e., logjams) can be used as a tool for river restoration increasing flow and bed heterogeneity. However, the transportation of LW may significantly increase during floods and LW jams can form at river infrastructure, creating an additional flood risk, which needs to be accounted for in management strategies of rivers. An interdisciplinary effort is required to improve our understanding of the complex interactions of wood with flow and sediment in fluvial ecosystems.

In this session we wish to highlight a broad range of research on methods, success/failure, and follow-up of river and catchment restoration and management. We are particularly interested in studies related to restoration with a changing baseline of climate conditions as well as aspects associated with LW; however, there are also many basic questions on how to manage and restore rivers that also need to be addressed, including time-to-recovery, resilience, relationships between different river facets, the impact of different spatial scales of restoration, etc. We hope this session will spark discussion among an interdisciplinary group of researchers of how to take into account a changing climatic baseline in future river restoration and evaluation of restoration success.

Co-organized by BG4/HS13
Convener: Lina Polvi Sjöberg | Co-conveners: Isabella SchalkoECSECS, Maciej LiroECSECS, Elizabeth FollettECSECS, Gemma Harvey
Presentations
| Wed, 25 May, 08:30–11:50 (CEST)
 
Room -2.32/33

Session assets

Session materials

Presentations: Wed, 25 May | Room -2.32/33

Chairpersons: Lina Polvi Sjöberg, Elizabeth Follett
08:30–08:35
08:35–08:45
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EGU22-7443
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solicited
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On-site presentation
Virginia Garófano-Gómez, Florent Arrignon, Franck Vautier, Eric Tabacchi, Elisabeth Allain, Anne Bonis, Sébastien Delmotte, Eduardo González, Frédéric Julien, Luc Lambs, Francisco Martínez-Capel, Anne-Marie Planty-Tabacchi, Erwan Roussel, Johannes Steiger, Jean-Pierre Toumazet, Irène Till-Bottraud, Olivier Voldoire, Romain Walcker, and Dov Corenblit

River ecosystems are spatiotemporally and intimately tied to physicochemical and biological processes, driven by strong feedbacks between riparian vegetation dynamics and hydrogeomorphic processes and fluvial landforms. Climatic and hydrogeomorphic constraints to vegetation determine a naturally shifting habitat mosaic dynamism, fostering high habitat heterogeneity and biodiversity, and providing multiple ecosystem services to society. However, most European river systems have lost their inherent highly dynamic character after major human-induced impacts, such as river channelisation and altered flow and sediment regimes. In March 2019, the United Nations designated the period of 2021–2030 as the "Decade on Ecosystem Restoration", and river ecosystems will be a significant target. Consequently, river restoration practitioners will need robust decision-making tools to guide their deliberations and subsequent management actions. Recommendations are to avoid merely reproducing river features and instead restoring geomorphic, hydrological, and ecological processes, but river science has not fully understood yet how processes develop and interact following restoration interventions. Integrative modelling of feedback mechanisms between riparian vegetation dynamics and hydrogeomorphic processes is critical for making predictions that enable river managers to optimise the use of the natural self-regulation potential of riparian corridors whilst maximising human benefits. Today’s existing models, however, do not fully reflect the interactions between river hydraulics and vegetation succession. In particular, the role of vegetation needs to be included through its impact in modulating river landforms and their evolutionary trajectories. Here, we present the conceptual and methodological framework, preliminary results, and the perspectives of the NUMRIP project, funded by the French National Research Agency. Along the project, a numerical (cellular automata) model of fluvial landscape dynamics will be developed, integrating physical, biological, and human components. The project focuses on riparian vegetation, from individual plants to communities. It explicitly considers vegetation as a dynamic component of the system, both responding to and affecting hydrogeomorphic processes and fluvial landforms. Accordingly, NUMRIP builds upon the conceptual fluvial biogeomorphological succession model and recent advances in remote sensing techniques of plant-geomorphology interactions. The NUMRIP project will explicitly associate plant functional traits (e.g., physiological, morphological, and biomechanical characteristics) to hydrogeomorphic processes and fluvial landforms, using plant functional trait approaches, remote sensing- and numerical modelling techniques. The lower course of the Allier River (France) is used as a case study. It is one of the last remaining free meandering river segments in Europe, and thus, constitutes an opportunity to investigate riparian succession processes of a dynamic, temperate river system. Despite its natural character, it is also experimenting an increase of stability (i.e., a reduction in channel migration and progression/retrogression of vegetation patches), because of a concomitant decrease of high and moderate magnitude floods due to current global climate change. The model could be used as a research tool in river science as well as a decision support system for river managers. It will be able to predict potential future evolutionary trajectories of fluvial corridors, adjusting for example to a changing hydrological regime or river restoration works.

How to cite: Garófano-Gómez, V., Arrignon, F., Vautier, F., Tabacchi, E., Allain, E., Bonis, A., Delmotte, S., González, E., Julien, F., Lambs, L., Martínez-Capel, F., Planty-Tabacchi, A.-M., Roussel, E., Steiger, J., Toumazet, J.-P., Till-Bottraud, I., Voldoire, O., Walcker, R., and Corenblit, D.: Trait-based numerical modelling of feedbacks between river morphodynamics and riparian vegetation for sustainable river management in a changing climate, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7443, https://doi.org/10.5194/egusphere-egu22-7443, 2022.

08:45–08:51
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EGU22-8326
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Highlight
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On-site presentation
Paul Hudson

New field data are reported for overbank sedimentation generated by the extreme flood event of summer 2021 along the Maas River, an intensively managed lowland river in the Netherlands. Flood duration was short (3-4 days) but flood magnitude was extreme, the highest stage and discharge (3,2650 m^3/s) recorded in more than 100 years.

Flood sediments were sampled at 108 sites from the NL-BE border to the delta (195-km distance) over a range of depositional environments, including artificial flood basins created for the Room for the River flood management program. Flood deposits were sampled in August and September using conventional field sampling procedures, which included identifying recent sediment deposited atop buried soil and organic layers using field texture and density, and differences in soil color (recorded). The modal Munsell soil color value for flood deposits and the darker underlying soil were brown (2.5 Y 3/2) and light olive brown (2.5 Y 5/3), respectively. Sedimentation thickness (mm) of each of the 108 reported values is an average of three individual thickness measurements obtained within a ~0.5 m radius at each field site. Minimum flood water height was measured by identifying silt and trash lines in vegetation and fencing at multiple locations and ranged from 3.5-m to 0.3-m above low and high floodplain surfaces, respectively. Particle size of 84 flood sediment samples was determined by hydrometer analysis and wet sieving.

Average flood deposit thickness was 21 mm, and varied significantly according to geomorphic setting: low floodplains (28 mm), high floodplains (6 mm), channel banks (31 mm), inset banks (11 mm), and flood basins (42 mm). Maximum sedimentation was associated with discreet sand sheets (295 mm). Floodplain stripping (erosion) at some low floodplain sites included reworking and deposition of large clasts (gravel, cobble). Pronounced lateral decreases in sedimentation thickness persists despite flood water height, and rapidly declines beyond about ~30 m from the channel bank. Lateral changes in particle size, however, are less abrupt, and along some reaches very fine sand was deposited to the distal margins of the embanked floodplain. Some laterally distant sites > ~200 m from the channel bank underwent high amounts of sedimentation (38 mm, 25 mm, 43 mm) with pronounced vertical fining (very fine sand to silt) of flood deposits associated with slackwater sedimentation within basins engineered for the Room for the River flood management program. In contrast to many prior sedimentation studies a pattern of downstream fining (along same geomorphic surface) does not exist, likely due to high stream power and reworking of older channel bed deposits.

The overall thickness of the 2021 flood deposits are considerably less than reported for large flood events in 1993 and 1995. This may be due to the shorter duration of the 2021 flood event, as well as the persistent decline in Maas River sediment loads since about the early 1950s, as well as differences in sampling strategy. Study results are further contextualized by considering corresponding event-based discharge – suspended sediment dynamics as well as sediment province.

How to cite: Hudson, P.: Sedimentation from an extreme event along an intensively managed fluvial system: Summer 2021 flooding along the Maas River, Netherlands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8326, https://doi.org/10.5194/egusphere-egu22-8326, 2022.

08:51–08:57
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EGU22-13365
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ECS
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Virtual presentation
Giuditta Trinci, Geraldene Wharton, and Nicola Bartoletti

In recent decades, the number of urban river restoration projects has grown considerably, with schemes designed to daylight rivers and reconnect them to their floodplains and deliver a range of environmental, social and economic benefits including building flood resilience in a changing climate. However, the limited pre and post-project appraisal continues to have implications for evaluating the success of projects and improving future schemes. In this presentation we share an example of a river restoration project aimed to tackle the urban river syndrome, loss of aquatic biodiversity and habitat degradation and present the results from several post-project appraisals carried out between 2013 and 2018 that examined different aspects of the river habitat. The lessons learned from combining the findings of several studies not only informs on-going management of the Wandle but the approach can help guide the appraisal of urban rivers more widely. In particular, we show the potential of Citizen Science surveys as for identifying early warning signs of deteriorating river condition and as a foundation for long term affordable monitoring of river restoration schemes.

How to cite: Trinci, G., Wharton, G., and Bartoletti, N.: Restoring urban river habitats. Lessons learned for monitoring, appraisal and management from the River Wandle, South London, UK., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13365, https://doi.org/10.5194/egusphere-egu22-13365, 2022.

08:57–09:03
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EGU22-315
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ECS
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Virtual presentation
qing deng and xinhua zhang

Maintaining riverine habitat connectivity for important ecological processes like fish reproduction is essential for conserving endangered migratory species in regulated river. The unique reproductive behavior of migratory fish, which has a potential effect on habitat connectivity assessment, is the key for the success of population restoration in a changing climate conditions. However, existing analytic connectivity models mostly focus on broad-scale terrestrial studies tested with landscape features and large-scale riverine hydrological cases, they are not able to describe aquatic micro-habitat connectivity and cannot incorporate effects of multiple pathways linking spawning function areas with altered hydrological conditions. Here, we developed an ecological functional connectivity model that overcame these obstacles by borrowing from electrical circuit theory and highlighting functional attributes of habitat patches. It was the first time for circuit theory to apply in water ecosystem environment for habitat protection and population rebuilding. In this model, a function path tree restricted to patch connectivity constraints was first proposed for micro-habitat connectivity index. The model greatly improves aquatic habitat suitability predictions because it incorporates patch function attributes to account for habitat status and simultaneously integrates all possible pathways connecting spawning function areas for a more reliable connectivity assessment. When applied to data from Chinese sturgeon (a well-known endangered anadromous fish) in the Yangtze River, our model outperformed conventional aquatic habitat models, revealing that the low functional connectivity in spawning function areas, especially between dispersal area and incubation area, was a limiting factor for Chinese sturgeon reproduction. Results also demonstrated that contributions of global warming on increasing stream temperature intensified spawning habitat fragmentation, which would further hampered fish breeding activities. The proposed model is transferrable to fish species with different life histories, and holds much promise in habitat restoration, river management and conservation planning to reduce future ecological impacts of climate change.

How to cite: deng, Q. and zhang, X.: Maintaining functional connectivity is essential for reducing negative effects of climate change on endangered species , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-315, https://doi.org/10.5194/egusphere-egu22-315, 2022.

09:03–09:09
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EGU22-3566
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ECS
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Presentation form not yet defined
Coleen Carranza, Nard Onderwater, Annegret Larsen, Jasper Candel, Victor Bense, Ton Hoitink, Jakob Wallinga, and Martine van der Ploeg

Longitudinal dams (LD) are novel engineering structures built parallel to the river channel that support sustainable river management. The recent replacement of groynes by longitudinal dams in low-land rivers such as the Waal has been successful in restoring the ecological river functions while simultaneously achieving its navigation, recreation, and flood-protection functions. However, the impact of the LD on the riverbanks is still unknown despite recent investigations on the flow dynamics in the side channel behind it. We fill this knowledge gap by investigating initial bank responses and quantifying changes in sediment dynamics over five years since the completion of the LD in the Waal at Wamel. We rely on available annual high-resolution LiDAR-derived DTMs, orthophotos, and in situ measurements to estimate erosion and deposition rates and their changes over the study period. A two-stage initial response is revealed with the largest bank erosion (~140 x 103 m3/yr) and deposition (~20 x 103 m3/yr) confined in the first year after installation, as the banks adjust to a new hydrogeomorphic equilibrium. This is followed by successively lower rates of surface-level changes (<70 x $103 m3/yr eroded and <10 x 103 m3/yr deposited) as a response to the hydrogeomorphic dynamics in the new system. The overbank deposits from recent floods have a similar distribution with those prior to LD construction based on the DTMs. However, higher volumes of sandy deposits are found post- compared to pre-LD construction for floods of similar magnitude and duration. This increase is caused by the additional contribution of the bank sediments that have been made available through the removal of groynes. Although eroding banks may be a threat to infrastructure and navigability, they have a positive effect on restoring ecological diversity and floodplain connectivity.

How to cite: Carranza, C., Onderwater, N., Larsen, A., Candel, J., Bense, V., Hoitink, T., Wallinga, J., and van der Ploeg, M.: Releasing the banks: initial morphological responses after removal of groynes and installation of a longitudinal dam, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3566, https://doi.org/10.5194/egusphere-egu22-3566, 2022.

09:09–09:15
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EGU22-13254
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On-site presentation
Sarah Höfler, Irene Pilz, and Clemens Gumpinger

Dam removals are currently experiencing a hype as a measure to restore water bodies to a more natural and thus more resilient state. Following the implementation of major projects in North America and some EU countries in particular, an inventory regarding planned and implemented projects has been carried out in Austria for the first time. A total of 53 cross barriers are known to have been removed to date. The characteristics and also problems in the definition of these projects will be presented.

The second part will deal with the challenges in the practical implementation of such measures. Case studies on the Maltsch and the Aschach show which resistances of the local population, hydraulic considerations and practical implementation risks are to be expected.

Finally, the significance of such measures will be evaluated in the overall consideration of river restoration measures and solutions in terms of climate change adaptation.

How to cite: Höfler, S., Pilz, I., and Gumpinger, C.: Dam Removal in Austria – Current status, lessons learned from implementation, and potential contribution of the measure in climate change adaptation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13254, https://doi.org/10.5194/egusphere-egu22-13254, 2022.

09:15–09:21
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EGU22-11359
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On-site presentation
Marco Redolfi, Mattia Carlin, and Marco Tubino

Understanding the possible geomorphic trajectory of rivers on the scale of decades is crucial for a successful design of river restoration interventions, especially in the contest of a changing climate. In this contribution we focus on river alternate bars, large bedforms that appear as a repeating sequence of diagonal depositional fronts and scour holes. Downstream-migrating alternate bars can spontaneously form due to a well-known process of riverbed instability and are frequently found in channelized river reaches. We considered two study reaches of the Alpine Rhine River in Switzerland, characterized by similar hydrological and sedimentological characteristics, but different channel width. Expected hydrological changes until 2100, depending on the Representative Concentration Pathways for greenhouse gases, were evaluated by considering the recents projections from the Hydro-CH2018 project. The bar evolution was reproduced through the novel mathematical model developed by Carlin et al. (2021), which allows for simulating the temporal variability of the reach-averaged bar height in the long-term. Model’s results clearly show that the expected response of the river bed strongly depends on channel conditions with respect to the relevant morphodynamics threshold for bar formation. The first reach, which is sufficiently wide to allow for a full development of migrating alternate bars, turns out to be weakly sensitive to the projected hydrological alterations. Conversely the second, narrower reach, which is currently close to the threshold conditions, is expected to experience a remarkable alteration in bar dynamics. Specifically, the average bar height is expected to significantly increase, while its variability during flood events will probably drastically reduce. Ultimately, this work reveals a noteworthy example of a more general property of near-threshold geomorphic systems, which are potentially fragile and highly susceptible to changes of their hydrological and ecological conditions, in contrast to systems that being far from threshold conditions are more likely to maintain their physical characteristics in the long term.

How to cite: Redolfi, M., Carlin, M., and Tubino, M.: The geomorphic response of river alternate bars to climate change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11359, https://doi.org/10.5194/egusphere-egu22-11359, 2022.

09:21–09:27
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EGU22-6134
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ECS
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On-site presentation
Georgios Maniatis, Richard Williams, and Trevor Hoey

Recent developments in generating High Resolution Topography (HRT), such as UAV photogrammetry, LiDAR and dGPS, have been extensively used in fluvial settings. Most data generation methods are based on commercial sensing and pre-processing tools that are tested by geoscientists in a trial-and-error manner for clarifying: a) their accuracy; and b) their applicability in field settings that are generally outside the range of their factory calibration. For many applications, this involves the concurrent deployment and the cross comparison of more than one sensing techniques. Despite the above, HRT techniques reduce surveying time and costs significantly. The frequency of surveying has increased to a point where it is now common to monitor the development and survival of in-stream bed forms with high resolution Digital Elevation Models (DEMs) on a monthly to annual basis.

In parallel, river scientists have developed dedicated GIS workflows for: a) parameterising the errors during DEM differentiation, thus producing better constrained DEMs of Difference (DoDs); and b) delineating automatically (or semi-automatically) DEMs for the coherent identification of Geomorphic Units (GUs), a term used to distinguish in-stream bed forms and morphological features within the 3 Tier Classification of Wheaton et al., (2015, https://doi.org/10.1016/j.geomorph.2015.07.010).

Here, we use the outputs from the GUT (Geomorphic Unit Tool, Riverscapes consortium) GU delineation as a proxy to predict the change of in-stream geomorphic variability. More specifically, we present a Markov-Chain (MC) model with a state incorporating all the observed GUs and transition matrices built using observed GU changes. The models are then left to converge to a set of probabilities that demonstrates what would happen to the stream if subjected to the observed hydrological forcing for a period that exceeds the surveying plan. To validate the model, we apply it for three successive post-restoration surveys (between 2012-2017) of a 700 m long reach of the Allt Lorgy restoration scheme (Scotland). The first two surveys are used to parametrise the MC transition matrix and the initial states and the third to test the predictions. The results show that the observed GU probabilities are within the predicted uncertainty ranges when the MC chain is modified and a proxy for sediment input is introduced as an additive term.

The MC model is intended to describe post-restoration morphological evolution, and subsequently to provide a tool for predicting morphological change and the end state, assuming constant hydrological forcing.

How to cite: Maniatis, G., Williams, R., and Hoey, T.: A High Resolution Topography (HRT) based stochastic model for  multi-year river adjustment post restoration, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6134, https://doi.org/10.5194/egusphere-egu22-6134, 2022.

09:27–09:33
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EGU22-9081
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On-site presentation
Lina E. Polvi and Richard J. Mason

River restoration is essential to reverse biodiversity decline and improve river resilience to climate change. In northern Sweden, virtually all rivers were historically timber-floated and thus channelized with all complexity elements (e.g., boulders, islands, side channels) removed. In these rivers restoration design is determined in the field by a team leader directing an excavator driver. This efficient methods allows restoration of 100s of river kilometers annually; however, there is little to no monitoring of restoration outcomes. Thus, the influence of restoration on channel morphology and habitat complexity is unknown. Furthermore, response of semi-alluvial rivers constrained by glacial legacy sediment (e.g., boulders) to restoration is poorly understood and expected to differ from their alluvial counterparts. In this study, we followed up eight reaches in the Lögde River catchment (~64° N, DA: ~1600 km2) restored as part of the EU LIFE project ReBorN. Reaches were equally divided above and below the former-highest coastline (FHC), demarcating different glacial histories and surficial geologies (semi-alluvial vs. alluvial channels). To evaluate the influence of river size on channel response to restoration, half of the reaches were located on tributaries and half of the reaches were on the mainstem of the Lögde River. We surveyed all reaches with a total station or RTK-GPS prior to restoration and 1-year and 3-years post-restoration. Hydromorphologic characteristics and complexity metrics were calculated and compared among years to determine changes during and post-restoration.

As expected due to the nature of the restoration methods, channel size increased, with significant increases in channel width and planform area. Although channel complexity showed increasing trends, few were significant except three metrics describing the longitudinal profile (α= 0.10); one complexity metric showed a significant decrease (thalweg planform sinuosity). In the 3-year period following restoration, channel width, planform area, and depth decreased. Complexity metrics either showed no change or a similar trend of decreasing, with significant decreases in three metrics (width SD, thalweg concavity, and thalweg R2). There were no significant differences between reaches above and below the FHC or between the mainstem and tributaries.

Overall, these reaches were over-dimensioned during restoration and post-restoration adjustment shows slight narrowing. Inset bankfull channels started forming with vegetation establishing below the designed bankfull channel. An over-dimensioned channel reduces overbank flooding and thus lateral channel-floodplain connectivity, negating a restoration design aim. The decreased post-restoration complexity indicates a smoothening of the longitudinal profile and planform bankfull profile through sediment settling and preferred areas of erosion/deposition, rather than the artificial complexity created by the excavator. Although eight reaches were too few to reveal many significant changes, many post-restoration studies make conclusions based on a single reach, thus the trends shown here indicate similar processes acting across several reaches. Similarly, three years is a short time period to evaluate post-restoration channel adjustment, particularly in semi-alluvial boulder-bed rivers. Ideally, river restoration should be followed up for at least a decade, allowing the river to experience high flows and potentially varied winter ice conditions.

How to cite: Polvi, L. E. and Mason, R. J.: Adjustment of channel morphology and complexity following restoration of timber-floated rivers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9081, https://doi.org/10.5194/egusphere-egu22-9081, 2022.

09:33–09:39
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EGU22-7845
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ECS
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On-site presentation
Baptiste Marteau, Kristell Michel, and Hervé Piégay

Gravel augmentation has become common practice to mitigate the effects of decline in upstream sediment supply in gravel-bed rivers. However, functional aspects of river systems such as thermal functions are often left out of rehabilitation monitoring programmes. Despite temperature being a fundamental parameter determining the general health of rivers, a limited number of studies have tested whether gravel augmentation can aid restoring thermal functions. Using airborne thermal infrared (TIR) imagery, this paper explores potential feedbacks through the monitoring of gravel augmentation on 3 rivers in France. To overcome the lack of pre-rehabilitation data, we used hydromorphological indicators within a trajectory-based Before-After-Control-Impact (BACI) framework to assess the success of rehabilitation on thermal functions. This design, combining long-term geomorphic evolution with TIR-based CI strategy, indicated that restoring forms was not sufficient to restore thermal functions. Nonetheless, hydromorphological indices mesures on historical aerial photographs can be used to estimate long-term evolution of groundwater-surface water interactions. We emphasise the benefits of trajectory-based BACI assessment to identify current conditions, understand the past evolution (trajectory) of the system to define the framework within which rehabilitation can objectively be assessed.

How to cite: Marteau, B., Michel, K., and Piégay, H.: Assessing the effects of gravel augmentation on thermal processes in gravel-bed rivers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7845, https://doi.org/10.5194/egusphere-egu22-7845, 2022.

09:39–09:45
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EGU22-9135
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Virtual presentation
Marta Crivellaro, Livia Serrao, Walter Bertoldi, Simone Bizzi, Alfonso Vitti, and Guido Zolezzi

Besides their environmental values, near-natural rivers offer the opportunity to observe and investigate riverine processes as they would occur under limited anthropic pressures, representing fundamental references for river management and restoration. Even so, few large near-natural rivers can still be found in Europe and worldwide, and their knowledge is often scarce due to a lack of hydromorphological monitoring and baseline studies. Among them, the Vjosa/Aoos River (GR, AL) has been recently recognized as a key large fluvial corridor and a significant model ecosystem. We investigate the catchment-scale recent morphological trajectories of the Vjosa river and its tributaries, coupling the reconstruction of channel adjustments over the past 50 years from remote sensing images with the analysis of possible drivers of change at the catchment and reach scale. We considered eight reaches in the main course of the Vjosa river as well as in some major tributaries (Sarandaporo, Drinos, Shushica) with different morphologies and confinement degrees. Our results underline the sensitivity of the Vjosa system to both hydrological alterations and human pressures. Specifically, it is possible to observe a response  of the system passing from an intense period of high magnitude, frequency, and duration of flood events in the 1960s to a drier period in the following decades. To study the morphological response, three time periods are considered: 1968-1985, 1985-2000 and 2005-2020. In the first examined decades, river trajectories highlight the narrowing of the active channel as a primary response to the hydrological change in the majority of selected reaches, with a 20-50% active width reduction with respect to 1968. In the following time periods, the narrowing rate decreases at the catchment scale, while in the last phase the effect of human pressures in some reaches can be observed. Indeed, from the late 1980s, human pressures at different spatial and temporal scales can be identified, locally altering the natural trajectory of the affected reaches. Such pressures include sediment mining and extensive bank protection of the lowland reaches, together with flow regime alteration occurring in one headwater sub-catchment.  However, our analysis reveals primarily a high sensitivity of the Vjosa system to recent climatic variations, suggesting the importance of accounting for future projected changes in rainfall regime in shaping morphological trajectories. The baseline knowledge on the morphological sensitivity and recovery time developed in this work provides an important reference for the management of highly dynamic river corridors in temperate and Mediterranean climates.

How to cite: Crivellaro, M., Serrao, L., Bertoldi, W., Bizzi, S., Vitti, A., and Zolezzi, G.: Morphological response to climatic and anthropic pressures of the Vjosa river, a reference system for river management and restoration, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9135, https://doi.org/10.5194/egusphere-egu22-9135, 2022.

09:45–09:55
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EGU22-1552
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solicited
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On-site presentation
Ellen Wohl

A river corridor includes the active channel(s), floodplain, and underlying hyporheic zone. Geomorphic heterogeneity refers to the spatial distribution of geomorphic units within the river corridor. Heterogeneity can be conceptualized at different spatial scales, from bedforms such as pools and riffles in the active channel, to the distribution of subsurface paleochannels across the entire floodplain. Essentially, geomorphic heterogeneity describes the extent to which the river corridor is spatially non-uniform in the three dimensions of vertical, lateral, and longitudinal. Heterogeneity results from erosion and deposition caused by temporal and spatial variations in both inputs and boundary resistance, as well as modifications created by biota such as riparian vegetation or beavers (Castor spp.). In many river corridors, these variations and biotic influences reduce longitudinal connectivity but enhance lateral and vertical connectivity within the river corridor. Resilience is the ability to absorb disturbances without diminishing or changing river corridor function. Resilience can be conceptualized as occurring along a continuum dependent on time and space scales, especially when applied to a system such as a river corridor that includes individual components with different levels of resilience. Changing climate will affect averages and extremes such as floods and wildfires. I use case studies from mountain streams in Colorado, USA to illustrate how a geomorphically heterogeneous river corridor is more resilient to extremes of high and low flow and large inputs of either sediment or solutes. Geomorphic heterogeneity promotes resilience because the spatial non-uniformity of the river corridor provides more opportunities for transient storage over diverse timespans, which attenuates downstream fluxes, and diffuses the energy inputs resulting from a disturbance.

How to cite: Wohl, E.: Geomorphic Heterogeneity in River Corridors as a Source of Resilience to Changing Climate, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1552, https://doi.org/10.5194/egusphere-egu22-1552, 2022.

09:55–10:00
Coffee break
Chairpersons: Isabella Schalko, Elizabeth Follett
10:20–10:30
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EGU22-3974
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ECS
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solicited
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Highlight
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On-site presentation
Janbert Aarnink, Virginia Ruiz-Villanueva, and Marceline Vuaridel

Large wood (10cm diameter & 1m long) gets recruited into a mountain river system from surrounding forested areas. Instream large wood positively influences the diversity of the river system, creating habitats for terrestrial and aquatic species. However, the corresponding risk to the presence of instream large wood is a more controversial topic in river management. On the one hand, large wood increases the riverbed roughness, partly dissipating energy during a flood. On the other hand, its transport during floods might cause damage to infrastructure. Direct observations or monitoring stations are scarce and knowledge on how and when wood is transported remains far from complete.

In order to quantify a river’s instream wood transport regime, we are developing a video-based wood tracking system that counts the number of pieces that pass a certain point and estimates their sizes. We use a DeepSORT algorithm that uses machine learning to identify individual pieces of instream wood and draws a bounding box around every piece. Subsequently, it uses a Kalman filter to estimate the piece’s trajectory. To prevent counting the same pieces multiple times, the projected trajectory is compared to the detections in the subsequent frame. The system is designed so that it can be applied to different datasets and will be available to the increasing wood monitoring efforts around the world. For a more detailed look into the large wood regime at one of our main study sites (Vallon de Nant, Switzerland), and to calibrate our video-based wood tracking system, we have installed RFID tags into all pieces of large wood (approximately 1000 pieces) over a stretch of 3 km. A stationary RFID antenna registers the tagged pieces that pass by, of which the size and origin are known.

First results show that the custom trained DeepSORT algorithm can not only identify pieces of instream wood, but also largely follow the pieces in subsequent frames. The approach seems to outperform current computer vision solutions. In our ongoing research, we aim to make the system more robust and expand the observation network to other rivers. With an expanding dataset, containing (manually) labelled training samples from different locations, and the low-cost measurement set-up, the system promises to aid successfully to an intercomparison of river systems in the context of the wood management debate.

This work is supported by the SNSF Eccellenza project PCEFP2-186963 and the University of Lausanne.

How to cite: Aarnink, J., Ruiz-Villanueva, V., and Vuaridel, M.: Machine learning and RFID-based large wood tracking in rivers , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3974, https://doi.org/10.5194/egusphere-egu22-3974, 2022.

10:30–10:36
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EGU22-4251
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Highlight
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Presentation form not yet defined
Andrés Iroumé, Karla Sanchez, Lorenzo Martini, Giacomo Pellegrini, and Lorenzo Picco

Large wood (LW), both as individual pieces and in accumulations (WJ), plays an important role in the morphology, hydrology, and ecology of rivers. However, LW dynamics in rivers affected by volcanic eruptions has been little studied. This study aims to investigate the changes of LW volumes along a segment of the Blanco-Este River (southern Chile) affected by the 2015 Calbuco volcanic eruption. The following research questions were addressed: a) what are the drivers that explain the spatial and temporal variability of the amount of LW along the river active channel? b) what is the level of connection between the potential source areas of wood and the channel? c) is it possible to infer a relationship between recruitment sources and floods, with fluctuations in the amount of wood along the channel? The study was conducted in two reaches, the upstream one more proximal to the volcano (hereinafter R1) and the downstream more distal from the volcano (R2). LW and WJ volume were calculated using the structure from motion (SfM) technique for several sampling campaigns performed between 2017 and 2020 using a drone. Data from a fluviometric station near the Blanco-Este River and time lapse camera records were used to interpret the dynamics of wood during floods. Finally, the stability of WJs was used to indirectly evaluate the mobility of LW in the study reaches. Results show that the amount of LW (n°/ha), WJ (n°/ha) and total wood volume (m3/ha) are considerably higher in R2 than in R1. In both reaches, the main recruitment source of LW to the channel is associated with erosions of the forested margins, but for R2 a tributary and erosions of old laharic deposits are also recruitment sources. LW volume in R1 did not vary much between campaigns (1.9-5.1 m3/ha) which would indicate that this reach is in an equilibrium condition of LW loading. Since the wood volume in R2 showed important variations between sampling campaigns (9.1-73.9 m3/ha), this reach does not seem to have reached this equilibrium condition yet. Results showed that there is no clear relationship between the wood fluctuations and the flood intensities (volume increases and decreases indistinctly associated to low or high peak flows), a fact confirmed from the time lapse cameras. However, wood supply appears, as might be expected, somehow controlled by floods, as well as wood transport. But, apparently, the floods competent to move logs are of lower magnitude than those generating bank erosions and subsequent wood recruitment. From the analyses of the drone images, it was observed that the stability of the WJs was very low in the Blanco-Este, which indicates a high LW mobility. A connection between the areas that supply LW to the river channel appears to occur during major flood events with sufficient competence to erode forested streambanks. The latter calls for the need to incorporate the analysis of longitudinal wood connectivity in channel studies. This study is part of the FONDECYT 1200079 project.

How to cite: Iroumé, A., Sanchez, K., Martini, L., Pellegrini, G., and Picco, L.: Impact of a volcanic eruption on the wood fluctuation along a Chilean river basin: the Calbuco study case , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4251, https://doi.org/10.5194/egusphere-egu22-4251, 2022.

10:36–10:42
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EGU22-5495
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ECS
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Presentation form not yet defined
Elizabeth Follett, Isabella Schalko, and Heidi Nepf

Engineered logjams with a gap at the bed are used in engineering practice to provide natural flood management and ecological benefits while preserving river connectivity at base flow. In addition, logjams with a gap at the bed form naturally in small streams with river width less than log length. The accumulation of wood pieces acts as a porous obstruction, and the distribution of flow through and beneath a jam with a lower gap satisfies a two-box, momentum-based model constrained by drag generated in the jam, momentum loss in flow through the lower gap, and net pressure force. Accumulation of brush and fine material upstream of logjams occurs naturally as small wood pieces and leaves are transported to the river channel. However, the impact of accumulated upstream material on logjam-generated increase in backwater rise presents a potential concern for long term maintenance of engineered logjam projects. We present recent results demonstrating that initial accumulation of wood pieces upstream of a jam with a lower gap has little impact on backwater rise, but backwater rise increased during a simulated flood cycle as wood pieces blocked the lower gap. The effect of varying brush size and shape and impact on flow redistribution between the jam and gap is examined.

How to cite: Follett, E., Schalko, I., and Nepf, H.: Flow redistribution and backwater rise due to brush accumulation upstream of logjams with a lower gap, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5495, https://doi.org/10.5194/egusphere-egu22-5495, 2022.

10:42–10:48
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EGU22-8699
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ECS
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On-site presentation
Isabella Schalko, Elizabeth Follett, and Heidi Nepf

Wood is a key part of a river ecosystem and affects both flow conditions and channel morphology. Wood accumulations or logjams may generate important habitat by increasing the upstream water surface elevation (backwater rise) and creating a downstream region with reduced flow velocity. Depending on the logjam size and the flow conditions, the resulting backwater rise can also provoke a flood hazard. Therefore, the prediction of backwater rise due to logjams is required to inform river restoration as well as flood hazard assessment efforts. Backwater rise due to channel spanning logjams can be described based on analytical and empirical models. However, logjams can exhibit various shapes, including partially spanning logjams. The hydrodynamic response to logjams that partially span the channel lateral extent has not been studied so far. Therefore, a series of flume experiments was conducted at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at ETH Zurich to study how the flow depth and flow velocity are altered by partially spanning logjams with a lateral gap. The objectives were to determine how the jam relative width (jam width to channel width) influenced flow heterogeneity, described by flow velocity and turbulent kinetic energy, and to predict the backwater rise. Initial results demonstrated that logjams with a relative width Brel ≥ 0.5 created two distinct zones of velocity and increased flow heterogeneity. In addition, backwater rise increased with increasing relative logjam width. As a next step, the existing analytical model for channel spanning logjams will be adapted to describe backwater rise due to partially spanning logjams.

How to cite: Schalko, I., Follett, E., and Nepf, H.: Hydrodynamic Response to Partially Spanning Logjams, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8699, https://doi.org/10.5194/egusphere-egu22-8699, 2022.

10:48–10:54
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EGU22-5730
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ECS
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Virtual presentation
Josh Wolstenholme

Title: Catchment-scale geomorphological modelling of leaky dams using CAESAR-Lisflood

 

Joshua Wolstenholme              j.wolstenholme-2018@hull.ac.uk          1

David Milan      d.milan@hull.ac.uk      1

Christopher Skinner    chris.skinner@environment-agency.gov.uk 2

Daniel Parsons              d.parsons@hull.ac.uk               1

 

Affiliations:

  • University of Hull, Energy and Environment Institute, United Kingdom of Great Britain – England, Scotland, Wales (j.wolstenholme-2018@hull.ac.uk)
  • Environment Agency, Flood Hydrology Improvements, United Kingdom of Great Britain – England, Scotland, Wales

 

The introduction of large wood to fluvial systems is becoming increasingly popular as a method of natural flood management commonly referred to as leaky dams. These are often installed as semi-permanent features through live felling and anchoring in-situ. Currently, most natural flood management modelling is hydrological and focuses on flood risk without accounting for geomorphology of these ‘fixed’ features. We argue that the long-term effectiveness of NFM interventions require and understanding of the nested hydrogeomorphological processes at work within river catchments, particularly those related to bed scour, sediment transport and deposition, and the associated feedbacks following implementation of leaky dams. Leaky dams that are designed to attenuate the hydrograph and ‘slow-the-flow’, may cause sediment storage as well as scour, potentially impeding the effectiveness of a leaky dam to reduce flood risk after a single storm event. Using the new ‘Working with Natural Processes’ toolbox developed for CAESAR-Lisflood, the influence of different storm scenarios on a series of leaky dams in a hypothetical catchment based on a site in North Yorkshire is assessed. The effectiveness of the model at representing the influence of the dams on hydrogeomorphology is also assessed.

How to cite: Wolstenholme, J.: Catchment-scale geomorphological modelling of leaky dams using CAESAR-Lisflood, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5730, https://doi.org/10.5194/egusphere-egu22-5730, 2022.

10:54–11:00
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EGU22-5060
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ECS
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Virtual presentation
Diego Panici and Georgina Bennett

Large wood is an essential component of river systems, often considered as the third leg of riverine fluxes (together with water and sediment). Large wood can provide beneficial effects to river restoration and natural flood management (NFM) measures. At the same time, large wood can obstruct bridge openings and increase risk of failure to structures and risk of flooding to adjacent areas. The transport of large wood in rivers crucially affects all the above processes, but to date the importance of factors affecting displacement of large wood in rivers is still poorly understood. Past theories postulated that flow secondary cells may drive large wood trajectories, but have never been confirmed. In this work, we experimentally tested at the flume scale the hydrodynamic factors influencing the displacement of large wood at the river surface. Results showed that past theories were inconclusive, whereas large wood elements tend to follow well-defined trajectories mostly driven by localised changes of the flow velocity. Furthermore, large wood elements are very sensitive to changes in their trajectories at the onset of motion, although are much less prone to change once motion has fully developed. The results from this work will pave the way for better-defined motion models of floating large wood, and will be used to test and calibrate smart sensors for field-based applications.

How to cite: Panici, D. and Bennett, G.: An experimental study on the displacement of large wood in river channels, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5060, https://doi.org/10.5194/egusphere-egu22-5060, 2022.

11:00–11:06
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EGU22-10199
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ECS
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On-site presentation
Chloe Carter, Thomas Coulthard, Robert Thomas, and Stuart McLelland

In the UK Leaky wooden dams (LWD) have become an increasingly popular method of Natural Flood Management (NFM) and river restoration. LWD are in and/or across channel structures made from woody material designed to mimic naturally occurring woody debris that is often found in riverine environments. LWDs aim to reduce flooding downstream by holding back water and promoting flow onto the floodplain, increasing connection with the floodplain and infiltration by diverting water onto the floodplain. A key difference between woody debris and LWD are that LWD are usually secured and unable to move and adjust within the river and LWD are sometimes placed in areas where woody debris would not naturally occur. With the large scale and quick implementation of LWD there is a lack of critique or investigation into the geomorphic impacts of LWD. Instead, researchers and practitioners have been using what is known about the geomorphic impacts of natural woody debris to explain and predict the geomorphic impacts of LWD – even though it has been established that they are fundamentally different. This project investigates the geomorphic impacts of different styles and configurations of LWD through the use of analog physical models, surface velocimetry and structure from motion photogrammetry. Using these techniques this research aims to identify any patterns in flow and sediment dynamics both up and downstream of LWDs and to further our understanding of the specific geomorphic impacts of different LWD structures. Identifying the specific geomorphic impacts of LWD is important to be able to understand if they are having a detrimental impact to the river systems where they have been installed in the UK and to be able to inform best practice for the future.

How to cite: Carter, C., Coulthard, T., Thomas, R., and McLelland, S.: Understanding the geomorphic impacts of Leaky Wooden Dams (LWDs) through utilising analog physical models, structure from motion photogrammetry and surface velocimetry., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10199, https://doi.org/10.5194/egusphere-egu22-10199, 2022.

11:06–11:12
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EGU22-10651
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ECS
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Highlight
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Virtual presentation
Kenta Koyanagi, Taku Yamada, and Koji Ishida

Understanding the dynamic of instream large wood (LW) is essential for reducing hydrogeomorphic hazards in populated mountainous catchments. Quantifying the spatiotemporal distribution of LW is generally the most demanding process for investigating LW dynamics in rivers. Over the last two decades, multiple airborne sensors have been applied for mapping LW in relatively large alluvial rivers. However, those existing approaches are not necessarily suitable for remotely sensing LW in forested headwater streams, mainly due to canopy obstruction, weak illumination, and operational difficulty. Therefore, we tested the applicability of a 5-kilogram commercial backpack mobile laser scanning system for detecting and quantifying LW in a forested headwater stream of southwest Japan. Extremely dense point clouds (~15000 pts/m2) were continuously scanned within 150-meter reach of the 2nd-order stream (slope: 0.045) by a 6-minute walk following rainfall-triggered debris flows. Dimension and volume of LW measured from point clouds were compared to associated field and UAV photogrammetry-based mapping data. Based on a surface shape detection algorithm and subsequent manual filtering of falsely detected objects (e.g., riparian trees), 25 cylinders corresponding to 34.9 m3 total volume were delineated from point clouds. While the UAV photogrammetry-based approach was able to quantify only 2.4% of total LW volume, 75.1% of LW volume was successfully reconstructed by backpack mobile laser scanning. The visibility of the UAV photogrammetry-based approach was substantially limited by the dense riparian vegetation of our study reach. However, underestimation of wood piece length and overestimation of wood piece diameter consistently occurred for both remote sensing approaches. Therefore, further efforts would be made to evaluate the sensitivity of individual parameters used in point cloud processing for LW detection and quantification. Considering the mobility of sensors and data availability of near-surface objects, our case study indicates that backpack mobile laser scanning potentially provides a powerful alternative for more continuous, efficient, and frequent LW mapping, particularly in forested headwater streams.

How to cite: Koyanagi, K., Yamada, T., and Ishida, K.: Using backpack mobile laser scanning system for mapping large wood in a forested headwater stream of southwest Japan, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10651, https://doi.org/10.5194/egusphere-egu22-10651, 2022.

11:12–11:18
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EGU22-11911
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ECS
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On-site presentation
Théo Fernandez, Ingo Schnauder, Olivier Eiff, and Koen Blanckaert

The research concerns the hydrodynamic processes around obstacles of cylindrical shape installed across an open channel flow at a subcritical Reynolds number of ReD = 1 x 104 (based on the cylinder diameter), and the forces exerted by the turbulent flow on these obstacles. Based on field measurements performed on the Plizska River, Poland, this study is mainly on cylinders representing large wood trunks that traverse a river. 

The first aim of the study is to reproduce the flow pattern around an inclined single tree trunk of quasi constant diameter and without branches measured in the field and to enable a more detailed analysis of the underlying turbulent flow processes. These field measurements have shown that horizontal near bank recirculation zones, scour below the trunk and plunge scour overtopping it occurred.

The second aim is to compare the mean flow and vortex shedding around inclined and horizontal cylinders across the flow. The effects of inclined and horizontal cylinders on the flow field are very different: the former create a higher variability in flow processes.  These configurations differ in gap width below the cylinder and in approach velocity, as the inclined cylinder is located at different elevations in the bottom boundary layer. Both parameters affect the vortex shedding frequency and the wake structure. 

Results show that a transversally inclined cylinder generates more complex flow patterns and creates a high heterogeneity in the flow as well as the depth. The analysis of the dimensionless shedding frequency also suggests the suppression of vortex shedding near both banks when the gap ratio is small. However, vortex shedding characteristics in the central part of the cross-section are similar for the horizontal and inclined cylinders, i.e. the changing gap ratio below the inclined cylinder does not affect significantly the vortex shedding. In the central part of the cross-section, the wake flow is governed by the interaction of the nearly symmetrical shear layers generated above and below the cylinder. Near the banks, the shear layer near the bed or water surface is suppressed, which could explain the suppression of the vortex shedding.

How to cite: Fernandez, T., Schnauder, I., Eiff, O., and Blanckaert, K.: Hydrodynamics in the near-wake of cylindrical obstacles in a turbulent open channel flow, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11911, https://doi.org/10.5194/egusphere-egu22-11911, 2022.

11:18–11:24
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EGU22-12394
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On-site presentation
Ronald Pöppl, Hannah Fergg, and John Perez

In-stream large wood (LW) can have significant effects on channel hydraulics and thus water and sediment connectivity. The relationship between LW structures and their hydraulic function is generally quantified through drag force. Drag analyses, however, are often not straightforward, especially in complex debris jam settings where LW accumulations often consist of wood pieces of variable sizes. Here, we introduce simple LW (dis-)connectivity and sediment storage potential indices, especially developed for river management assessments. The LW (dis-)connectivity index (IDLW) is calculated based on visually estimated, field-derived parameters such as the degree of channel blockage. The LW sediment storage potential index (ISLW) is based on a classification scheme differentiating between different types of LW accumulation. Both indices were calculated and tested in two medium-sized mixed-load streams in Austria, further assessing fine sediment retention volumes behind LW structures. In both systems a variety of different types of LW accumulation with different degrees of blockage and storage potential have been detected. The larger system (river length = 5.7 km) had IDLW and ISLW values of 0,75 and 0,027, the smaller system (river length = 1.3 km) of 1,76 and 0,057. In the larger system in total 88.7 m³ fine sediment have been found to be retained by LW, while 4.7 m³ have been accumulated behind LW structures in the smaller river system. The application of the newly developed indices has shown to be a straightforward and valuable method to assess the effects of LW on water and sediment (dis-)connectivity, especially in a river management context.

How to cite: Pöppl, R., Fergg, H., and Perez, J.: Large wood (LW) and sediment (dis-)connectivity in river systems: Introducing the newly developed LW (dis-)connectivity and sediment storage potential indices and their application in river management contexts. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12394, https://doi.org/10.5194/egusphere-egu22-12394, 2022.

11:24–11:30
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EGU22-12515
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ECS
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Virtual presentation
Jérémy Marchesseau, Ana Lucía, Francesco Comiti, Emmanuel Mignot, and Virginia Ruiz-Villanueva

Large and relatively immobile sediment particles (i.e., boulders, usually defined with a diameter greater than 256 mm) are naturally delivered to rivers from hillslopes, transported by extreme floods, or produced by processes such as bed armouring. Boulder placement is also used as an artifical method for stabilizing channel beds and banks in river restoration projects. Natural or reintroduced boulders are important elements with a significant influence on channel hydraulics, erosion and deposition dynamics, and morphology. Still, little is known about their effect on large wood transported as floats along the river.

A field experiment was performed to track the mobility of cylindrical wood elements artificially placed in a reach of the Rienz River upstream from the city of Brunico, in South Tyrol (Northern Italy) and transported along a few kilometres over a period of three years. The Rienz River is a single thread sinuous gravel-bed river, characterized by the presence of several large boulders. Combining available field observations and 2D numerical modelling (coupling a 2D flow and a Lagrangian calculation of wood elements), this work aims to test the effect of boulders on both the river ecohydraulics and large wood transport. First, a detailed topography was obtained combining an available digital elevation model (2 m resolution) with topographical surveys. Second, the numerical model (i.e., Iber-Wood) has been calibrated with flow depths observations and the wood travel distances recorded during one high flow event were used for validation of the Lagrangian calculation. Finally, different scenarios with different boulder rearrangements are currently run to explore the effects of boulders size and location distribution on both wood transport and river ecohydraulics. This contribution will show preliminary results and discuss how boulder-rich channels differ from boulder-free channels in terms of large wood transport and deposition.

How to cite: Marchesseau, J., Lucía, A., Comiti, F., Mignot, E., and Ruiz-Villanueva, V.: Exploring the effect of instream boulders on large wood transport combining numerical modelling and field experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12515, https://doi.org/10.5194/egusphere-egu22-12515, 2022.

11:30–11:36
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EGU22-13483
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ECS
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Virtual presentation
Rebeca Mallqui, Juan Cabrera, and Arnold Lazóriga

The backwater caused by the accumulation of wood and large logs in rivers surrounded by tropical forests is determined by the characteristics of the floating material and the approaching flow. Density, as a characteristic of wood logs, determines their buoyancy and depends on the tree species, age, state of decomposition and water content, reaching values between 250 kg/m3 and over 900 kg/m3. Despite this apparent relationship, flood hazard studies in rivers with log transport usually do not consider the influence of density.

In the present study, the effect of wood density on the increase in backwater and the shape of the accumulation is evaluated by means of laboratory-scale simulation with pieces of artificial logs for different Froude numbers and approach flow heights. The pieces were manufactured on 3D printers to obtain certain density ranges (400 ±30, 600 ±30, 800 ±30 and 950 ±30 kg/m3), reduce the possible variation in the moisture content of the wood and facilitate its reuse. Backwater formation was forced by installing vertical steel rack in a control section installed downstream of the test channel. The results of the evaluation show a marked tendency in the increase of the backwater height with the increase of the density of the wood for each approach flow condition evaluated. Regarding the shape of the accumulations, the presence of a carpet form was observed only for the tests with subcritical approach flows, for the tests with supercritical flow, wedge or box shapes were observed for low densities and higher densities, respectively. Likewise, it was observed that the length of the carpet form decreases as the Froude number of the approach flow increases. On the other hand, it was observed that the percentage of retention of pieces of logs in the grid decreases when the density of the logs increases under subcritical flow conditions. The findings of the present investigation demonstrated the interaction between the density of the wood and the different forms of accumulations of logs and the relationship of the density of the wood with the increase in backwater.

How to cite: Mallqui, R., Cabrera, J., and Lazóriga, A.: Influence of Wood Density on Backwater Rise due to Large Wood Accumulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13483, https://doi.org/10.5194/egusphere-egu22-13483, 2022.

11:36–11:46
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EGU22-6472
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ECS
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solicited
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Highlight
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On-site presentation
Elisabetta Persi, Gabriella Petaccia, Stefano Sibilla, Lorenzo Picco, and Alessia Tonon

In low flow conditions, wood transport is limited but still important. In addition, low flows are significant to stress a numerical model of Large Wood (LW) transport and to assess its capacity in simulating LW displacement or non-displacement.  The solver ORSA2D_WT was employed and tested to improve the knowledge related to these thresholds (moving vs not moving). The software couples the solution of the 2D Shallow Water Equations to a dynamic Discrete Element Model that computes the hydrodynamic forces to calculate LW transport. To assess whether ORSA2D_WT can cope with the infrequent mobilization of LW in low flow conditions, it is applied to a reach of the Piave River (North-East Italy), where the wood budget was already investigated. Field data about LW position, mobilization, shape, size and orientation, flow conditions and morphological changes were collected.

The critical aspects that affect the model performance and that deserve an in-depth analysis are the wood-riverbed interaction and the log shape representation in the model. ORSA2D_WT works in fixed-bed conditions, computing a 2D force balance to determine wood entrainment. It considers only cylindrical forms or jams composed by cylindrical elements, whose relevant hydrodynamic parameters are the longitudinal cross-section and the hydrodynamic coefficients, that depend also on the log orientation to the flow.

Regarding wood-riverbed interaction, bed friction plays a significant role compared to the forces that trigger wood motion. This is especially true in low flow conditions when floatation is less important than rolling/sliding. The local erosion that occurs nearby wood pieces likely influences wood mobilization, as well as the presence of roots and/or branches.

To assess if the model schematizations are sufficiently accurate for low flow conditions and to overcome the model limitations, the friction and hydrodynamic coefficients are suitably corrected. In particular, the influence of the local water level on the friction coefficient is investigated, and the hydrodynamic coefficients are modified to include different LW shapes. The modified model is calibrated with the data available for one sub-reach and then applied to a different sub-reach, to assess its performance.

How to cite: Persi, E., Petaccia, G., Sibilla, S., Picco, L., and Tonon, A.: Modeling the effects of low flow on wood transport in the Piave River, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6472, https://doi.org/10.5194/egusphere-egu22-6472, 2022.

11:46–11:50