GM5.2

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The research concerns the hydrodynamic processes around obstacles of cylindrical shape installed across an open channel flow at a subcritical Reynolds number of Re_D = 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.

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 (ID_LW) is calculated based on visually estimated, field-derived parameters such as the degree of channel blockage. The LW sediment storage potential index (IS_LW) 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.

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.

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.

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.