The accumulation of floating large wood at bridge piers exacerbates flood risk. Climatic change, deforestation, soil erosion and expanding settlements cause increasing loads of wood into rivers. Besides naturally eroded wood, also harvested and treated wood, e.g. wood cut and stored on the floodplain that is mobilized during inundation events, is of concern. Cut wood is typically unbranched, more slender, dryer and often smoother than naturally eroded wood. Understanding how these different wood properties affect the jamming processes and identifying their governing control parameters is key for a bridge design with reduced jamming vulnerability. In this study, we therefore experimentally investigate the initiation and growth of wood jams from slender wood elements.

Flume experiments are conducted in a 1.7 m wide, fixed bottom flume at the TU Wien hydraulics lab. Flow depth was set to 0.30 m at a Froude number of 0.23 and a flow-Reynolds number of 1.16 x 10⁵. A cylindrical pier with 0.1 m of diameter was installed centrally in the flume. Unbranched cylindrical elements of 30, 45 and 60 cm length and 0.4 and 0.6 cm diameter were used to covered high slenderness regimes (l/d) of 50 - 100 and high relative lengths (l/D) of 3 – 6. The elements were produced from waterproofed pine dowels and plastic pipes sealed at both ends yielded elements with relative densities between 0.3 and 0.6 in water. A downward-looking camera recorded the jamming process.

Preliminary experiments focused on phenomenological observations of the jamming process. Approaching elements were only trapped, if their eccentricity (the lateral distance between their center and the center of the pier), was below one third of the element length. Within this range, slender long elements remained trapped for a long time – up to infinity in many cases. This first metastable regime is possible because of stabilizing compensatory movements, including rotational swaying around the bridge pier, vertical dipping and vibrations related to vortex-shedding. Hereby, swaying had the most stabilising effect as it exposed one end of the element into higher flow velocities upstream, thus increasing drag and initiating reverse rotation. The second stage of jam formation was governed by the interaction and collision of additional elements with the first element. At low eccentricity, the colliding element was rotated and attached parallel to the first element. At higher eccentricity, the collision destabilized the first element and rotated both elements. In this case, a third element was required to collide within a critical impact time to stop rotation and dislodgement. Thus, the stabilising mechanism shifted from compensatory movements to compensatory collisions. When collisions caused the trapping of elements, three (or more) elements formed a triangular, scissor-like pattern around the pier. This ‘scissor-pattern’ was a second metastable regime, typical for the tested slender long elements and observed throughout all runs. Experiments indicated, that friction between the elements and the pier surface controls the stability of the ‘scissor-pattern’, which is subject of ongoing analyses.

How to cite: Pratama, M. I., Schnauder, I., and Blanckaert, K.: Flume Study on the Process of Slender Wood Jamming at Bridge Piers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15085, https://doi.org/10.5194/egusphere-egu23-15085, 2023.

Instream large wood (LW) plays an important role in the geomorphic and ecological diversity of a fluvial ecosystem. However, during flood events, LW can also pose a risk to infrastructure and populations by blocking channels, damaging bridges, and other structures. The primary source of LW are forested areas established along rivers and their upstream network. Understanding the origin of LW and the factors that influence its dynamics is key for optimizing river and riparian forest management and reducing the risk associated with flood events.

We study a 50km reach of the Rhone River between the city of Geneva and Génissiat dam (France), where the wood material arriving is retained. In this 3000km² catchment, we aim to infer the origin of the LW that arrives at the reservoir by differentiating between the two main tributaries and providers of LW in the reach: The Arve River (coming from the Alps Mountains) and the Valserine River (from the Jura Mountains).

We have explored several methods for inferring the origin of instream wood. By combining them, we gain a more comprehensive understanding of the factors that influence the supply of LW to the watershed and its dynamics within the river system. These methods are based on:

• Stable isotopes in the cellulose coming from the water molecule (δD and δ¹⁸O): they present a spatial distribution due to fractionation happening during evaporation-precipitation processes. The tree absorbs the isotopic signal and stores it in the cellulose, that can be analyzed to distinguish between different source areas.
• Chemical composition of wood cellulose: it can be analyzed to provide information about the geology of the area where the tree grew. Techniques such as inductively coupled plasma optical emission spectrometry (ICP-OES) or X-ray fluorescence spectrometry (XRF) are used for this purpose.
• Riparian forest composition: some forest characteristics (e.g., tree species, tree diameter, forest density, dead wood present in the floodplain, lateral connectivity with the river, etc.) provide useful information on the areas that are supplying the wood that reaches or will potentially reach the dam.

By combining these approaches, it may be possible to distinguish between different source areas of LW within the catchment and to better understand the factors that influence the supply of LW to the river system. We aim to develop a method that can be applied to similarly scaled mountainous catchments to determine the origin of instream large wood.

How to cite: Gibaja del Hoyo, J., Monbaron, L., Vennemann, T., Vauridel, M., and Ruiz-Villanueva, V.: A dendroprovenance approach to fingerprint the origin of instream wood at the river basin scale, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8911, https://doi.org/10.5194/egusphere-egu23-8911, 2023.

Large wood is an essential component of the river channel. Channel morphodynamics are impacted, specific habitat for aquatic biota is created, and aesthetic or education function is enhanced by the presence of large wood in the river. The concept of ecosystem services can be applied to describe the functions of large wood. In the case of large wood, there can also be potential risks for human society connected with flooding or bank erosion. We inventoried large wood in 13 active meanders of the Odra (Oder) River, Czechia, and its potential ecosystem services. We surveyed large wood using aerial images (2012 and 2020) and field surveys (2016 and 2020). Based on previous literature research, we assessed each large wood piece separately and decided on the services (3 main categories and 13 subcategories) based on the functions by preselected indicators. Preliminary results show that the functions and provided services depend mainly on the residence time of large wood in the channel and its additional characteristics such as dimensions, type of large wood (e.g., whole tree with preserved crown, present/absent root wad) or its orientation against the flow. We provide a methodological approach of (i) possible assessment of large wood in the river reach to summarize its benefits and risks and (ii) to simplify the understanding of the presence of large wood and its promotion to river practitioners, city planners, and the broader public.

How to cite: Poledniková, Z. and Galia, T.: How does spatiotemporal dynamics of large wood impact its ecosystem services in a meandering river?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2536, https://doi.org/10.5194/egusphere-egu23-2536, 2023.

Large woody debris (LWD) is commonly used in river restoration projects because it provides important habitats. However, when used in urban creeks and rivers, these structures must be designed and installed in a way that balances the needs for flood safety with the goals of ecological restoration. To ensure the stability of LWD during floods, engineers must consider a range of characteristics such as the size of the structure relative to the flow cross-section, the ratio of length to diameter, shape, orientation, flow magnitude, and the overall hydraulic conditions. To properly design and fixate LWD, it is necessary to consider both lift and drag forces in different flow situations. There are several approaches available for calculating these forces, but many of them are often only applicable by simplifying the LWD into an idealized 1D or 2D cylinder case.

The general drag force equation uses an empirical drag coefficient, which is a function of the object's shape, size, and surface roughness, as well as the properties of the fluid and the flow conditions. However, the drag coefficient does not account for factors such as blockage ratio (the ratio of the object's area to the flow cross-section) and orientation (the angle at which the object is oriented relative to the flow direction). These factors can significantly affect the drag force, and their inclusion in the drag force calculation can lead to more accurate predictions.

To evaluate the role of blockage ratio and cylinder orientation, experiments were conducted in a 10 meter long, 79 cm wide glass flume with a rough bed to create a fully turbulent velocity profile. Smooth PVC cylinders, representing the woody structures, were placed in the flume with rotation angles between 0 and 90 degrees relative to the flow. Examined were three cylinders with a ratio of length to diameter from 3.16 to 9.48 and lengths between 20% and 60% of the channel width. A dynamic load cell was used to measure the drag forces on the cylinders in the flow direction, and various subcritical flow conditions with different depths and velocities were examined.

How to cite: Balmes, J. P.: Evaluating the Role of Blockage Ratio and Orientation in the Drag Force Calculation for Large Woody Debris, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11739, https://doi.org/10.5194/egusphere-egu23-11739, 2023.

Meander bends are perceived as sites with high large wood (LW) retention potential by trapping transported LW at their concave banks or on point bars. We employed five field inventories between 2016 and 2022 (2016, 2019, 2020, 2021 and 2022) in 13 subsequent meander bends of the Odra (Oder) R., Czechia, to assess spatiotemporal variations of LW in the period characterised by ordinary flow events including a flood of 5-year recurrence interval (10/2020, 308 m³/s). We found an increasing trend in LW volume (121.1 m³ in 2016 and 138.1 m³ in 2022, respectively) in a 3.65 km long study reach. We observed a high disproportion in the longitudinal LW distribution during individual surveys along the study reach. Furthermore, four upstream meander bends had stable LW volumes during the study period, whereas the bends located in the middle and downstream part of the study reach indicated large fluctuations of LW loads without any regular trend. These fluctuations are likely related to (i) chronic recruitment of LW from outer meander banks, (ii) burial of LW during floodplain accretion, and (iii) LW mobility during high flows. Independent variables representing the characteristics of the riparian trees (tree basal area and the length of the riparian forest at the bend) were detected as significant predictors of the LW volume at the meander scale. Future research will focus on the complex links among the migration rates of individual meanders, the characteristics of riparian stands, and the dynamics of LW in the channel and floodplain.       

How to cite: Galia, T., Škarpich, V., and Horáček, M.: Six years of spatiotemporal variations of large wood at the meander scale, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3080, https://doi.org/10.5194/egusphere-egu23-3080, 2023.

Globally, river surfaces release ~1.8 ± 0.3 PgC yr^-1 of carbon dioxide (CO₂). This is larger than the net removal of anthropogenic CO₂ to the land surface of 1.6 ± 0.5 PgC yr^-1, meaning that river CO₂ could act as a leak of carbon back to the atmosphere over the coming decades. To better understand the impact of this large flux on the carbon cycle, we must seek to connect the geomorphic, hydrological and ecological controls on the export of carbon from the terrestrial biosphere, soils and rocks to river networks. Despite the recognition that the release of CO₂ from river surfaces is substantial, we still lack insight on the source, delivery and/or production of CO₂ along rivers. Here we assess the source of riverine CO₂ along a ~250 km transect from the high Andes to the lowland Amazon floodplain, across the upper Madre de Dios basin in the wet season of March 2019. Using floating chamber methods, we quantify CO₂ release from river surfaces. To explore the competition of CO₂ sources from weathering (rock-derived C) and from the biosphere, we use a headspace method to trap CO₂ on zeolite sieves for isotopic analysis (stable carbon isotopes and radiocarbon). The major and trace element dissolved chemistry was also assessed to quantify the dominant weathering reactions. We find downstream variability in CO₂ release from river surfaces (ranging from ~650 to 2900 gC m^-2 yr^-1), with the mainstem of the Madre de Dios at our most downstream location having the highest flux. In contrast, the radiocarbon activity (reported as Fraction Modern, F¹⁴C) of the CO₂ varied much less, with the two major tributaries the Rio Manu and Rio Alto Madre de Dios having F¹⁴C values of CO₂ of 0.818 and 0.824, respectively, while ~150 km downstream the mainstem F¹⁴C of CO₂ was 0.809. The F¹⁴C of a lowland river, not sourced from the Andes, had a F¹⁴C of CO₂ of 0.954, suggesting old organic matter degradation may be underway upstream. Together with the stable C isotope composition and dissolved chemistry, these findings suggest a sustained release of old CO₂ from carbonate weathering sources across this tropical floodplain transect, but that the overall flux is dominated by CO₂ from the terrestrial biosphere that must be efficiently delivered to the river channel.

How to cite: Hilton, R., West, J., Garnett, M., Dellinger, M., and Burt, E.: Sustained old carbon dioxide release from river surfaces across an Andes to Amazon floodplain transect, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13379, https://doi.org/10.5194/egusphere-egu23-13379, 2023.

Microbial activity on Earth’s surface is key in the decomposition of organic matter and humus formation, carbon sequestration, cycling of rock-derived nutrients, and the development of soil structure and stabilization. While the role of microbial life in various weathering processes has been demonstrated experimentally and observed at the nanometer-centimeter scale, the obvious link between microbial life activity and landscape-scale geomorphic processes remains unexplored.

We examined the reciprocal relationship between microbial communities and rates of surface processes in recently deglaciated landscapes in the eastern Sierra Nevada Mountains, California, USA. Sampling along an elevational transect in a glacial basin in the high Sierra (between 2800 and 3050 masl), we quantify exposure ages and rates of soil production using cosmogenic nuclides and examine microbial community ecology in the same rocks and soils. Exposure ages from cosmogenic ¹⁰Be indicate rapid deglaciation around 12-13 ka and relatively fast soil production rates (~4.3-4.5 cm/kyr), independent of elevation.

To understand how these rapid soil production rates correlate with microbial community composition and diversity, we extracted and sequenced environmental DNA from near-surface soils, saprolite samples, and exposed surface rocks. Microbiome sequencing results constrain changes in microbial ecology from rock to soil, shed light on the complex relationship between microbial community dynamics, and weathering rates in the eastern Sierra, and help us to better understand the link between life and landscape evolution.

How to cite: Ben-Israel, M., Lukens, C. E., Peyakov, K., and Beman, J. M.: Landscape-scale links between microbial ecology and surface processes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-471, https://doi.org/10.5194/egusphere-egu23-471, 2023.

River planform results from the complex interaction between flow, sediment transport and vegetation, and can evolve following a change in these controls. Disentangling this complex causation path as a preliminary measure to devising restoration measures is not straightforward. We propose a modelling approach that can be used as tool for analysis of observed trajectories and to forecast future behaviours in dam- and vegetation- impacted braided rivers.

We focus two iconic braided river cases in New Zealand’s South Island: the Lower Waitaki River and the Waimakariri River. The Waitaki is impacted by the combined effects of exotic vegetation and a hydropower scheme that has altered the flow regime. As the Waitaki River is unable to clear vegetation efficiently, vegetation encroachment has promoted a shift towards a single-thread morphology. In contrast, the more active Waimakariri River, despite having been subjected to similar vegetation, retains a largely unvegetated channel due to its ability to naturally clear vegetation.

A two-dimensional physics-based numerical model capable of accounting for the evolution of morphology and vegetation in braided reaches is constructed and applied to the two rivers.

Calibration and validation of the vegetation parameter settings, which is critical to obtaining realistic planform styles, is carried out in applications to the two test cases by selecting the parameter values that allow the model to predict vegetation encroachment in the Waitaki and efficient vegetation clearing in the Waimakariri. The model responds sensibly to changes in parameters, showing that more aggressive vegetation types cause a sharper reduction of braiding.

The calibrated model is applied to reconstruct planform changes in the Lower Waitaki under a reconstructed natural flow regime, showing that, even in the absence of the hydropower scheme, the river would have suffered from vegetation encroachment due to its naturally steady hydrology.

Finally, summary metrics that represent vegetation presence in each model are computed and their dependence on the flood frequency is analysed. We find that vegetation presence across rivers and flow regimes can be explained as a function of the duration of periods of vegetation growth, intervening between floods that cause vegetation removal.

How to cite: Stecca, G., Measures, R., Hoyle, J., and Hicks, D. M.: Modelling the impact of dams and exotic vegetation in New Zealand braided rivers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2150, https://doi.org/10.5194/egusphere-egu23-2150, 2023.

Glacial retreat is a well-known effect of global warming. Where glaciers retreat, land becomes available for soil formation. The water that is produced by the melting of the glaciers forms a stream system in the newly available land, and together these form the proglacial area, or glacial forefield. Proglacial areas are interesting study areas for a negative feedback loop of global warming: where land becomes available, microbial and plant biomass are formed, taking up CO₂ from the atmosphere. For inland glaciers, dry soils generally cover most of the surface of proglacial areas, with only a very small fraction covered by wetlands.

Using detailed carbon stock data, CO₂ flux measurements, and GIS methods, we assessed the contribution of soils and wetlands to the valley-wide carbon storage in a proglacial valley in the Martellertal, Parco Nazionale dello Stelvio, Italy. We explored the relationship among the CO₂ flux, soil carbon content, and location factors such as slope steepness, rock and vegetation cover, and litter layer thickness. Furthermore, we studied the relationship between the soil age, or time since deglaciation, and carbon stocks and fluxes. Our data shows that wetlands are major carbon storage hotspots: not only was the carbon stock significantly higher at wetland sampling locations, also the CO₂ uptake per surface area was significantly higher than in dry soils. These findings suggest that despite their small spatial coverage, wetlands are key areas to consider when assessing proglacial carbon budgets, both from a carbon storage as well as a carbon flux viewpoint.

How to cite: Janssen, N., van Grinsven, S., and Temme, A.: Wetlands are the primary hotspots of carbon accumulation in proglacial areas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11446, https://doi.org/10.5194/egusphere-egu23-11446, 2023.

Glaciers are retreating worldwide due to climate change, creating extensive proglacial margins exposed to solar radiation and hence colonization by phototrophic organisms. The extremely dynamic nature of proglacial margins makes ecological colonization difficult. Whilst proglacial margins have received significant attention from the geomorphology community, their ecological functioning remains less intensively investigated. Classic research has shown that colonization depends on distance from the glacier terminus and on season. However, with current rates of glacier retreat, long downstream distances are becoming exposed in a relatively short time, questioning the validity of this longitudinal chronosequence model. In this research, we decrypt the physical habitat of periphyton in recently deglaciated floodplains and we demonstrate the role that periphyton plays in favoring embryonic ecosystem development.

First, we combine UAV based remote sensing with characterization of local environmental conditions (e.g., inundation extent, rates of disturbance). We show that in proglacial margins periphyton effectively develop extensively during windows of opportunity (i.e., spring and autumn) but they can also develop less extensive but still important extents in summer, during the season of most intense glacial melt. Such development may occur rapidly (timescale of days) in the active zone of the braidplain as access to water is secured. But high rates of morphodynamic reworking means that the periphyton are emphemeral. However, in smaller channels, often fed by hillslope tributaries and/or groundwater, away from the active zone, that are more stable, extensive perennial periphyton cover may develop. As the probability of access to water tends to be positively correlated with the probability of disturbance, extensive perennial periphyton development is spatially restricted.

Second, we deploy in-situ flume experiments to mimic the conditions of stable channels and use close-range photogrammetry and 3D hydraulic analysis. We show that periphyton development strongly modifies the streambed morphology but much less so the near-bed hydraulics. Most importantly, it reduces water vertical infiltration by clogging the streambed interstices. This autogenic response, a form of ecosystem engineering, explains why pioneer vegetation tends to develop in specific locations of a glacial floodplain, and reveals new patterns in primary succession in deglaciated terrains and the important role played by periphyton. However, whilst periphyton can improve local hydrological conditions, they do not appear to be able to counter the potential risks of geomorphic disturbance and it is that which determines the patterns of ecological succession.

How to cite: Roncoroni, M., Mancini, D., Ballu, A., Miesen, F., Müller, T., Gianini, M., Ouvry, B., Clémençon, M., Selitaj, A., Lardet, F., Battin, T., and Lane, S. N.: Highlights on periphyton in a recently deglaciated floodplain, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11318, https://doi.org/10.5194/egusphere-egu23-11318, 2023.