The Chaco Plain (South America) is an aggradational lowland (40-600 m asl) with divergent drainage patterns, at present under a tropical–subtropical climate. Late Quaternary coalescent fluvial megafans constitute the most extensive depositional system of the Central Andean foreland basin. A complex assemblage of macro- and mesoscale landforms of the Southern Chaco plain (22°–31°S) was analyzed at variable spatio-temporal scales, based on remote-sensing data coupled with field and sedimentological datasets. The Pilcomayo, Bermejo, Salado-Juramento and Dulce River megafans are between the largest distributive fluvial systems (DFS) in modern landscapes of Earth. The surface covered by these megafans reaches ca. 5,5 million km², deeply advancing basinward and achieving megafan radii of 600/750 km, most of these to the axial fluvial trunk (Paraná).  They are characterized by highly dynamic fluvial processes related to flooding and partial avulsions. A process-based geomorphological research focusing on Late Quaternary-Present fluvial and wetland dynamics from fluvial valley scale to DFS scale is presented. The spatio-temporal pattern of fluvial aggradation and progradation on these megafans is related to climatic and neotectonic forcings. The influence of the South American Summer Monsoon System during the Late Quaternary produced high seasonal fluctuations in the discharge in their Andean rivers, associated with the high sediment input and short-term sedimentation of the formative rivers. Fluvial distributive channels, alluvial ridges, aggradational lobes and crevasse and terminal splays are the typical landscape elements of the megafans. Foreland basin configuration determined the landscape patterns along these megafans. The mosaic of the Late Pleistocene and Holocene landforms influences the present hydro-geomorphological dynamic of each megafan. Extensive permanent to temporary wetlands were developed in the middle/distal reaches of these DFS, which are characterized by surfaces of extremely low gradients (0.03°–0.18°). These wetlands functioned as sediment sinks. Late Pleistocene (MIS 5/3) wetland sedimentary facies and hydromorphic palaeosols were analyzed from cores obtained from research boreholes in distal areas, considering the lack of exposed stratigraphy across the extensive and flat megafan surfaces. Partial avulsions are frequent in the Holocene river belts in middle/distal areas of the megafans, where large, floodplain-filling splays dominate (overbank processes).  The accelerated expansion in deforestation and agricultural land use on the Chaco megafans enhanced their flood vulnerability under climate change. Hence, understanding the landscape dynamics of the Chaco Plain is fundamental to risk mitigation strategies.

How to cite: Kroehling, D.: Process-landform relationships at variable spatio-temporal scales in the Southern Chaco fluvial megafans (Argentina) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-422, https://doi.org/10.5194/egusphere-egu24-422, 2024.

Rivers globally experiencing a regime shift in sediment load during the Anthropocene. Substantial alterations in sediment transport patterns were also reported in fluvial systems of Indian Peninsula, attributed mainly to the climate change and human activities. This study focuses on small-sized rivers originating from the Western Ghats, flowing over 50 km and debouching into the Arabian Sea with two major objectives: (i) identifying factors governing temporal changes in sediment load over recent decades, and (ii) to evaluate the potential geomorphic and ecological implications to the fluvial settings. To address these inquiries, hydro-meteorological data (discharge, suspended sediment concentration, and rainfall) from the Central Water Commission (CWC) and the India Meteorological Department (IMD) spanning from the mid-1970s to 2018 were examined. Various statistical methods such as the non-parametric Mann-Kendall test, Pettitt test, and double mass plot were employed to recognize trends, abrupt changes, and the interplay of climate and human activities. Results reveals a temporal variation of sediment load in most studied rivers unfolds in four significant episodes: (i) pre-dam construction before 1985, (ii) a phase marked by an equilibrium between rapid deforestation and dam construction during 1985-1995, (iii) reduced deforestation coupled with swift dam construction from 1995 to 2010, and (iv) a relatively stable period post-dam construction since 2010.   Our findings also show a declining trend in sediment load for all the southern rivers of Kerala. Nevertheless, it was found that Netravati, Chaliyar, Bharathapuzha, and Periyar rivers emerge as the leading contributors of sediment to the Arabian Sea among the 18 rivers studied.  This system response has wider implications, including effective management of water resources, sedimentation control, and understanding the ecological responses within the Western Ghats, influencing both human communities and biodiversity in the region.

How to cite: Das, S., Scaringi, G., Ali, Y., and Narayana, A. C.: Climate change and anthropogenic stresses on the sediment load variation in the Western Ghat Rivers, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4459, https://doi.org/10.5194/egusphere-egu24-4459, 2024.

Rivers and floodplains are hotspots of biodiversity that support a large and growing number of people with food, water, nutrients, and transportation. Within these floodplains, the dynamic interplay of erosion on the outer banks and sedimentation on the inner banks propels lateral channel migration, resulting in the creation of intricate and sinuous meandering river landscapes. A key question in river meandering research concerns the ongoing debate regarding the primary driver of lateral river migration. Is it initiated by outer-bank erosion, leading to localized inner-bank deposition (the process known as ‘bank pull’), or is it inner sedimentation that initially diverts the flow, subsequently triggering outer-bank erosion (‘bar push’)? This study introduces an innovative methodology that combines extensive time series analysis of remote sensing imagery with cloud computing to discern the prevalence of bar push versus bank pull across vast sections of the global river network. The methodology involves analyzing each image to pinpoint the precise timing of pixels undergoing erosion and/or sedimentation. For each river bend, we compare the years in which the outer bank undergoes erosion with the years in which the inner bank experiences sedimentation, thereby determining the predominant process—erosion and bank pull, or sedimentation and bar push. Additionally, we explore whether these processes change over time and whether they are correlated with factors such as river sediment load, bank vegetation, river curvature, and other relevant physiographic metrics. By extending this methodology to diverse rivers worldwide, we systematically test the bar push versus bank pull theories in a variety of real environmental settings. This unprecedented, large-scale analysis advances our understanding of meandering rivers and their complex dynamics.

How to cite: Nagel, G., Darby, S., and Leyland, J.: Towards a Global Assessment of Bar Push Versus Bank Pull using Remote Sensing and Cloud Computing., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1364, https://doi.org/10.5194/egusphere-egu24-1364, 2024.

Drainage networks are responsible for water, sediment, and nutrient fluxes across landscapes. In the Amazon region, the freshwater system also houses an unparalleled aquatic biodiversity. Therefore, understanding the origins of drainage network reorganizations across variable spatial and temporal scales is fundamental for various disciplines. To elucidate the evolution of river systems in continent interiors, it is necessary to constrain the mechanisms that regulate the upstream propagation of base-level changes.  Features such as drainage captures, knickpoints, paleochannel and paleovalleys, wind gaps, and asymmetric drainage divides are widespread and have long been observed in the Amazon region. These features are typically thought to result from climate change or intraplate tectonics whereas the influence of rock type as a trigger is largely overlooked. In this study, we link the spatial patterns of landscape transience to lithologic variations in the Amazon Craton. Before their confluence with the Amazon River, the largest left-margin tributary systems exit the Amazon Craton and cross sedimentary rocks of the Amazon Sedimentary Basin (ASB). This lithologic transition is marked by an expressive escarpment formed over resistant sandstones of the basal units of the ASB. Through quantitative geomorphologic analysis of the topography, drainage divides, and rivers, we investigate if this sharp lithologic transition contributed to the observed patterns of drainage rearrangement. The results revealed that rivers with larger drainage areas, lower mean elevations, and that flow shorter distances over the resistant rocks systematically capture neighboring basins. We suggest that this pattern produces the observed ‘ladder-like’ topography where the smallest basins are perched at higher elevations and vulnerable to river captures and divide migrations. We argue that, as tributaries draining the shield respond to downcutting and/or base-level fall of the Amazon River, bedrock incision and knickpoint propagation are differentially slowed down by the resistant rocks according to their incision capacity, generating the observed systematic landscape transience. The observed widespread and systematic distribution of geomorphic transients suggests that lithology could be an important autogenic control of drainage network rearrangement in the Amazon region as well as in other post-orogenic landscapes. The protracted exhumation of resistant rocks in cratons and continental interiors offers an exceptional natural laboratory for studying landscape dynamics associated with rock type.

How to cite: Fadul, C., Oliveira, P., and Val, P.: Rock type as a driver of drainage network reorganizations in the Amazon region, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21442, https://doi.org/10.5194/egusphere-egu24-21442, 2024.

This study proposes a free energy centred approach to surface runoff and morphological development of hillslopes and rivers. The starting point is the strong analogy between the functioning of engines on the one hand and watersheds on the other hand. Like an engine converts energy input into motion/kinetic energy, watersheds and hillslopes convert potential energy inputs by rainfall into potential energy and kinetic energy of surface runoff. The latter determines the maximum work water can perform on the sediments. Similar to the energy efficiency of an engine, relating the free energy/work output to the energy input, we define energy efficiencies for each process in the aforementioned cascade of rainfall-runoff formation and sediment transport. The efficiency in generating potential energy of surface runoff depends on precipitation, the runoff coefficient, topography and landforms. While the vast amount of the runoffs potential energy is dissipated, a minute amount sustains the kinetic energy of surface runoff and stream flow, driving erosive changes in watersheds. Here the efficiency depends on the controls of driving and frictional forces: the geo-potential gradient, material roughness and hydraulic radius of the river. We applied this framework to surface runoff at the hillslope scale and the Amazon basin. 

At the hillslope scale we found that typical morphological stages of hillslope forms and related transitions of dominant erosion processes (from soil creep, rain splash, to soil wash) evolve towards a declining energy efficiency in surface runoff. This implies a reduction in power to trigger future landform changes. However, rill and river networks do essentially the opposite. By reducing the specific dissipation, they increase the efficiency in the conversion of potential into kinetic energy of overland and streamflow. In several cases, rill networks were found to even maximize total power of surface runoff in the sheet and rill domains. 

For the Amazon and its tributaries, we found distinct self-similar patterns of stream flow potential energy along each river course. Starting from the source, potential energy in stream flow was growing with downstream distance, up to a maximum value, and exhibited from there an almost linear decline to the river mouth. This implies that the maximum work the river can perform is growing from its source to this maximum, as the rapid growth in the stream flow mass over-weights the steep downstream decline in geopotential. We found the same behavior at the hillslope scale. 

In a third step, we found that for the largest terrestrial river networks on the world that Horton’s laws of stream area and length were close the Feigenbaum constants characterising bifurcations of logistic growth at related deterministic chaos. This suggest parallels between the interplay of growth and mortality of populations, and the interplay of stream power generation and its turbulent dissipation.

How to cite: Zehe, E., Schroers, S., Kleidon, A., Eiff, O., and Savenije, H.: Linking energy efficiency of surface runoff, logistic growth and landform changes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18107, https://doi.org/10.5194/egusphere-egu24-18107, 2024.

Channel belts, floodplains and fluvial valleys form by the mobilization and deposition of sediments during the lateral migration of rivers. The width of these surfaces is determined by the speed of lateral migration and the average time that the channel migrates laterally before switching direction. Here, we introduce a recent physics-based model of channel-belt width and explore its consequences for transient channel-belt evolution. The model builds on the assumption that the switching of the direction of lateral migration of a channel can be described by a Poisson process, with a constant rate parameter related to channel hydraulics. As such, the lateral migration of the channel can be viewed as a non-standard random walk. We derive an exponential equation to describe the mean approach to the steady state channel belt or valley width. Further, we exploit the properties of random walks to obtain equations for the increase of area visited by the channel (squareroot scaling), the “safe” distance from a channel that is unlikely to be inundated in a given time interval (law of the iterated logarithm), and the mean lateral drift speed of steady state unconstrained floodplains as well as constrained fluvial valleys in uplifting landscapes. All of these equations can be directly framed in terms of the channel hydraulic properties. The results are compared to experimental observations of the inundated area in a large-scale sand box. Finally, we show that the distribution of floodplain ages follows a power law scaling with a scaling exponent of -1.5, close to what has been observed in natural systems. This observation implies that the mean and variance of floodplain ages are infinite, with implications for storage times and chemical alteration of floodplain sediments.

How to cite: Turowski, J., McNab, F., Bufe, A., and Tofelde, S.: Evolution of channel belts, flood plains and fluvial valleys, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9534, https://doi.org/10.5194/egusphere-egu24-9534, 2024.

Since the establishment of the classic Theory of Alpine Glacial Cycles, a close relationship between glacial activities and fluvial terrace development has been observed. However, problems such as the extent and mechanisms through which glacier advances influence downstream fluvial aggradation, are not fully understood. The Aba Basin, located on the eastern Tibetan Plateau, features well-developed river terraces. In conjunction with this, the upstream Nianbaoyeze Mountains have undergone intense glacial activities during glacial periods. The integration of these features make this source-to-sink system an ideal site to study these problems. In this work, we utilized Luminescence and Radiocarbon dating methods to reconstruct terrace sequences in the Aba Basin. Furthermore, geochemical analyses were undertaken to delineate trends in provenance variation during terrace aggradation periods, and subsequently to assess the impact of sediment supply from the Nianbaoyeze Mountains. Integrating our analysis of fluvial evolution in the Aba Basin with glacier activities from the Nianbaoyeze region and correlating them with regional and global paleoclimate data, we present detailed insights into how glacial activities have driven terrace formation in the Tibetan Plateau since the late Pleistocene. Our research offers new perspectives on the fluvial processes in periglacial regions, enhancing the understanding of the interplays between fluvial landform dynamics and glacial-interglacial cycles.

How to cite: Shen, Q., Kang, H., Tao, Y., and Zhang, H.: Fluvial aggradation and incision processes in eastern Tibetan plateau and their relationship to glacial activities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7521, https://doi.org/10.5194/egusphere-egu24-7521, 2024.

Poyang Lake is the largest freshwater lake in China, yet mechanisms controlling its storage capacity variations remain poorly investigated. Here we show that lake storage capacity dynamics are mainly driven by jacking force and outlet channel erosion, based on 39-year daily in situ observations (1980-2018). A lower water level at the lake outlet and a diminished jacking force in the Yangtze River can be attributed to the upstream dams storing water between August and October; consequently, more water from Poyang Lake flows out, causing the impairment of storage capacity. Furthermore, channel degradation near the outlet is likely due to the severe sand mining and hungry-water-driven Yangtze channel erosion, the latter of which implies an enhanced outlet channel scour. As a result, the lake storage capacity has been substantially weakened. Our findings further the understanding of the downstream lake storage capacity responses to dam operation and human activities and have important implications for lake ecology and flood management in large dammed river-lake systems.

How to cite: Hu, Y., Li, D., Deng, J., Li, Y., and An, Z.: Mechanisms controlling storage capacity dynamics in China’s largest freshwater lake, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2690, https://doi.org/10.5194/egusphere-egu24-2690, 2024.

Vertical connectivity between groundwater and river discharge is fundamental in maintaining river health. Groundwater sustains baseflow in rivers during lean period, helps in maintaining e-flow, and facilitates the transfer of nutrients and organisms across the hyporheic zone. The present study has been conducted in a ~300 km long alluvial reach of the Yamuna river from the Himalayan mountain front to Delhi National Capital Region (NCR), India. The Yamuna river is the largest tributary of the Ganga River system. It originates from the Higher Himalaya at an elevation of 6387m with an average annual rainfall of 906 mm. Seasonal and downstream variability in vertical hydrological connectivity along long profile was assessed using field measurements and laboratory analysis of stable isotopes.

A total of 71 samples from both groundwater and river water were analyzed for δd, δ¹⁸O, and d-excess values during both pre-monsoon and post-monsoon seasons of the year 2021. The average δ¹⁸O isotopic composition of most of the post-monsoon river water samples exhibit depletion in heavy isotopes in comparison to pre-monsoon samples. Depleted δ¹⁸O values indicate increase in rainfall contribution to river water during monsoon period. The low slope of the pre-monsoon groundwater in the graph plotted between δ¹⁸O and δd indicate lack of groundwater recharge during pre-monsoon period. This is responsible for higher values of δ¹⁸O isotopic composition of the groundwater, which is especially significant in the upstream reaches of Delhi NCR. Seasonal changes are evident only in the reaches upstream of Delhi NCR with similar pattern in groundwater and river water isotopic values. Data indicates a well-developed vertical hydrological connectivity in the upstream reaches. Our field based measurement of river and groundwater head validates the existence of hydrological connectivity in this reach. However, vertical connectivity is not apparent in the downstream reaches around Delhi NCR, as the seasonal changes in the isotopic composition of both river and groundwater are not prominent. Integrating these findings with water depth measurements in the field facilitated that out of 4 sub-reaches of our study area loose water to the groundwater, while 3 sub-reaches are gaining streams. This study will help to identify areas of concern and develop an effective management and conservation strategies to protect and restore the health of the river.

How to cite: Karnatak, N., Jain, V., Shekhar, S., Padhya, V., and Deshpande, R. D.: Investigating Hydrological Connectivity inthe Yamuna River System, India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19826, https://doi.org/10.5194/egusphere-egu24-19826, 2024.

The Loire River is France's largest river, with a watershed covering 1/5 of the country's total area. However, we know very little about the reajustment of the Loire River fluvial forms to climate change. To better predict future consequences, we study the Loire River system in the past, and more specifically during the Little Ice Age (LIA) (14^th-19^th c.). In the light of studies on other European rivers, we assume that the LIA intensified the Loire River hydrological activity and sediment transport, and consequently modified its fluvial pattern. The use of documentary archives enabled us to characterize phases of intense hydrological activity during the LIA (Mesmin et al., accepted¹). The aim here is to study the impacts of the LIA on the readjustment of fluvial forms and on the sedimentary construction of the floodplain using a morpho-sedimentary approach.

This study is essentially based on the analysis of historical Loire River paleochannels, combining two approaches. The first involves studying the evolution of channel geometry using Lidar images, old maps (since first half of the 18^th century) and geophysical measurements (ERT). This approach enables us to precisely characterize the fluvial paleo-forms and calculate the discharges associated with the major floods of the LIA. The second approach focuses on the study of the sedimentary filling of paleochannels in order to precisely characterize the deposits of major floods. Over thirty boreholes were drilled in the Loire River paleochannels.  Grainsize measurements (every one cm) were carried out, coupled with magnetic susceptibility and XRF measurements, to determine variations in the type of sedimentary deposits and assess sedimentation rates as a function of fluvial unit type. Deposits has been dated by C14 and OSL dating. The complementary nature of the method helps to reconstruct the evolution of the Loire River floodplain over the last millennium. The results show that LIA discharges were much larger than current flood discharge, explaining the construction of very large paleo-channels. The paleochannels filling revealed the importance of sandy deposits in paleochenal formation during the LIA. However and surprinsingly, silty overbank deposits on the floodplain are relatively thin. Finally, while fluvial metamorphosis of the Loire downstream has been documented during the LIA, in our study area further upstream, fluvial metamorphosis may not have occurred.

¹ Mesmin, E., Gautier, E., Arnaud-Fassetta, G., Foucher, M., Martins, G., Gob, F., accepted. Characterization of periods of high and low hydrological activity in the Loire River, France, between the 14th and mid-19th centuries. Journal of Hydrology.

How to cite: Mesmin, E., Gautier, E., Arnaud-Fassetta, G., Saulnier-Copard, S., and Virmoux, C.: Study of the impacts of the Little Ice Age period on the Loire River floodplain: contributions of the morpho-sedimentary approach., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16631, https://doi.org/10.5194/egusphere-egu24-16631, 2024.

Large anabranching lowland river banks comprise sediments of varying strength, resulting from erosional and depositional processes that act over geological timescales. Although bank strength variability is known to affect channel morphodynamics, it often remains unquantified and its effect on the migration of large sand bed rivers is poorly understood. Measurements from a ~100 km long reach of the Solimões River, the upstream part of the Brazilian Amazon River, show that cohesive muds in Pleistocene terraces along its right bank have bank strengths up to three times greater than Holocene floodplain deposits that comprise the left bank. Image analysis reveals that these resistant outcrops control channel-bar dynamics: channel widening and bar deposition are inhibited, which reduces topographic forcing and stabilizes the opposing erodible bank. Analysis of the 1,600 km long Solimões River shows that where the channel is associated with older terraces, lower rates of bank erosion and deposition rates between 1984-2021 are evident. In locations where the channel has migrated away from the resistant terrace, further change analysis finds that channel has a strong tendency to rapidly migrate back. Analysis of water surface slope for the Solimões River finds scant correlation between water gradient and migration. Bank strength heterogeneity is thus the primary control on the large-scale morphodynamics of the world’s largest lowland river.

How to cite: Aalto, R., Brückner, M., Best, J., Paes de Almeida, R., Nicholas, A., Ashworth, P., and Ianniruberto, M.: Bank Strength Variability controls the system-scale Morphodynamics of the Solimões River, Brazil, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13875, https://doi.org/10.5194/egusphere-egu24-13875, 2024.

Currently, in the northern hemisphere, ice cover forms annually on rivers located in high latitudes. However, in the context of future climate changes, this natural phenomenon may undergo alterations, resulting in later ice formation and earlier ice breakup compared to traditional patterns. These changes affect the flow characteristics of rivers and, consequently, impact river environments, especially sediment transport, nutrient transport, and aquatic habitats. While there has been an increase in field measurement-based studies during past decades, our understanding of the effects of river ice cover on flow dynamics within natural environments remains constrained. Specifically, the influence of ice cover on near-bed flow characteristics has been relatively underexplored due to the difficulties in obtaining accurate data in such conditions.

When ice cover forms in a river, it creates an additional roughness layer compared to open-channel conditions. This roughness layer alters flow conditions by creating an asymmetrical flow structure. Under a fixed ice cover, flow velocity increases, and maximum velocities might be higher than under similar discharge conditions in an open channel. The flow gradient near the riverbed is notably different in an ice-covered channel than in an open one. In this study, we aim to 1) detect the impacts of ice cover on near-bed flow direction and 2) compare the effects of different riverbed roughness on near-bed flow conditions. The aim is to assess how ice cover and riverbed roughness alter flow characteristics and quantify the effects that might occur when river ice is not present in a similar form to before.

The study site is a meander bend of the sub-arctic Pulmanki River, located in Northern Finland. The Pulmanki River undergoes annual freezing, with the ice-covered season typically extending from October through May. Field measurements have been conducted during the ice-covered low flow period spanning from 2016 to 2024 using ADCP (Acoustic Doppler Current Profiler) and ADV (Acoustic Doppler Velocimeter). Near-bed flow directions are analysed based on the comparison of different field measurements. Analysis of the effects of different riverbed forms, roughness,  and prevailing ice conditions on flow characteristics is conducted. The results of this study contribute to the understanding of river ice processes and, therefore, help improve the management of river systems under a changing climate.

How to cite: Lintunen, K., Lotsari, E., Takala, T., Blåfield, L., Kasvi, E., and Alho, P.: Near-bed flow directions under varying ice cover of a meander bend in 2016–2024, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2094, https://doi.org/10.5194/egusphere-egu24-2094, 2024.

River functions and morphology are strongly impacted by the amount of sediments in a river, which are often supplied by river bank erosion. To better understand rivers as a whole it is thus important to know the timing and causes of river bank erosion, which are difficult to assess with current sensor technology. Especially in cold-climate regions where a large variety of processes occur that contribute to river bank erosion, identifying the timing and causes of bank erosion is challenging. We employed sensors, originally developed for the agricultural sector, in a novel way to obtain a one-year dataset of soil moisture, soil temperature and soil movement in real-time and with a high temporal resolution on three banks of northern rivers with different geographical, climatological and landscape characteristics. Thus, this research used a new type of dataset of soil temperature, moisture and movement never used before. We compared the timing of soil movement events with the soil temperature, soil moisture, air temperature and with discharge information. Specifically, the distribution of the soil movement events in time at each field site was analyzed. Furthermore, the distribution of time-lag between changes in these variables and a soil movement event were considered. The analysis of the results showed that there is no clear temporal distribution of bank movement in the Southernmost investigated site, while in the more Northern field sites soil movement was most frequent around the freezing and thawing periods (e.g. spring and autumn). At the northernmost field site an additional period with an increased frequency of soil movement events is likely caused by reindeer during summer months. The time-lag analysis also shows that freeze-thaw is likely the main driver of soil movement events in the investigated sites. The novel sensors allowed us to obtain a unique dataset which we used to identify individual soil movement events and helps to better understand river bank erosion and by extension fluvial systems in cold-climates.

How to cite: van Rooijen, E. and Lotsari, E.: Distributions of river bank erosion in cold-climate regions identified using novel real-time sensors, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7517, https://doi.org/10.5194/egusphere-egu24-7517, 2024.

The morphological trajectory of river bifurcations is commonly investigated through one-dimensional models. In this approach, the two-dimensional topographic effects exerted by the bifurcation are simply accounted for by a nodal point relation, which quantifies the amount of sediment that is transported towards each downstream branch. The most widely-adopted nodal point relation is based on considering two computational cells located just upstream the bifurcation node, which laterally exchange water and sediments. The results of this approach strongly depend on a dimensionless parameter that represents the ratio between the bifurcation cell length and the main channel width, whose value needs to be empirically estimated. An interesting possibility is the calibration of this parameter on the basis of the analysis of existing  two-dimensional linear models, which directly solve the momentum and mass conservation equations. Following this idea, I demonstrate that a full consistency between the one-dimensional approach and the two-dimensional models can be directly achieved by adopting different scaling for the bifurcation cell length, which results in a theoretically-defined and constant dimensionless cell length parameter. Comparison with experimental observations reveals that this physically-based scaling yields more accurate predictions of bifurcation stability and discharge asymmetry. This constitutes  a starting point for incorporating other factors that are typically observed in natural settings, such as flow variability and non-trivial plainform configuration. In conclusion, this work provides a physically-based method for parameterizing one-dimensional bifurcation models, easily incorporable in existing models of braided networks, channel deltas or individual channel loops.

How to cite: Redolfi, M.: A physically-based estimation of the length parameter in river bifurcation models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18108, https://doi.org/10.5194/egusphere-egu24-18108, 2024.

River bedforms affect bed roughness and, with that, fluvial hydraulics. With increasing flow velocity, subaqueous bedforms grow from flat beds to ripples to dunes, before diminishing again to an upper stage plane bed. Previous studies report an increase in the standard deviation of bedform height with increasing transport stage (a measure of flow strength), and rapid switches in time between contrasting bed configurations. Not much attention has been given to this phenomenon despite its importance in, for example, flood prediction. This study reanalyzes experimental data from earlier experiments. We show that there are statistically strong indications that the increase in standard deviation is due to the emergence of bimodal distributions in river dune height for transport stages larger than 18. This is consistent with our understanding of the physics, as time series of observed dune heights exhibit flickering between low and high dunes, suggesting critical transitions between two alternative morphological states. We hypothesize that local sediment outbursts drive temporary shifts from suspended- to bed load conditions, causing dunes to form transiently before returning to an upper stage plane bed. Flickering behavior of dunes at high transport stages implies that one single snapshot is not enough to capture the state of a system, with far-reaching implications for field measurements and experimental designs. The possibility of alternative dune states also calls for a reconsideration of classical equilibrium relations. This study implies a presence of tipping points in geomorphology and calls for further research to understand and quantify flickering behavior in sediment beds at high transport stages.

How to cite: van de Vijsel, R. C., de Lange, S. I., Torfs, P. J. J. F., and Hoitink, A. J. F. (.: Bimodality in river bed state suggests critical transitions at high flow, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2103, https://doi.org/10.5194/egusphere-egu24-2103, 2024.

Self-organising phenomena play a crucial role in river systems, from the catchment scale to the single morphological unit. Many studies on morphological patterns in straight channels have been conducted, but full regime diagrams in fundamental parameters (e.g. aspect ratio and mobility parameter) are not known yet.  However, determining such morphological regimes is crucial for prediction of river dynamics and consequently for a range of applications, from flood risk assessment to ecological management and restoration projects.

This study employs a 2D numerical model based on spectral methods to explore these morphodynamic regimes in a straight channel, a first approximation of most existing managed rivers. The model couples the depth-averaged shallow-water and continuity equations for the flow and the Exner mass-conservation equation for the sediment. The morphological patterns arising from the simulations are explored in a wide range of parameter space, beyond the critical values for bar formation used in perturbation methods. A thorough taxonomy of planforms and their complexity is generated, resulting in a morphodynamic regime diagram for straight channel flows. Moreover, both the spatial and temporal characterisation of the solutions is carried out, and the presence of quasi-periodic and chaotic behaviour is investigated.

How to cite: Weber, F., Toffolon, M., Dijkstra, H. A., Colombini, M., and Siviglia, A.: Numerical Exploration of the Morphodynamic Regime Diagram for Straight Channel River Flows, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12442, https://doi.org/10.5194/egusphere-egu24-12442, 2024.

The geometrical characteristics of subaqueous dunes may exert a strong control on hydraulic roughness. Conventionally, the prediction of dune existence and geometry relies on phase diagrams and empirical equations tailored for uniform, cohesionless sediments. However, in deltas, estuaries, and lowland rivers, mixtures of sand, silt, and clay are prevalent, which hamper dune prediction. We study the impact of fine sand-silt mixtures on the geometry of dunes. We built a sediment recirculation facility in the Water and Sediment Laboratory at Wageningen University, capable of recirculating mixtures of sediment. The system is composed of a 15-m long tilting flume and a low reservoir where sediment is kept in suspension using a caterpillar system, from where sediment is pumped back into the flume. We systematically varied the sand and silt content for different flow rates. We measured flow velocities profiles with an acoustic Doppler velocimeter (UB-Lab 2C by Ubertone), and captured the bed geometry using a line laser scanner. The mobility of the bed is clearly influenced by the bimodal characteristics and the level of cohesion. When non-cohesive fine sand or coarse silt were introduced to medium-sand base material, we infer that the hiding-exposure effect amplified the mobility of the coarser material. This resulted in increased dune lengths, affecting dune steepness. Conversely, the addition of weakly cohesive fine silt reduced sediment mobility and suppressed dune length. As a consequence, sediment composition has an indirect influence on hydraulic roughness, which was significantly related to leeside angle. During the high flow rates, our results suggest flickering between alternative stable river bed states.

How to cite: Hoitink, A., de Lange, S., Niesten, I., van Veen, S., Lammers, J., Waldschlager, K., Baas, J., and Boelee, D.: Fine sediment in mixed sand-silt environments impact bedform geometry by altering sediment mobility, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12699, https://doi.org/10.5194/egusphere-egu24-12699, 2024.

Groins are devices used for riverbank protection. These macrostructures change the river flow, which impacts sediment and nutrient transport. Furthermore, in the inter-groin space, flow recirculations are observed. Aiming at characterizing the flow field in and along a groin field, a comprehensive experimental program is put forward by considering, at this stage, a river reach modeled with a fixed bed and targeting to characterize different flow parameters: mean velocity distribution, streamlines in the function of the flow rate.

Using a Lagrangian technique, an analysis is carried out to determine relevant flow statistics to compute turbulent variables. Additionally, we assess the probability of particle entrapment in the inter-groin area and the distribution of residence times of particles in that area. 

The scale effects and limitations of the experimental approach are presented and discussed. Future lines of work regarding the interaction between the groin field and a loose bed are discussed, improving the understanding of the transport and distribution of suspension load, contaminants, and nutrients in the river regions of groins.

How to cite: Fuchs, Y., Rüther, N., Chen, M., and Aleixo, R.: Velocity field in a groin field: implications for sediment and nutrients transport, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19752, https://doi.org/10.5194/egusphere-egu24-19752, 2024.

Braided river and meandering river are possible to form a braided-meandering transitional river, which has been concerned by scholars from sedimentology, geomorphology and hydrology. Traditional studies based on satellite data, field outcrop and drilling data while lacking understanding of its formation process and mechanism. In this study, sedimentary numerical simulation based on hydrodynamics is adopted to reproduce the development process of braided-meandering transitional river and compare it with modern river sedimentary data for verification. Through analyzing different sedimentary elements such as bar, channel and bank, The development process of sandy braided-meandering transitional river is revealed as follows: 1) Due to the extension and lateral accretion of the tail of the unite bar, a large number of secondary braided channels are filled, forming a compound bar, and the main channel gradually emerges, controlling the morphology development and evolution of the river bed; 2) The sediment and discharge of the channel on both sides of the compound bar are asymmetrical, and the sediment of the main channel is carried by denudation to the secondary channel near the bank and then unloaded and filled. Finally, the compound bar is connected with the embankment, and the main channel gradually presents a curved shape. 3) The main channel constantly adjusts and transforms the banks and bars. The concave bank is eroded and the sediments are transferred to the convex bank due to tranverse circulation. At this time, the compound bars develop multi-stage lateral deposits and the main channel migrated laterally. This understanding is of significance to the characterization of palaeochannel sediments.

How to cite: Li, R. and Zhang, X.: Sedimentary numerical simulation of the development mechanism in sandy braided-meandeing transitional river, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2527, https://doi.org/10.5194/egusphere-egu24-2527, 2024.