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GM5.1

EDI
Fluvial systems from source to sink: dynamics and interactions across scales

Fluvial systems cover much of the Earth’s surface; they convey water, sediments, and essential nutrients from the uplands to the sea, intermittently transferring these materials from the river channel to the adjacent floodplain. The routing of sediment and water through the channel network and into the fluvial-tidal transition zone initiates complex process-form interactions as the river bed and banks adjust to changes in flow conditions. Despite their ubiquity, little is known about the landform-driven morphodynamic interactions taking place within the channel that ultimately determine patterns of sedimentation and changes of channel form. Furthermore, an understanding of how these process-form interactions scale with the size of the fluvial system is also currently lacking. Recent technological and methodological advances now afford us the opportunity to study and to quantify these process-form interactions in detail across a range of spatial and temporal scales.

At the coast, complex physical and ecological interactions coupled with anthropogenic and climate driven changes in sediment supply and hydrological behaviour control net changes in the filling of accommodation space. Sustainably managing these coastal regions requires a holistic understanding of how these processes interact over different timescales.

This session aims to bring together interdisciplinary researchers working across field, experimental, and numerical modelling approaches who are advancing methods and providing new insights into: (i) sediment transport and morphodynamic functioning of fluvial and coastal systems, (ii) evaluating morphological change at variable spatial and temporal scales, such as at event vs. seasonal scales, (iii) investigating the sedimentology of these river systems, and iv) understanding how anthropogenic and climate changes may influence the evolution of these systems. We particularly welcome applications which investigate the morphodynamic response of fluvial systems in all types and sizes and we would specifically like to encourage submissions from early career researchers and students.

Convener: Joshua AhmedECSECS | Co-conveners: László BertalanECSECS, Christopher Hackney, Eliisa Lotsari, Anne BaarECSECS
Presentations
| Fri, 27 May, 08:30–11:50 (CEST), 13:20–16:02 (CEST)
 
Room D1

Fri, 27 May, 08:30–10:00

Chairpersons: László Bertalan, Anne Baar

08:30–08:33
Welcome and Introduction

08:33–08:43
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EGU22-6082
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solicited
The role of bed morphology and channel curvature in the redistribution of momentum in a series of meander bends, Pearl River, USA
(withdrawn)
Kory Konsoer et al.
08:43–08:50
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EGU22-12139
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ECS
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Highlight
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Virtual presentation
Rucha Sanjay Deshpande and Sanjay Kumar Mandal

Rivers are one of the most dynamic features on the Earth's surface. Over time, river channels migrate across its floodplain either gradually or rapidly in response to erosion, accretion, sediment transport, or high-water flow events, respectively. These lateral migrations of the river channel could be detrimental to the human settlements, infrastructure, and ecological elements in the floodplain region. Indo-Gangetic plain is the world's largest alluvial tract, drained by rivers such as Ganga, Brahmaputra, Indus, and their tributaries. Most of these rivers are known to be very dynamic and have the potential of affecting a large population. Previous studies focused on individual rivers to understand the spatiotemporal patterns of channel migration. However, regional-scale analysis becomes necessary to understand the large-scale controls on river dynamics and determine their response to future climate change and anthropogenic activities. This study intends to map and measure migration rates of all the major river channels in the Himalayan foreland basin using Landsat imagery from 1990 to 2020. We generated annual active channel binary masks from Landsat imagery using Google Earth Engine. We delineated the centerline of channels and calculated channel migration rates between consecutive years using the RivMAP toolbox in MATLAB. Here we show that the elevation difference between the river channel and its floodplain acts as a spatial constraint and controls the relationship between channel patterns and migration rates. Channel segments with higher elevation differences correspond to less channel movement and vice versa. Additionally, we explore the effects of anthropogenic activities on river dynamics in the study area.

How to cite: Deshpande, R. S. and Mandal, S. K.: Spatiotemporal migration patterns of rivers across the Himalayan foreland basin, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12139, https://doi.org/10.5194/egusphere-egu22-12139, 2022.

08:50–08:57
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EGU22-8795
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On-site presentation
Rebecca Hodge et al.

The 3D morphology of a river channel is a key control on the resistance experienced by the flow, affecting the channel hydraulics, sediment transport and in-stream habitats. In bedrock channels, the morphology is a combination of multiple components including exposed bedrock, overlying sediment cover, and boulders. Furthermore, the morphology of exposed bedrock is expected to depend on the lithology, rock structure and the dominant erosion processes. In order to improve predictions of flow resistance in bedrock channels, we first need to understand how these different components contribute to the overall bed morphology. Here, we present high-resolution (cm-scale) topographic data collected using terrestrial laser scanning from multiple bedrock channel sections on different lithologies. We compare how roughness varies between these channels, and at different spatial scales. We identify the dominant wavelengths at which roughness occurs, and consider how they correspond to the different channel components. We also consider the extent to which lithology and sediment grain size appears to affect channel morphology and roughness.

How to cite: Hodge, R., Beer, A., and Asher, B.: Decoding bedrock channel morphology using high-resolution topographic data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8795, https://doi.org/10.5194/egusphere-egu22-8795, 2022.

08:57–09:04
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EGU22-25
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ECS
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Virtual presentation
Adeyemi Olusola et al.

The Niger River, the third-largest river on the African continent with a length of about 4,200 km, is a major transboundary river that flows through 9 countries. It is a mainstay of the economy in West African countries with over 20 million people directly or indirectly depending on it for their livelihoods. Although the geomorphology of the Niger Delta is well studied, comparatively little is known about the fluvial portions of the system, despite their considerable value to local communities. Here, we focus on a ~100 km braided segment of the Lower Niger River located between Lokoja and Idah, downstream of the Niger-Benue confluence in south-central Nigeria. The hydrogeomorphology of the segment is largely controlled by the Kainji Dam (Niger tributary) and the sediment supplied from the Benue catchment (Benue tributary). Our study aims to assess changes in river planform by applying semi-automated satellite imagery analyses. We use the cloud-based computing platform Google Earth Engine (GEE) to analyse multi-temporal collections of Landsat, Sentinel and Planet satellite imagery acquired between 1987 and 2021. At decadal time intervals, we classify the active river channel (including water and exposed alluvial deposits) using a HSV colour representation of the RGB imagery, combined with conventional multispectral indices. We quantify areas of erosion and accretion to identify laterally dynamic reaches during the analysis period. We link these findings to changes in the hydrological regime using discharge estimates from nearby gauging stations. Findings are useful for predicting and building resilience to river-related hazards in dynamic landscapes and will support sustainable river management interventions in the area.

How to cite: Olusola, A., Boothroyd, R., and Adelabu, S.: Geomorphic characterization of the Lower Niger River using Google Earth Engine , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-25, https://doi.org/10.5194/egusphere-egu22-25, 2022.

09:04–09:11
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EGU22-472
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Virtual presentation
Rahul Devrani et al.

The high and low flood events in a braided river system cause perpetual changes in the channel morphodynamics and make it difficult to understand a channel response.  Therefore, the management of braided channel morphodynamics becomes a challenging issue for flood mitigations and river restoration purposes. The lower Brahmaputra river basin has the world’s most extensive braided river system, and every year it confronts reoccurring monsoon driven flooding causing widespread flood inundation and changes in channel morphodynamics. Such adequate conditions promise a natural laboratory to understand dynamics channel morphodynamics changes with high and low flood events. The present work integrates the mutual effects of varying channel area, width and sinuosity, and sediment bar area along a braided channel reach in the selected reach in the Brahmaputra River in thirty events during high discharge months of 2018, 2019 and 2020. To observe detailed channel morphodynamics changes, we developed 100 grids with a width of ~6.25 km enclosing the selected reach. These grids are maintained stationary for each year and were used to extract the Brahmaputra River channel area (BRCA)  and Brahmaputra River sediment bar area (BRSBA) and average Brahmaputra River channel width (BRCW) for each grid. We developed a site-specific Google Earth Engine algorithm to delineate the channel and sediment bar in the selected reach to perform supervised classification on Sentinel-1 SAR GRD and Sentinel-2 level-1C.

The results show that grids upstream of the selected reach have a high BRCA, BRSBA and BRCW, and these grids are located around high clustered flood inundated hotspot regions.  We also found that the world's largest river island  (Majuli) is also located in this zone. In the present study, we also observe that if we consider BRCA, BRCW and BRSBA for the same event, the BRSBA has a high correlation with the BRCW compared to the BRCA. Further, we compared the impact of the sinuosity on BRCA, BRCW and BRSBA of regular and influential flood events. During influential events, the sinuosity has only a good impact on the BRCA, and during regular events, it has a higher impact on BRCA and BRSBA. We conclude that in the braided river system of the Brahmaputra River, the channel and bar area and channel width are highly correlated during flood events, and the channel sinuosity also controls channel and bar area and channel width during regular and influential flood events.

How to cite: Devrani, R., Srivastava, P., Kumar, R., and Kasana, P.: Flood induced channel morphodynamics variability of a braided river system in the lower Brahmaputra river basin, India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-472, https://doi.org/10.5194/egusphere-egu22-472, 2022.

09:11–09:18
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EGU22-2464
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ECS
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On-site presentation
Aditya Kumar Anand and Sarada Prasad Pradhan

In the middle and upper reaches of the Brahmani River, Odisha is prone to riverbank erosion and the avulsion process. The river devours a colossal landmass due to major floods and rainfall in the monsoon season. In the present study, morphodynamics of the Brahmani river has been studied for the past two decades (2000-2019). The study has been carried out using LANDSAT data to determine the changes in the channel belt of the river. Channel area, bar area, sinuosity, and braid-channel ratio have been determined to understand the avulsion and bank line shifting. The Brahmani River has been classified into ten classes based on their geomorphological and geological characteristics and further subdivided into 87 reaches. It has been observed that the middle and upper reaches (2000-2015) of the river depict variation in channel area and bar area. The variation in the channel area and bar area is less during 2015-2019 due to the construction of embankments and groyne on river banks. The higher sinuosity values are observed in the lower reaches of the Brahmani River. The major flood of 2011 had significantly affected the sinuosity pattern of the river. Similar trends have been observed in the braid channel ratio, which has been observed in the channel area. These parameters have been correlated with India Meteorological Department's gridded rainfall data (0.25°*0.25°) and discharge data collected from the Department of Water Resources, Odisha. Avulsion threshold index (ATI) was determined to classify the 87 reaches into stable, moderately stable, Critical, and most critical zones. The soil samples have been collected from the critical zones of the Brahmani River to determine their shear strength properties. A detailed geotechnical investigation of riverbank soil samples has been done in the laboratory. The shear strength properties of soil samples have been determined in consolidated undrained (CU) and unconsolidated undrained (UU) conditions. The numerical simulation of bank slopes has been done using PLAXIS 2D software under different conditions. The results obtained from numerical simulation determined the potential zones of failures.

How to cite: Anand, A. K. and Pradhan, S. P.: Morphodynamics of Brahmani River, Odisha, and its implication on Riverbank failure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2464, https://doi.org/10.5194/egusphere-egu22-2464, 2022.

09:18–09:25
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EGU22-4418
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ECS
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On-site presentation
Qing Li et al.

River patterns in the tropics usually exhibit seasonal changes because of strongly seasonal rainfall and its impact on geomorphological processes. However, studies on tropical rivers are much less than those on temperate rivers, and so these seasonal patterns have not been quantified. To fill some of this research gap, this paper employs machine learning methods using Sentinel-2 multispectral remote sensing images to classify geomorphological units in Bislak River, Laoag River and Abra River in west Luzon, the Philippines. In this study, we firstly designed a workflow for river pattern classification, which was validated for the three rivers at different spatial and temporal scales. Then, 5.5 years of river patterns, defined using three morphological units, in the three rivers were generated for further geomorphological analysis. The classification results were analysed in terms of both spatial and temporal aspects. The results show a variety of relationships between channel width and each landform unit (wetted channel; exposed sediment bar; vegetated bar). The analysis shows that channel width has an impact on the area occupied by vegetation (the bigger the river, the stronger the correlation between channel width and vegetation). We present a way to analyse interactions between geomorphic units at seasonal scales using time series of correlations. The rivers were divided into sub-reaches based on observed patterns of water frequency and confinement, and then temporal analysis was undertaken for each sub-reach. This analysis used Ensemble Empirical Mode Decomposition (EEMD) which decomposed the time series and precipitation. The EEMD results indicate that areas occupied by water and vegetation commonly show synchronised fluctuations with precipitation, while sediment bars have an anti-phase oscillation with precipitation. The results suggests that deviations from periodic consistency in patterns may reflect the influence of extreme events and/or human disturbance. Correlation results show that the total area of unvegetated bars is usually the most stable landform unit in all three rivers, and that the vegetated area changes less in narrower channels. Confinement, due to hillslope and terrace topography, and the impact of fault are also considered. The methods for generating time series of landform unit data and time series analysis used here provide a framework for analysis of tropical rivers that are subject to regular, frequent and dynamic changes of planform.

How to cite: Li, Q., Barrett, B., Boothroyd, R., Hoey, T., and Williams, R.: Inter-annual river patterns change detection using machine learning, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4418, https://doi.org/10.5194/egusphere-egu22-4418, 2022.

09:25–09:32
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EGU22-9970
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ECS
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On-site presentation
Riccardo Bonanomi and Marco Tubino

In the last two decades several models have been proposed to analyse the evolutionary trajectories of meandering rivers (e.g., Seminara et al., 2001; Camporeale et al, 2007; Frascati et al., 2009; Frascati & Lanzoni, 2013; Eke et al., 2014; Bogoni et al., 2017; Monegaglia et al., 2019). These models are based on the assumption that channel migration, which is locally driven by the differential excess of flow speed at the banks, is globally governed by the average bankfull geometry. Previous studies suggest that bankfull parameters strongly affect meander development. More specifically, the planform shape depends on the width ratio falling below or above a resonant threshold: sub-resonant meanders are typically downstream skewed, while super-resonant meanders exhibit upstream skewed loops and are prone to evolving much faster. Therefore, the model adopted to define, at each time step, the variation of bankfull parameters fundamentally affects the morphodynamic regime of meanders. A common strategy to initialize the simulations and to set the reference values of bankfull parameters is the use of a quasi-straight configuration. This is a legitimate way to obtain a fully developed meandering planimetry; however, this initial configuration is often used in conjunction with bankfull parameters derived from field data, which implies that the values of the external independent variables, water discharge and sediment supply, that characterize the simulated configuration are similar between such initial state and the fully developed one. However, when combined with the widely adopted assumption that the channel slope must decrease proportionally to meander elongation, this leads to significant variations of bankfull parameters, with a dramatic drop of the transport capacity, as the channel length can increase by two-four times. Therefore, the values of bankfull parameters of the statistical steady-state that the system eventually achieves in long-term simulations (Camporeale et al., 2008; Bogoni et al., 2017) can be quite different from those selected as initial reference values, which may lead to simulating unrealistic evolutionary scenarios and shifts of the morphodynamical regime. However, such a strong variation of bankfull parameters must be viewed as a gimmick introduced by the initial quasi-straight configuration. Based on these considerations, results of long-term simulation need to be revisited taking the bankfull parameters of the statistical steady-state as reference values.

In our work we analyze planimetry features, such as the sinuosity, and their oscillations, when we vary these bankfull parameters. Moreover, we look into the dependency on the aspect ratio of the fully developed state to better understand how super or sub-resonant regime affects the planimetrical configuration.

How to cite: Bonanomi, R. and Tubino, M.: Bankfull parameters of meandering rivers in long term average state, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9970, https://doi.org/10.5194/egusphere-egu22-9970, 2022.

09:32–09:39
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EGU22-10729
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On-site presentation
Andrew Wickert and B. Nilay İşcen

Alluvial rivers aggrade and incise by moving sediment while simultaneously evolving their hydraulic geometries. For both gravel- and sand-bed rivers, stress-based criteria for equilibrium channel width in turn maintain a constant bed shear stress and therefore linearize the sediment-transport response to changing river discharge. Here we demonstrate that realistic sediment-transport and width-closure relationships yield a stream-power form for sediment discharge. Differentiating this in space (i.e., taking the divergence) yields a slightly nonlinear diffusion equation that describes long-profile evolution. This simple equation-coupling work suggests that a single equation may suffice to describe river long-profile evolution from the bedrock--alluvial transition to the point at which backwater effects become significant.

How to cite: Wickert, A. and İşcen, B. N.: Quasi-universal relationship for alluvial river long-profile evolution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10729, https://doi.org/10.5194/egusphere-egu22-10729, 2022.

09:39–09:46
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EGU22-6403
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ECS
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On-site presentation
Yuniarti Yuskar et al.

The relationship between alluviation and environmental change in tropical fluvial environments remains extremely poorly known, especially over late Pleistocene to Holocene time scales. At the same time, new developments in dating allow more precise quantification of associated deposits, opening up the opportunity to determine the alluvial history and relate it to environmental change. The Kampar River is a meandering tropical river in Sumatra island, Indonesia. This river has a gentle slope, gravel bed-river, and their bends migrate because of bank erosion. Fluvial dynamics are recorded by vertically and laterally changing sediment facies due to bend growth. This study develops a chrono-stratigraphical framework at the time scale of the Late Quaternary for the Kampar River to better understand its long-term fluvial dynamics and flooding frequency relative to changing climate and vegetation. We applied sedimentological analysis utilizing XRD and granulometry analysis to investigate mineralogy and facies changes. However, sedimentological analysis is insufficient to correlate sediment facies because this river is highly dynamic. Therefore, the reconstruction of morpho-dynamics in the meandering river is a challenge. Satellite images and drone imagery are utilized to document the geometry of the river. Then, to clarify the chrono-stratigraphy, sediment dating becomes essential. Due to the challenging characteristics of quartz optical stimulated luminescence (OSL), we applied infra-red stimulated luminescence (IRSL) of K-feldspar to constrain a chronology for the fluvial deposits. As a result, we concluded that sediment deposition began with a sediment channel dominated by gravel deposits in this study. It is then gradually transformed to low amplitude point bars reflected in lateral accretion deposits characterized by fining-upward sandy gravels. The onset of large-scale flooding is reflected in more intensive bend-growth and also evidence of overbank deposition. Chute bar formation was also identified, characterized by gravelly sand with a basal lag deposit. Preliminary dating suggests fluvial aggradation spanning Late Pleistocene – Holocene time scales. We highlight the potential for these new dating methods in studying Holocene fluvial dynamics in highly active tropical river systems sensitive to climatic shifts.

How to cite: Yuskar, Y., Bartz, M., Schmidt, C., Choanji, T., Revanda, M., Lane, S. N., and King, G. E.: Fluvial Morphodynamics of Kampar River, Sumatra, Indonesia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6403, https://doi.org/10.5194/egusphere-egu22-6403, 2022.

09:46–09:53
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EGU22-9159
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ECS
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On-site presentation
Melissa Latella et al.

European watersheds have been subjected to different anthropic disturbances affecting their sediment budget and the morphodynamic asset in the last century, such as extensive mining, flow regulation, damming, land-use change, and embankment. As a response, gravel-bed rivers went through planform shifts, typically from multi- to single-thread configurations, and dramatic bed degradation.

The regulation of mining activity and the occurrence of major floods can (partially) restore river dynamism by redistributing sediments in the floodplain and reactivating secondary channels. Since the flood pulsing is an intrinsically random process, the overall behaviour of mined gravel-bed rivers is not always straightforward to understand, therefore hampering river management and restoration.

This work focuses on the Orco River (northwest Italy) case to study its short-term response to anthropic and hydrological forcing. For this purpose, we performed extensive field measurements and Light Detection and Ranging (LiDAR) data acquisition, integrating these data to reconstruct the morphological configuration of the selected site from 2019 to 2021 and map riparian vegetation biomass. Exploring the river plano-altimetric evolution, the sediment budget and the biomass variations, we investigated the prevalent eco-morphological processes to provide valuable indications for the Orco River management and a general benchmark for studies on gravel-bed rivers.

How to cite: Latella, M., Notti, D., Baldo, M., Giordan, D., and Camporeale, C.: Investigating the short-term ecomorphological evolution of a gravel-bed river, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9159, https://doi.org/10.5194/egusphere-egu22-9159, 2022.

09:53–10:00
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EGU22-11741
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ECS
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Virtual presentation
Joshua Ahmed and Adriana Skrzypczynska

Cutoffs represent an intrinsic process by which meandering rivers regulate their sinuosity through time. Meanders can be terminated by one of two distinct mechanisms: neck cutoff occurs when two meanders migrate into one another; chute cutoff occurs where a shorter, steeper bypass channel is incised across the floodplain between two bends. The latter process can be influenced by a number of factors including floodplain roughness and lithology, in-channel obstructions, and the propensity for the channel to generate overbank flows. Contrastingly, neck cutoff is controlled by lateral channel migration and the hyporheic conditions within the bend. Here we explore the role of antecedent floodplain topography in promoting the development of meander cutoff using a combination of optical and topographic timelapse imagery. We observe that both neck and chute cutoff formation is enhanced by pre-existing floodplain depressions originating from meander migration, floodplain channels, and historic cutoffs. These observations suggest that rivers with greater floodplain complexity – particularly with respect to relief – may experience greater cutoff frequencies, thus impacting channel evolution, and concomitant sediment and nutrient cycling.

How to cite: Ahmed, J. and Skrzypczynska, A.: Assessing the role of floodplain topography in enhancing cutoff formation on meandering rivers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11741, https://doi.org/10.5194/egusphere-egu22-11741, 2022.

Fri, 27 May, 10:20–11:50

Chairpersons: László Bertalan, Joshua Ahmed

10:20–10:23
Welcome and introduction

10:23–10:33
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EGU22-8749
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solicited
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On-site presentation
Ann V. Rowan et al.

Fieldwork is an important part of teaching in Geography and the Geosciences that can add value to a student’s career progression. Making field classes and research fieldwork as accessible as possible requires taking an enabling approach to their planning, from the location of the field site to details of the daily schedule. Such an approach can promote the progression of a diverse cohort of students and allow the leadership of field classes to be distributed equitably amongst teaching staff. An open dialogue between all participants is important to develop fieldwork that embodies the principles of Equality, Diversity, and Inclusion (EDI). As well as at the planning stage, maintaining such dialogue during the fieldwork enables students and teaching staff to raise concerns and ask for support as needed. However, this approach can be difficult to implement without a wider understanding of the diverse needs of students and researchers and how these affect their ability to participate in fieldwork.

The Department of Geography at the University of Sheffield recently developed an EDI policy for taught field classes to help teaching staff plan accessible field classes in partnership with our students. This talk will introduce key ideas and resources that can be used in planning both taught field classes and fieldwork for postgraduate research projects to enable students to be safe, secure, and productive in the field.

How to cite: Rowan, A. V., Olund, E., Pickerill, J., and Souch, C.: Incorporating Equality, Diversity and Inclusion (EDI) into field-based teaching and research fieldwork, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8749, https://doi.org/10.5194/egusphere-egu22-8749, 2022.

10:33–10:40
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EGU22-8443
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ECS
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On-site presentation
Alex Andréault et al.

Morphological classifications are often poorly adapted to complexity of rivers. One way of improving them is based on new technologies such as topo-bathymetric LiDAR (LTB). This tool captures a large quantity of data (more than 10 pts.m-2) with a centimetric accuracy. A survey, carried out on the Loire River between Nevers and Nantes (ca. 450 km) allowed to test the potential of the LTB to define morphological signatures in contrasted river sectors (Garcia-Lugo et al., 2015). As a working hypothesis, we propose that the morphology of a river reach on its active width presents a specific morphological signature indexed on elevations and slopes.


Five sites were retained for this work 1) anabranching, 2) sinuous single channel, 3) braided, 4) formerly trained by groynes and 5) trained by groynes. 


The morphological signature corresponds to the statistical distribution of dimensionless elevations. Density curves were calculated for elevation and slope data of nondimentionalized detrended Digital Elevation Models. Simplification of complex density curves was reached using a Gaussian Mixture Model to divide the signal. These, simplified signals were compared to corresponding DEMs and correlation was established between statistical signal and spatial data. 


Distributions are varying in terms of shape and location on the x-axis. The shape gives an information of lateral connectivity of the system while the location of the curve on the x-axis indicates the predominance of low or high elevations regarding elevation magnitudes. As an example, the description of the anabranching site leads to the identification of the secondary channels network that is also recognized by the GMM. The latter appears to be efficient for identifying the morphological units of a river. For the braided reach, the signature is different from the previous and highlight a system with relative homogeneous elevations . For the site with groynes, the signature clearly highlights 2 morphological disconnected units. 


Results of this study are in line with literature concerning both braided and trained rivers (Ashmore et al., 2013 ; Campana et al., 2011). Methodology proposed here allows to identify and assess the meaning of the statistical signal by directly projecting it on a DEM. GMM shows its relative capacity to discretize complex topobathymetrical signal and link it with morphological units. The methodology presented herein is promising for the understanding of river morphodynamics and their classification. It also opens perspectives for river restoration and management.

How to cite: Andréault, A., Rodrigues, S., and Gaudichet, C.: Multi-channel rivers characterization using Gaussian Mixture Model applied to topobathymetric LiDAR data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8443, https://doi.org/10.5194/egusphere-egu22-8443, 2022.

10:40–10:47
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EGU22-9236
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ECS
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On-site presentation
Ellie Crabbe et al.

The introduction of river dams has resulted in a deleterious simplification of the geomorphological and hydrological characteristics within fluvial environments. To ensure riverine geomorphic diversity in dammed rivers, a variety of watercourse management techniques have been utilised. Flushing flows (the intermittent release of water and sediment from the dam reservoir) have been demonstrated to improve the longitudinal continuity of sediment flux, through the partial re-instatement of periodic flooding. Those flushes are designed to emulate hydrological disturbance events that correspond to natural flow regimes. However, few investigations have sought to characterise nor quantify the geomorphic impacts of flushing flows on channel planform morphology.

Prior to the operation of the Satsunaigawa Dam in 1998, the Satsunai River (Hokkaido, Japan) was renowned for its geomorphic diversity, featuring a plethora of braided channels with double-row bars. After the reported depletion of such features, flushing flows have been used as part of a wider management (2012-ongoing) towards improving the geomorphic diversity of the river.

To quantify the geomorphic impacts of the flushing flow on the planform morphology, a chronology of satellite images (between August 2014 and May 2020) were used to track the lateral mobility of the channel centreline across a 10.4 km reach of the river. Centreline lateral mobility tracking is then used as a proxy for the geomorphic response and differentiate between geomorphic change cause by: a) natural hydrological forcing and, b) artificial annual flushing flows as part of the restoration program.

Results from statistical tests of difference identified significant changes in centreline lateral mobility, both spatially and temporally, within the study reach. The intense natural hydrological forcing is also linked to statistically significant increase of the variability for the lateral mobility of the channel centreline.

Spatial comparisons indicate certain sub-reaches exhibiting high lateral mobility consistently in contrast to areas with statistically constant median values over the entire study period. In this context, the position of the sub-reaches in relation to the damn is formally tested as a predictor for the intensity of geomorphic change.

By contextualising the patterns of centreline lateral mobility within the wider hydrology of the Satsunai catchment, the geomorphic implications of the flushing flow can be critically assessed. This is the missing link between previous studies that focused either on the local study of hydraulic forcing or on the assessment of the ecological impact of the artificial flushing within the Satsunai catchment.

How to cite: Crabbe, E., Maniatis, G., and Tsubaki, R.: The beautiful River Satsunai: Quantifying the effect of artificial flushing flows on channel planform morphology. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9236, https://doi.org/10.5194/egusphere-egu22-9236, 2022.

10:47–10:54
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EGU22-12417
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ECS
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On-site presentation
Pascal Pirlot et al.

Bifurcations are key elements shaping a variety of surface water streams such as river deltas, channel loops, anastomosing and braided rivers. Their geometry interacts in retroactive feedback loops with the upstream and downstream channels and other nodal elements (surrounding bifurcations and confluences).

The easiest way to analyse the dynamics of such bifurcations is by focusing on their planimetric geometry, i.e. the width of each anabranch, the deviation angle at the node along with the typical longitudinal and transverse extensions of a bifurcation unit. We are gathering imagery from laboratory experiments and, mostly, using remote sensing technique and are developing a procedure to measure the key features of bifurcation systems found in their diverse environments mentioned above.

This acquisition method does not yet respect the flow variation since the pictures represent only a single stage of the system evolution. Moreover, most of the data comes from low flow conditions, the formative discharge events being associated with bad weather and scarce optical quality. The dataset is being upgraded to a few time series of single short river reaches containing bifurcations.

Observing the already acquired data, we conclude that the majority of observed river bifurcations are asymmetrical, and they expose two characteristics: the first being the width of the upstream channel increases by a factor δW > 1 to match with the sum of the widths of the downstream branches (i.e. “channel enlargement”, quantitatively verifiable by the measuring procedure). The second concerns the slope variation in the longitudinal axis and is for now only qualitatively verifiable from satellite data by observing the water reflection: the slope of the upstream channel increases by a factor δS > 1 to meet the average of the downstream channel slopes.

The bifurcation system can be analysed in a pure mathematical way by applying the conservation of water and sediment masses and momentum over each channel, constituting the “BRT-model” [Bolla Pittaluga et al., 2003]. The BRT-model discriminates two cases in which the flux partition towards the downstream branches finds equilibrium, depending on whether the width-to-depth ratio (β0) of the upstream channel crosses a threshold value βcr : only one balanced equilibrium exists provided that β0 < βcr. In the other case, the bifurcation finds three equilibria: one unstable balanced and two reciprocal stable, highly unbalanced flux partitions. The BRT-model can be upgraded to account for the channel width and slope adapting to the flux partition along the bifurcates. Hereby, the “Miori-model” [Miori et al. 2006] is built and conserves the stable unbalanced configuration described by the BRT-model. 

 

Such a model seems to reproduce qualitatively the features of the observed bifurcations, which are the unbalanced flux partition, the channel enlargement and averaged slope uplift.

 

Bolla Pittaluga, M., R. Repetto, and M. Tubino, (2003), Channel bifurcation in braided rivers: Equilibrium configurations and stability, Water Resour. Res., 39(3), 1046, doi:10.1029/2001WR001112

Miori, S., R. Repetto and M. Tubino (2006), A one-dimensional model of bifurcations in gravel bed channels with erodible banks, Water Resour. Res., 42, W11413, doi:10.1029/2006WR004863.

How to cite: Pirlot, P., Redolfi, M., and Tubino, M.: River Bifurcations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12417, https://doi.org/10.5194/egusphere-egu22-12417, 2022.

10:54–11:01
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EGU22-9761
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ECS
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On-site presentation
Gabriele Barile et al.

River bifurcations play a crucial role in the morphodynamics of multi-thread channel systems such as braiding or anastomosing rivers, deltas and alluvial fans, as they guide the downstream distribution of water and sediment fluxes. Several experimental and theoretical studies have highlighted the unstable character of bifurcations, even in the case of a symmetric planform configuration and steady boundary conditions, which results in a differential erosion/deposition in the downstream channels and leads to equilibrium states where the flow distribution can be highly unbalanced, sometimes causing the complete closure of one of the anabranches. However, the dynamics of natural bifurcations are always influenced by external unsteady forcing factors, such as water discharge variations due to the hydrological regime, changes in downstream water depth (e.g. because of tidal excursions or interactions between discharge variations and local constraints) and the presence of migrating bedforms. The effect of such factors on the bifurcation’s evolutionary process velocity and equilibrium states’ flow balance remains widely unexplored: in this work we seek to address this gap, also focusing on the interplay between the factors’ characteristic timescales and the bifurcation’s “intrinsic” timescale (i.e. the one related to its autogenic instability mechanism). In particular, we investigate the effect of a time-dependent downstream water surface elevation Hd on the behaviour of a simple bifurcation. To this purpose, we model the upstream channel and the two anabranches employing a 1-D shallow-water numerical scheme, coupled with the two-cell nodal point relationship proposed by Bolla Pittaluga et al. (2003) to determine water and sediment partition at the bifurcation node. Starting from a stable, unbalanced equilibrium configuration, we let Hd vary according to linear and sinusoidal functions of time, with the aim of reproducing –in a very simplified fashion– natural phenomena such as backwater effects in braided rivers. Bifurcations response shows a strong dependence on the forcing timescale: specifically, the system reacts accordingly to the ratio between the rate of change of Hd and the bifurcation’s intrinsic timescale. In particular, when the two scales show comparable values, the system behaviour is governed by the competition between the external forcing and the intrinsic dynamic response. Such competition allows the bifurcation to reach a regime configuration, whose water and sediment partitionings differ from the initial conditions: specifically, a steady increase of Hd leads to a more balanced configuration, while a decrease of Hd enhances system asymmetry. On the other hand, when variations of Hd are fast with respect to the intrinsic timescale, the bifurcation response increases in magnitude, often leading to an avulsion. This dual behaviour is closely related to the width-to-depth ratio β; specifically, the rate of change at which avulsion occurs is lower for higher values of β. Ultimately, this modelling framework can be extended to model the unsteady response of fluvial bifurcations to a variety of possible deterministic and stochastic forcing conditions, including hydrological variations of flow discharge.

How to cite: Barile, G., Redolfi, M., and Tubino, M.: Exploring river bifurcations response to time-dependent external forcings, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9761, https://doi.org/10.5194/egusphere-egu22-9761, 2022.

11:01–11:08
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EGU22-13022
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ECS
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Virtual presentation
Endalkachew Abebe Kebede et al.

Dams impose changes in water flow and sediment transfer that cause large-scale alterations in the downstream river morphology. The Lower Zambezi River's hydrology and morphology regime changed due to the two large impoundments in the middle part of the basin. The main goal of this study is to analyze the long-term effect of damming on the Lower Zambezi River and its delta based on analytical methods and 1D morphological modeling. The geographical, hydrological, and morphological data are analyzed to describe the current and past river conditions and infer morphological trends. The water and sediment balances of the basin developed by Carimo (2020) form the basis for the present study. The land cover of the Lower Zambezi river basin from 1989 to 2019 is determined using Google Earth Engine (GGE), a web-based image analysis tool. The long-term morphological changes of the river are assessed using the Modified Normalized Difference Water Index (MNDWI). The satellite image analysis revealed a deposition trend in Zone B and Zone C, while the Zambezi delta remained stable between 1986 to 2019.  Data analysis shows that the river's width increased significantly after the dam (2007), with the highest river width change observed in Zone C. Besides, a reduction of thalweg was observed in Zone B, while the average bed level increased in most sections of the river. There has also been a reduction in bed levels in Zone D after the construction of the dam. The impact of damming on the river is further analyzed using a 1D morphological model. Appropriate flow and sediment boundary conditions, grid size, and initial conditions are provided to the model based on measured data complemented by indirect assessments where data are missing. The model calibration based on Chézy's coefficient results in good agreement between measured and simulated water levels. The model output revealed that it could reproduce the river's average bed level for 1962 and 2007. The simulations of future developments have been carried out for 300 years (2007 to 2307), starting from the 2007 bed level profile and cross-sections. The discharge regimes of the Zambezi River and tributaries have been modified based on published discharge projections for 2100 to include the impact of climate change. The downstream boundary condition has also been adjusted based on IPCC mean sea level rise scenarios. The model predicts that there will be erosion in the first 200 km downstream of Cahora Bassa, but no significant bed level changes are expected in the other reaches. Deposition in the bifurcation channel in the delta does not cope with sea-level rise for both scenarios. This shows a "river drowning" trend due to the delta's lack of sediment input to cope with the predicted future sea-level rise. In general, river bed erosion due to the effect of the Cahora Bassa dam will be limited to the first 200 km of the river.

How to cite: Kebede, E. A., Crosato, A., Paron, P., and Sloff, K.: 1D morphological adaptation of Lower Zambezi River to dam construction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13022, https://doi.org/10.5194/egusphere-egu22-13022, 2022.

11:08–11:15
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EGU22-8754
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ECS
Ian Delaney et al.

As the climate warms, increases in glacier melt and altered glacier dynamics will result in changes to the dynamics of the Greenland Ice Sheet, impacting the sediment delivery in these rivers. In turn, examining the processes by which proglacial rivers transport sediment delivered by the ice sheet has important implications for the delivery of sediment to the oceans. Applying current knowledge of sediment transport from glacially-fed catchments in alpine regions is difficult, given several pronounced differences compared to glacially-fed catchments in the Arctic. These differences include elevated water discharge and reduced amplitude in diurnal variations of water discharge. Thus, it is imperative that we understand the differences in sediment dynamics between these two regions and evaluate the processes responsible for sediment transport between the ice sheet and ocean. To pursue this understanding, we installed seismic stations to measure bedload transport near the terminus of Russell Glacier during the summer of 2021.

 

We convert the seismic signal from these stations to a bedload transport rate by evaluating several environmental variables, including the transported grain size and ground properties near the river. One station was close to the glacier, whilst the other is 1.5 km downstream. The distance between the stations allows us to evaluate the timing of proglacial sediment transport and deposition. Additionally, the operation of the instruments from early June through mid-August allows us to evaluate seasonal characteristics in sediment discharge. Lastly, we compare these results to the current knowledge of sediment transport from glacierized catchments in mountain regions.

How to cite: Delaney, I., Mancini, D., Anderson, L. S., Belloti, B., Bauder, A., Lane, S. N., and Herman, F.: Bedload transport from a glacially-fed river in Greenland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8754, https://doi.org/10.5194/egusphere-egu22-8754, 2022.

11:15–11:22
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EGU22-7706
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On-site presentation
Iris Niesten et al.

After major storm surge protection works in the Rhine-Meuse Delta, referred to as the Delta Works (Vellinga et al., 2014), the New Waterway has become the only remaining open channel connecting the estuary to the North Sea. Like in many harbour areas, continuous deepening of this channel for navigation purposes has led to strong stratification and often salt wedge conditions, which likely has a strong impact on the marine sediment import. The sediment balance for various fractions is highly uncertain (Cox et al., 2021). Based on field measurements and sediment transport modelling, we aim to unravel the mechanisms controlling residual sediment fluxes in highly stratified estuarine channels, by focusing on the New Waterway.

A measurement campaign was set up consisting of two 13-hour surveys, one during spring tide and one during neap tide. A measurement frame was equipped with a LISST-100x, a Seapoint turbidity meter and a CTD probe. Suspended sediment samples are collected every hour at three depths, next to water temperature, salinity and turbidity. The flow was monitored continuously based on a vessel-mounted ADCP transects across and along the channel .

The ADCP-measurements show a clear distinction in flow magnitude and direction between the upper fresh water layer and lower saline layer, confirming the high degree of stratification especially during neap tide. After low water slack, most suspended sediment is found in the lower half of the water column. Suspended sediment concentrations (SSCs) increase during the flood acceleration phase, suggesting local resuspension during this phase of the tide. When reaching high water slack, SSCs decrease with flow velocity. At high water slack, the ADCP-backscatter profiles indicate settling of the suspended sediment on top of the pycnocline. During the ebb phase, SSCs increase again, and the water column is better mixed compared to the flood phase. Preliminary results of the grain size analysis indicate coarsening of the suspended sediment at the end of the flood acceleration and ebb acceleration phases. Ongoing analysis of these data and numerical modelling of SSC will provide more insight in the suspended sediment transport processes under various degrees of stratification.

 

Cox, J. R., Huismans, Y., Knaake, S. M., Leuven, J. R. F. W., Vellinga, N. E., van der Vegt, M., et al. (2021). Anthropogenic effects on the contemporary sediment budget of the lower Rhine-Meuse Delta channel network. Earth's Future, 9, e2020EF001869. https://doi.org/10.1029/2020EF001869

Vellinga, N. E., A. J. F. Hoitink, M. van der Vegt, W. Zhang, en P. Hoekstra. ‘Human Impacts on Tides Overwhelm the Effect of Sea Level Rise on Extreme Water Levels in the Rhine–Meuse Delta’. Coastal Engineering 90 (1 augustus 2014): 40–50. https://doi.org/10.1016/j.coastaleng.2014.04.005.

How to cite: Niesten, I., Hoitink, T., and Huismans, Y.: Residual sediment transport in a stratified estuarine channel, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7706, https://doi.org/10.5194/egusphere-egu22-7706, 2022.

11:22–11:29
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EGU22-11317
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ECS
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Virtual presentation
Ashkan Pilbala et al.

Sediment transport in rivers consists, at a moderate discharge stage, of individual grains that undergo a series of step movements and rest periods (bedload). In this study, we exploited available data representing resting time and jump length of particles involved in bedload processes. Following the entropy approach based on Shannon and Tsallis theories, we got formal probability functions describing the distribution of the above-mentioned kinematic quantities. Finally, accepting the Einstein assumptions and exploiting the experimental data, we found the values of the constants involved in the entropy functions and complete the analysis. A comparison between experimental and theoretical distributions is showing encouraging matches.

An indirect, but quite relevant, way to prove the validity of the obtained probability distribution, is worked out by looking at the dispersion of traced grains, originally located in well-bordered pillows at different depths within the bed. The application of a stochastic model able to move the grains of the bed with prescribed frequencies in space and time allowed us to further appraise a good behavior of the entropy-based distributions versus the experimental ones.  

Future directions of this research would address the important goal nested in the detection of river flow with or without bedload, by using entropy information based on the measurement of velocity field and/or flow depths over cross-sections.

How to cite: Pilbala, A., Tosato, L., Fraccarollo, L., Bahmanpouri, F., and Moramarco, T.: Shannon and Tsallis entropy results for particle resting times and jump lengths in river bedload , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11317, https://doi.org/10.5194/egusphere-egu22-11317, 2022.

11:29–11:36
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EGU22-6556
Sagy Cohen et al.

Bedload flux is notoriously challenging to measure and model. The dynamics of bedload, therefore, remains largely unknown in most fluvial systems worldwide. We present a global scale bedload flux model as part of the WBMsed modeling framework. Our results show that the model can very well predict the distribution of water discharge and suspended sediment and well predict bedload. We analyze the model’s bedload predictions sensitivity to river slope, particle size, discharge, river width, and suspended sediment. We found that the model is most responsive to spatial dynamics in river discharge and slope. We analyze the relationship between bedload and total sediment flux globally and in representative longitudinal river profiles (Amazon, Mississippi, and Lena Rivers). We show that while, as expected, the proportion of bedload is decreasing from headwater to the coasts, there is considerable variability between basins and along river corridors. The latter is largely responsive to changes in suspended sediment and river slope due to dams and reservoirs. We provide a new estimate of water and sediment fluxes to global oceans from 2,067 largest river outlets (draining 67% of the global continental mass). Estimated water discharge (30,579 km3/y) corresponds well to past estimates however sediment flux is considerably higher. Of the total 22 Gt/y estimated average sediment flux to global oceans, 19 Gt/y is transported as washload, 1 Gt/y as bedload, and 2 Gt/y as suspended bed material. The largest 25 rivers are predicted to transport over 55% of total sediment flux to global oceans.

How to cite: Cohen, S., Syvtski, J., Ashley, T., Lammers, R., Fekete, B., and Li, H.-Y.: Trends and Drivers of Bedload and Suspended Sediment Fluxes in Global Rivers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6556, https://doi.org/10.5194/egusphere-egu22-6556, 2022.

11:36–11:43
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EGU22-4392
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ECS
Fate of the grain size gap material in river bed sediments
(withdrawn)
Elizabeth Dingle and Jeremy Venditti
11:43–11:50
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EGU22-2779
The impacts of near-bed flow characteristics on sediment transport under varying ice-cover conditions of a seasonally frozen river
(withdrawn)
Eliisa Lotsari et al.

Fri, 27 May, 13:20–14:50

Chairpersons: Anne Baar, László Bertalan

13:20–13:23
Welcome and introduction

13:23–13:33
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EGU22-840
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solicited
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On-site presentation
Maarten Kleinhans et al.

Some estuaries gained their planform shape and size by sedimentation of mud flats and establishment of ‘coastal floodplains’ with saltmarsh or mangrove, while others are laterally confined by valley walls. For rivers, the channel dimensions and bar and bend shapes and sizes are related to upstream discharge, but for estuaries where most of the flow comes from the sea, there is no such fluid flux limit on size. This poses a riddle: what determines estuary planform shape, size and channel-bar patterns? Is there such a thing as an equilibrium or are multiple states possible under the same boundary conditions? A steady state  is a convenient concept in assessments of effects of human interference and changing boundary conditions.
We created sandy estuaries with mud and vegetation in numerical biogeomorphological modelling and in the Metronome tidal flume. The partial answers to the riddle include infilling by mud and flow-focussing by vegetation, but also the characteristics of the feeder river and tidal friction. For systems with plenty of sediment input, an `ideal' convergent estuary shape is one endmember equilibrium state. Here, the river, however small, mainly determines estuary size. Another equilibrium state is a system with mid-channel bars, where the collective multiple subtidal channels also develop a convergent shape as floodplains form. Likewise, unfilled estuaries with bay-head and flood deltas also tend to convergent shapes but filling can be arrested by lack of fine sediment, floodplain formation and vegetation settlement. Which of these quasi-steady states is reached depends in part on the system history of filling, wave energy, and (past) rate of sea-level rise.

How to cite: Kleinhans, M., Weisscher, S., Xie, D., and Boechat Albernaz, M.: What determines estuary planform shape, size and channel-bar patterns?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-840, https://doi.org/10.5194/egusphere-egu22-840, 2022.

13:33–13:40
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EGU22-2672
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ECS
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Highlight
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Virtual presentation
Jaap Nienhuis and Derrick Vaughn

Delta plains trap some of the sediments and carbon that are carried down rivers. Trapping keeps deltas above sea level, prevents carbon oxidation, and makes their soils fertile. Quantifying the fraction that is trapped (or retained) is tricky because it is a ratio of two uncertain numbers: delta plain accumulation and fluvial supply. Here we present our work that aims to overcome this issue by estimating trapping on large space and time scales. We quantified sediment and particulate organic carbon (POC) trapping for many coastal delta plains (n=4700), from the time or their inception 7000 years ago to the present, using modern global DEMs, estimates of lowstand channel profiles, and model-derived long-term fluvial fluxes. Our preliminary calculations suggest that modern delta plains, on average, have trapped 25% of the supplied sediment and now contain 30,400 Pg (~1.9.104 km3). They trapped 19% of the supplied POC, totalling 119 Pg C. It seems that sediment trapping has increased with time, as delta plain grew larger and gained more space to accumulate sediment. POC trapping might have decreased with time. Deltas grew out of their protective incised valleys and exposed POC to marine processes on continental shelves. Trapping will likely change in the future, but our long-term averages provide a useful baseline against which the additional effects of humans, sea-level rise, and other changes can be assessed.

How to cite: Nienhuis, J. and Vaughn, D.: Sediment and carbon sequestration on global river delta plains, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2672, https://doi.org/10.5194/egusphere-egu22-2672, 2022.

13:40–13:47
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EGU22-9294
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ECS
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Virtual presentation
Frances Dunn et al.

The Mekong delta in Vietnam, home to over 17 million people, is rapidly losing elevation due to groundwater-extraction-induced subsidence, natural compaction and global sea-level rise. These combined processes result in salinity intrusion, erosion and land loss. The delta has been extensively poldered for agri- and aquaculture, largely cutting off the land from the dwindling supplies of fluvial sediment which would otherwise accumulate on the delta during seasonal floods and increase land elevation. Considering the current state of the Mekong delta and sediment delivery from upstream, the future sustainability of the delta cannot rely on natural delta-building processes and instead must be planned and managed to prevent major degradation of the system.

Sedimentation strategies are methods of ensuring that the limited sediment available is retained to increase elevation where it is desirable for the population and sustainability of the delta. Considering the potential disruption to land uses and livelihoods caused by sedimentation strategies, it is vital to discuss the possibilities for sedimentation strategies with stakeholders to ensure that people, particularly vulnerable groups, are not disadvantaged, and ensure that delta management is sustainable from all perspectives. In this research we engage with stakeholders to start an explorative dialogue on the potential of sedimentation strategies in the Mekong delta considering physical, socio-institutional, governance and legal aspects. We present an analysis of interviews and workshop discussions with farmers, officials, and regional experts in the provinces of Soc Trang (coastal, the most downstream part of the delta) and An Giang (inland, the most upstream part of the delta) to gain insights into local perspectives in different areas of the delta, with different physical environments, land use histories and constraints.

Preliminary findings suggest that the perceived role of sediment varies across different stakeholders e.g. farmers focus more on the role of sediment for their agricultural activities while experts also express their concerns on the elevation of the delta. Interestingly, while sediment is perceived to be important in the upstream area, it is considered an inconvenience in the downstream part of the delta due to, among other factors, different types of livelihoods. It is therefore recommended that the sedimentation strategies need to be designed based on the typical livelihoods of locals and communication strategies need to be enhanced to raise the awareness of local actors on the role of sediment.

How to cite: Dunn, F., Quee, N., Luu, T., Tang Phuong, G., Triyanti, A., Du, H., Wyatt, A., Minderhoud, P., and Sieben, E.: Sustainable floods: Exploring stakeholder perceptions of sedimentation strategies for the sinking Mekong delta, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9294, https://doi.org/10.5194/egusphere-egu22-9294, 2022.

13:47–13:54
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EGU22-9333
John Shaw et al.

River deltas and the vast marshes that they host are impossible to separate. However, the sediment dynamics of rivers (channel- and lobe-based deposition) and marshes (elevation-based deposition) have not been investigated as a coupled system. We investigate this coupling by comparing a laboratory delta experiment with proxy marsh accumulation to a proxy-less control. The proxy adds just 8% mass to the system but clearly influences delta slopes and mass partitioning. Slopes in the marsh window (elevations around sea level where marsh accumulates) are reduced by 40%, as marsh deposition away from channels smooths topography. While riverine sedimentation continues to be 85% of the deposit in the marsh window, the reduced slopes increase the area in this zone such that 1.3 times more clastic volume is deposited in this window. The area above the marsh window and the mass fraction deposited at these high elevations is correspondingly reduced as if the marshes are “stealing” riverine sediment from upstream. Comparing experimental elevation distributions to the field, we show that large deltas might also exhibit this signature. Given that coastal risk is tied to elevation, these findings show that the coupling between marshes and deltas significantly impacts how they should be managed.

How to cite: Shaw, J., Sank, K., Zapp, S., Silvestre, J., Dutt, R., and Straub, K.: Marsh-Delta Interactions: The strong influence of marsh deposition on delta slopes and mass partitioning, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9333, https://doi.org/10.5194/egusphere-egu22-9333, 2022.

13:54–14:01
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EGU22-10627
Emergence, migration and welding of a sand barrier-spit over the last century driven by estuarine delta evolution and storm events
(withdrawn)
Michael Kinsela and David Hanslow
14:01–14:08
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EGU22-9871
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ECS
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Virtual presentation
Quan Le Quan et al.

Deltas are home to 4.5% of the global population and support a range of ecosystem services that are vital to lives and livelihoods. As low-lying regions, deltas are also amongst the most vulnerable areas to the threat climate change and relative sea-level rise, which are being exacerbated by ongoing local resource exploitation. Anthropogenic activities such as riverine sand mining, construction of flood embankments, deforestation and changes of land use and hydropower dams are disrupting the natural evolution of deltaic systems, with many of the world’s large deltas now being sediment starved. This is important because changes of the sediment flux into large deltas can have implications for the evolution of the morphology of delta bifurcations and their function at routing water and sediment seaward. This can amplify flood hazard and risk for riparian communities and intensify processes such as bank erosion, presenting hazards to human lives and exacerbating land loss. The present study focuses on the Chaktomuk junction at the apex of the Mekong delta, connecting the Mekong with the Tonle Sap Lake and the downstream delta. The junction is important as it provides the connection between the Mekong and the largest freshwater lake in Southeast Asia and because of the proximity of the junction to the rapidly expanding urban centre of Phnom Penh. We present a combined 2D hydrodynamic and sediment transport model for the Chaktomuk junction, constructed and based on high-resolution bathymetric data obtained with multibeam echosounders. A series of established sediment transport equations are adopted and tested through a sensitivity analysis to identify the most appropriate sediment transport solver for the model, which is then validated against field observations. The model was forced with a series of scenario combinations including changes of water and sediment flux and rates of sand mining. Simulation runs are presented that project the future evolution of the apex of the Mekong delta, including changes in bifurcation morphology, water and sediment routing seaward through delta distributary channels and changes in water and sediment exchanges between the Mekong and the Tonle Sap. The implications of these future trajectories will be discussed in terms of the sustainability of the delta to future change.

How to cite: Le Quan, Q., Vasilopoulos, G., Hackney, C., Parsons, D., Nguyen Nghia, H., Darby, S., and Houseago, R.: Sediment routing though the apex of a mega-delta under future anthropogenic impacts and climate change , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9871, https://doi.org/10.5194/egusphere-egu22-9871, 2022.

14:08–14:15
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EGU22-11035
Jacopo Composta et al.

In microtidal environments, sedimentation occurring during high freshwater discharges is essential to prevent floodplains from drowning with rising sea levels. Vegetation has long been considered as one of the main drivers of overbank deposition because it reduces flow velocity on the floodplains (trapping effect). However, it has recently been shown that dense vegetation patterns can behave as a barrier for sediments and water fluxes (buffering effect), thus reducing the mass of sediments flowing from the river to the floodplains and increasing the seaward export of sediments through the channel. The buffering effect has been shown to prevail over the trapping effect only in the Wax Lake Delta and only for a few specific hydrographs. Therefore, there is a need to systematically investigate the impact of floodplain vegetation on sediment trapping and buffering. To this purpose, we conduct numerical simulations with the Deltf3D model to analyze sediment deposition over floodplains for several different rivers and floodplains geometries, vegetation characteristics, and flood conditions (i.e., peak magnitude, duration, and hydrograph skewness). The model domain consists of a simplified riverine environment, constituted of a rectilinear channel surrounded by rectangular floodplains. Our results indicate that besides the trapping and buffering effects, there is a third important effect, which we name piling-up effect, consisting of a general increase of water level along the river for higher vegetation densities and heights. This increase favors higher fluxes of water and sediments from the river into the floodplains. We identify the parameter space for which trapping and piling-up effects are larger (or smaller) than the buffering effect, thus leading to more (or less) deposition in the vegetated case than in the unvegetated one. We also identify the vegetation characteristics that maximize floodplain deposition given the river and floodplain geometries. This information can be used for targeted floodplain restoration strategies.

How to cite: Composta, J., Pinton, D., Canestrelli, A., and Carniello, L.: Influence of vegetation and flood regimes on deltaic floodplain deposition, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11035, https://doi.org/10.5194/egusphere-egu22-11035, 2022.

14:15–14:22
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EGU22-702
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ECS
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On-site presentation
Jana Cox et al.

Estuaries are often dredged for navigational access to ports and harbours. Dredging alters the natural dynamics and morphology of estuaries, tending to create deep channels and high intertidal bars and shoals. Estuaries will face many challenges in the future including sea-level rise (SLR) which will influence both estuarine hydrodynamics and morphology. Whilst the effects of SLR have been studied through numerical modelling and have been inferred from empirical relations for natural estuaries, little is known about whether dredged systems will react differently to SLR compared to natural estuary systems. Our objective is to quantify effects of dredging, SLR and the interaction of both processes on estuarine morphology. 

 

We performed scale experiments in a tilting tidal flume (The Metronome)to indicate the proposed effects of SLR on estuarine morphology. Previously, dredging was induced in the flume and the response of morphology compared well with real-world examples such as the Western Scheldt. In the current study, simulated SLR was induced in both a sandy natural (undredged) estuary and a dredged estuary in otherwise the same conditions. These were compared with control experiments without SLR to isolate the effects of SLR in both types of system.  

 

Overall, both maintenance and capital dredging volumes increase with SLR and dredging locations tend to shift upstream. The experiments indicate that channels under SLR tend towards a new equilibrium by changing their hypsometry (width and depth) in response to excess water. This has the overall effect of increasing channel mobility and channel migration speeds. In dredged systems, banks become unstable and collapse into channels which is compounded with rapid erosion of intertidal bars and shoals.  

 

Intertidal areas in non-dredged systems tend to maintain their elevation and extent under SLR, though their locations shift in the upstream direction.  In contrast, dredged systems typically show a decrease in total intertidal area, which means the loss of valuable intertidal habitat area and reduction of flood storage. 

 

In the long-term, dredged systems without SLR have an almost fixed morphology, such that the dredged channel persists even after dredging ends. But SLR overwhelms this fixation and remobilizes sediment, enhances channel meandering and migration and induces lateral expansion. If estuaries are constrained by dikes, bank protection or other flood measures, this excess energy, which is not all dissipated in meandering and migration, may have negative effects on infrastructure. In undredged systems, shallower channels have more space to deepen and widen, reducing the braiding index and providing more adaptation capacity. Infrastructure along urban dredged systems (e.g. flood protection measures, dikes) will be at higher risk under SLR than systems with floodplains and intertidal areas which have space to adapt.  

 

How to cite: Cox, J., Lingbeek, J., Weisscher, S., and Kleinhans, M.: The effects of sea-level rise on estuary morphology in dredged and undredged systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-702, https://doi.org/10.5194/egusphere-egu22-702, 2022.

14:22–14:29
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EGU22-9984
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ECS
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On-site presentation
Joshua Johnson et al.

Many global deltas are heavily populated and ecologically important landforms that exist due to a balance between basin sediment supply, relative sea-level rise and coastal erosion. This balance is being increasingly disrupted by anthropogenic activities, through sediment impoundment behind dams, riverine sediment mining, accelerating eustatic sea-level rise and enhanced delta subsidence through groundwater and oil and gas extraction.

This study utilizes a morphodynamic model, Delft3D, to examine how a range of sedimentological boundary conditions can influence the response of deltas to combined pressures of sediment supply reductions and differing rates of relative sea-level rise. A group of baseline scenarios were created by running the model with a range of different fluvial sediment cohesivities, where the proportions of incoming river sediment defined in the model as cohesive was varied systematically . In addition, a second suite of baseline models were developed where the receiving basin substrate type, over which the deltas evolved and prograded, was varied in terms of its cohesive sediment content and threshold bed shear stress required for erosion. Across these two baseline series of simulations, the prograding deltas were then exposed to a suite of relative sea-level rise scenarios and a set of runs with reductions in fluvial sediment supply. A baseline control scenario was also run in which sediment supply and relative sea-level were kept constant.

The resulting deltas were analysed using channel identification algorithms that quantified the channel geometries and morphodynamics through time. The resultant morphologies and rates of morphological evolution were quantified for each run and scenario. In all cases, sediment starvation was found to be a more significant driver of morphodynamic change than sea-level rise, with reduced deltaic land area and channel mobility resulting from reductions in sediment supply. Deltas forming over more resistant receiving basin substrates, analogous to consolidated clays or glacial till, were found to be more vulnerable to changes in sediment supply than those forming over less resistant substrates. The implications of these findings for both managing deltas and understanding delta deposits in the rock record will be outlined and discussed. 

How to cite: Johnson, J., Parsons, D., Hackney, C., Coulthard, T., Best, J., and Edmonds, D.: The effect of sediment qualities on the resistance of deltas to anthropogenic pressures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9984, https://doi.org/10.5194/egusphere-egu22-9984, 2022.

14:29–14:36
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EGU22-3792
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ECS
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On-site presentation
Miyo Yoshizaki et al.

Climate change puts estuaries at higher risk of flooding from the land, river, and coast by increasing sea levels and extreme weather events such as heavy rainfall. Meanwhile, sediment delivery from catchments and the seas is expected to play a crucial role in estuarine to keep up with sea-level rise (SLR), which will consequently affect estuarine hydrodynamics and flood risks in the future. Nevertheless, the actual impact of fluvial inputs, and specifically, the role of catchment management for the long-term estuarine development, is less known due to a lack of integrated catchment and coastal modelling works. Therefore, this study built a modelling framework that combined catchment hydrological and estuary hydro-morphological processes to understand the impact of catchment management, particularly natural flood management, on long-term estuarine evolution. A cellular automata model (Bentley, 2016) and HEC-HMS were applied for estuary and catchment modelling, respectively. The reforestation scenario with and without SLR (3 mm/year) for 100 years on a hypothetical catchment and estuary were tested. The reforestation effect was captured as the proportionally reduced catchment discharge and sediment delivery at the rainfall events compared to the baseline (non-reforestation) scenario. Preliminary results, however, showed the estuarine morphology is less sensitive to reforestation cases. In both management scenarios, SLR cases resulted in a 50 % increase in sedimentation in the estuary compared to non-SLR. Though due to rapid SLR, that sedimentation was not sufficient to keep tidal prism constant, and it was increased by 13% in 100 years as a result. The above results/sensitivities are based on simulation runs, with no tidal pumping, incorporating this may change these results. Further work such as introducing tidal asymmetry to the estuary model will provide a more comprehensive view of the fluvial impact on estuaries' long-term evolution. 

Bentley, I. (2016) A novel cellular automata based estuarine morphodynamic model. PhD. University of Glasgow. Available at: http://theses.gla.ac.uk/6821/.

How to cite: Yoshizaki, M., Schuerch, M., Mao, L., and Macklin, M.: Modelling the long-term impact of catchment and coastal management on estuarine morphology, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3792, https://doi.org/10.5194/egusphere-egu22-3792, 2022.

14:36–14:43
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EGU22-7463
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ECS
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On-site presentation
Riccardo Xotta et al.

Depositional landforms, such as tidal marshes and deltas, formed by sediment deposition over the last centuries to millennia. They are complex, vulnerable, and dynamic systems with important roles from environmental and human points of view. The survival of these lowlying landforms is threatened by multiple stressors, e.g. sea level rise and reduction in sediment supply. In addition to these external factors, natural compaction of the sedimentary bodies may have an important role on the elevation dynamics because of the large porosity and compressibility that characterize the shallow deposits. A three-dimensional (3D) finite element simulator (NATSUB3D) has been recently developed to model the long-term dynamics of transitional landforms. The model couples a 3D groundwater flow module to compute over-pressure dissipation with a 1D compaction module based on the elasto-plastic Terzaghi theory. NATSUB3D properly accounts large deformations thanks to an accreting/compacting mesh that follows the grain movements (Lagrangian approach). The NATSUB3D formulation is updated to account for viscous deformations using the NEN-Bjerrum constitutive relationship. Indeed, creep may represent an important process in fine unconsolidated deposits forming Holocene coastal landforms. With the new constitutive model, soil deformation is effective stress and time dependent. The new simulator has been applied on 1D synthetic cases mimicking long-term accretion of sedimentary columns. Hydro-geomechanical properties typical of sediment classes composing depositional landforms are used. A sensitivity analysis has been performed on sedimentation rates and secondary compression coefficients, which are the main parameters affecting the viscous deformation, leading to significantly different elevation dynamics. In the simulation of these processes (i.e., the formation of a sedimentary landform), the overconsolidation ratio (OCR), which is the geomechanical parameter most difficult to quantify and highly impacting soil compaction, can be simply set equal to 1 for the newly formed soil layer. Indeed, OCR is then properly updated by the model itself because the simulation follows the soil deformation since time of sediment deposition, with soil experiencing compaction because new sedimentation occurs on the landform top.

How to cite: Xotta, R., Minderhoud, P. S. J., Zoccarato, C., and Teatini, P.: Simulating the impact of creep in natural consolidation and subsidence in depositional environments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7463, https://doi.org/10.5194/egusphere-egu22-7463, 2022.

14:43–14:50
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EGU22-2221
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ECS
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Virtual presentation
Reinier Schrijvershof et al.

Current tidal input reduction approaches applied for accelerated morphological simulations aim at capturing the dominant tidal forces in a single or double representative tidal cycle, often referred to as a "morphological tide''. The existing methods may provide appropriate boundary conditions to simulate representative residual transport fluxes and the resulting morphological changes of the tidal channels. However, heavily simplified tidal signals fail to represent the tidal extremes. They poorly represent intertidal areas, which exert a major impact on the development of tidal asymmetry and the associated residual transport fluxes.

Here, we aim to develop a generic method to construct a synthetic representative tidal signal that incorporates tidal extremes. Such a synthetic cycle should adhere to several criteria to make it applicable for long-term (i.e. subdecadal) accelerated simulations. The synthetic signal should: (1) represent the original signal, particularly preserving asymmetries present in the original signal; (2) be exactly periodic, to ensure coherency between consecutive cycles and to control the relative phasing with other types of forcings (e.g. wind, waves, discharge); and (3) remain valid during the long-term simulations in which the bathymetry in the modelling domain changes shape.

The starting point for the construction of the synthetic signal is a fortnightly modulation of the semi-diurnal tide to represent spring-neap variations, while conserving periodicity. Diurnal tides and higher harmonics of the semi-diurnal tide are included to represent the asymmetry of the tide. The amplitudes are then scaled to give a best fit to the full tidal signal. Statistical measures of the synthetic signal show that it gives a better representation of the amplitude variation and asymmetries present in the original signal, compared to existing approaches of tidal input reduction. A hydrodynamically validated depth-averaged model of the Ems estuary (The Netherlands) demonstrates the effects of different tidal input reduction techniques on residual sediment transport patterns. Adopting the new approach, the shape of the tidal wave is better represented over the entire length of the estuary, and inundation of shallow parts of the basin is modelled in closer resemblance to the real-world intertidal dynamics.

How to cite: Schrijvershof, R., van Maren, B., and Hoitink, T.: A synthetic representative tidal signal for long-term morphological modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2221, https://doi.org/10.5194/egusphere-egu22-2221, 2022.

Fri, 27 May, 15:10–16:40

Chairpersons: László Bertalan, Anne Baar

15:10–15:13
Welcome and introduction

15:13–15:20
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EGU22-270
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ECS
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Virtual presentation
Anshul Yadav and Sumit Sen

The flow velocity and discharge estimation are important in fluvial geomorphology and other wide variety of scientific purposes in river research applications. However, in steep coarse-bed streams dominated by large sediments and irregular channel beds, the conventional equations fail to consider the additional losses, leading to the overestimation of flow velocity. In such cases, the science of flow resistance, particularly during low relative submergence (y/D84), and active transport conditions, needs to be revisited. Since the poorly sorted sediments on the channel bed also increases the resistance in the flow (Yadav et al., 2021). This work examined the effects of geometric standard deviation (σg) of the bed material on flow resistance in steep streams using the dataset reported in the literature and conventional flow resistance equations. The flow resistance estimates in poorly sorted sediments were observed to be unreliable for the non-uniformity () range 7.5-10 for all the equations, however relatively better flow velocity estimates were observed for σg greater than 10. This distinct response of flow resistance equations for σg ranging between 7 to 10 was probably due to additional losses occurring due to armouring and formation of sediment clusters and reticulate structures in this subset of the data. The dimensionless shear stress (τ*) exerted on the channel bed for the dataset with σg between 7.5 to 10 was in agreement to develop sediment clusters as suggested by various researchers. Furthermore, when the geometric standard deviation exceeds 10, the unbiased flow velocity estimates using the conventional flow resistance equations indicate the reduced resistance in the flow field. This behaviour may be attributed to the smoothing of bed or change in bed conditions either due to disintegration of bedforms and sediment clusters at higher discharge or structural instability. 

Keywords- Flow resistance, sediment sorting, non-uniformity, sediment clusters, armouring, shear stress

 

Yadav, A., Sen, S., Mao, L., & SchwanghartW. (2021), Evaluation of flow resistance equations for high gradient rivers using geometric standard deviation of bed material, Journal of Hydrology (accepted)

How to cite: Yadav, A. and Sen, S.: Effects of sediment clustering on flow resistance in steep coarse-bed streams, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-270, https://doi.org/10.5194/egusphere-egu22-270, 2022.

15:20–15:27
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EGU22-10014
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On-site presentation
Simone Bizzi et al.

River channel patterns and associated morphology are determined by a variety of factors: sediment supply and grain sizes are significant factors together with channel confinement and channel gradient. From decades we develop empirical and hydraulic equations which help us to link water discharge and channel geometry (and then indirectly its pattern) with transported sediment. Water discharge determines mostly the channel dimensions (width and depth), slope provides the rate of energy expenditure, and river morphology is eventually shaped by amount of sediment supply and its caliber. Notwithstanding that, our ability to quantify functional links between sediment connectivity, meant as amount, frequency, sizes and origins of transported sediment, and channel morphology is still pretty limited and mostly qualitative. For instance, we hardly can provide thresholds in sediment transport regimes for channel type shifts from braided to single channel or vice-versa. More in general, we still have limited ability to quantify the sensitivity of channel morphological alterations to changes in water and sediment fluxes. 

In this contribution, we aim to discuss our findings linking the output of a network-scale sediment transport model (CASCADE) to river morphology in two different basins: the braided Vjosa River in Albania and a predominantly mixed load river system the Richmond River Catchment, in New South Wales, Australia. In the Richmond River we identified various controls linked to the simulated sediment fluxes: in-stream sediment storage units, junctions between different geomorphic river types, tributary confluences and sediment storage units within partly confined floodplain units. Such analysis lays the foundation for network scale identification and quantification of potential hotspots of geomorphic adjustment.  In the Vjosa river we used the modeled sediment fluxes as input to a set of theoretically derived functions that successfully discriminate between multi-thread and single-thread channel patterns. This finding proves a clear connection between modeled sediment concentrations and observed river morphology. We were also able to observe that a reduction in sediment flux of about 50% (e.g., due to dams) would likely cause existing braided reaches to shift toward single thread morphology.    

Our results highlight opportunities and limits, which arise integrating the outputs of recently available network-scale sediment transport models with river morphology mapping derived by emerging remote sensing technology. These new data and methods can potentially significantly advance our ability to understand and formally quantify functional links between water and sediment fluxes and associated channel morphology, and use this understanding for management applications.

How to cite: Bizzi, S., Tangi, M., Khan, S., Schmitt, R., Fryirs, K., Castelletti, A., Piegay, H., and Pitlick, J.: Linking sediment transport and river morphology , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10014, https://doi.org/10.5194/egusphere-egu22-10014, 2022.

15:27–15:34
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EGU22-608
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ECS
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On-site presentation
Ţuţuianu Laureţiu-Florin et al.

The stratigraphy of the lower reaches and floodplains store both mineral sediments and organic matter which are very important for understanding the environmental evolution and organic carbon storage over time. By deciphering the sedimentation patterns of floodplains is important for understanding how they adapt to Holocene sea level and climate changes.

The present study is based on 10 cores from the Lower Danube floodplain between Braila and Tulcea, 50 14C ages and over 1000 samples which were sedimentologically and geochemically analyzed. These data allowed us to reconstruct sedimentation and environmental evolution patterns of the floodplain during the Middle and Late Holocene. Two major phases have been desciphered: I) between 8000 and 5500 years BP - a floodplain with a dynamic (rapid-changing) landscape characterized by interchangeable wet and dry areas), developed in a period with a decelerated sea level rise. In that time we find a decrease in the sedimentation rate (from 6.7 to 1.2 mm / year) and the grain mean-size (from ~ 4.75phi to ~ 6.5phi) and an increase of the organic matter content by about four times (from 2.5 to 10%) and II) between 5500 BP and the middle of the 20th century - a relatively-stable and wet floodplain (with large lakes and wetlands) which was partially silted by small channels). This latter phase was developed in a time with a quasi-stable sea level and it can be subdivided into two sub-phases: IIa) (5,500 - 2,200 BP) and IIb (2,200 - XX century). In the first, the sedimentation rate decreases slightly (up to 0.7 mm / year) while the organic matter content becomes almost double (~ 19%, 2200 years ago) and in the second, once with the rise of the Roman Empire, the sediments become finer with a much lower content of organic matter (~ 12%) and the sedimentation rate become doubles (~1.7 mm / year in the last millennium), all due to increased anthropogenic influence in the Danube river basin.

How to cite: Laureţiu-Florin, Ţ., Alfred, V.-S., Ionel, S., Mihaela, D., Luminita, P., Tiberiu, S., and Cristian, P.: Lower Danube Floodplain: Middle to Late Holocene sedimentation rates and organic matter sink (processes and patterns), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-608, https://doi.org/10.5194/egusphere-egu22-608, 2022.

15:34–15:41
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EGU22-13152
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On-site presentation
Marcin Słowik

Meandering and anabranching rivers shaped postglacial and loess areas of Europe since the Late Pleniglacial. These landscapes inherited landforms and sediments left by the glaciations and loess formation, however, the influence of this inheritance on the evolution of the rivers is not fully understood. The main goal of this study is to determine the influence of deposits and landforms inherited from the glaciations and loess formation on processes forming meandering and anabranching rivers. The following research tasks were realized: i) identification of types of floodplain sedimentary architecture, ii) determination of grain-size properties of sediments forming alluvial fills, iii) determination of differences between channel planform changes of postglacial and loess rivers since the Late Pleniglacial, and iiii) creation of a model describing the influence of inherited sediments and landforms on the evolution of anabranching and meandering rivers in postglacial and loess landscapes of Europe.

This research work is based on data collected from literature on the evolution of 60 rivers of western, central and eastern Europe. During the literature review, attention was paid to sedimentary structures preserved in channel and floodplain sediments, types and grain-size of deposits forming alluvial fills, and channel planform changes since the Late Pleniglacial. Data regarding periods of river incision and increased deposition were also collected.

The inheritance of landforms and sediments from the last glaciation (glacial till, sands and gravels), and a deposition of loess at the forefront of glaciated areas drive the major differences between the evolution of anabranching and meandering rivers of postglacial and loess landscapes of Europe. Point bar accretion forms meandering rivers in postglacial zone whereas oblique accretion influences the formation of meandering courses in loess areas. Anabranching rivers of postglacial zone evolved through the formation of crevasse channels, meandering anabranches, and switch from multi- to single-thread planform in periods of low water levels. Anabranching rivers of loess zone formed sustained bifurcations and soft avulsions. The inherited landforms (such as e.g. ice-marginal valleys and subglacial tunnels in postglacial areas) influenced the rivers’ evolution by the formation of bifurcations and multi-channel river confluences.

The most distinct differences between channel planform changes in postglacial and loess areas were found in the period of the last 4000 years, characterized by increased humidity and deposition. Meanders of postglacial zone formed alluvial islands in their courses or were transformed into anastomosing rivers. Anabranching rivers in ice-marginal valleys sustained their multi-channel courses until the major hydro technical works in the 19th century. Anabranching rivers of loess zone evolving in small catchments evolved into meandering courses. Low-energy meandering rivers turned into wetlands. Rivers evolving in large valleys with high stream power formed in loess areas maintained meandering planforms throughout the Holocene. Further research on rivers on subarctic zone, and large rivers of Europe (i.e. the Danube River) is required to develop the understanding of processes forming rivers in both zones.

How to cite: Słowik, M.: The influence of types of sediments and landforms on the evolution of anabranching and meandering rivers in postglacial and loess landscapes of Europe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13152, https://doi.org/10.5194/egusphere-egu22-13152, 2022.

15:41–15:48
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EGU22-11824
Gilles Rixhon et al.

In parallel to standard optically stimulated luminescence (OSL) dating, portable OSL readers have been increasingly employed in a wide range of geomorphological settings over the last decade. In fluvial landscapes, most of the OSL signal intensities measured by the portable reader were successfully used either to explore bleaching characteristics of river deposits or to rapidly gain new insights into alluvial stratigraphy via luminescence profiling (Munyikwa et al., 2020). However, going beyond the mere one- (or sometimes two-) dimensional sedimentary screening, the use of portable readers shall steer toward the production of three-dimensional chronostratigraphical information. Against this background, the high lateral mobility of the lowermost Bruche reach (directly upstream of Strasbourg), documented at the decadal scale by Jautzy et al. (2022), thus represents a suitable setting to explore the potential of field-based portable luminescence profiling to provide new insights into both lateral and vertical fluvial dynamics.

Here, the sampling approach with the portable reader using both blue and infra-red stimulations (BSL and IRSL) is twofold:

  • testing the ability of the reader to measure signals of varying intensities in morpho-sedimentary units of different ages, i.e. an early Holocene terrace, historical palaeomeanders and a modern swale-and-ridge system;
  • investigating the gradual lateral shifting and incision of a single palaeomeander in the floodplain recorded by a succession of palaeochannels and former point bar deposits.

Preliminary results (i) show that the older the landform, the higher the BSL/IRSL signal intensity, and highlight (ii) a consistent pattern of downward increasing BSL/IRSL signal intensities in the homogeneous fine-grained upper part of all profiles. However, BSL/IRSL signal intensities measured in the sandy fraction (i.e. lower parts of the alluvial sequences or in the swale-and-ridge system) usually record a larger scatter that requires further investigations. This study underlines the potential of the portable reader as a rapid and efficient tool for tracing historical overbank deposition in floodplains; these results will be complemented soon by standard luminescence dating to constrain sedimentation rates.

References:

Jautzy, T., Schmitt, L., Rixhon, G., (2022, in press). Historical geomorphological adjustments of an Upper Rhine sub-tributary over the two last centuries (Bruche River, France). Géomorphologie, Relief, Processus et Environnements.

Munyikwa, K., Kinnaird, T.C., Sanderson, D.C.W. (2020). The potential of portable luminescence readers in geomorphological investigations: a review. Earth Surface Processes and Landforms DOI: 10.1002/esp.4975.

How to cite: Rixhon, G., Laible, J., Jautzy, T., and Schmitt, L.: Investigating historical floodplain dynamics using portable luminescence profiling in a Rhine sub-tributary (Lower Bruche, E. France), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11824, https://doi.org/10.5194/egusphere-egu22-11824, 2022.

15:48–15:55
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EGU22-7490
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ECS
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On-site presentation
Jeffery Valenza et al.

Evidence of an enigmatic and unidirectional shift in fluvial architecture coincides with the proliferation of early vascular plants in the Silurian period. Unlike post-Silurian fluvial deposits, pre-Silurian fluvial strata lack significant alluvial mudrock and are most often characterized by broad sheet-like sandstones. This sedimentary architecture is traditionally interpreted as a pre-Silurian global prevalence of broad, shallow rivers with unstable channel banks, in stark contrast to the modern-day prevalence of low-sloping, meandering rivers. The Silurian stratigraphic shift has been attributed to the profound influence of vegetation on river geometry. Rooted plants are thought to have dramatically increased channel bank strength, decreased the ability of rivers to rework overbank deposits, and increased mud production rates through enhanced chemical weathering. Recent paleohydraulic reconstructions of pre-Silurian channel-bodies from around the world reveal that deep, single-threaded rivers were likely common during the pre-Silurian period, challenging the traditional paradigm. However, the mechanisms that provided pre-Silurian rivers the necessary bank strength to sustain deep-channeled flows have yet to be quantitatively explored using geologic observations. Here, we integrate recent advances in paleohydrological methods with original and published field data to reconstruct paleohydraulics and channel planforms of Mesoproterozoic fluvial deposits in NW Scotland.

Specifically, we combine geological observations of ~1.2 Ga Stoer Group channel fill deposits in NW Scotland with mechanistic theories that describe the formation of river dunes to quantitatively assess the dominant channel planform style. Our results indicate that Stoer Group fluvial strata represent formative channels with 2-6 meters bankfull flow depth and bed slopes ranging from 6x10-5-2x10-3. We combine these estimates with measurements from modern channels to show that the Stoer Group rivers plot alongside modern single-thread, meandering rivers and high-sinuosity, wandering rivers in a quantitative channel-planform discriminant space. Furthermore, using a mechanistic theory that describes the formation of single-threaded rivers without plants, we show that the deep Stoer Group rivers could have been sustained by the cohesive bank strength provided by mixed siliciclastic sediments with 25-40% mud content—a range consistent with field observations of mud content in putative floodplain facies of the Stoer Group. Finally, we relate our findings to modern environments by considering width to depth ratios and bank cohesion thresholds in a large set of modern rivers, showing that only a small fraction of global rivers requires vegetation to maintain bank stability.

How to cite: Valenza, J., Ganti, V., Whittaker, A., Lamb, M., and Fischer, W.: Pre-vegetation, single-thread rivers sustained by cohesive, fine-grained bank sediments: Mesoproterozoic Stoer Group, NW Scotland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7490, https://doi.org/10.5194/egusphere-egu22-7490, 2022.

15:55–16:02
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EGU22-10307
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On-site presentation
Jakob Pamminger and Ronald Pöppl

A flooding event in 2014 caused a prominent bank erosion along the Salza River in Styria, Austria. The bank became instable and its further development was uncertain, which raised concern for further progressing bank erosion at this location, as well as in other sections in the entire studied river reach. The major aim of this study was to monitor bank erosion rates at this specific bank erosion hotspot as well as to survey the entire study area for past, present and potential future channel changes. Past river changes have been mapped using historical maps and orthophotos. For a geomorphological evaluation of the current system state field mapping has been applied using the approach developed by Wheaton et al. 2015. Bank erosion monitoring was done by using erosion pins and photogrammetry, while potential future channel changes have been assessed via landscape evolution modelling using CAESAR-Lisflood developed by Coulthard et al. 2013.

Historical map (1678 - 1887) and orthophoto (2004 - 2017) analyses have shown that the Salza River has altered noticeably throughout the past through anthropogenic impacts and natural processes, with two prominent natural changes in the recent years one being the prominent bank erosion which initiated this study. Mapping the river course in the field in May 2017 has shown, that the most common in-channel river shapes are in descending order transition zones, planar, concave and convex. Monitoring the bank erosion hotspot using erosion pins has shown a mean change of the whole bank of -1.63 cm from 15.06.2017 - 11.07.2017. The photogrammetric approach, a 2D distance analysis to the erosion hotspot for the timeframe 06.05.2017 - 11.06.2018, resulted a mean change of roughly -2 m for the whole bank, while a 2.5D volume change analysis for the same timeframe has shown an eroded volume of 319 m³. Modelling the Salza River using four different discharge scenarios with the landscape evolution model CAESAR-Lisflood has shown four potential hot spot areas for a lateral shift of 20 m to 130 m from 2019 to 2050.

How to cite: Pamminger, J. and Pöppl, R.: Mapping, monitoring and modelling past, present and potential future channel changes in an Alpine River system in Austria, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10307, https://doi.org/10.5194/egusphere-egu22-10307, 2022.