GM5.1

Fluvial systems: 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 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.

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 systems, (ii) evaluating morphological change at variable spatial and temporal scales, such as at event vs. seasonal scales, and (iii) investigating the sedimentology of these river systems. We particularly welcome applications which investigate the morphodynamic response of fluvial systems in all types and sizes and we specifically would like to encourage submissions from early career researchers and students.

Co-organized by HS13/SSP3
Convener: Christopher HackneyECSECS | Co-conveners: Joshua Ahmed, László BertalanECSECS, Eliisa Lotsari
vPICO presentations
| Fri, 30 Apr, 09:00–12:30 (CEST)

vPICO presentations: Fri, 30 Apr

Chairpersons: Joshua Ahmed, Eliisa Lotsari, Christopher Hackney
09:00–09:05
Fluvial processes
09:05–09:07
|
EGU21-4568
|
ECS
|
Highlight
Richard Mason and Lina Polvi

Research to understand the drivers of river form and processes has focussed on alluvial sand and gravel-bed channels. However, boulder-bed rivers are also an abundant channel type, particularly in previously glaciated and mountainous regions. Understanding boulder distribution in rivers is important because of their effects on  channel hydraulics and sediment transport processes. Boulder-bed channels in low-relief, previously glaciated landscapes may be considered semi-alluvial since the boulders likely were not deposited by fluvial processes (unlike in e.g., step-pool mountain channels). However, the relative importance of glacial legacy sediment and fluvial processes as drivers of boulder-bed river morphology is poorly understood. This is especially true in northern Sweden where channel clearance for timber floating has resulted in the removal of boulders from most rivers. Restoration of these rivers involves the replacement of boulders but is challenged by a lack of geomorphological understanding.

This study aimed to quantify the morphological characteristics of northern Swedish boulder-bed streams and determine the association between fluvial and glacial legacy controls on these channels. We undertook a large-scale field campaign surveying 20 rivers (drainage area: 15 - 112 km2) that have not been cleared for timber floating. At each reach, we measured channel morphology using a total station over approximately 100 m river length, surveying the channel planform, thalweg and 5 cross sections. In addition, we measured the location, diameter and protrusion of every boulder (> D84) within each reach. We also conducted a survey of the size and density of boulders on the floodplain to compare to in-channel boulder distributions. We coupled this field campaign with analysis of digital elevation models, surficial geology, glacial landform maps, and hydrological data to investigate potential landscape controls on reach-scale geomorphology. Associations between drainage area, channel slope, width and D84 as well as longitudinal clustering of boulders into fluvial bed-forms would indicate fluvial rather than legacy glacial drivers.

Preliminary results show high variability in the morphology of reference sites, from low-gradient reaches with high floodplain connectivity to steep and narrow channels (Slope ranged 1.1 - 8.8%). D84 ranged from 0.4 m to 2.1 m with some sites having as many as 500 large boulders (> 1 m diameter) in a 100 m reach. D84 was not associated with channel slope and boulders were not clustered longitudinally in most reaches. This suggests that boulder spacing is the result of glacial legacy controls. These results are important for understanding geomorphic processes in boulder-bed channels and how channel form relates to reach- and landscape-scale controls. The relative importance of fluvial versus glacial legacy controls on boulder-bed channel morphology is also important to help restoration practitioners more accurately identify reference states of boulder-bed channels in previously glaciated landscapes.

How to cite: Mason, R. and Polvi, L.: Drivers of channel morphology in semi-alluvial boulder-bed streams & implications for river restoration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4568, https://doi.org/10.5194/egusphere-egu21-4568, 2021.

09:07–09:09
|
EGU21-719
|
ECS
Adeyemi Olusola and Adetoye Faniran

Over the years, there has been tremendous growth in the literature as regards river channel classifications, however, very few studies have been able to engage the use of remote sensing products in channel classification at the reach-scale level especially by combining reflections from satellite sensors with channel morphological variables. This study aims to identify discriminating spatio-morphological variables using machine learning algorithms and classify site-specific channel types at the reach scale. Each reach was broadly classified based on valley settings (confined, partly confined and unconfined) and channel types (alluvial or bedrock). However, variations and site observations were recorded for site-specific classification purposes. For each reach, Global Positioning System devices were used to geo-locate their endpoints. Standard field instruments were used for cross-sectional measurements and established hydraulic equations for the derived variables. A total of 249 points across 83 reaches were sampled during the fieldwork. Landsat 8 and Sentinel-1 bands were retrieved for days the fieldwork was carried out/for days close to those dates using Google Earth Engine (GEE) platform. Hierarchical cluster analysis, HCA, using Ward’s linkages was used to provide a classification for the channel types. For the identification of important variables in predicting channel unit types, the random-forest - recursive feature elimination (RF-RFE) algorithm was used using the rfe() function. To identify the best machine learning algorithm, random-forest (rf), support vector machines (svm), multivariate adaptive regression spline (mars) extreme gradient boosting (xgb) and adaptive boosting (adaboost) were used on the training and test data to identify the best performing algorithm. The rfe() feature selection identified five (5) variables that can significantly help in channel unit type identification. The top five variables are dimensionless stream power, slope, width, wetted perimeter and Band 4. Using ROC curve, sensitivity, and specificity, the mars model has the highest ROC curve. Hence, it appears to be the best performing out of the five. However, if the argument is to be based on positive prediction, then any of the models except adaboost will be preferred given their high sensitivity. The HCA using illustrated the clustering structure of the studied reaches by producing five distinct channel classification types distinguished based on width-depth ratio values (high and low). The five distinct channel types are listed as  M1e, M5e, B1, E5b, and E. These codings are based partly on Rosgen’s classification while, the capital letters (M, B and E) represent mixed channels, bedrock with moderate width-depth ratio and alluvial channels with low width-depth ratio respectively. Numbers 1 and 5 represent bedrocks and sandy beds based on slope variation respectively. The identified channel unit types are a result of the underlying lithology, process-form dynamics and confinement. As streams are expected to respond differently to shocks and recover from damages, it becomes essential to understand these differences in classification which will go a long way in establishing watershed and streamside management guidelines.

How to cite: Olusola, A. and Faniran, A.: Interpreting reach-scale classifications and the role of spatial-morphological variables in river channel mapping using machine learning algorithms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-719, https://doi.org/10.5194/egusphere-egu21-719, 2021.

09:09–09:11
|
EGU21-1287
Janet Hooke

Active meandering rivers have the advantage of exhibiting morphodynamic changes at such a rate that changes are detectable on a range of timescales, from event, through decadal, to centennial.  The evidence from river reaches that have been investigated directly for 40 years by various methods is used to examine the morphodynamics over differing spatial and temporal scales. Several questions can be addressed, including:   what channel changes are produced by different events, discharge periods and sequences; how do events and phases combine to produce changes in position and in morphology; to what extent are such changes coherent, systematic and predictable. The combination of evidence from detailed cross-sectional surveys, field mapping, drone and aerial photographic surveys and use of historical maps demonstrates how the morphodynamics at different scales combine and interact. Analysis indicates some systematic changes at cross-sectional and bend scale but contrasts between different reaches, bends and within bends, and reveals complex patterns and lags in propagation of change. Survey and monitoring requirements to detect and understand the interactions are identified. Research into these active meandering rivers may be indicative and helpful for our wider understanding of less dynamic rivers, where changes are less easily detected. It raises the question of whether the mechanisms are similar in more stable rivers and reaches.

How to cite: Hooke, J.: Morphodynamics of active meandering rivers in a hierarchy of spatial and temporal scales, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1287, https://doi.org/10.5194/egusphere-egu21-1287, 2021.

09:11–09:13
|
EGU21-7957
Mattia Carlin, Marco Redolfi, and Marco Tubino

Alternate bars are large bedforms, characterized by an ordered sequence of scour and deposition zones, which often appear in rivers. It is well proved by many experimental, theoretical and numerical works that the formation of migrating alternate bars results form an intrinsic instability mechanism occurring when the width-to-depth ratio of the channel is larger than a critical threshold. Although a large amount of literature is available to describe equilibrium bar properties under steady flow conditions, much less information exists about the evolution of bars when flow discharge is variable in time. In a recent work we investigated how the long-term, average properties of bars respond to changes of the hydrological regime. This average state represents the net result of a multitude of flood events, each of them producing a different morphological alteration. However, a systematic description of how changes of the bar properties depend on the characteristics of the individual floods is still missing, as existing studies are limited to a small number of flood events, not sufficient to make a statistical description of the riverbed response. In this work, we aim at studying the time evolution of the bar amplitude in a relatively straight, channelized reach of a gravel bed river. Specifically, we considered a 10 km-long reach of the Alpine Rhine River, for which a detailed record of flow stages is available for the period from 1984 to 2010. This is accomplished by modelling the bed evolution through the theoretically-based model of Colombini et al. (1987), here applied by considering a time-varying basic flow and numerically integrating the bar amplitude. Compared with classical approaches based on numerically solving the two-dimensional shallow-water equations, our procedure allows for calculating the bar response over long periods of time with a very low computational cost. This enables for modelling different scenarios of hydrological alterations, due to dam constructions or climate changes, and to statistically analyse the expected impact on bar evolution. Assuming that bars cannot evolve when the flow is too low to fully submerge the bar crests, we identify 200 morphologically-active flood events, covering about 1.1% of the total duration of the flow series. Model results reveal that moderate flow events tend to increase the bar amplitude, while larger floods reduce the bar height. However, the value of the peak discharge alone is not sufficient to explain the morphological changes, as an important (and opposite) role is also played by the duration of the events. Specifically, longer floods tend to promote an increase of the bar height during the receding phase, which implies that a strong reduction of the bar amplitude requires intense, but relatively short flood events.

How to cite: Carlin, M., Redolfi, M., and Tubino, M.: The effect of flood events on the altimetric response of river alternate bars, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7957, https://doi.org/10.5194/egusphere-egu21-7957, 2021.

09:13–09:15
|
EGU21-12728
|
ECS
|
Highlight
Roberto Fernández

Experimental work on small-scale meltwater meandering channels over ice and field observations have identified similarities and differences between their planform morphologies and those of meandering channels in other media (e.g. alluvial, bedrock). Qualitatively and quantitatively, planform characteristics, including sinuosity, wavelength-to-width ratios, coefficient of skewness and fatness, suggest that most meandering channels behave in certain ways and within certain ranges. However, what makes meltwater meandering channels over ice unique? In this contribution, I highlight the different aspects that set meltwater meandering channels over ice apart from meandering channels in other media and share ongoing work focusing in their planform morphologies, curvature signals, and cross section geometry.

How to cite: Fernández, R.: Meandering channels over ice: Cooler and unique?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12728, https://doi.org/10.5194/egusphere-egu21-12728, 2021.

09:15–09:17
|
EGU21-8178
|
ECS
Gabriele Barile, Marco Redolfi, and Marco Tubino

River bifurcations play a crucial role in the morphodynamics of multi-thread channel systems such as braided or anastomosed 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, which tend to produce a differential erosion/deposition in the downstream channels, even in the case of symmetric planform configuration and boundary conditions. This leads to equilibrium states where the flow distribution can be highly unbalanced, depending on the channel width-to-depth ratio. The analyses performed so far have mainly focused on equilibrium configurations, while little information exists about the time evolution of the instability process. In particular, there is no systematic analysis of how the bifurcation timescale depends on the controlling parameters, such as the channel aspect ratio, and the length of the downstream channels. Evaluation of this “intrinsic” time scale is fundamental to study the response of river bifurcations to time variations of any “external” factor that influences the bifurcation dynamics, such as the water discharge, the downstream conditions, or the presence of migrating bedforms. In this work we consider a simple bifurcation, consisting of a straight channel with mobile bed but fixed banks, which splits in two bifurcates that diverge with the same angle. We employ a 1-D shallow-water model for the upstream channel and downstream bifurcates, which are connected by means of the nodal point relation proposed by Bolla Pittaluga et al. (2013). We then numerically analyse the development of the bifurcations instability, starting from an initially-small perturbation of the bed elevation. Finally, we extensively investigate the effect of the key controlling parameters, including the model calibration coefficients, also allowing them to vary in time. The evolution of water discharge asymmetry shows a first exponential growth stage, followed by an asymptotic behaviour that leads to the equilibrium configuration. Model results reveal the key role of the width-to-depth ratio in determining the speed of the bifurcation evolution. Specifically, the evolution is very slow when the system is close to marginal stability conditions, while it becomes increasingly fast when increasing the width-to-depth ratio. Moreover, the timescale of the bifurcation increases with the length of the downstream channel, unless their length-to-depth ratio is sufficiently high.

How to cite: Barile, G., Redolfi, M., and Tubino, M.: Analysing the intrinsic time scale of river bifurcations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8178, https://doi.org/10.5194/egusphere-egu21-8178, 2021.

09:17–09:19
|
EGU21-8017
Niccolò Ragno, Marco Redolfi, and Marco Tubino

The morphodynamics of multi-thread fluvial environments like braided and anastomosing rivers is fundamentally driven by the continuous concatenation of channel bifurcations and confluences, which govern the distribution of flow and sediment among the different branches that are reconnecting further downstream. Almost all studies performed to date consider the two processes separately, although they frequently appear as closely interconnected. In this work, we tackle the problem of analyzing the coupled morphodynamics of such bifurcation-confluence systems by studying the equilibrium and stability conditions of a channel loop, where flow splits into two secondary anabranches that rejoin after a prescribed distance. Through the formulation of a novel theoretical model for erodible bed confluences based on the momentum balance on two distinct control volumes, we show that the dominating anabranch (i.e. that carrying more water and sediment) is subject to an increase of the water surface elevation that is proportional to the square of the Froude number. This increase in water surface elevation tends to reduce the slope of the dominating branch, which produces a negative feedback that tends to stabilize the bifurcation-confluence system. A linear analysis of the coupled model reveals that the stabilizing effect of the confluence depends on the ratio between the length of the connecting channels and the average water depth, independently of the channel slope and Froude number. Furthermore, the effect of the confluence is potentially able to stabilize the channel loop in conditions where the classic stabilizing mechanism at the bifurcation (i.e. the topographical effect related to the gravitational pull on the sediment transport) is very weak, as expected when most of the sediment is transported in suspension. The identification of a characteristic length scale that produces a coupling between the confluences and bifurcations opens intriguing possibilities for interpreting the self-adjustment of the planform scale of natural multi-thread rivers.

How to cite: Ragno, N., Redolfi, M., and Tubino, M.: Morphodynamic stability and characteristic length scales of bifurcations and confluences loops, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8017, https://doi.org/10.5194/egusphere-egu21-8017, 2021.

09:19–09:21
|
EGU21-7478
Chris Tomsett and Julian Leyland

Interactions between riparian vegetation and river morphology are complex as they are often co-dependent, highly dynamic, and vary across both space and time. Vegetation diversity can be partially attributed to factors such as flood regimes and morphology, whilst simultaneously influencing the flow of water and sediment, ultimately impacting morphology and floodplain connectivity. As such, the importance of vegetation within the river corridor is well recognised and has been the subject of a considerable volume of research. However, within ecogeomorphology, most studies to date have been scale invariant, focusing either on characterisation of fine scale hydraulic roughness (e.g. using Terrestrial Laser Scanning; TLS) or on >reach scale patterns of riparian vegetation (using airborne or satellite imagery). Similarly, less attention has been paid to the temporal dynamics of vegetation beyond some appreciation of seasonality in controlling flow dynamics. This leaves a number of unresolved questions relating to the nested spatial and temporal (i.e. 4-dimensional; 4D) interactions of riparian vegetation and river flow.

In this study we seek to establish the temporal and spatial scales of riparian vegetation interaction within a river corridor using a traits based framework. Traits based research characterises plants with similar functional traits into guilds (groups) as opposed to by species or types, and as such provides a more useful basis to group vegetation according to the potential geomorphic impact that they exhibit. Traits based research for ecogeomorphic processes is relatively new in fluvial geomorphology, but has shown promise in its applicability, albeit existing applications are yet to investigate the temporal changes in vegetation. The need for extensive ground survey currently limits the application of traits based methods at reach scale and greater, highlighting the requirement for an approach that is able to classify a range of vegetation sizes and types into appropriate guilds.

Using a novel ULS and multispectral imaging systems, we have collected repeat high resolution (~1000 points per m3) surveys over a 1 km reach of the River Teme, UK, which has a wide variety of seasonally dynamic riparian vegetation. For each survey we use the point cloud data and multispectral imagery to classify vegetation into guilds. We use these in conjunction with the morphological data from the survey to create spatially varying surfaces of ecogeomorphic interactions, allowing us to establish links between guild coverage and morphological evolution across the reach throughout the year. The results show that vegetation-morphological co-evolution exists across scales and that high resolution survey methods are highly beneficial for resolving such interactions. The methods are designed to be transferable to other eco-geomorphic domains in any morpho-climatic regions, highlighting the flexibility and potential of a high resolution 4D traits based approach.

How to cite: Tomsett, C. and Leyland, J.: Exploring the 4D scales of vegetation-morphological interactions along a river corridor using repeat UAV Laser Scanning (ULS), multispectral imagery, and a functional traits framework., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7478, https://doi.org/10.5194/egusphere-egu21-7478, 2021.

09:21–09:23
|
EGU21-1465
Suresh Modalavalasa, Vinay Chembolu, Ketan Kumar Nandi, Vinayak Kulkarni, and Subashisa Dutta

A natural riverine corridor has several curls based on its physical and geomorphological characteristics. In most of the scenarios, the bridge construction on a meandering channel aligns along the convergent section. The enhanced secondary flow at convergent sections and the effect of meandering curvature bring the complexity in river turbulent characteristics. This effect may become predominant inside the main channel with variability in size and shape of the bridge pier. The present work discusses the turbulent structures in the main channel due to the variability in pier diameter (1inch and 2 inch ϕ) and a number of bridge piers on floodplain with inclusive of vegetation. Three-dimensional flow vertical and transverse velocity measurements were carried with acoustic Doppler velocimeter (ADV) 100Hz, at apex cross-section in a low sinuous channel. The results of the analysis showed that the combined effect of pier and vegetation on floodplain significantly altered the shear layer mechanisms in the channel with varying flow patterns. The comparison of the difference in secondary velocities between the pier with 1 inch and 2 inch ϕ  is 57% more in the case of lesser diameter pier. Further, the effect of size and number of piers on transverse velocity, Reynold’s shear stress is more susceptible to the mainstream. The convergence induced contraction of the meandering channel along with the bridge pier on its floodplain is observed to affect the turbulent structures formed in the main channel.

How to cite: Modalavalasa, S., Chembolu, V., Nandi, K. K., Kulkarni, V., and Dutta, S.: Effect of bridge pier induced turbulence on vegetated meander river morphology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1465, https://doi.org/10.5194/egusphere-egu21-1465, 2021.

09:23–09:25
|
EGU21-10900
|
ECS
Jyotirmoy Barman, Jyotismita Taye, and Bimlesh Kumar

The study of turbulence in a compound channel would address the nature of sedmient transport and bank erosion activity. The study would also give insights of embankment and levee breaches at the time of high flood. Experimental investigations were conducted on two compound channels of 310 and 450 bank angle in the laboratory flume to study the turbulece scale. Velocity data were recorded with Nortek Velocimeter at seven different locations (3 locations at the upstream, 3 locations at the downstream and 1 location at the middle) of the compound channel. Turbulence scale like Taylor microscale (λT) estimates the length scale of the inertial sub- range. The Taylor scale is calculated as:

The Taylor microscale analysis showed dominance in the main channel for 450 bank angle as compared to 310 bank angle. In the location of slope midpoint and floodplain region of the compound channel, Taylor microscale was more dominant for 310 bank angle. Another important observation in both the compound channels (310 and 450 bank angle) is the dominance of Taylor microscale at the upstream section of the channel as compared to the downstream part of the channel. The results from the study would help us to get a better understanding of the role of turbulence in the morphological changes in a compound channel with different bank angles.

How to cite: Barman, J., Taye, J., and Kumar, B.: Turbulence Scale in Compound Channel, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10900, https://doi.org/10.5194/egusphere-egu21-10900, 2021.

09:25–09:27
|
EGU21-1630
|
ECS
Laxman Rathod, Bandita Barman, and Bimlesh Kumar

Estimation of sediment transport has significant implementation on water resources and hydraulic engineering. Transport of sediment is affected by flow and sediment properties and also climatic variation of the region. To examine the behaviour of sediment transport, wide range of experiments have been performed in laboratories. Most of the developed sediment transport formulations are empirical or semi empirical in nature. These days, the development of computer-aided programs such as MATLAB has opened the way for researchers to quickly study the generation mechanism. The “Artificial Neural Networks (ANN) and Adaptive Neuro-Fuzzy Inference System (ANFIS)” can be used widely for developing sediment model. In this research, Feed Forward Back Propagation (FFBP) sort of ANN and Hybrid type based on the Sugeno approach of ANFIS is used to develop a model for bed material load transport using parameters like “channel discharge, width of the channel, flow depth,  friction/energy slope, mean size of sediment, bed shear stress, critical shear stress, gradation coefficient of the sediment particles, specific gravity, and viscosity”. Subsequently, the relationship between the expected and observed values is presented. The proposed approach showed superior results based on various statistical parameters, like the coefficient of determination (R2), Nash-Sutcliffe coefficient (NSE), Root mean square error (RMSE) and Mean absolute error (MAE). Correlation (R2), higher than (~0.90) indicates that ANN and ANFIS are compatible and capable of measuring the total bed material load.

Keywords: Sediment transport, Bed material Load, ANFIS, ANN, FFBP

How to cite: Rathod, L., Barman, B., and Kumar, B.: Estimation of Bed Material Load using Artificial Intelligence Techniques, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1630, https://doi.org/10.5194/egusphere-egu21-1630, 2021.

09:27–09:29
|
EGU21-2141
Kristen Cook, Jens Turowski, and Niels Hovius

In mixed bedrock-alluvial rivers, the response of the system to a flood event can be affected by a number of factors, including coarse sediment availability in the channel, sediment supply from the hillslopes and upstream, flood sequencing, and coarse sediment grain size distribution. However, the impact of along-stream changes in channel width on bedload transport dynamics remains largely unexplored. We combine field data, theory, and numerical modeling to address this gap. Observations from two flood events in the Daan River gorge in western Taiwan suggest that coarse sediment evacuation and re-deposition can cause intra-flood changes of up to several meters in channel bed elevation that are distinct from measured before/after bed changes. We hypothesize that this could be related to the abrupt change in width between the 1 km long bedrock gorge and the river upstream and downstream. An analysis of the theoretical relationships between discharge, channel width, and bedload transport capacity shows that for a given slope, narrow channels transport bedload more efficiently than wide ones at low discharges, while wider channels are more efficient at high discharges. We used the model sedFlow to explore this effect, running a random sequence of floods through a channel with a narrow gorge section bounded upstream and downstream by wider reaches. Channel response to imposed floods is complex, as high and low discharges drive different spatial patterns of erosion and deposition, and the channel may experience both of these regimes during the peak and recession periods of each flood. Our modeling suggests that width differences alone can drive substantial variations in sediment flux and bed response, without the need for variations in sediment supply or mobility. Further, the deposition or erosion that takes place within a flood is often not reflected in the before/after changes to the bed, and this disconnect increases with increasing flood size.

How to cite: Cook, K., Turowski, J., and Hovius, N.: Width control on event scale bedload dynamics in bedrock-confined channels, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2141, https://doi.org/10.5194/egusphere-egu21-2141, 2021.

09:29–09:31
|
EGU21-10377
|
ECS
Mel O. Guirro, Rebecca A. Hodge, Fiona Clubb, and Laura Turnbull

Sediment transport in rivers depends on interactions between sediment supply, topography, and flow characteristics. Erosion in bedrock rivers controls topography and is paramount in landscape evolution models. The riverbed cover indicates sediment transport processes: alluvial cover indicates low transport capacity or high sediment supply, and bedrock cover demonstrates high transport capacity or low sediment supply. This study aims to evaluate controls on the spatial distributions of bedrock and alluvial covers, by analysing scaling geometric relations between bedrock and alluvial channels. A Principal Component Analysis (PCA) was conducted to evaluate correlations between river slope, depth, width, and sediment size. The two principal components were used to implement a clustering analysis in order to identify differences in alluvial and bedrock sections. Spatial distributions of mixed bedrock-alluvial sections were investigated from two datasets - Scottish Highlands (Whitbread 2015) and the San Gabriel Mountains in the USA (Dibiase 2011)-, representing different environmental conditions, such as erosion rates, lithology, tectonics, and climate. The rock strength of both areas is high, and therefore it is excluded as a factor that explains the difference between the areas. The results of the cluster analysis were different in each environment. The main sources of variation among river sections identified by PCA were slope and width for the San Gabriel Mountains, and drainage area and depth for the Scottish Highlands. The rivers in the Scottish Highlands formed clusters that differentiate bedrock and alluvial patches, showing a clear geometric distinction between channels. However, the river analysis from the San Gabriel Mountains showed no clusters. Bedrock rivers are typically described as narrower and steeper than alluvial rivers, as demonstrated by rivers in the Scottish Highlands (e.g. slope was around 0.1 m/m for bedrock sections and 0.01 m/m for alluvial sections). However, this may not be always the case: both bedrock and alluvial sections in San Gabriel Mountains presented similar slope around 0.1 m/m. The inability to demonstrate significant geometry differences in bedrock and alluvial sections in the San Gabriel Mountains may be due to the frequency and magnitude of sediment supply of that region, which are influenced by tectonics and climate. A major difference in the supply of sediment in rivers of the San Gabriel Mountains is the frequent occurrence of debris flow. Non-linear interactions between hydraulic and sediment processes may constantly modify the geometry of bedrock-alluvial channels, increasing the complexity of analysis at larger temporal and spatial scales. This study is part of the i-CONN project, which links connectivity in different scientific disciplines. A sediment connectivity assessment in different environments and scales may be useful to evaluate the controls on the spatial distribution of bedrock and alluvial rivers.

 

Dibiase, R.A. 2011. Tectonic Geomorphology of the San Gabriel Mountains, CA. PhD Thesis. Arizona State University, Phoenix, 247pp.

Whitbread, K. 2015. Channel geometry data set for the northwest Scottish Highlands. British Geological Survey Open Report, OR/15/040. 12pp.

How to cite: Guirro, M. O., Hodge, R. A., Clubb, F., and Turnbull, L.: Analysis of geometric relationships of bedrock and alluvial channels: a comparison between rivers from the Scottish Highlands and San Gabriel Mountains (USA), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10377, https://doi.org/10.5194/egusphere-egu21-10377, 2021.

09:31–09:33
|
EGU21-12453
Matteo Saletti and Marwan Hassan

In gravel-bed rivers the relation between the magnitude and frequency of sediment input, the threshold for motion and channel stability is still not fully understood.

Here we present results from a 280-hour long flume experiment, in which poorly sorted sediment was fed episodically in an 18-m long, 2.2%-steep channel. The experiment included 7 consecutive runs lasting 40 hours each characterized by a constant water discharge but different sediment supply regimes (i.e., with no feed, constant feed and sediment pulses). Several measurements of sediment transport, flow depth and bed structures were taken along the flume, to assess how changes in sediment supply influence particle mobility and channel stability.

Our results show that the surface grain‐size distribution coarsened quickly, developing an armored layer that persisted throughout the entire experiment with only short-lived changes after sediment pulses. Grain clusters and other bed structures developed continuously during the experiments, changing dynamically in response to sediment pulses.

We estimated the thresholds of motion with three different methods, all of which yielded consistent results. Overall, the threshold for motion increased during the experiment, fluctuating in response to changes in sediment input. Our results provide further evidence to the idea that the threshold for motion in gravel-bed rivers is not a constant, but changes as a state parameter. These changes in our experiments are controlled by (a) the sediment supply regime, (b) the degree of bed structuring, and (c) the history of bed evolution. These outcomes suggest that sediment supply regime is a primary control on bed surface evolution and the channel stabilizing function played by surface structures.

How to cite: Saletti, M. and Hassan, M.: Feedbacks between sediment input, bed state and threshold for motion in gravel-bed rivers: an experimental study, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12453, https://doi.org/10.5194/egusphere-egu21-12453, 2021.

Flood processes
09:33–09:35
|
EGU21-9362
|
ECS
Chandan Pradhan, Suman Padhee, Subashisa Dutta, and Rishikesh Bharti

River recovery is the process that describes the capacity of the river to adjust to the limiting boundary conditions. In the regulated rivers, altered flow-sediment regime controls the trajectory of adjustments along the geomorphic and vegetative attributes. The present study is focused on recovery potential assessment of Mahanadi River, which shows a gradual emergence of in-channel vegetation in the post-dam period. The study area encompasses (i) 10 km reach of Mahanadi River (M1) having bedrock exposed, anabranching channel pattern and (ii) 102 km reach of Ong river (O1) with alluvial, compound channel form. In this study, Google Earth Engine cloud computing platform is used to process the Landsat images (1980-2010) and vegetation, water, and floodplain geomorphic classes are derived by Normalized Difference Vegetation Index (NDVI) and modified Normalized Difference Water Index (mNDWI). Finally, the intensity disorder index (IDI) is computed to represent the ‘system state’ in the post-monsoon periods and the influence of vegetation growth on the channel recovery. The results show that M1 is relatively stable, with cumulative vegetation area increased from 2% in 1980 to 8% in 2010. However, O1 demonstrates an accelerated increase in vegetation area i.e., 10% in 1980 to 30% in 2010. The system state (IDI) varies between 0.2 and 0.6 and follows a decreasing trend along M1 and O1. The findings establish that both regulated reaches may approach channel recovery in the near future, and prevailing boundary conditions indirectly influence the rate and direction of IDI.        

How to cite: Pradhan, C., Padhee, S., Dutta, S., and Bharti, R.: An entropy-based investigation on the river recovery potential in a regulated river basin , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9362, https://doi.org/10.5194/egusphere-egu21-9362, 2021.

09:35–09:37
|
EGU21-14124
|
ECS
Haiqing Xu, Shailesh van der Steeg, and Raymond Torres

Intermittent floodplain channels are low‐relief conduits etched into the floodplain surface and remain dry much of the year. These channels comprise expansive systems and are important because during low‐level inundation they facilitate lateral hydraulic connectivity throughout the floodplain. Nevertheless, few studies have focused on these floodplain channels due to uncertainty in how to identify and characterize these systems in digital elevation models (DEMs). In particular, their automatic extraction from widely available DEMs is challenging due to the characteristically low‐relief and low‐gradient topography of floodplains. We applied three channel extraction approaches to the Congaree River floodplain DEM and compared the results to a channel reference map created through numerous field excursions over the past 30 years. The methods that we tested are based on flow accumulation area, topographic curvature, and mathematical morphology, or the D8, Laplacian, and bottom‐hat transform (BHT), respectively. Of the 198 km of reference channels the BHT, Laplacian, and D8 extracted 83%, 71%, and 23%, respectively, and the BHT consistently had the highest agreement with the reference network at the local (5 m) and regional (10 km) scales. The extraction results also include commission “error”, augmenting the reference map with about 100 km of channel length. Overall, the BHT method provided the best results for channel extraction, giving over 298 km in 69 km2 with a detrended regional relief of 1.9 m.

How to cite: Xu, H., van der Steeg, S., and Torres, R.: Automatic Extraction of Intermittent Channel Systems of a Low‐Relief, Low‐Gradient Floodplain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14124, https://doi.org/10.5194/egusphere-egu21-14124, 2021.

09:37–09:39
|
EGU21-7216
|
ECS
Ervis Krymbi and Dritan Rustja

In the last years, Albania underwent a rapid development, which resulted in an uncontrolled building boom and general land degradation. For these reasons, an ever-greater portion of the Albanian population is exposed to natural risks, whose major threats are represented by floods and earthquakes. Spatial planning and hydraulic risk management are a worldwide necessity which is best achieved when natural and artificial elements located closely to watercourses are known in detail. A geodatabase is a practical tool to store and manage such information. Land use and land cover changes have negative consequences on watershed management in Buna River Basin. They increase impervious ground surfaces, decrease infiltration rate and increase runoff rate, hence causing flood during the dry seasons. This study was undertaken to achieve the natural and artificial elements connected to hydraulic risk and fluvial dynamics in Buna River. Through a GIS overlay and GPS measurements where mapped elements include buildings, hydraulic works, weirs, drainage outlets, riverbanks, structural damages, fluvial bars and eroding banks. Consequently, a GIS geo database was built to visualize the spatial distribution of the mapped elements and to store a series of technical data, including the present preservation condition for man-made objects. GPS data was integrated in GIS to examine the extent of land use and cover change in the sub catchment of Buna River. Both quantitative and qualitative data were used for this study.  The geo database provides an overview of the territories connected with the fluvial dynamics, highlighting that in the studied territory; the more is urbanized, the more it is exposed to hydraulic risk.

Key word; spatial planning, natural hazards, relief drill, Buna river, Shkoder.

How to cite: Krymbi, E. and Rustja, D.: Spatial planning and flood risk: The case of the Buna River, Shkoder, Albania, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7216, https://doi.org/10.5194/egusphere-egu21-7216, 2021.

09:39–10:30
Break
Chairpersons: László Bertalan, Eliisa Lotsari, Joshua Ahmed
11:00–11:02
|
EGU21-15498
Paul Hudson, Franklin Heitmuller, Samuel Muñoz, and Jansen Costello

Flood sedimentary deposits vary due to upper basin and lower basin controls. In this study we focus on overbank sediment thickness, which over longer periods drives changes to riparian aquatic habitat and floodplain construction. The study setting is a ~25 km long segment of the lower Mississippi alluvial valley, between Natchez, MS and Red River Landing, LA. We report new field data for overbank sedimentation generated by compound flooding over 2018 and 2019 hydrologic years, and compare with sedimentation data from prior large flood events. Overbank conditions in 2018 and 2019 persisted for 286 days (at Natchez, MS). During the 2019 hydrologic year the Mississippi was overbank for a record duration of 216 days, resulting in a much greater duration of overbank sedimentation than the 2011 (53 days) and 1973 (90 days) flood events.

The thickness of overbank deposits are reported for 48 field sites across a range of depositional environments typical of large lowland meandering river floodplains. Flood deposits were sampled in October 2019 using conventional field sampling procedures, including sedimentation traps (artificial grass mats installed in October 2017) and recognition of recent sediment deposited atop buried organic layers. The thickness of each reported sample is an average of three measurements obtained at each field site.

The average thickness of flood deposit samples over 2018-2019 hydrologic years is 71 mm, with variability according to distance from channel and floodplain depositional environment. Maximum sedimentation was associated with crevasse (750 mm) and sand sheet (1,430 mm) deposition along the crest of natural levees. Sedimentation thickness decreases within ~250 m of the channel, but remains high at a distance of ~3.5 km (30 mm). Beyond the range of sand sheet deposition, overbank deposition is likely influenced by variability in floodplain hydrology and geomorphology across natural levee (181 mm), meander scroll (30 mm), old channel (77 mm), and backswamp (108 mm) environments. High backswamp sedimentation at the study site is likely influenced by historic hydraulic engineering for flood control, which has altered local sedimentation patterns.

The 2018-2019 sedimentation data are contextualized by comparison with field data from the record 2011 magnitude flood (peak Q of 65,978 m3/s at Vicksburg, MS, USGS 0728900) and the historic 1973 flood (55,558 m3/s).  Average sediment thickness for the 2011 and 1973 overbank deposits was 42 mm (n=49) and 230 mm (n=31), respectively. The 2018-2019 daily sedimentation rate (0.25 mm/day) is much less than 2011 (0.75 mm/day). Thus, the much thicker sedimentary deposits for the 2018-2019 events suggests the greater importance of flood duration – rather than flood magnitude – to overall floodplain processes and alluvial fill chronologies along lowland rivers. The much lower flood sedimentation rate for 2018-2019 in comparison with 1973 (2.49 mm/day) may reveal the persistent decline in Mississippi suspended sediment loads since the early 1950s. Study results are further contextualized by considering corresponding event-based discharge – suspended sediment dynamics, sediment province, as well as flood hydroclimatology.

How to cite: Hudson, P., Heitmuller, F., Muñoz, S., and Costello, J.: Contextualized sedimentation rates for large floods along the lower Mississippi River: the importance of flood duration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15498, https://doi.org/10.5194/egusphere-egu21-15498, 2021.

11:02–11:04
|
EGU21-9075
|
ECS
Diego Alves de Oliveira, Cristina Helena Ribeiro Rocha Augustin, Trevor Hoey, and Cristina Persano

The presence of wetlands as a result of local fluvial and hydrological conditions constitutes a frequently observed feature of such rivers. Therefore, they are important elements of the basin, because besides functioning as buffer zones for CO2 and sediments they also house important ecosystems, playing an important role in the control of water circulation. Brazilian wetlands have different typologies and sizes, varying from huge swamplands such as the Pantanal do Mato Grosso, to flooded savannas called “veredas” or oxbow lakes. Their distribution in inland areas depends on the variety of flood pulses mainly linked to seasonality with the presence of distinct dry and wet seasons (Junk et al., 1989). This strong seasonality affects the São Francisco River (SFR), the 4th largest river in Brazil, which has frequent marginal lakes and swamps as it passes through five Brazilian states. This research aims to analyze the effect of the variation of the SFR level from 1925 to 2018, on the flow of the Pandeiros River which is one of many tributaries on the left side of SFR and on its wetland (“Pantanal Mineiro”). This wetland is hydrogeomorphologically linked to the SFR and receives water inputs during SFR flood periods. Measurements of the SFR water level performed once daily in the morning were obtained from gauging station no 44200000 belonging to the Companhia de Pesquisa de Recursos Minerais (CPRM) [altitude 445 m; 15°56'57.84"S; 44°52'4.68"W. The hydrological year starts at the end of the dry season on October 1st. Time series analyses (level duration curve, Seasonal Trend Decomposition (STL) of the daily level data, monthly level, mean, maximum, minimum level for each day of the year) were conducted to describe the hydrological regime and to assess temporal changes of the SFR levels and how these affect the magnitude, frequency and duration of flooding of the Pandeiros’s River wetland. Field observations (March 14, 2018) show that when SFR, which is Pandeiro’s base level, reaches a level of 5.0 m this leads to flooding conditions of the Pandeiros River wetland. Over the full period of record (1925-2018) the average level of the SFR was 3.86 m, with a minimum annual average of 2.43 m during the dry season (winter) and maximum of 5.98 m during the wet season (summer), with an average annual range of 3.55 m between both seasons. The SFR was above the 5.0 m threshold flooding level for 20% of the time 1925-2018, which corresponds to an average of 77.8 days of flooding per year in the wetland. The longest period of inundation was 178 days in 1926, when the SFR reached its maximum recorded level, and the shortest was 1 day in 2015, when it reached its minimum. The number of days per year of inundation have decreased over the full record, but that this is mainly due to a significant decrease since 1985. Prior to this, cyclic differences between wetter (1925 and 1985) and drier periods (1925 to 1945, 1945 to 1965) are observed.

How to cite: Alves de Oliveira, D., Helena Ribeiro Rocha Augustin, C., Hoey, T., and Persano, C.: Wetland dynamics at the transition between humid and semiarid environments of inland Brazil: São Francisco river morphodynamics and implications for the Pandeiros wetland., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9075, https://doi.org/10.5194/egusphere-egu21-9075, 2021.

Fluvial sedimentology
11:04–11:06
|
EGU21-6652
|
ECS
Jandessa Jesus, Fabiano Pupim, and André Sawakuchi

The Tocantins River is the easternmost fluvial system of the Amazon region, with a watershed draining both the Amazon rainforest and the Cerrado dry forest. This condition makes the Tocantins a natural laboratory to investigate the effects of past climate variability along with the rainforest-savanna ecosystems because its watershed drains both the Amazon rainforest and the Cerrado dry forest, and it is influenced by the Equatorial and central-northeast Brazil hydroclimates. Despite these unique physiographic and climatologic conditions, the Quaternary history of the Tocantins River is poorly known due to a lack of geochronological data about its sedimentary record. Here, we use optically stimulated luminescence (OSL) dating applied to quartz sand grains combined with geomorphological and sedimentological techniques to reconstruct the morphosedimentary evolution of the middle reach of the Tocantins River during the Late Quaternary. Three main geomorphological units were mapped: (i) fluvial plain, (ii) fluvial terraces, and (iii) paleo-alluvial fans. The OSL ages of 33 sediment samples retrieved from these geomorphological units range from 661 ± 42 years to 160 ± 16.3 ka, allowing to reconstruct of the depositional-erosion periods during the Mid-Late Pleistocene and Holocene. Our data indicate three stages of fluvial aggradation and two stages of incision. The older aggradational stage is represented by sediments from Upper Terrace (T1) and the paleo-alluvial fan deposited between 160 and 32 ka. Subsequently, a major incision event occurred at ~31 ka, which resulted in the abandonment of T1. The second phase of aggradation is recorded in the Lower Terrace (T2) and it also promotes reactivation of the paleo-alluvial fans from 31 to 6 ka. A new incision occurred from about 6 to 5 ka, allowing the abandonment of the T2 and reducing the local base level to its current position. The modern floodplain was built from 5 ka to the present, with sediment deposition due to lateral migration of the Tocantins River channel. The phases of aggradation and incision were correlated with regional paleoclimatic data, suggesting that precipitation changes related to the South American Summer Monsoon (SASM) as the main driver of the evolution of the Tocantins river in the last 160 ka. Disturbances in the supply of sediments and the flow of these rivers promote phases of depositional (drier periods) and incision (wetter periods). These depositional and incision phases of the Tocantins River appear to be synchronous with changes recorded by rivers from central and western Amazon, suggesting that the SASM is the main control of the Amazon fluvial systems. The continuous change in the Tocantins River dynamics has molded a high heterogeneity of habitats in the associated floodplains and terraces, which is a fundamental factor to support the diversity of fauna and flora in this transitional environment between Amazon and Cerrado biomes. 

How to cite: Jesus, J., Pupim, F., and Sawakuchi, A.: Quaternary climate variability as the main driver of the fluvial evolution of the Middle Tocantins River, eastern Amazonia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6652, https://doi.org/10.5194/egusphere-egu21-6652, 2021.

11:06–11:08
|
EGU21-9928
|
ECS
Caio Breda and Fabiano Nascimento Pupim

The last 30 ka is a period marked by well-documented abrupt environmental changes on Earth. Despite the growing efforts to investigate the effects of past environmental changes in the fluvial dynamics, there is a lack of studies in intraplate tropical regions. Here, we applied geomorphological, sedimentological, and optically stimulated luminescence dating (OSL) technics to investigate the effects of environmental factors on the evolution of the Upper and Middle Tietê River during the Late Quaternary. Tietê River is one of the most important rivers of the southeast of Brazil, flowing from steepest to low-relief intraplate terrains, and under tropical climate. In order to understand the responses of the Tietê River system to environmental changes during the Late Quaternary, two main questions were tentatively answered: (i) what are the most important allogeneic factors for the evolution of this system?; (ii) how did climatic fluctuations affect river dynamics over time? We recognized a sequence of seven terraces, from 2 to 105 m above the channel, in the Middle Tietê valley. These terraces are formed by thin deposits (< 10 m), composed of sandy and conglomeratic sediments. The high and intermediate terrace levels of the Middle Tietê River are strath, while the low terraces of the middle reach are cut-and-fill. The formation of seven terrace levels in the Middle Tietê River was controlled by the combination of low erosion resistance of the lithological substrate and high stream power and coarse bedload that increase the erosion efficiency of the channels. OSL dating of sedimentary deposits in different terrace levels indicate 5 periods of aggradation in the Middle Tietê valley since the Last Glacial Maximum: 18.5 ± 2.0 ka; 9.8 ± 1.0 to 8.6 ± 0.8 ka; 7.1 ± 0.7 to 5.8 ± 0.5 ka; 4.2 ± 0.4 to 3.1 ± 0.3 ka; and 0.6 ± 0.06 ka. The results indicate that the activity of the South American Monsoon System induced the occurrence of climatic fluctuations and changes in vegetation cover in the river valleys of southeastern Brazil over the past 20 ka. The aggradation periods are correlated with more arid environmental conditions and sparser vegetation, while the incision events in the valley developed under transitions to humid environmental conditions and stimulated by vegetation recovery.

Key-words: Tietê River, fluvial evolution, fluvial terraces, Quaternary geochronology.

How to cite: Breda, C. and Nascimento Pupim, F.: Change of channel pattern and construction of fluvial terraces driven by SAMS since the LGM in southeastern South America: records from Tietê River, Brazil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9928, https://doi.org/10.5194/egusphere-egu21-9928, 2021.

11:08–11:10
|
EGU21-11713
|
ECS
Stefania C. Oliveira, Fabiano N. Pupim, José C. Stevaux, and Mario L. Assine

The Paraná River has being extensively studied in terms of its hydrological and sedimentological aspects, but the geological history of its fluvial plain is still poorly understood due to the lack of geochronological data. Most of the published studies are focused on the low and middle reaches, and the region upstream of the Porto Primavera dam is an area almost unknown in terms of geomorphic evolution. Here, we aim to investigate the succession of geological events responsible for the evolution and current configuration of the Upper Paraná River fluvial system, in the stretch between the Jupiá and Porto Primavera Hydroelectric Plants (São Paulo and Mato Grosso states). Thus, we used an integrated approach including remote sensing data, geomorphology, sedimentology, bathymetric profiles, and chronological methods. Four geomorphological compartments were identified and three of these compartments were dated by Optically Stimulated Luminescence (OSL): Unit 1, raised terrace with circular and semicircular ponds (~150 ka); Unit 2, intermediate terrace with small ponds and waterlogged areas (~60 ka); Unit 3, low terrace with preserved paleochannels (~39–35 ka); and Unit 4, current river plain (>9 ka). The geomorphological units were correlated with previous studies downstream of the studied area and their sedimentary characteristics and depositional ages suggest that their genesis is linked to changes in climatic and hydrological conditions during the Late Quaternary. Units 1, 3, and 4 are considered extensions of the geomorphological units Taquaruçu, Fazenda Boa Vista, and Rio Paraná, respectively. Unit 2 is a compartment with unique morphological characteristics, therefore not correlated with units presented in previous works. Further, two main knickpoints were identified, suggesting an important control in the sedimentation and development of the terrace levels. Thus, this work brings new data on the evolutionary history of the Paraná River, which allows us to understand that the development of the terrace levels and the floodplain of the upper reaches are strongly controlled by the climatic changes that occurred during the Late Quaternary.

How to cite: Oliveira, S. C., Pupim, F. N., Stevaux, J. C., and Assine, M. L.: Late Quaternary evolution of the Upper Paraná River, southeast Brazil: a new geomorphological and chronological database, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11713, https://doi.org/10.5194/egusphere-egu21-11713, 2021.

11:10–11:12
|
EGU21-16498
Patricia Mescolotti, Fabiano do Nascimento Pupim, Francisco Sérgio Bernardes Ladeira, André Oliveira Sawakuchi, Amanda Santa Catharina, and Mario Luis Assine

Climate, tectonic and sea level factors contribute to the geomorphic evolution of large rivers. Rivers draining settings where the tectonic activity and sea level influence are minors allow clearer signals from climate variation to be identified. Thus, rivers that run exclusively in tectonically quiescent areas and away from coastal regions are the most suitable targets to understand the climate drivers. In northeastern Brazil, the São Francisco River is one of the largest cratonic rivers in South America, with an extension of 2,900 km, and its thousand years evolution is registered by Quaternary deposits preserved as terraces. With the upper course in semi-humid settings, the São Franciscos flows northward, but with a watershed mostly under semi-arid conditions. Hence, the São Francisco River’s deposits are an excellent fluvial sedimentary record to shed light on how large tropical rivers responded to climatic changes of the Quaternary. We studied a 200 km stretch of the middle course of the São Francisco in the State of Bahia by using remote sensing methods and field surveys for geomorphological and sedimentological analyses combined with optically stimulated luminescence dating (OSL). We recognized at least four phases of fluvial aggradation (>90 ka; 65 to 39 ka; 18 to 9.5 ka and 380 years to recent) and three phases of incision (I1 - 85 to 65 ka; I2 - 39 to 18 ka and I3 - 9.5 to 1.0 ka). Two aggradation events and the incision event I2 are also observed in the upper course of the São Francisco River. The river incision events agree with precession insolation cycles (~25 ka) at latitude 10° S, which influence the rainfall in the area. The incision events occurred probably due to increased fluvial discharge produced by intensification of the South Atlantic Convergence Zone (SACZ), which has great influence on precipitation over the upper São Francisco river. Thus, we conclude that the aggradation-incision cycles of the São Francisco River during the last 100 ka are likely products of millennial precipitation variation, possibly related to precession cycles. The events of high sedimentation rate in the São Francisco river mouth are partially correlated with incision phases in its middle course. This suggests that sedimentation in plains of large plateau rivers can be decoupled from the coastal area.

Keywords: Late Quaternary, fluvial response, OSL dating, aggradation-incision cycles, precession cycles

How to cite: Mescolotti, P., do Nascimento Pupim, F., Bernardes Ladeira, F. S., Oliveira Sawakuchi, A., Santa Catharina, A., and Assine, M. L.: How Quaternary climate changes build and erode sedimentary deposits in an intraplate large fluvial system: the São Francisco River, Brazil, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16498, https://doi.org/10.5194/egusphere-egu21-16498, 2021.

11:12–11:14
|
EGU21-7592
Hans von Suchodoletz, Christoph Zielhofer, Mathias Ulrich, Azra Khosravichenar, Jan Miera, Pierre Fütterer, Ulrich Veit, Peter Ettel, Lukas Werther, Helen Ballasus, and Ulrike Werban

Fluvial sediments are valuable archives of late Quaternary landscape evolution, paleoenvironmental changes and human-environmental interactions. However, given their complex and non-linear character their correct interpretation requires a good understanding of the fluvial architecture. The fluvial architecture describes the spatial arrangement and genetic interconnectedness of different types of fluvial sediments in a floodplain such as channel and overbank deposits. To properly map the different fluvial forms, their variations in composition and geometry must be understood in three dimensions. However, whereas investigations of the fluvial architecture are relatively easy in cohesive floodplain types with incised channel beds and large natural exposures, these are challenging in floodplains with buried stratigraphies where artificial exposures or corings are required.

We studied three cross sections through the floodplain of the middle and upper course of the Weiße Elster River in Central Germany by means of geophysical Electrical Resistivity Measurements (ERT) and closely spaced drillings. These 2D investigations were complemented by spatial geophysical 3D measurements of Electromagnetic Induction (EMI) in the surrounding areas of the cross sections. The latter technique allows fast mapping of larger areas, and was only rarely applied to fluvial systems so far. Our novel and cost-effective combination of core drillings with multidimensional geophysical measurements allowed to systematically reconstruct the fluvial architecture of larger areas of the Weiße Elster floodplain with high resolution, and thereby demonstrates its high value for fluvial geomorphology. Furthermore, in combination with ongoing numerical datings of the fluvial sediments these investigations form the base for precise conclusions about possible climatic and human drivers of the Holocene fluvial dynamics of the Weiße Elster River.

How to cite: von Suchodoletz, H., Zielhofer, C., Ulrich, M., Khosravichenar, A., Miera, J., Fütterer, P., Veit, U., Ettel, P., Werther, L., Ballasus, H., and Werban, U.: A novel combination of core drillings with 2D and 3D geophysical measurements helps to decipher the fluvial architecture of buried floodplain sediments of the Weiße Elster River (Central Germany), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7592, https://doi.org/10.5194/egusphere-egu21-7592, 2021.

11:14–11:16
|
EGU21-6701
|
ECS
Mubarak Abdulkarim, Stoil Chapkanski, Damien Ertlen, Claire Rambeau, Laurent Schmitt, Louis Le Bouteiller, and Frank Preusser

The Alsatian Upper Rhine floodplain (northeastern France) is characterized by a complex anastomosing network of paleochannels inherited from Late Glacial braided fluvial pattern of the Rhine system. These paleochannels are filled by mixed or stratified clastic and organic sediments originating from different sediment sources. Identifying these sediments' provenance is critically important for understanding past surface processes and reconstructing the Upper Rhine Valley evolution in the course of the Holocene. This study employed mid-infrared spectroscopy to determine the source of sediments and, therefore, understand which rivers may have contributed to the paleochannel infilling and establish the main patterns of filling through time. Sediment samples with unknown sedimentary provenance were collected in 16 sites consisting of paleochannels and the Ill River's levees. Mid-Infrared spectroscopic analyses were carried out on powdered (< 2 mm) samples using a Frontier Spectrometer (PerkinElmer) equipped with Diffuse Reflectance Infrared Fourier Transform accessory. Statistical analysis (Discriminant Analysis - DA) was performed to compare the spectral signatures obtained from the samples and a previously established reference spectral dataset (Chapkanski et al. 2020) covering potential sediment sources in the Upper Rhine area (the Rhine, Ill, and Vosges tributaries). The results showed well-contrasted sediment sources, with multiple rivers contributing to the paleochannel infilling history. The sediments were found to originate from the Rhine and Ill River systems and, to a lesser extent, Vosges tributaries. Some channels have an exclusively Alpine Rhine catchment origin while others showed purely Ill signatures. However, for most of the channels, infilling sources changed over time, presenting a relatively complex mixture of multiple sediment sources, indicating the lateral shifting of the two rivers within the alluvial plain. The results confirm that Mid-infrared spectroscopy (MIRS), combined with discriminant analysis, can give highly-specific determinations to sediments' sources. Thus MIRS-DA technique shows the potential of its being applicable as a rapid, low-cost, and efficient alternative method for provenance analysis of fluvial deposits in large, complex floodplains.

How to cite: Abdulkarim, M., Chapkanski, S., Ertlen, D., Rambeau, C., Schmitt, L., Le Bouteiller, L., and Preusser, F.: Provenance Determination of Paleochannel Infillings in the Alsatian Upper Rhine Floodplain Using Mid-Infrared Spectroscopy-Discriminant Analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6701, https://doi.org/10.5194/egusphere-egu21-6701, 2021.

11:16–11:18
|
EGU21-4979
Eva Mencin Gale, Petra Jamšek Rupnik, Miloš Bavec, Mirka Trajanova, Luka Gale, Flavio S. Anselmetti, and Andrej Šmuc

This study focuses on the Pliocene-Quaternary sedimentary evolution of the fluvial systems in the Slovenj Gradec, Nazarje, Velenje, Celje, Drava-Ptuj and Krško Basins in the south-eastern Alpine foreland, Slovenia. The main aim was to determine the composition, morphostratigraphy, provenance, sedimentary environment and age of the deposits using geomorphological, sedimentological, geochemical, mineralogical and biostratigraphical methods. Pliocene-Quaternary sediments were deposited in fluvial (braided and wandering river systems) and alluvial/colluvial fan environments. The sediments are preserved in the terrace staircase sequences, formation of which is strongly controlled by tectonic activity. Based on geomorphological analyses, low-, middle- and high-level terrace groups were constrained and tentatively attributed to Late Pleistocene, Middle Pleistocene, and Plio-Early Pleistocene, respectively. The provenance analyses focused on the Plio-Early Pleistocene sediments and included lithological and microfacies analyses of the clasts. Based on the provenance analyses and published data, the long-term development of the drainage network was interpreted. Major changes occurred during the transition from Miocene-Pliocene and at the latest at Plio-Early Pleistocene the drainage network reached conformity with the present one. Overall, the spatial distribution of the Pliocene-Quaternary landforms revealed tectonic activity in intramontane basins during their development, from which the landscape evolution was deduced. 

How to cite: Mencin Gale, E., Jamšek Rupnik, P., Bavec, M., Trajanova, M., Gale, L., Anselmetti, F. S., and Šmuc, A.: Pliocene-Quaternary river-terrace sequences in intramontane basins in the south-eastern Alpine foreland (Slovenia): characterization of morphostratigraphy and provenance, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4979, https://doi.org/10.5194/egusphere-egu21-4979, 2021.

11:18–11:20
|
EGU21-12502
|
ECS
Kim Genuite, Carole Nehme, Daniel Ballesteros, Dominique Todisco, and Damase Mouralis

The Seine river (France) drains today a catchment area of 80,000 km2 covering almost the northern part of France. Despite its importance, few studies focused on the Seine catchment and its landscape evolution, unlike the Somme basin, which remains a European reference for the Quaternary, because of to the numerous archaeological sites it contains. The middle and lower Seine valley in Normandy shows nevertheless a particular meandering fluvial dynamic and a succession of fluvial terraces over 120 m height, dated back to Early Quaternary. Previous works focused on the stratigraphy of alluvial sequences and led to the accurate characterization of lower fluvial and estuarine levels from Marine Isotope Stage (MIS) 1 to MIS 11. The alluvial terraces comprise also various Acheulean industries, showing human settlements in the valley for at least 400,000 years. Such archaeological remnants were retrieved in Saint-Pierre-Lès-Elbeuf, Tourville-la-Rivière, Vernon and La Celle.

Nowadays, the Seine connects to the drowned lower Seine course which continues in the Channel. This submerged part was subaerial during the last glacial cycle. Presently, the lower Seine course is still under the influence of marine tidal effects up to la Bouille (around 30 km from the coast). Additionally, estuarine deposits filled the valley up to Les Andelys (around 80 km from the coast) during the Holocene transgression and cover the penultimate and last glacial alluvial terraces. Nevertheless, the dynamic of the Seine river is broadly identified with few chronological constraints, but without any morphometric analysis combined with stratigraphical study.

This work provides a review of the stratigraphy of the quaternary alluvial deposits in the lower part of the Seine Valley, together with new morphometrical analysis of the paleo-meanders located at higher altitudes. The analysis of the paleo-morphologies compared with high-resolution digital elevation model (DEM), provides new means for constraining the fluvial incision and deposition over long distances and periods, and helps to discuss the river evolution related with quaternary uplift, catchment evolution and glacio-eustatic dynamics.

How to cite: Genuite, K., Nehme, C., Ballesteros, D., Todisco, D., and Mouralis, D.: Morphological evolution of the middle and lower Seine valley during the Quaternary period., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12502, https://doi.org/10.5194/egusphere-egu21-12502, 2021.

11:20–11:22
|
EGU21-14718
Alessandro Fontana, Livio Ronchi, Kim Cohen, Esther Stouthamer, Timme Donders, Kim Hissink, and Sandra Donnici

At the end of LGM the alluvial plains extending along the southern side of the Alps experienced a strong phase of fluvial entrenchment because of the impressive decrease of sedimentary input, related to the withdrawn of the Alpine glaciers within their valleys. Since 19 ka cal BP and up to Early Holocene, few incised valleys formed from the apex of the alluvial megafans to their distal sector but, along the northern Adriatic, the mid and late Holocene fluvial and coastal depositions have largely buried these landforms. During the Late Glacial the incised valleys were the only fluvial corridors where transport and deposition of sediments could occur in the whole plain.

We investigated the distal sector of the alluvial megafan of Tagliamento River through the analysis of a dataset consisting of ca. 2300 mechanical and hand-made cores. These data, compared with LIDAR-derived DEM, radiocarbon and paleoenvironmental analyses, allowed a detailed reconstruction of the formation and evolution of the buried incised valley characterizing the area of Portogruaro and Concordia Sagittaria. The valley has been traced for over 25 km, is up to 1.2 km wide and with a depth of 20 m below the top of LGM surface.

The erosive valley has been mainly formed between 19 and 14 ka cal BP, leading also to its partial infill with about 10 m of gravels, that arrived up to the present coast. The fluvial activity has been after limited to the deposition of fine sediment almost until the end of Late Glacial and, according to paleobotanical information, for the first time in the Venetian–Friulian Plain, these deposits recorded the vegetation of the Younger Dryas period.

After the disconnection from active Tagliamento, swampy environments occupied the valley bottom and the Holocene marine transgression started to indirectly affect the valley around 9.5 ka cal BP, contrasting the drainage and favouring the formation of widespread lacustrine environments. Since 8 ka cal BP lagoon entered in the valley and, following the sea-level rise, led to the deposition of a ca. 15 m thick unit of lagoon muds up to historical time. The infill of the valley documents the evidence of anthropogenic activity since 6-5 ka cal BP, probably in relation to wood clearance and soil degradation. Anyhow, significant human impact occurred during Iron and Roman Age, when Concordia became an important city. In 6th century AD high-magnitude floods deposited up to 5 m of sediments and largely obliterated the valley.

The detailed 3D reconstruction of the valley of Concordia allowed also to highlight the importance of the groundwater-fed streams in affecting the formation of this and other large incised valleys of Tagliamento. In particular, we produced evidence that river piracy by minor rivers triggered the creation of other incised valleys in the distal sector of Tagliamento megafan.

The buried incised valley of Concordia can represent a reference model also for describing the fluvial evolution of the other main Alpine rivers in the coastal sector of the whole Venetian-Friulian Plain during Late Glacial and Early Holocene.

How to cite: Fontana, A., Ronchi, L., Cohen, K., Stouthamer, E., Donders, T., Hissink, K., and Donnici, S.: Formation and infill of the Late Glacial incised valley of Concordia Sagittaria (Tagliamento River, NE Italy), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14718, https://doi.org/10.5194/egusphere-egu21-14718, 2021.

11:22–11:24
|
EGU21-13533
|
ECS
Mohammed Lghamour, Lhoucine Karrat, Vincenzo Picotti, Irka Hajdas, Negar Haghipour, Giulia Guidobaldi, and Karin Wyss Heeb

      The Inaouène wadi is a river located in the northern region of Morocco. Its catchment area covers about 5124 km² with an average altitude of 800 m. The tributaries drain the marly reliefs of the Prerif in the northern side, as well as its southern ones are crossing the liasic carbonate and the Paleozoic crystalline rocks of the last Middle Atlas foothills. This region is characterised by a semi-arid Mediterranean climate influenced by the ocean oscillations, the average annual rainfall records 600 mm with a very significant spatial and interannual irregularity.

Along the major part of its flow, the Inaouène river has cut its bed between the Prerif and the Middle Atlas belts, by following the foreland corridor that separates them. From a pass (Touaher) that marks the corridor closing, the river valley widens from East to West, forming an alluvial plain with a maximum width of 5 km incised by a meandering and highly sinuous stream.

      Alluvial deposits in this valley are more developed on the Atlas side than at the Prerif foot; At least five levels representing the vestiges of the Lower and Middle Pleistocene terraces are present in the landscape.

More recent deposits occupy the valley floor, they constitute a more homogeneous surface showing low terraces abrupts and lateral limits between different sedimentary units. These alluvial deposits correspond to the terminal Pleistocene, middle and upper Holocene epoch. About 30 samples of charcoal and TOC have been selected and analysed using the  AMS 14C dating. Due to the scarcity of organic matter, some of the samples contained less than 0.1 mg of carbon and had to be analysed using the gas ion source (GIS) interface of the MICADAS (Haghipour et al., 2019; Wacker et al.,2013). 12 sections were described in the field and of which 8 sections were analysed regarding grain size, mineralogical composition, carbonate content as well as organic matter in soils and sediments.

      The analysis results indicate that the late Pleistocene is characterised by a high fluvial activity reflected by the development of braided system river and so coarse material, while fine deposits of floodplains are more abundant during the Holocene.

……...........

Haghipour, N., Ausin, B., Usman, M. O., Ishikawa, N., Wacker, L., Welte, C., Ueda, K., and Eglinton, T. I., 2019, Compound-Specific Radiocarbon Analysis by Elemental Analyzer-Accelerator Mass Spectrometry: Precision and Limitations: Analytical Chemistry, v. 91, no. 3, p. 2042-2049.

Wacker, L., Fahrni, S., Hajdas, I., Molnar, M., Synal, H., Szidat, S., and Zhang, Y., 2013, A versatile gas interface for routine radiocarbon analysis with a gas ion source: Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions With Materials and Atoms, v. 294, p. 315-319.

How to cite: Lghamour, M., Karrat, L., Picotti, V., Hajdas, I., Haghipour, N., Guidobaldi, G., and Wyss Heeb, K.: Alluvial deposits evolution in the Inaouene river valley (Morocco) during late Pleistocene and Holocene epoch., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13533, https://doi.org/10.5194/egusphere-egu21-13533, 2021.

11:24–11:26
|
EGU21-13188
|
ECS
Peter Wooldridge, Robert Duller, Rhodri Jerrett, and Kyle Straub

Basin-scale fluvial architecture is, to a large extent, determined by the ability of river systems to migrate and avulse across their own floodplain. River avulsion takes place when a river aggrades by one channel depth to achieve super-elevation above the surrounding floodplain. However, peat enhancement of floodplain aggradation is likely to affect this fluvial behaviour and has received little attention. The interaction between river channels and peat-dominated floodplains is likely to have the effect of inhibiting or prolonging the conditions required for river avulsion, and so will impact on basin scale architecture during prolonged peat accumulation on floodplains. To elucidate and quantify the nature of this channel-floodplain interaction we investigate the coal-bearing clastic interval of the Carboniferous Pikeville Formation, Central Appalachian Basin, USA. Using a combination of well data and outcrop data, two coal horizons and intervening sand bodies, were mapped across an area of 5700 km2 to ascertain overall basin-scale architecture. Comparison of the accumulation rate of the coal units (corrected for decompaction) with the synchronously deposited sand bodies suggests that extensive and rapid peat accumulation can increase avulsion timescales by 3 orders of magnitude and dramatically alter basin-scale fluvial architecture.

How to cite: Wooldridge, P., Duller, R., Jerrett, R., and Straub, K.: Quantification of fluvial-peat interactions in the Pikeville formation Central Appalachian Basin, USA., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13188, https://doi.org/10.5194/egusphere-egu21-13188, 2021.

11:26–12:30