GM5.2

EDI
Advancing theory and modelling of river systems and erosion mechanics

Rivers in most parts of the world are experiencing ever strong disturbances of humans, which, in combination with climate change, have made river systems adjust their morphologies and boundaries significantly, resulting in a wide range of degradation in aquatic habitats, extinction of fish species, loss of flood-retaining areas etc. To minimize these negative effects, it is necessary to provide convincing predictions of the adjustments of river systems to the public and decision makers. However, rivers are dynamic systems that are too variable and behave in very complex manners. A lot of theoretical and numerical modelling frameworks have been proposed and practiced for quantitatively predicting the self-adjustments of river morphologies over the last several decades, and it is necessary to evaluate the physical/empirical bases and practical applicabilities of available theoretical and modelling frameworks so as to advance theory and modelling of river systems. This session aims to explore advances in modelling of river systems responding to environmental change, and identify possible links between simulated or projected changes, and the erosion mechanics that are in part responsible for these changes.

Public information:
Rivers in most parts of the world are experiencing ever strong disturbances of humans, which, in combination with climate change, have made river systems adjust their morphologies and boundaries significantly, resulting in a wide range of degradation in aquatic habitats, extinction of fish species, loss of flood-retaining areas etc. To minimize these negative effects, it is necessary to provide convincing predictions of the adjustments of river systems to the public and decision makers. However, rivers are dynamic systems that are too variable and behave in very complex manners. A lot of theoretical and numerical modelling frameworks have been proposed and practiced for quantitatively predicting the self-adjustments of river morphologies over the last several decades, and it is necessary to evaluate the physical/empirical bases and practical applicabilities of available theoretical and modelling frameworks so as to advance theory and modelling of river systems. This session aims to explore advances in modelling of river systems responding to environmental change, and identify possible links between simulated or projected changes, and the erosion mechanics that are in part responsible for these changes.
Co-organized by HS13, co-sponsored by IAG and I
Convener: Shawn Chartrand | Co-conveners: He Qing Huang, Paul Carling, Ian D. Rutherfurd, Alexander BeerECSECS, Claire MastellerECSECS, Matteo SalettiECSECS
vPICO presentations
| Tue, 27 Apr, 13:30–15:00 (CEST)
Public information:
Rivers in most parts of the world are experiencing ever strong disturbances of humans, which, in combination with climate change, have made river systems adjust their morphologies and boundaries significantly, resulting in a wide range of degradation in aquatic habitats, extinction of fish species, loss of flood-retaining areas etc. To minimize these negative effects, it is necessary to provide convincing predictions of the adjustments of river systems to the public and decision makers. However, rivers are dynamic systems that are too variable and behave in very complex manners. A lot of theoretical and numerical modelling frameworks have been proposed and practiced for quantitatively predicting the self-adjustments of river morphologies over the last several decades, and it is necessary to evaluate the physical/empirical bases and practical applicabilities of available theoretical and modelling frameworks so as to advance theory and modelling of river systems. This session aims to explore advances in modelling of river systems responding to environmental change, and identify possible links between simulated or projected changes, and the erosion mechanics that are in part responsible for these changes.

vPICO presentations: Tue, 27 Apr

Chairpersons: Paul Carling, Alexander Beer, Shawn Chartrand
A. River system evolution
13:30–13:32
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EGU21-13485
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ECS
Camilla Santos, Leonardo Dantas Martins, Kenia Sousa da Cruz, and Jonas Otaviano Praça de Souza

Rivers on semiarid landscapes typically are characterised by sandy geomorphic units and riverbanks, a natural factor that enhances lateral mobility. Vegetation cover is a crucial factor on lateral instability due to its impact on riverbank and geomorphic units erosion resistance. Nevertheless, riparian vegetation on intermittent and ephemeral channels show growing patterns directly affect by the flow temporality, that controls the water availability. Extended dry intervals hinder the succession ecological on geomorphic units, like bars and islands, and riverbanks and retard the growing process. This work analysed the effects of hydrological changes, caused by one water transfer project, on the bio-geomorphological patterns on riverbanks of a main intermittent river of Brazilian Drylands. Flow data series was used to understand the hydrological pattern changes; Google Earth images and UAV surveys to analyse the vegetation and riverbank behaviour from 2008 to 2020.  Lastly, the identification of riverbank material resistance was based on sedimentology analysis.  The water transfer Project PISF (Projeto de Integração do São Francisco), operating since 2017 March, increase the average flow days from 137,5 to 260/300 days and decreasing the continuous dry period from 200 to 30/45 days. The impact on average annual discharge was slightest, whereas the average water transfer volume was 3m3/s. It is essential to highlight the short period of data posterior to the water transfer and the non-regulatiry of water volume transferred; what limits the temporal representativity of the results. There were different types, and level of impacts depending on the river reach characteristics. However, in general, the longer flow permanence increases riparian vegetation density, vertical incision, and lateral stability. Riparian vegetation cover increase, from 20% to 100% on the 9 reaches analysed, across the entire channel, including bedrock reaches, with riverbanks having some rock outcrops percentage. The main changes were on sand bed reaches, that used to have, before 2017, a dynamic braiding pattern, without a clear main incised channel and thalweg shifting. Afterwards, the flow permanence, due to the water transfer project, enabled herbaceous stratus temporal continuity, contributing to surface stability and progressive bushes/trees cover growing. Lastly, the increase in lateral stability, mainly on thalweg position, facilitates the vertical incision on the sand bed reaches, representing 85% of this channel. As a secondary impact, there were necessary, to the road network, built floodway crossings at several points, which changes the channel morphology and the (dis)connectivity process. It can generate distinct channel position and morphology changes causing water and sediment retention upstream and erosion downstream. Lastly, there were slight differences in textural characteristics on riverbanks and geomorphic units, with a rise in fine sediment on the most vegetated areas/units. This analysis reveals that a fast response of riparian vegetation and sand bed reaches morphology, affecting the bio-geomorphological process and all environmental dynamic. It points to fundamental elements which need monitoring after hydrological changes, especially to intermittent and ephemeral rivers.

How to cite: Santos, C., Dantas Martins, L., Sousa da Cruz, K., and Otaviano Praça de Souza, J.: Riparian Vegetation Dynamic and River Stability on Intermittent Rivers: Impact of Water Transfer Project on Tropical Drylands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13485, https://doi.org/10.5194/egusphere-egu21-13485, 2021.

13:32–13:34
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EGU21-14337
Jonas Souza, Fernando Alexandre, and Gabriel Monteiro

Intermittent and ephemeral rivers, prevalent in dryland areas, have less monitoring data than perennial rivers worldwide. It hinders studies about hydro-geomorphology dynamics on these streams, which is especially complex in rain-fed flow regime on tropical rivers. Irregular rain patterns characterise the tropical drylands, which reverberate in the hydro-geomorphological dynamic. Unmanned Aerial Vehicles (UAV) survey is an efficient and cheap technic to monitor these streams since the dry periods expose the riverbed surface. This research aimed to analyse the hydro-geomorphology dynamic on sandy bed reaches of an intermittent tropical river. Five UAV surveys were realised on eight sandy reaches, from headwater to the outlet, between 2020 January 7 and December 9 in the Tigre River – Brazilian Drylands –, a 30Km ephemeral/intermittent. The UAV photos from all the surveys were co-aligned to create matching DEMs. We compared the DEMs to identify channel morphology changes, calculating differences in the riverbed and riverbank. The DEMs comparison enabled to calculate the erosion and sedimentation volume to each reach. Simultaneously, we installed crest stages gauges to monitor the peak water level between the surveys. Lastly, we used five rain gauges to identify the necessary rain volume that generates flow events. The 2020 annual rain volume was close to the historical average, between 530mm and 700mm, on the pediments, up to 1000mm on highland headwaters. The average potential evapotranspiration is around 1400-1800mm/year, due to the tropical climate. There was an average of 3.4 extreme rainfall daily events (over 50mm/day) during the year and the rainest period was between March 15 to 26th when rained from 134mm to 376mm around the watershed. The surveys between January 18 and March 8 identified insignificant morphology changes on eight reaches. The peak water levels were between no flow to 0.49m; only the outlet reach showed slight erosion and water level reaching 1.1m. The rain events between March 15/26th generate the water level annual peaks at all the reaches, from 1.9m to 5.4m (outlet reach). Seven reaches increased the vertical incision around 20/30cm to 80cm, and localised pools were eroded to up 1.7m deep. The outlet part exhibit around 30 to 40cm of sedimentation even with a water level peak of 5.4m. This unusual response could be caused by backwater effect from the Espinho River flood, which Tigre River is a tributary, that trapped sediment in the Tigre River. These results highlight how dynamic intermittent/ephemeral tropical rivers and showed how low-cost UAV High-Resolution DEMs and stage crests are workable and efficient techniques to monitor ungauged intermittent/ephemeral rivers. Simultaneously, narrow the surveys timespan (Covid-19 pandemic hindered most of the monthly planned surveys) is essential to identify which flow events caused erosion and sedimentation and which rain events trigger flow events.  

Keywords: Sand-bed rivers; UAV HR-DEMs; Brazilian Drylands; Water Level Stage Crest, riverbed erosion

How to cite: Souza, J., Alexandre, F., and Monteiro, G.: River bed dynamic monitoring on ephemeral/intermittent rivers – sand-bed  response on topical drylands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14337, https://doi.org/10.5194/egusphere-egu21-14337, 2021.

13:34–13:36
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EGU21-7033
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ECS
Changjin Wang and Peng Hu

Physics-based models have been increasingly developed in recent years and applied to simulate the braiding process and evolution of channel units in braided rivers. Braided rivers are the river network system characterized by the staggered distribution of bars and channels. In the numerical calculation, the grid scale affects the behavior process and morphological description of braided rivers. In this paper, a 2D numerical model is used to simulate the evolution of the braided rivers where the transport of load bed sediment plays a dominant role. In the natural scale braided rivers evolution modeling, the difference of the braided rivers' morphological characteristics under different grid scales is discussed, and the influence of different distribution of topographic disturbance caused by grid scale difference on the morphological characteristics of braided rivers is discussed. The study shows that the grid scale does not affect the description of braided rivers evolution process, and braided rivers evolve in the same way regardless of grid scale (within a reasonable range). However, the statistical characteristics of braided rivers are greatly affected by the grid scale. The braiding index increases as the grid scale decreases, but when the grid scale decreases to a certain extent (20m in this paper), the braiding index no longer increases. The number and average height of bars in braided rivers increase with the decrease of grid scale, and the average area of bar near riverbed also increases with the decrease of grid scale, but the average area of bar near water surface does not change with the change of grid scale. In general, the higher the grid resolution is, the more similar the bar morphology in the numerical model is to that in natural rivers. In addition, the different distribution of topographic disturbance caused by the grid scale difference has an influence on the braiding intensity and the bar morphology of the braided rivers, but the influence degree is much smaller than that caused by the grid scale difference.

How to cite: Wang, C. and Hu, P.: Effects of grid scale and resulting initial bed disturbance differences on the evolution of braided rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7033, https://doi.org/10.5194/egusphere-egu21-7033, 2021.

13:36–13:38
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EGU21-4561
Hossein amini, Guido Zolezzi, Federico Monegaglia, Emanuele Olivetti, and Marco Tubino

This study investigates the dependency of meander lateral migration rates on the spatial distribution of channel centerline curvature in both synthetic and real meandering rivers. It employs Machine Learning techniques (hereafter ML) to relate observed local lateral meander migration rates with the local and the upstream/downstream values of the centerline curvature. To achieve this goal, it was primarily essential to identify the feasibility of using ML in the meandering river's morphodynamics. We then determined the ability of ML to predict the excess near bank velocity based a set of input data using different regression techniques (linear and polynomial, Stochastic Gradient Descent, Multi-Layer Perceptron, and Support Vector Machine). We then moved forward to study the upstream-downstream influence on local migration rate. Synthetic meandering river planforms, as obtained through the planform evolution model of Bogoni et al. (2017), which is based on Zolezzi and Seminara (2001) meander flow model, were used as test cases for the calibration and check of the different adopted ML algorithms. The calibrated algorithms were then applied to multi-temporal information on meander planform dynamics obtained through the PyRiS software (Monegaglia et al., 2018), to quantify to which extent the upstream and downstream distribution of meander centerline curvature affects the local meander migration rate in real rivers.

References 

1- Zolezzi, G., & Seminara, G. (2001b). Downstream and upstream influence in river meandering. Part 1. General theory and application overdeepening. Journal of Fluid Mechanics, 438(September 2015), 183–211. https://doi.org/10.1017/S002211200100427X

2- Monegaglia, F., Zolezzi, G., Güneralp, I., Henshaw, A. J., & Tubino, M. (2018). Automated extraction of meandering river morphodynamics from multitemporal remotely sensed data. In Environmental Modelling & Software (Vol. 105, pp. 171–186). https://doi.org/10.1016/j.envsoft.2018.03.028

3- Bogoni, M., Putti, M., & Lanzoni, S. (2017). Modeling meander morphodynamics over self-formed heterogeneous floodplains. In Water Resources Research (Vol. 53, Issue 6, pp. 5137–5157). https://doi.org/10.1002/2017wr020726

4- Benozzo, D.,  Olivetti, E., Avesani, P. (2017). Supervised Estimation of Granger-Based Causality between Time series. In Frontiers in Neuroinformatics. 

https://doi.org/10.3389/fninf.2017.00068 

5- Sharma A., Kiciman, E. (2020). DoWhy: An End-to-End library for Causal Inference. arXiv preprint arXiv:2011.04216. 

https://arxiv.org/abs/2011.04216

How to cite: amini, H., Zolezzi, G., Monegaglia, F., Olivetti, E., and Tubino, M.: Upstream and downstream morphodynamic influence in simulated and real meandering rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4561, https://doi.org/10.5194/egusphere-egu21-4561, 2021.

13:38–13:40
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EGU21-8168
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ECS
Mengzhe Sun, Peng Hu, and Youwei Li

Fujiangsha waterway is located in the tidal reach of Yangtze River, which is one of the key sections for channel regulation. The channel condition of the waterway is governed by the evolution of the channel bar and point bar. Groins are consequently set on both sides of the channel bar and the left edge of Fujiangsha island. To explore the impact of the regulation works on the evolution of bars and channels, a numerical research is carried out based on a depth-integrated hydro-sediment-morphodynamic model, using the method of nesting large-scale model with local model. The non-negligible impact on the quality and momentum of water flow caused by enormous sediment transport and drastic change of topography, as well as the complex flow condition in both tide and runoff working together, has been taken into account. The simulation successfully reproduces the hydrological process and changes of topography in Fujiangsha waterway. Results show that: 1) there is a silting trend at the head of the channel bar, and the effect of the regulation works in bar protection and sand stabilization is remarkable; 2) The erosion on both sides of the channel bar improves the channel condition, and the hydrodynamic performance of shallow area at the entrance of the south branch has been enhanced; 3) The control on the evolution of point bar is still weak, which will have an adverse effect on channel condition of north branch.

How to cite: Sun, M., Hu, P., and Li, Y.: Numerical Research of Fujiangsha Waterway Based on a Depth-integrated Hydro-sediment-morphodynamic Model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8168, https://doi.org/10.5194/egusphere-egu21-8168, 2021.

13:40–13:42
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EGU21-12234
He Qing Huang, Min Zhang, Teng Su, and Guoan Yu

Taking the width/depth ratio of an alluvial channel as an independent variable, a variational analysis of basic flow relationships shows that flow is able to achieve stationary equilibrium by adjusting channel geometry when the condition of maximum flow efficiency (MFE) is satisfied. To examine if this theory of self-adjusting channel morphodynamics can be practically applied to large river systems heavily loaded with sediment, this study examines the degree of correspondence between theoretically determined equilibrium channel geometries and actual measurements along the lower Yellow River. Using the Meyer-Peter and Müller bedload relation modified on the basis of MFE theory and relations of flow continuity and resistance we present a detailed investigation of the potential physical causes and main factors resulting in the correspondence. 

How to cite: Huang, H. Q., Zhang, M., Su, T., and Yu, G.: Application of equilibrium theory on alluvial channel-form adjustment in a large river heavily loaded with sediment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12234, https://doi.org/10.5194/egusphere-egu21-12234, 2021.

13:42–13:44
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EGU21-10910
Guo-An Yu, He Qing Huang, and Weipeng Hou

Incised valleys or steep slopes in tectonic active mountain areas are normally in a critical equilibrium state which is highly fragile and prone to deviate under exotic disturbances (e.g., earthquake, heavy precipitation, or even human activities), inducing mass movements (e.g., landslides, avalanche, and/or debris flows). Mass movements have great impacts on fluvial processes and may even reshape valley morphology, hence are powerful drivers to river evolution in those environments. Unfortunately, compared to the mass movements themselves (e.g., occurrence time, volume, dynamics and underlying mechanisms), less attention has been paid to the fluvial processes (in a short/intermediate-term) and the long-term evolution of river morphology corresponding to (and after) those mass movements (especially catastrophic ones). This motivates the current work.

The southeast Tibet, located on the east Qinghai-Tibet Plateau, is one of the most active regions globally in terms of tectonic motion and rates of uplift. Rivers in the lower Yalung Tsangpo basin in this area are investigated to understand the morphodynamics influenced by modern and historical mass movements and examine the feedbacks of fluvial processes to mass movements. River reaches influenced by typical mass movements were chosen for detailed field surveys, including: (1) the upper part of the Yalung Tsangpo Grand Canyon which has been seriously impacted by avalanches and debris flows from tributary gullies originating at glacial mountains of Namcha Barwa and Gyala Peri; (2) the lower reach of the Yigong River covering the Yigong Landslide from the Zhamunong Gully; (3) the lower reach of the Palong River influenced by debris flows from Guxiang and Tianmo gullies; and (4)  the upper and middle reaches of the Palong River (extending roughly from Ranwu Lake to the upstream of Guxiang Lake) influenced by glacial processes and other induced mass movements since the last glacial maximum. Remote sensing images before and after the large-scale mass movements in recent decades were also used to track the corresponding river morphology variation.

Due to very high transport rate and volume of sediment incoming, mass movements have caused dramatic channel processes in east Tibet. Some even dammed the river, forming knickpoints and reshaping valley morphology. The morphology of the valleys in this area normally show alternating sections of gorges and wide valleys, with a staircase-like longitudinal profile. The gorge sections exhibit single and deeply incised channels with a high-gradient channel bed and terraces. In contrast, the wide valley sections consist of lakes, braided or anabranching channels, gentle bed gradients, and thick alluvial deposits. In recent decades, mass movements (mostly debris flows), occurred more frequently through gullies in the reaches of gorge sections than through gullies along the wide valley sections. Mass movements deviate river morphology and slope from (quasi-)equilibrium to non-equilibrium state, however, with attendant rapid sediment incoming, valley bottom siltation and erosion benchmark rising, it triggers a negative feedback which drives the river morphology to a new round of development towards equilibrium.

How to cite: Yu, G.-A., Huang, H. Q., and Hou, W.: River morphology evolution driven by mass movements in tectonic active regions – A negative feedback response of transient landscape, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10910, https://doi.org/10.5194/egusphere-egu21-10910, 2021.

discussion
B. Bedrock rivers - part 1: bedrock erosion
13:44–13:46
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EGU21-9137
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ECS
James Buckley, Rebecca Hodge, and Louise Slater

Active incision of bedrock rivers exerts a vital control on landscape evolution in upland areas. Previous research found that bedrock rivers were typically steeper and sometimes narrower than alluvial rivers. However, most of the literature on partially-exposed bedrock rivers has employed small samples mostly from mountainous regions, so their geomorphological properties remain poorly understood. In contrast with the existing literature, a large-sample analysis of bedrock river channel properties would allow the controls on bedrock river width and slope to be unpicked and reveal whether or not the existing literature is biased towards pristine, mountainous bedrock rivers. Overall, such an analysis could improve the reliability of upland landscape evolution models.

Here we present an analysis of 1,924 river sites from the EPA National Rivers and Streams Assessment to assess the geomorphological differences between bedrock and alluvial rivers. The influences of lithology and uplift on bedrock channel properties are examined using external datasets. We find bedrock rivers to be significantly steeper and wider than alluvial rivers. Sedimentary bedrock rivers were seen to be significantly wider than igneous/ metamorphic bedrock rivers, consistent with findings from Ferguson et al. (2017). We estimated shear stress and critical shear stress for each river site and assessed correlation with bedrock exposure. We found that exposed bedrock could not always be explained by local sediment transport exceeding local sediment supply, indicating that bedrock exposure may be controlled by other factors in some bedrock rivers. Currently, uplift data are being compiled for further analysis.

How to cite: Buckley, J., Hodge, R., and Slater, L.: Compiling and Analysing Bedrock River Data Across the USA to Unpick Bedrock River Geomorphology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9137, https://doi.org/10.5194/egusphere-egu21-9137, 2021.

13:46–13:48
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EGU21-6784
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ECS
Hui Chen and Jongmin Byun

Bedrock river is rock-bound, its bed and banks are composed mainly of in-place bedrock. Bedrock channel reaches, commonly short and intermittent, often occur where transport capacity exceeds bedload sediment flux. Despite the abundant research on the typical patterns of alluvial channel reaches, the distribution of bedrock channels has not been well studied. Rock type may affect the occurrence of bedrock channels because the strength, joint density, and erosion process of bedrock vary depending on the rock type. Previous studies have viewed the bedrock channel occurrence in the aspect of the excessive sediment transport capacity, but the influence of lithology has not been considered in the literature. To understand the influence of lithology on bedrock channel occurrence in a drainage basin-scale, we investigated the distribution of bedrock channels in relation to varying lithology and unit stream power along the Seogang River in South Korea. We used satellite images with high resolution for the identification of bedrock channel reaches and then verified them through field surveys. Geological maps and 1 arc-second SRTM DEMs were used to analyze lithological effects and calculate unit stream power.  As a result of the analysis, we identified 94 bedrock channels in the studied river, varying depending on lithologies. The frequency of bedrock channels in granitic gneiss areas (0.73/km) is much higher than those in the other rock type areas (granite areas, 0.57/km; limestone areas, 0.16/km). In the more frequent granitic gneiss areas, the bedrock channels are steepened (average channel slope: 0.0074 m/m) and narrow (average channel width: 65 m) and mainly reside within steepened and narrow (average valley width: 123 m) rock-bound valleys so that their occurrence is mainly associated with high unit stream power. In contrast, the bedrock channels over the other lithologies are wider (89 m) and lower-gradient (0.0056 m/m) and occur along flat and broad valleys (391 m). Consequently, the bedrock channels in the studied river were divided into two types: confined and unconfined bedrock channels. The confined bedrock channels are within the steepened and narrow valleys composed of resistant granitic gneiss and show the evidence for recent bedrock incision processes. However, the unconfined bedrock channels are mainly within the broad and flat valleys of weak saprolites and limestone with high joint density have lower unit stream power and don't show any marker for bedrock incision. In conclusion, high-relief landscape mainly composed of more resistant rocks generates steep and narrow valleys, which leads to the formation of continuous and actively incising bedrock channels. However, low-relief landscape underlain by non-resistant rocks shows wider and lower-gradient channels, with intermittent bedrock channels due to locally more resistant rock bodies.

How to cite: Chen, H. and Byun, J.: Effects of lithology on bedrock channel occurrence: an examination from the Seogang River in South Korea, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6784, https://doi.org/10.5194/egusphere-egu21-6784, 2021.

13:48–13:50
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EGU21-14546
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ECS
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Highlight
Rivoningo Khosa, Stephen Tooth, Jan Kramers, Vela Mbele, Lee Corbett, and Paul Bierman

The world’s largest meteorite impact crater, the Vredefort Dome, has been the subject of extensive studies relating to its age, geology and geomorphology. However, there are no studies pertaining to the rate at which the rocks in the crater remnant are eroding, which can provide insight into the development of the landform over time. This study used the cosmogenic nuclides 10Be and 26Al, extracted from purified quartz samples, to investigate erosion rates along the Vaal River as it traverses the impact crater. The Vaal River flows in mixed bedrock-alluvial terrain through the dome, crossing two different bedrock lithologies. The river is multi-channelled (anabranching) atop the granitoids exposed in the core of the dome, then downstream flows as a single channel through a narrow canyon cut into the quartzites that form the rim of the dome. We collected 14 samples from the two rock types to assess lithologic controls on erosion rate and determine landscape erosion history. Results from the analysis of both isotopes were in close agreement; here, we report outcrop erosion rates based on the 10Be. The average 10Be-determined erosion rates (± 1 SD) along the active river channel for the quartzite (n = 4) and granitoid (n = 6) regions are 1.90 ± 0.12 and 2.19 ± 0.14 m/Ma respectively.  Additional samples from older, now elevated (>5 m) strath terraces developed atop quartzite (n = 4) indicate slightly lower average apparent erosion rates of 1.65 (± 0.09) m/Ma.  The data demonstrate that the erosion rates along the active river channel are similar between the two lithologies despite differences in rock hardness.  The resistant, slowing eroding quartzites serve as the local base level for the river upstream, promoting the development of anabranching, which disperses bedrock erosion over a wider area of the crater. We infer that both bedrock hardness and channel characteristics are important controls on erosion rates along the river.  Collectively, the dataset further illustrates the low bedrock erosion rates that prevail across large areas of the southern African interior.

How to cite: Khosa, R., Tooth, S., Kramers, J., Mbele, V., Corbett, L., and Bierman, P.: Quantifying the erosion of the world’s largest impact crater using cosmogenic nuclides: the Vredefort Dome, South Africa., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14546, https://doi.org/10.5194/egusphere-egu21-14546, 2021.

13:50–13:55
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EGU21-3304
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ECS
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solicited
Tingan Li, Jeremy Venditti, and Leonard Sklar

Bedrock walls can be undercut by saltating bedload particle impacts that are deflected by alluvial cover. Continued undercutting of the lower wall creates an imbalance on the wall and may cause the upper part to collapse and to widen the whole channel. Compared with vertical erosion rates, less is known about lateral erosion (undercutting) rates that are thought to dominate when river beds are alluviated. Here, we derive an analytical model for lateral erosion by saltating bedload particle impacts. The analytical model is a simplification of the Li et al. (2020) numerical model of the same process. The analytical model predicts a nonlinear dependence of lateral erosion rate on sediment supply, shear stress and grain size, revealing the same behaviour observed in the numerical model, but without tracking particle movements through time and space. The analytical model considers both uniformly distributed cover and patchy partial cover that is implemented with a fully alluviated patch along one bank and a bare bedrock along the other. The model predicts that lateral erosion rate peaks when the bed is ~70% covered for uniformly distributed alluvium and when the bed is fully covered for patchy alluvium. Vertical erosion dominates over lateral erosion for ~75% and >90% of sediment supply and transport conditions for uniformly distributed cover and patchy cover, respectively. We use the model to derive a phase diagram of channel responses (steepening, flattening, narrowing, widening) for various combinations of transport stage and relative sediment supply. Application of our model to Boulder Creek, CA captures the observed channel widening in response to increased sediment supply and steepening in response to larger grain size.

How to cite: Li, T., Venditti, J., and Sklar, L.: A mechanistic model for bedrock undercut from saltating bedload particle impacts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3304, https://doi.org/10.5194/egusphere-egu21-3304, 2021.

discussion
C. Bedrock rivers - part 2: sediment dynamics
13:55–14:00
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EGU21-14033
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ECS
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solicited
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Highlight
Susannah Morey, Katharine Huntington, David Montgomery, Michael Turzewski, and Mahathi Mangipudi

Quaternary megafloods (106 m3/s) sourced from valley blocking glaciers on the Tibetan Plateau have long been implicated in the evolution of Yarlung-Tsangpo Gorge on the Yarlung-Siang River. However, past estimates of megaflood erosion in this region have relied on back of the envelope estimates of peak discharge and shear stress. This makes it difficult to fully understand how megafloods shape the landscape. Here, we use 2D numerical simulations of megaflood hydraulics over 3D topography to examine the legacy of these massive floods on this confined, sinuous mountain river. First, to assess erosional potential in the Gorge, we calculate flood power and compare it to measurements of annual stream power. We find that the simulated megaflood produces peak flood power up to three orders of magnitude higher than the stream power of the annual river. Compared to stream power, flood power in the Gorge is disproportionately higher than it is downstream of the Gorge. Additionally, in the Gorge, a larger proportion of the inundated valley experiences high flood power and shear stress for long periods of time (5-10 hrs) compared to the valley downstream of the Gorge. These results support previous hypotheses that megafloods can erode more material (both alluvium and bedrock) than the annual monsoon—potentially enough to “reset” the mountain valley by removing most of the sediment and fractured bedrock in the system. However, we hypothesize that this erosional effect is felt primarily in the Gorge region. In contrast to the erosive power in the Gorge, there is an order of magnitude decrease in average peak flood power downstream of the Gorge. We hypothesize that megafloods are predominantly depositional in this downstream domain. Here, we observe few locations that experience sustained (>5 hrs) high (>10 kPa) shear stress and those locations are often isolated and vary through time. At locations that do experience these higher shear stresses, megafloods could move and deposit large (>3 m) boulders, which subsequent annual flows or smaller historical outburst floods would be incapable of moving. These large boulders could then armor the bed and prevent erosion, which could have lasting consequences for the modern river. Most of the shear stress and flood power of the simulated megaflood outside of the modern channel boundaries are much lower, capable of moving gravel to sand sized sediment at most. This is particularly true where we observe significant amounts (>10 km) of megaflood backflow up tributaries. Instead of resetting the system, we predict our megaflood will overwhelm this downstream flood domain with the deposition of coarse- and fine-grained sediment. For the Yarlung-Siang River to incise into the bedrock in a post-megaflood landscape, it must first make its way through these megaflood deposits. Together, our results suggest that the legacy of a megaflood in the region is both erosional and depositional. We predict wide-spread megaflood erosion in the Gorge, potentially enough to reset the system, but would expect exceptional deposition downstream of it, possibly enough to overwhelm this downstream domain.

How to cite: Morey, S., Huntington, K., Montgomery, D., Turzewski, M., and Mangipudi, M.: Do megafloods reset mountain valleys?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14033, https://doi.org/10.5194/egusphere-egu21-14033, 2021.

14:00–14:02
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EGU21-5815
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ECS
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Highlight
Manel Llena, Tommaso Simonelli, and Francesco Brardinoni

River canyons are transient geomorphic systems shaped by river incision into bedrock and coupled by instability of the adjacent valley walls. Investigating the evolution of river canyons is typically challenging due to the geologic time scales involved. In this context, the Marecchia River, which hosts in its intermediate portion a 6-km canyon, developed since the early 1950’s following intense gravel mining, may be instructive. Indeed, this setting offers the opportunity to: (i) document canyon development through highly erodible pelitic rocks; and (ii) evaluate relevant upstream and downstream effects on fluvial morphodynamics. To these ends, we subdivide the 50-km stretch of the Marecchia River main stem into 22 homogeneous reaches and evaluate decadal geomorphic changes through analysis of LiDAR-derived digital elevation models (i.e., 2009 and 2019) in conjunction with planimetric changes of active channel width delineated on orthophoto-mosaics (i.e., 2009, 2012, 2014, 2017, 2019). The estimation of patterns and rates of fluvial erosion into bedrock and its geomorphic effects are essential for understanding landscape evolution and for applying sustainable sediment management plans.

In terms of volumetric changes, the entire river stretch recorded a decadal degradation of 2,516,150 m3 (57%) and 1,884,700 m3 of aggradation (43%), with a corresponding net volume loss of -631,450 m3. Highest specific volumes of aggradation were observed in a homogeneous reach located in the lower part of the study segment (0.5 m3/m2), while highest values of degradation were observed in the upper reach of the canyon (-2.3 m3/m2). During the 2009-2019 period, knickpoint headward migration within the canyon has progressed for approximately 500 m, producing an average bedrock incision of about 10 m. As documented by area and volume changes, both rates of fluvial incision and canyon widening, as modulated by landslide activity and valley wall collapses, are highest in proximity of the main knickpoint and tend to decrease progressively downstream. By March 2019, when the second LiDAR survey was conducted, the main knickpoint had reached the foundations of a major check dam, which eventually collapsed two months later. Upstream of the canyon, channel reaches displayed narrowing dynamics with an alternation of degradation and aggradation processes. In terms of total volumetric changes, these reaches presented an indirect correlation with confinement, with the most confined reaches acting as sediment transfer zones. In contrast, the segment downstream of the canyon displayed widening dynamics (+ 11 m on average) together with an increase of aggradation processes. Due to the pelitic nature of the hosting bedrock, despite the high geomorphic change observed, most of the material supplied by the canyon walls gets transported in suspension, contributing very little to the estimated budget of the Marecchia River's distalmost reaches. In this way, we argue that most part of the aggradation observed in this segment was originated upstream, bypassing the canyon.

How to cite: Llena, M., Simonelli, T., and Brardinoni, F.: Decadal sediment dynamics of a perturbed fluvial system: the case of the man-made Marecchia River canyon, Northern Apennines , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5815, https://doi.org/10.5194/egusphere-egu21-5815, 2021.

14:02–14:04
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EGU21-10160
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ECS
Jiamei Wang, Marwan A. Hassan, Matteo Saletti, Xingyu Chen, Xudong Fu, Hongwei Zhou, and Xingguo Yang

Steep step-pool streams are often coupled to adjacent hillslope, directly receiving episodic sediment supply from mass movement processes such as landslides and debris flows. The response of step-pool channels to the variations in sediment supply remains largely unexplored. We conducted flume experiments with a poorly sorted grain-size distribution in an 8%-steep, 5-m long flume with variable width at the University of British Columbia, to study the effects of episodic sediment supply on channel evolution. After a conditioning phase with no feed, the channel was subjected to sediment pulses of different magnitude and frequency under constant flow discharge. High-resolution data of hydraulics, bedload transport, bed surface grain size, and channel morphology were collected every 10-20 minutes and an additional time at the end of each pulse.

In response to sediment pulses, we recorded an increase in bedload transport rates, channel aggradation, bed surface fining, and continuous step formation and collapse. In between pulses, bedload rates dropped by several orders of magnitude, net erosion occurred, the bed surface gradually coarsened, and steps became more stable. The small-magnitude high-frequency pulses caused smaller but more frequent spikes in bedload transport, bed surface evolution, and thus step stability. Instead, the large-magnitude low-frequency pulses cause larger changes but provided a longer time for the channel to recover. This suggests that in step-pool channels pulse magnitude is a key control on channel rearrangement, while pulse frequency controls how fast and strong the recovery is.

The frequency and stability of steps varied as a function of local channel width, showing that channel geometry is a primary control on step formation and stability even under episodic sediment supply conditions. Instead, the effect of sediment pulses is less important because the total number and average survival time of steps were similar among runs with different pulses. The critical Shields stress decreased following sediment pulses, then increased immediately after, and fluctuated until the next pulse. The variations in sediment supply caused cycles in bedload transport rate, surface and bedload texture, thus controlling the variability in the threshold for motion.

Our results indicate that episodic sediment supply is a primary control on the evolution of step-pool channels, with sediment feed magnitude affecting mostly morphological changes, and sediment feed frequency controlling channel stability.

How to cite: Wang, J., Hassan, M. A., Saletti, M., Chen, X., Fu, X., Zhou, H., and Yang, X.: Effects of episodic sediment supply on experimental step-pool channel morphology, bedload transport and channel stability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10160, https://doi.org/10.5194/egusphere-egu21-10160, 2021.

14:04–14:06
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EGU21-10291
Jens Turowski

Fluvial bedrock incision is driven by the impact of moving bedload particles. Mechanistic, sediment-flux-dependent incision models have been proposed, but the stream power incision model (SPIM) is frequently used to model landscape evolution over large spatial and temporal scales. This disconnect between the mechanistic understanding of fluvial bedrock incision on the process scale, and the way it is modelled on long time scales presents one of the current challenges in quantitative geomorphology. Here, a mechanistic model of fluvial bedrock incision that is rooted in current process understanding is explicitly upscaled to long time scales by integrating over the distribution of discharge. The model predicts a channel long profile form equivalent to the one yielded by the SPIM, but explicitly resolves the effects of channel width, cross-sectional shape, bedrock erodibility and discharge variability. The channel long profile chiefly depends on the mechanics of bedload transport, rather than bedrock incision. In addition to the imposed boundary conditions specifying the upstream supply of water and sediment, and the incision rate, the model includes four free parameters, describing the at-a-station hydraulic geometry of channel width, the dependence of bedload transport capacity on channel width, the threshold discharge of bedload motion, and reach-scale cover dynamics. For certain parameter combinations, no solutions exist. However, by adjusting the free parameters, one or several solutions can usually be found. The controls on and the feedbacks between the free parameters have so far been little studied, but may exert important controls on bedrock channel morphology and dynamics.

How to cite: Turowski, J.: Upscaling sediment-flux-dependent fluvial bedrock incision to long timescales, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10291, https://doi.org/10.5194/egusphere-egu21-10291, 2021.

discussion
14:06–15:00