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GM5.1

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 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.

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Co-organized by HS13/SSP3
Convener: László BertalanECSECS | Co-conveners: Joshua Ahmed, Christopher HackneyECSECS, Eliisa Lotsari
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| Mon, 04 May, 08:30–10:15 (CEST)

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Chat time: Monday, 4 May 2020, 08:30–10:15

Chairperson: Laszlo Bertalan, Josh Ahmed, Chris Hackney, Eliisa Lotsari
D1158 |
EGU2020-4303
| Highlight
Minhui Li, Baosheng Wu, and Yi Chen

Tibetan Plateau is the source of many major rivers in Asia. Drainage networks of these rivers vary in shapes and features due to complex climatic and geomorphic conditions. In this study, we extracted drainage networks in the source area of Yellow River, Yangtze River and Yarlung Zangbo River from 90-m-resolution SRTM DEM. We chose 62 sub-basins in the Yellow River, 96 sub-basins in the Yangtze River and 120 sub-basins in the Yarlung Zangbo River and tested self-similarity of drainage networks in two ways. First, we tested self-similarity for traditional Horton laws. Based on Horton-Strahler order, the results indicate that rivers with low levels generally obey Horton laws while rivers with high levels show deviation. Second, we tested statistical self-similarity in the topology of river networks. Random self-similar networks (RSN) model which combines self-similarity and randomness shows topological features of river networks statistically. Real networks were decomposed into generators that produce the network. The results demonstrate that the generators of RSN model obey a geometric distribution and the parameter p, which describes the distribution of generators, ranges from 0.401 to 0.587. Self-similarity holds in a statistical sense in the selected basins in the Tibetan Plateau. Motivated by the need to understand the controlling factors of drainage networks in Tibetan Plateau, these sub-basins were divided into groups according to possible controlling factors, such as climate, tectonic and geology. Analysis shows that Horton ratios and generators of low-level rivers are affected by precipitation, but the relationship between these parameters of high-level rivers and these factors is not obvious. In order to further explore the controlling factors, we analyzed three typical rivers (Tao River, Yalong River and Lasa River) in more details. For Yalong River, Tao River and Lasa River, bifurcation ratios are 4.46, 5.00 and 4.37 while the length ratios are 2.35, 2.71 and 2.30 respectively. The Normalized Concavity Index for Tao River, Lasa River and Yalong River are -0.129, -0.082 and 0.009 respectively, indicating that the profiles of the first two rivers are concave-up and that of Yalong River is convex-up. The influence of climate is well reflected in the structure and longitudinal profiles of the drainage network in the Tibetan Plateau. Strong tectonic activities in the eastern margin of the Tibetan Plateau destroy the network of Yalong River, resulting in river capture to maintain equilibrium.

How to cite: Li, M., Wu, B., and Chen, Y.: Features and controlling factors of drainage networks in the Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4303, https://doi.org/10.5194/egusphere-egu2020-4303, 2020

D1159 |
EGU2020-1292
Peter Ashmore

Morphology, bedload and sedimentology of morphologically active gravel bed rivers interact in fundamental ways. In braided and wandering rivers these interactions have distinct characteristics.  In these cases much of the bedload transfer is tied up in morphological change so that the bar and channel scale morpho-dynamics are, in effect, the bedload transport process. Physical models and field data reveal several inter-related aspects of this interaction.  We can define the morphological active layer as that in which erosion, deposition and bed particle exchange occur during channel-forming flows. The dimensions, complexity, and lateral and longitudinal connectivity of this layer increase with discharge in a given river and with channel-forming stream power between rivers. Bedload flux correlates strongly with the dimensions of the active layer and temporal variability of bedload at a given discharge is a consequence of bar-scale  variation in morphological change in complex morphology. Rates of planimetric change in braided channels also follow this morphological-bedload relationship. Higher rates of morphological change also correlate with greater bed material mobility, approaching equal mobility at the highest rate of change and the highest morphological active layer dimensions. Bed particle transfer distances and burial depths are also strongly controlled with the length scale and depth of the bar-scale morphology and active layer. The sedimentology reflects the channel morphological scale and processes in defining sedimentary unit thicknesses and geometry. The deposits of the active channel belt are almost homogenous with respect to particle size because of the ‘turnover’ of the bed material.  Morphology, bedload and sedimentology of morphologically active gravel bed rivers interact in fundamental ways that help to define the characteristics of these channel types. To what extent are these observations applicable in other channel types?

How to cite: Ashmore, P.: Morphology, bedload and sedimentology of active gravel bed rivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1292, https://doi.org/10.5194/egusphere-egu2020-1292, 2019

D1160 |
EGU2020-1262
Elizabeth Dingle and Jeremy Venditti

An abrupt transition in river bed grain size occurs from gravel to sand over a short downstream distance, often only a few channel widths, and is termed the gravel-sand transition. At this point, the bed structure also changes from framework- to matrix-supported. Whether the gravel-sand transition is externally imposed, a result of internal dynamics (sediment sorting, abrasion, suspension deposition) or due to some other emergent property is unclear. Interestingly, there is a general absence of rivers beds with median surface grain sizes between ~1 and 5 mm. Here we present a new global compilation of gravel-sand transition characteristics across a diverse range of settings. We identify commonalities in the location of gravel-sand transitions, finding they occur at upstream extents of externally imposed backwater effects, where the gravel supply is exhausted (i.e. downstream of mountain ranges), or where both effects are coincident. A series of laboratory channel experiments, examining changes in fluid and sediment dynamics across a gravel-sand transition, show systematic changes in near bed turbulence that control sand deposition patterns. Gravel coarser than ~10 mm prevents sand deposition at the bed surface. We also find that gravel-sand transitions cannot form where river beds contain substantial amounts of ~1 to 5 mm particles, because these grain sizes enhance the mobility of coarser gravel, preventing a shift to a sand bed.

How to cite: Dingle, E. and Venditti, J.: Experiments on the grain size gap across gravel-sand transitions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1262, https://doi.org/10.5194/egusphere-egu2020-1262, 2019

D1161 |
EGU2020-4079
Andrew Wickert and Taylor Schildgen

Gravel-bed rivers cross and sculpt Earth's upland regions. Field, flume, and theoretical studies together provide governing equations for these rivers. Building upon this rich background, we quantitatively link catchment-scale hydrology, sediment transport, and morphodynamics into a model of river long-profile change over time. We focus on the transport-limited case (i.e., alluvial rivers), as most rivers around the world expend the majority of their geomorphic work by moving sediment rather than eroding the underlying substrate. Morphologically, this "transport-limited" category includes all alluvial rivers as well as those bedrock rivers for which bedrock erosion is easy relative to sediment transport. This model provides predictions for how such systems respond to changes in water supply, sediment supply, and base level – which are often linked to climate, land use, and tectonics. After deriving the central equation for long-profile evolution, we demonstrate that river concavity is strongly determined by the attrition rate of gravel, which can occur by either hillslope weathering or downstream fining. This dependency creates the potential for significant feedbacks between climate, tectonics, lithology, and river morphology. Furthermore, the equation predicts that oscillations in sediment and water supply will lead to net river incision when compared to steady means of both quantities. If true, this theoretical prediction could help to explain the near-ubiquitous presence of river terraces around the world.

How to cite: Wickert, A. and Schildgen, T.: Long-profile evolution of transport-limited gravel-bed rivers: Implications for sediment and landscape dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4079, https://doi.org/10.5194/egusphere-egu2020-4079, 2020

D1162 |
EGU2020-12435
Ricardo Carrillo and Luca Mao

Geomorphic changes in rivers often happen after either single high magnitude floods or several following ordinary flood events. Erosion and deposition have been well documented in all types of rivers, as well as the formation and destruction of step-pool sequences. However, there are less evidence available on the link between erosion-deposition at the reach scale and the formation-destruction of geomorphic units. This work is based on a series of field surveys carried out in a small glacierized basin in the central Chilean Andes. The location and extent of erosion/deposition were quantified using the photogrammetric technique with a drone before and after a high magnitude flood occurred during autumn 2016. High-resolution Digital Elevation Models (DEMs) were computed to generate erosion-deposition maps (DoD; Difference of DEMs). Also, orthomosaics were used to derive maps of geomorphic units for a 100-m long study reach, before and after the studied flood event. Results show an overall deposition of sediments in the study reach, but a decrease in the number of step-pool sequences from 20 to 14. Step-pool destruction is linked to depositional patterns, whereas the formation of new step-pool sequences is more likely to occur in erosional zones. Rapids and cascades also change in number, increasing from 1 to 4 units, and their formation was related to the deposition of sediments. These results may have larger implications in terms of ecological habitat dynamics and are also important for planning and management in civil projects like bridges and hydropower water intakes

How to cite: Carrillo, R. and Mao, L.: Linking erosion-deposition to geomorphic units changes in a high-gradient stream in the Central Chilean Andes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12435, https://doi.org/10.5194/egusphere-egu2020-12435, 2020

D1163 |
EGU2020-13821
| Highlight
Maarten Bakker, Florent Gimbert, Clément Misset, Laurent Borgniet, and Alain Recking

Alpine environments are responding to accelerated climate warming through the release and mobilization of large amounts of unconsolidated sediment. Sediment fluxes delivered to Alpine streams may be buffered, filtered and/or modulated as they pass through braided river reaches, which play a key role in the downstream transfer and dynamics of bed material. The functioning of these braided reaches is however still poorly understood, particularly during high magnitude events whose effects are very difficult to monitor but play an ever more prominent role in river system evolution.

In this study, we investigate the transfer of bedload material and river bed morphological change in a braided reach of the Séveraisse River (France), over the course of the melt season and two large flood events with an estimated return period of 5 and 50 years. To quantify braided reach dynamics, a multi-physical approach is employed that combines both temporally and spatially resolved techniques. We use bank-side geophones and locally derived parameters that describe seismic wave propagation in the subsurface to accurately quantify bedload transport and gain a unique insight in its temporal dynamics, particularly during the flood events. River bed elevation changes are determined from intermittent UAV-based LiDAR and photogrammetric acquisition. These are complemented with hourly (daytime) time-lapse images that register planform changes during the flood events.

Our results show strongly contrasting morphodynamic behavior with different flow conditions. During ‘normal’ bedload transport conditions driven by annual snow-melt, channel aggradation occurs leading to progressively lower bedload export from the reach for a given discharge. During the flood with a 5 year return period, which occurred at the end of the melt season, the braided riverbed morphology is rearranged and net sediment export took place. Most interestingly, in the autumn an extreme flood event led to the development of a single channel, meandering planform with significant outer bend erosion on alternating banks. Although this morphological change may be only temporary, i.e. a braided configuration may be expected to be gradually re-instated, it has important implications on the general functioning and morphological evolution of the reach and the downstream transfer of sediment.

How to cite: Bakker, M., Gimbert, F., Misset, C., Borgniet, L., and Recking, A.: Spatiotemporal response of an Alpine braided river reach to snow melt and flood events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13821, https://doi.org/10.5194/egusphere-egu2020-13821, 2020

D1164 |
EGU2020-10211
Aaron Bufe, Jens Turowski, Douglas Burbank, Chris Paola, Andrew Wickert, and Stefanie Tofelde

Lateral movements of alluvial river channels control the extent and reworking rates of alluvial fans, floodplains, deltas, and alluvial sections of bedrock rivers. These lateral movements can occur by gradual channel migration or by sudden changes in channel position (avulsions). Whereas models exist for rates of river avulsion, we lack a detailed understanding of the rates of lateral channel migration on the scale of a channel belt. Here we develop, for the first time, an expression that describes the lateral migration rate of braided alluvial channels in non-cohesive sediment. On the basis of photographic and topographic data from laboratory experiments of braided channels performed under constant external boundary conditions, we first explore the impact of autogenic variations of the channel-system geometry (i.e., channel-bank heights, water depths, channel-system width, and channel slope) on channel-migration rates. In agreement with theoretical expectations, we find that, under such constant boundary conditions, lateral channel-migration rates scale inversely with the channel-bank height. Furthermore, when changes in channel-bank heights are accounted for, lateral migration rates appear independent of channel slope, channel-system width, and water depth. These constraints allow us to derive two dimensionally consistent expressions for lateral channel-migration rates under different boundary conditions. We find that migration rates are strongly sensitive to channel-bank heights and water discharges and more weakly sensitive to sediment discharges in braided equilibrium channel systems. In addition, the strong dependence of lateral migration rates on channel-bank heights implies that external perturbations (for example, perturbations of sediment and water discharges) that modulate the depth of channel incision and can indirectly affect lateral channel-migration rates.

How to cite: Bufe, A., Turowski, J., Burbank, D., Paola, C., Wickert, A., and Tofelde, S.: Controls on the lateral channel migration rate of braided alluvial channel systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10211, https://doi.org/10.5194/egusphere-egu2020-10211, 2020

D1165 |
EGU2020-15504
Rebecca Hodge, Marcus Buechel, and Sophie Kenmare

Sediment grains in bedrock-alluvial channels can be entrained from bedrock surfaces or from alluvial patches. Field tracer data has shown that grains entrained from different surfaces can have very different critical shear stresses, which will affect bedload transport rates, the stability of sediment cover and bedrock incision. We hypothesise that the topography of the bedrock surface affects the critical shear stress of a sediment grain in at least three ways: the pivot angle through which the grain must move to be mobilised; the extent to which the grain is sheltered by upstream bedrock protrusions; and the impact on the flow profile via the roughness length z0. Here we quantify how bedrock topography affects these three different components, and their overall impact on critical shear stress.

Our analysis is based around six samples of bedrock river topography, from rivers with different degrees of roughness and structural characteristics. Each surface was 3D printed at a reduced scale, and pivot angles were measured by dropping grains of different sizes at different locations, and tilting the surface until the grain moved. For the surface with bedrock ribs, experiments were repeated with the ribs parallel and perpendicular to the downslope direction. Further experiments were performed after incrementally covering 25% through to 100% of the surface with fixed sediment cover. Bedrock sheltering and z0 were estimated from analysis of surface topography.

Overall, we find that measured pivot angles decrease with increasing surface roughness, similar to previous relationships from alluvial channels. However, we find that the pivot angle for a grain at any particular location cannot be predicted from the local surface topography, because of the complex interaction between grain shape and the different scales of roughness present on the surface. Rib direction also has a significant influence on mean pivot angle. The impact of sediment cover depends on the relative roughness of the cover and the bedrock surface.

We calculate critical shear stress using Kirchner’s force balance model, parameterised using our measurements of pivot angle, sheltering and z0. We find that z0 has the largest impact on the predicted median values of critical shear stress. Including the measured pivot angles reduces the lowest values of critical shear stress, with implications for the onset of sediment transport. Overall, our data represent the first attempt to quantify fully how bedrock topography influences the critical shear stress of sediment grains in bedrock-alluvial channels.

How to cite: Hodge, R., Buechel, M., and Kenmare, S.: The influence of bedrock topography on grain entrainment in bedrock-alluvial channels, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15504, https://doi.org/10.5194/egusphere-egu2020-15504, 2020

D1166 |
EGU2020-7598
Fei Peng, Cornelis Kasse, Maarten Prins, John Van der Woude, Nathalie Van der Putten, Hessel Woolderink, Simon Troelstra, Christiaan Beets, and Ronald Van Balen

Lateglacial climatic oscillations exerted profound impacts on the earth surface. In the Lower Meuse Valley (southern Netherlands), geomorphological studies in the last decades mainly centered on Lateglacial vegetation evolution, channel pattern changes and river terrace formation. Little information has been reported with respect to the paleohydrology and its relation with local and regional climate system. This study investigates a sediment core that contains flood sediments deposited from the Allerød to the middleHolocene. We conducted grain-size analysis, thermogravimetric analysis (organic matter and calcium carbonate content), pollen counting, macro fossils analysis, and oxygen and carbon stable isotopes analysis of the biogenic carbonate. Plant species variations in each pollen assemblage zone represent the local and regional vegetation development. The pollen and macro fossil studies reveal that the core site was in a lake and marsh environment through the Allerød-early Holocene period. The oxygen isotope record is believed to have captured the intra-Allerød Cold Period, its synchronous variation with the carbon isotope record indicates a dominant evaporation effect on the lake during the warm Allerød period. By highlighting the coarser components (flood signal) of the fine and coarse end members, two flooding energy indexes were constructed separately. The hydrological processes in the first phase of the Younger Dryas were characterized by rapidly increased flooding conditions and high accumulation rates. In the second phase of the Younger Dryas, an addition of aeolian sediments to the core site complicates the paleoflood identification. This work expands the paleoflooding reconstruction to a more broadly deposition setting where only fine or coarse fluvial sediment is the dominant component. The nearly synchronous changes of the increased flooding with the abruptly enhanced westerlies at the Allerød-Younger Dryas transition indicates a link between the Lower Meuse catchment and the regional North Atlantic climatic system

How to cite: Peng, F., Kasse, C., Prins, M., Van der Woude, J., Van der Putten, N., Woolderink, H., Troelstra, S., Beets, C., and Van Balen, R.: Paleoflooding and paleoclimate of the Lower Meuse during the Allerød-early Holocene, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7598, https://doi.org/10.5194/egusphere-egu2020-7598, 2020

D1167 |
EGU2020-13216
| Highlight
Chris Skinner, Nadav Peleg, Tom Coulthard, and Peter Molnar

The geomorphic activity of fluvial systems at any scale is ultimately driven by precipitation often in the form of rain. In numerical models, such as landscape evolution models, an input dataset of rainfall is commonly used to drive the model, often using a coarse spatial and/or temporal averaging. Beyond availability, characteristics of the rainfall data itself are frequently overlooked and the impacts of these on the results of the model not considered. However, landscape evolution models are sensitive to spatial and temporal variations in rainfall data and rainfall observations themselves contain spatial and temporal uncertainties, the nature of which varies between different observation methods.

This presentation synthesises the results of several linked studies highlighting the role rainfall can play in the modelling of geomorphology. First, we examine how the spatial and temporal resolution of the driving rainfall data is applied at influences the model outputs, with more than 100% difference in simulated sediment yields between the coarsest and finest resolutions used. Secondly, the role the source of the rainfall data plays, through comparison of observations from different methods, is explored showing that the uncertainty between the observations propagates non-linearly to simulated sediment yields.

To investigate these sensitivities the CAESAR-Lisflood model was used in combination with the STREAP weather generator to produce high-resolution estimates of rainfall, conditioned by observations, for the longer timescales required for landscape evolution studies. This pairing opened up the opportunity to investigate changes of geomorphic response to future predicted changes to rainfields due to climate change, showing that this is more complex than when considering changes to rainfall volumes alone.

How to cite: Skinner, C., Peleg, N., Coulthard, T., and Molnar, P.: Blame it on the Weatherman: How critical is rainfall to geomorphology?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13216, https://doi.org/10.5194/egusphere-egu2020-13216, 2020

D1168 |
EGU2020-4254
Michael Dietze, Sophie Lagarde, Eran Halfi, Lina E. Polvi, Eliisa Lotsari, Jens M. Turowski, Jonathan B. Laronne, and Niels Hovius

Constraining bedload flux in rivers is a challenging objective, especially when the data need to be continuous, beyond-point estimates. Seismometers are potentially valuable alternatives to in‐stream devices, which involve extensive measurement infrastructure or labour‐intensive manual sampling that can be potentially dangerous. We present a Monte Carlo-based inverse approach to deducing hydraulic and bedload transport dynamics continuously, with high temporal resolution, from seismic data, that averages the system’s behaviour over tens of metres. Water depths and bedload fluxes can be reproduced with average deviations of 0.10 m and 0.02 kg/sm, respectively. The method is validated against synthetic data sets and independently measured metrics from several challenging streams: we show applications of the technique from a flash flood-dominated catchment in Israel (Nahal Eshtemoa), from an ice-covered subarctic river (Sävarån, Sweden), and from a typhoon-driven major mountain river in Taiwan (Liwu River). The presented approach is a generic method implemented in the R package ‘eseis’ that can be used with off-the-shelf seismic equipment, installed at safe distances from potentially hostile conditions with minimum site disturbance.

How to cite: Dietze, M., Lagarde, S., Halfi, E., Polvi, L. E., Lotsari, E., Turowski, J. M., Laronne, J. B., and Hovius, N.: Hydraulic and sediment transport metrics of river systems from inverse modelling of seismic ground motion data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4254, https://doi.org/10.5194/egusphere-egu2020-4254, 2020

D1169 |
EGU2020-5533
Maggie J. Creed, Elizabeth H. Dingle, Hugh D. Sinclair, Dilip Gautam, Noel Gourmelen, Alistair G.L. Borthwick, and Mikael Attal

Rivers sourced from the Himalayas support ~10% of the global population living on the Indo-Gangetic Plain. These rivers can be a source of devastating floods. Flood hazard maps used to inform early warnings systems in the Terai region in southern Nepal are based on static, outdated DEMs, which may not reflect the current river and floodplain topography. Sediment dynamics can change the river course and the distribution of flow down large bifurcation nodes, affecting flood inundation extent. These processes are rarely considered in flood prediction models for this region. In this study, using a 2D depth-averaged hydrodynamic model, several flood scenarios for the Karnali River are investigated, including different DEMs, variable bed elevations, and a scenario with bed levels modified at an important bifurcation node to reflect field observations. Inundation extent varied by upto 14% between scenarios for a 1-in-20 year flood discharge. Our results suggest that combining regular field measurements of bed elevation, with updated DEMs, could help to improve future flood prediction maps. Updating model input parameters is particularly important following large flood events and/or large landslides in the upstream catchment, which could increase bed aggradation and provoke channel switching in highly mobile, alluvial river systems.

How to cite: Creed, M. J., Dingle, E. H., Sinclair, H. D., Gautam, D., Gourmelen, N., Borthwick, A. G. L., and Attal, M.: Numerical modelling of dynamic flood topographies in the Terai region, Nepal. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5533, https://doi.org/10.5194/egusphere-egu2020-5533, 2020

D1170 |
EGU2020-21984
| Highlight
Julio Isaac Montenegro Gambini and Magaly Cusipuma Ayuque

The Madre de Dios river basin belongs to the Amazon river basin, until the study area has an approximate area of ​​47070 km2. In recent years serious problems of lateral undermining are occurring on the right bank of the river, bringing with it problems in nearby towns and the layout of the interoceanic highway that connects to the country of Brazil. The case study is the Meander “La Pastora”, whose right margin is constituted by a compact clay material, which was affected by local undermining phenomena, also the approach of thalweg and sedimentation in the left margin was caused by causes of deforestation of the basin and increased solid contribution. Since 2015, measures have been built that have the function of mitigating erosion on the right bank and recovering the affected area. Using two-dimensional numerical modelling, ADCP/multibeam bathymetric surveys and limnimetric records, the hydrodynamic conditions and sediment transport will be evaluated by the hand of a results from physical modelling and inclusion of structural measures, estimating erosion and sedimentation areas that may have been in the meander. BASEMENT and IRIC NAYS2DH-FASTMECH models were used which simulated the flow conditions in different minimum and maximum hydrological scenarios compared with physical modelling results and field data, considering sediment flux corrections in curved channels with significant secondary flow motions and lateral erosion to precisely capture the complex flow field induced by channel curvature and riverbank gravitational effects.

How to cite: Montenegro Gambini, J. I. and Cusipuma Ayuque, M.: Assessment of meander hydro-morphodynamics using modelling approaches in an amazonian river, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21984, https://doi.org/10.5194/egusphere-egu2020-21984, 2020

D1171 |
EGU2020-7238
| Highlight
Anne Baar, Elena Bastianon, Lisanne Braat, and Daniel Parsons

Alluvial estuaries are dynamic landscapes that are very sensitive to changes in boundary conditions such as river discharge and sediment supply. A better understanding of the influence of upstream river discharge and sediment input on the development of estuaries under various scenarios requires long-term morphodynamic models, to both predict future changes and improve geological interpretations by storing the stratigraphy. Past 1D model studies have shown that upstream river discharge has a significant effect on the equilibrium bed profile of estuaries, but these studies ignore the effect of 2D bar and channel formation. Using 2D numerical models to predict the development of these systems on the scale of millennia proved to be difficult, since the modelled morphology is very sensitive to the choice in e.g. sediment transport predictor and bed slope effect. In this study, we use the knowledge of previous research that determined best parameter settings for realistic river and bar patterns to model long-term and large-scale estuary morphodynamics in Delft3D. Our objective is to quantify the effects of river discharge and sediment supply on the shape of estuaries and its deposits. Firstly, we systematically varied upstream river width and tidal amplitude to examine the relation between upstream river pattern and estuary dimensions. We quantified e.g. braiding index, bar dimensions, and tidal excursion length. Results show that flood flow velocities and tidal prism are less influenced by river discharge than suggested by 1D models, and are significantly influenced by the braiding index of the river. With relatively high tides, estuary bar patterns depend on tidal amplitude, while with lower tides estuary depth and braiding index are related to upstream river width and discharge. Next steps will include varying discharge to study the effect on the rate of adaptation of the river and estuary, and varying the grain size of the sediment input at the upstream boundary. We will input coarse sediment to explore differences between fluvial deposits and tidal currents, and fine sediment to use the model for research related to biofilm.

How to cite: Baar, A., Bastianon, E., Braat, L., and Parsons, D.: Influence of river pattern and sediment input on estuary morphology, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7238, https://doi.org/10.5194/egusphere-egu2020-7238, 2020

D1172 |
EGU2020-175
Paweł Mikuś and Bartłomiej Wyżga

Quantifying delivery and mobility of large woody debris in small mountain streams requires long-term and repeatable observations, so far very scarcely described. Such observations have been conducted in the upper course of Kamienica Stream, Polish Western Carpathians, where a sample of 429 trees growing along three separated sections of  the stream was tagged with metal plates and monitored during 10 years. The monitoring of standing and fallen trees has been conducted a few times per year, especially after heavy rainfall and windstorms. In this period, 96 trees (22.4% of the tagged sample) were recruited to the channel during high-intensity meteorological and hydrological events, mostly as a result of bank erosion during floods and windthrow, with recent bark beetle infestation of the riparian forest considerably accelerating the turnover of riparian trees. Large wood inventory performed in 2012 in the second- to fourth-order stream reaches and of the 10 years-long monitoring of tagged trees indicated variable mobility of large wood along the upper course of the stream. Wood mobility was negligible in the second-order reach, very small in the third-order reach, and higher, but still limited in the fourth-order reach. 46 trees were subjected to transport during five significant floods, and mean lengths of displacement of the tagged trees were small, not exceeding 32 m in sections A and B, whereas in section C they were a few times longer. However, an advanced state of decay of most pieces leads to their disintegration during floods, rather than to distant transport, and thus large wood retained in the upper stream course within a national park does not constitute an important flood hazard to downstream, inhabited valley reaches.

How to cite: Mikuś, P. and Wyżga, B.: Long-term monitoring of the recruitment and dynamics of large wood in Kamienica Stream, Polish Carpathians, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-175, https://doi.org/10.5194/egusphere-egu2020-175, 2019

D1173 |
EGU2020-374
Sana Khan and Kirstie Fryirs

Contemporary geomorphic river behaviour can only be understood with a sound knowledge of the historical range of river adjustment. This is particularly the case for rivers that have experienced anthropogenic alterations. Using a case study of Richmond River, New South Wales, Australia, we track the history of geomorphic river adjustment from the time of European colonisation in the late 18th Century. We use this study to develop a novel framework, called the ‘Behavioural sensitivity logical tree’ which can be applied to any catchment for assessing and quantifying reach scale behavioural sensitivity, defined as the ease with which geomorphic units and associated water, sediment, vegetation interactions adjust within the expected behavioural regime of a river. We use this framework to develop a behavioural sensitivity index and categorise rivers as Fragile, Active sensitive, Passive sensitive, Insensitive and Resilient. When applied across a catchment, such analyses highlights hotspots of river adjustment and sensitivity.

Fragile rivers have a behavioural sensitivity index > 0.85 and have the propensity to undergo wholesale river change such that a new river type and behavioural regime is created. For example, change from discontinuous or absent channels (e.g. swamps) to continuous channelised fills. Active sensitive rivers have a behavioural sensitivity index of 0.50-0.85 and have the ability to re-configure within their contemporary behavioural regime. For example, by reducing sinuosity via abrupt chute cut-off or progressive channel straightening. The behavioural sensitivity index of Passive sensitive rivers is between 0.20-0.50. These rivers have the ability to maintain their behavioural regime and withstand adjustment. Insensitive rivers have a behavioural sensitivity index of 0.05-0.20. They do not readily adjust and may contain significant resistance elements such as fine-grained sediments that limit geomorphic adjustment.  Resilient rivers have a behavioural sensitivity index < 0.05 and tend to be confined reaches where the capacity for adjustment is controlled by bedrock or other antecedent controls, such that the river cannot readily adjust.

We further demonstrate the evolutionary nature of behavioural sensitivity itself. The behavioural sensitivity of a river is not set in space and time, rather, rivers can dynamically evolve and shift to a different sensitivity category over time and in response to different forms of direct and indirect disturbance. Analysing a rivers’ behaviour sensitivity and identifying hotspots of geomorphic adjustment, can help inform process-based river management practice.

How to cite: Khan, S. and Fryirs, K.: Tracking post-colonisation geomorphic river sensitivity: A framework for identifying hotspots of river adjustment across a catchment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-374, https://doi.org/10.5194/egusphere-egu2020-374, 2019

D1174 |
EGU2020-400
Elad Dente, Nadav Lensky, Efrat Morin, and Yehouda Enzel

Meandering rivers and valleys are dominant landscape features on Earth and Mars, and central to a geomorphological debate: do sinuous channels actively develop during steepening of regional slope or whether they inherited the sinuosity of an ancient meandering channel through vertical incision? This and related questions were studied by field-scale case studies of channel evolution, numerical simulations, and physical laboratory experiments. Here, we document and investigate decadal- and field-scale formation of meandering valleys in perennial channels. These channels have incised into a homogeneous erodible substrate in response to the progressive Dead Sea level fall in recent decades (>30 m over 40 years). This unique study area enabled analysis of three clusters of adjacent elongating and incising channels with stable confined discharge, that evolved through an active increase in regional and channel slopes. The emerged slopes greatly vary along the study area and channels, allowing the test of slope impact under three primary settings: (a) relatively long and low gradients on shelf-like margins, (b) sharp basinward gradient increase on a shelf-slope transition, and (c) steepening slopes. These clusters triggered different channel and valley response by means of stream incision depth, channel sinuosity, and valley width. The sinuosity of the channels was actively increased only following steepening in the valley slope. During stable valley slope, the channels were mainly incising vertically, inheriting previous sinuous pattern. The highest sinuosity was developed in the channel within the most steepening slope, that was also developed the deepest and widest valley. Together with the Jordan River response to the same Dead Sea level fall in recent decades, these insights promote the interpretations regarding the evolution of incised meandering channels under changes in regional slope. Abundant evidence for chute cutoffs along an incised channel can imply that frequent overbank floods prevailed in the channel during the incision, whereas the absence of or rare evidence for such cutoffs can be the result of infrequent high-magnitude floods during the evolution.

How to cite: Dente, E., Lensky, N., Morin, E., and Enzel, Y.: Out of equilibrium sinuosity: The development of incised meandering channels in response to base-level fall, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-400, https://doi.org/10.5194/egusphere-egu2020-400, 2019

D1175 |
EGU2020-991
Pamela Louise Tolentino, John Edward Perez, Esmael Guardian, Carlos Primo David, Richard Boothroyd, Kirstie Fryirs, Gary Brierley, Trevor Hoey, and Richard Williams

The design and implementation of water management strategies in the Philippines, where  precipitation is abundant and groundwater reserves are substantial, are compromised by extreme hydrometeorological events that create hazards such as flooding, bank erosion and landslides. Additionally, structural and institutional factors, such as responsibility for land and water management being divided among 38 agencies, inhibit integrated land and water management. Such a fragmented context threatens the sustainability of water resources and provide challenges for risk management. Integrated river basin management and master plans have been formulated to address catchment-related concerns which include water resources, disaster risk, biodiversity, mineral resources, and socio-economic development. These plans typically include assessment on physical variables such as hydrology and geology. One critical aspect that is missing is baseline understanding of dynamic river geomorphology. Such understanding of river character, behaviour and pattern is required to underpin scientific guidance from a rational evidence base that informs management applications. The Bislak Catchment (593 km2), north-western Luzon Island, is underlain by interbedded clastic sedimentary and volcanic rocks. It has a Type I climate which is described as having distinct dry and wet seasons. Early this year, the region suffered a prolonged drought which resulted to huge agricultural damage. In 2018, two severe tropical storms hit the area that caused destructive flooding to communities and infrastructure. In response, flooding and erosion are currently being mitigated by new and repaired defences such as gabion walls and concrete dikes. Satellite images from 1970 to 2019 show spatially variable channel change, in response to channel network and valley geometry. Here, the morphodynamic units throughout the catchment are described using the River Styles Framework which provides a geomorphic template to assess management trajectories. This approach is demonstrated for the Bislak Catchment, and is proposed as a template wider use in the Philippines.

 

 

How to cite: Tolentino, P. L., Perez, J. E., Guardian, E., David, C. P., Boothroyd, R., Fryirs, K., Brierley, G., Hoey, T., and Williams, R.: Characterising river character, pattern and behaviour in the Bislak Catchment: towards a geomorphic template to inform river management in the Philippines, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-991, https://doi.org/10.5194/egusphere-egu2020-991, 2019

D1176 |
EGU2020-3594
Chris Tomsett and Julian Leylan

River corridors are greatly influenced by vegetation, whether it be through direct interactions with flow, influencing the stability of banks, or contributing to floodplain roughness. With vegetation present across many of the world’s river corridors in one form or another, it is a vital component of the active river corridor that receives relatively less attention than the flow and morphological components. This is partly because the routine monitoring of the very complex and temporally dynamic structure of vegetation is challenging.  Terrestrial Laser Scanning (TLS) and Airborne Laser Scanning (ALS) have been used to monitor fluvial vegetation across scales. However, whilst UAVs and Structure from Motion (SfM) techniques have recently bridged the gap between fine scale local surveys and coarse larger surveys for fluvial morphology, they are not well suited to characterising complex vegetation.

A UAV based laser scanning and imagery system has been developed which enables the collection of high resolution (> 300 points m2) point cloud data (first and last return) to analyse vegetation structure alongside simultaneous multispectral imagery data, including the red edge band. Such data can be collected on scales from metres to kilometres depending on the needs of the user, and is capable of picking out vegetation structure using metrics such as stand height, vertical distribution, canopy health, plant density etc. Moreover, the collection of this data through time will allow the evaluation of how these factors change across seasons, subsequently filling a void in data collection between spatially limited TLS and temporally limited ALS. Here we show some examples of how the data can be used to establish interactions between vegetation, flow and fluvial morphology from a series of flights over a 1 km reach of the River Teme, UK. These examples highlight how the data enables us to begin to establish a more detailed conceptual understanding of temporally evolving fluvial-vegetation interactions along river corridors.

How to cite: Tomsett, C. and Leylan, J.: Reach scale analysis of riparian vegetation interactions with fluvial morphology using UAV based laser scanning and multispectral imaging , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3594, https://doi.org/10.5194/egusphere-egu2020-3594, 2020

D1177 |
EGU2020-5206
Chenwei Zhao

Large eddy simulation of incompressible turbulent flow over a loose bed of spherical particles are investigated in an open channel. Eulerian and Lagrangian point-particle methods is applied to solve the Navier-Stokes equations and particle motion respectively and the particle-flow interaction is also considered. A new method solving particle-particle collisions is utilized for the first time to reduce the computational time spending on calculating the pairwise distances between particles. A turbulent fluid condition from the experiment of Robert and Uhlman (2001) are chosen of which the corresponding sediment patterns are ‘ripple’. Flow over the formed bed is considered and it is found that double-averaged Reynolds stresses including shear stress and three normal stresses reach their peak values near the bed. However, affected by the movable bed, they decay quickly as the height increases. The flow direction slightly rises over the stoss-side of ripples and falls after the crest and the velocity magnitude of time-averaged flow accelerate and decelerate before and after the crest of ripples as well. Hence, recirculation zones and clockwise vorticity appear at the trough of the bed where kolk boil vortices like hairpins and elongated streamwise vortices is also evident. Coherent structures, in the form of high- and low-speed streaks near the bed are also affected by the bed formation. The near-bed low-speed streaks entrain into the main flow domain over the stoss-side of ripples and the high-speed fluid streaks from the main flow rush toward the bed over the leeside of ripples. The bedload transport rate is well represented by previous empirical formulas. The bed surface elevation changes from upstream to downstream with time and there is a difference in the direction of sediment transport which is obvious three-dimensionality. In addition, it is also found that the bed surface elevation shows a positive correlation with particle streamwise velocity and entrainment rate which means higher bed elevation leads to larger bedload transport rate.

How to cite: Zhao, C.: Large eddy simulation of bed formation in subaqueous bedload , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5206, https://doi.org/10.5194/egusphere-egu2020-5206, 2020

D1178 |
EGU2020-5274
Huan Jing, Deyu Zhong, and Hongwu Zhang

The channel geometry in a fluvial river is significantly affected by the flow and sediment regimes, and the response behavior of channel dimensions usually varies widely to different management strategies from the upstream reservoir. Therefore, it is significantly crucial to investigate the variation of the channel geometry in response to changing flow and sediment conditions and quantify the influence of the latter in the sedimentation reduction and flood releasing in lower reaches downstream of the dam. In this study, three laboratory experiments on the physical model covering the typical braided reach HGK—JHT downstream of the Xiaolangdi Reservoir in the lower Yellow River are carried out, under the discharge of 2000 m3/s, 3000 m3/s, and 4000 m3/s respectively and with the corresponding constant suspended sediment concentration of 8.0 kg/m3. Results indicate that (i) spatially, the erosion and deposition in studied channel reach distributed alternately along the course which performs typical evolution properties of the braided river, corresponding to the total erosion amount of 2.27×106 m3, 10.29×106 m3, and 7.98×106 m3 for three magnitude of discharges; and (ii) four representative adjustment patterns are listed based on the observed cross-sectional geometry after each experiment, including the lateral widening pattern, vertical incision pattern, composite pattern and geometrical stable pattern where sectional geometry rarely changes during the period of experiment; and (iii) the quantity ξ=B1/2/H where B and H is the width and depth of the main channel zone is selected as the typical indicator to determine the variation of the channel stability. It is discovered that ξ in the reaches upstream of section FJS have rather larger values, implying relatively wider and shallower sectional geometry and lower channel stability which is closely associated with the levee safety. And moreover, the quantity ξ generally has lower values, that is, higher channel stability with the increase of experiment discharge; Besides, through the method of nonlinear regression analysis, the empirical relations for HGK—JHT Reach are developed between the main channel dimensions and incoming flow erosion intensity F=(Q2/S)/106 where Q is the discharge and S is the corresponding sediment concentration. In general, the calculated results are generally consistent with the measured values, as the riverbed degradation and the variation of sectional area increase exponentially with a stronger erosion intensity F.This paper may provide some practical basis for the study of channel evolution in sediment-laden rivers.

How to cite: Jing, H., Zhong, D., and Zhang, H.: Response of channel geometry to flow and sediment conditions in the lower Yellow River, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5274, https://doi.org/10.5194/egusphere-egu2020-5274, 2020

D1179 |
EGU2020-5780
Eliisa Lotsari, Maria Kämäri, Petteri Alho, and Elina Kasvi

Macro-turbulent flows during ice-covered and open-channel conditions, and their impacts on the total sediment transport, have not been studied widely in northern rivers. Previous studies have detected these processes, for example, only at the inlet area of one meander bend, or only during low discharge conditions. Thus, for understanding their impacts on the total sediment transport, it is needed to detect these macro-turbulent flow structures from a variety of cold region rivers, from multiple years, and also from a variety of different flow magnitude conditions. The pulses of high flow velocities related to these macro-turbulent structures may be important for determining the seasonal total sediment amount transported to the arctic ocean.

 

The aim is 1) to detect the macro-turbulent flow in a meandering river at ice-covered low flow condition, and compare it to both high and low magnitude open-channel flow conditions. 2) Within a meander bend, the macro-turbulent flow will be compared between its inlet, apex and outlet sections. 3) The shear forces will be analyzed to detect the effects of macro-turbulent flow on potential sediment transport and channel development. The analyses are based on 5–10 minutes long moving boat Acoustic Doppler Current Profiler (ADCP) measurements from a meandering sub-arctic river. The measurements have been done in February and May during 2016–2019, and in September during 2016-2018. The preliminary results of this study are presented. The hypothesis is that the sediment transport potential of a sub-arctic river could be higher during all seasons than previously expected due to the pulses of high velocities related to macro-turbulent flow structures.

How to cite: Lotsari, E., Kämäri, M., Alho, P., and Kasvi, E.: Impacts of macro-turbulent flow on sediment transport potential during ice-covered and open-channel conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5780, https://doi.org/10.5194/egusphere-egu2020-5780, 2020

D1180 |
EGU2020-9790
Luděk Bureš, Radek Roub, and Petra Sychová

Various techniques can be used to create a river terrain model. The most common technique uses 3D bathymetric points distributed across the main channel. The terrain model is then created using common interpolation techniques. The quality of this terrain depends on the number of the measured points and their location.

An alternative method may be an application of a set of cross-sections. Special interpolation algorithms are used for this purpose. These algorithms create new bathymetric points between two adjacent cross-sections that are located in a composite bathymetric network (CBN). Common interpolation techniques can be used to create a river terrain model. The advantage of this approach is a necessity of smaller dataset.

We present a comparison of four different algorithms for creating a river terrain model based on measured cross-sections. The first algorithm (A1) adopts a method of linear interpolation to create CBN [1]. The second algorithm (A2) reshapes the cross-sections and then applies linear interpolation. This reshaping allows better take into the account the thalweg line [2]. The third algorithm (A3) uses cross-sectional reshaping and uses cubic hermit splines to create CBN [3]. The last algorithm (A4)  implies the channel boundary and the thalweg line as additional inputs. Additional inputs define the shape of the newly created river channel [4].

Three different distances among individual cross-sections were used for the performance tests (50, 100 and 200 meters). The quality of topographic schematization and its impact on hydrodynamic model results were evaluated. Preliminary results show that there is almost no difference in the performance of the algorithms at cross-section distance of 50 m. The A4 algorithm outperforms/surpass its competitors in the case that input data (the cross-section distance is) are in 200 m spacing.

This research was supported by the Operational Programme Prague – Growth Pole of the Czech Republic, project No. CZ.07.1.02/0.0/0.0/17_049/0000842, Tools for effective and safe management of rainwater in Prague city – RainPRAGUE.

[1]       Vetter, M., Höfle, B., Mandelburger, G., Rutzinger, M. Estimating changes of riverine landscapes and riverbeds by using airborne LiDAR data and river cross-sections. Zeitschrift für Geomorphologie, Supplementary Issues, 2011, 55.2: 51-65.

[2]       Chen, W., Liu, W. Modeling the influence of river cross-section data on a river stage using a two-dimensional /three-dimensional hydrodynamic model. Water, 2017, 9.3: 203.

[3]       Caviedes-Voullième, D.; Morales-Hernández, M.; López-Marijuan, I.; García-Navarro, P. Reconstruction of 2D river beds by appropriate interpolation of 1D cross-sectional information for flood simulation. Environ. Model. Softw., 2014, 61, 206–228.

[4]       Merwade, V.; Cook, A.; Coonrod, J. GIS techniques for creating river terrain models for hydrodynamic modeling and flood inundation mapping. Environ. Model. Softw., 2008, 23, 1300–1311.

How to cite: Bureš, L., Roub, R., and Sychová, P.: Comparison of different algorithms used for creating river terrain model based on the cross-sections., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9790, https://doi.org/10.5194/egusphere-egu2020-9790, 2020

D1181 |
EGU2020-10489
Ingo Schnauder

In the last decade, the perception of large wood in rivers has shifted from a hazard perspective towards a valuable and required component of the river ecosystem. Consequently, there is a demand to quantify and predict the effects of large wood on flow, morphology and retention.

The research programme ‘Large Wood Hydraulics’ investigates the flow and turbulence characteristics of instream large wood. Within the programme, field measurements and lab experiments are conducted and cover different wood types (tree morphology, branching pattern), their position / orientation in the cross-section and single or multiple elements (wake interference). Field measurements were carried out in river Mulde, Germany within the BMBF-project ‘Wilde Mulde’ and flume experiments in the hydraulics lab of  TU Vienna.

The aim of the study is to predict the effects of different wood configurations to promote the use of wood in river restoration schemes.

How to cite: Schnauder, I.: Large wood hydraulics - a fluvial process of growing interest, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10489, https://doi.org/10.5194/egusphere-egu2020-10489, 2020

D1182 |
EGU2020-11470
Timothée Jautzy, Pierre-Alexis Herrault, Valentin Chardon, Laurent Schmitt, and Gilles Rixhon

A majority of European rivers have been extensively affected by diverse anthropogenic activities, including e.g. channelization, regulation and sediment mining. Against this background, the planimetric analysis based on remotely-sensed data is frequently used to evaluate historical planform changes, eventually leading to quantification of migration rates. However, geometric spatially-variable (SV) error inherently associated with these data can result in poor or even misleading interpretation of measured changes, especially on mid-sized rivers. We therefore address the following issue: What is the impact of spatially-variable error on the quantification of surfacic river planform changes?

Our test river corresponds to a 20 m wide meandering sub-tributary of the Upper Rhine, the Lower Bruche. Within four, geomorphologically-diverse sub-reaches, the active channel is digitised using diachronic orthophotos (1950; 1964) and the SV-error affecting the data is interpolated with an Inverse Distance Weighting technique based on an independent set of ground control points. As a second step, the main novelty of our approach consists in running Monte-Carlo (MC) simulations to randomly translate active channels according to the interpolated SV-error. This eventually allows to display the relative margin of error (RME) associated with measured eroded and/or deposited surfaces for each sub-reach through MC simulations, illustrating the confidence level in the respective measurements of our river planform changes.

Our results suggest that (i) SV-error strongly affects the significance of measured changes and (ii) the confidence level might be dependent not only on magnitude of changes but also on their shapes. Taking SV-error into account is strongly recommended, regardless of the remotely-sensed data used. This is particularly true for mid-sized rivers and/or low amplitude river planform changes, especially in the aim of their sustainable management and/or restoration. Finally, our methodology is transferrable to different fluvial styles.

How to cite: Jautzy, T., Herrault, P.-A., Chardon, V., Schmitt, L., and Rixhon, G.: Measuring river planform changes from remotely-sensed data: A Monte-Carlo approach to assess the impact of spatially-variable geometric error., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11470, https://doi.org/10.5194/egusphere-egu2020-11470, 2020

D1183 |
EGU2020-12295
Luca Mao, Ricardo Carrillo, Francesco Brardinoni, Matteo Toro, and Luigi Fraccarollo

Coarse bed load transport is a crucial process in river morphodynamics, but it is difficult to monitor in mountain streams. Predicting bed load is a difficult task especially in steep step-pool streams, where the critical dimensionless shear stress is affected by local channel slope and relative submergence, and only part of the flow energy is available to entrain and transport sediments as some is dissipated in local hydraulic plunging and jumps. Here we present a new sediment transport dataset obtained from two years of field-based monitoring (2014-2015) at the Estero Morales, a high-gradient stream in the central Chilean Andes. This stream features step-pool bed geometry and a glacier-fed hydrologic regime characterized by abrupt daily fluctuations in discharge. Bed load was monitored directly using Bunte samplers and by surveying the mobility of passive integrated transponder (PIT) tags. We used the competence method to quantify the effective slope, which is the fraction of the total slope responsible for bed load transport. This accounts for only 10% of the total slope, confirming that most of the energy is dissipated on macroroughness that characterize step-pool stream. We used the displacement lengths of PIT tags to derive the statistics of flight and resting times, observing that the average length of a flight scales inversely with grain size.

How to cite: Mao, L., Carrillo, R., Brardinoni, F., Toro, M., and Fraccarollo, L.: Coarse particle motion and resting times during bedload in a glacier-fed mountain stream of the central Chilean Andes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12295, https://doi.org/10.5194/egusphere-egu2020-12295, 2020

D1184 |
EGU2020-12903
Colin Phillips, Eric Lajeunesse, Kimberly Hill, and Chris Paola

Sediment transport is an inherently challenging process to predict due to a variety of granular and hydrodynamic phenomena. These challenges are only enhanced in natural systems where the forcing of the hydrograph and the availability of sediment is decidedly unsteady. Here we show through several field and laboratory experiments comprised of sediment flux and tracer displacement under unsteady hydrographs that their dynamics can be understood through the application of an integrated forcing metric (impulse), where the impulse represents the integrated excess transport capacity of a flood or a sequence of floods. When viewed through this framework we show that the cumulative bed load flux and tracer displacement from the particle flight length scale up to multi annual timescales are linearly related with the impulse parameter despite highly unsteady forcing. By considering the integrated forcing and sediment flux the transience of the hydrograph can be recast into a simple linear relation with parallels to long term landscape evolution models, where the details of the hydrograph are approximated as a characteristic flood stress times an intermittency factor. Through the use of an impulse metric we gain new insights that are obscured when only considering the instantaneous fluxes.

How to cite: Phillips, C., Lajeunesse, E., Hill, K., and Paola, C.: Understanding the impacts of hydrograph transience on sediment transport, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12903, https://doi.org/10.5194/egusphere-egu2020-12903, 2020

D1185 |
EGU2020-12935
Lina Polvi

Many streams in northern Fennoscandia are considered semi-alluvial in that they contain abundant coarse sediment (cobbles and boulders) deposited during continental glaciation in the form of moraines, eskers, and erratics. These streams contain sensitive trout and salmon populations, and restoration efforts (after channelization from over a century of timber-floating) strive to re-create essential habitat and spawning beds. However, little is known about controls on sediment transport processes in boulder-bed streams that can affect both channel evolution and restoration of spawning gravel. Prior flume and field research of boulder-bed channels in mountainous areas show that boulder protrusion and proximity can alter critical shear stress; however, in contrast to mountain streams, boulder-bed channels in northern Fennoscandia have relatively low bed slopes (S0: 0.5-6%) and low-magnitude flow regimes (buffered by upstream lakes).

In order to determine controls on gravel and cobble sediment transport in semi-alluvial boulder-bed streams, a sediment tracer experiment was conducted in the Vindel River catchment in northern Sweden. Approximately 1500 tracer clasts (b-axis: 2.5-15 cm), with imbedded RFID tags, were placed in five stream reaches (121-556 per reach) with a range of channel slopes (S0: ~2-6%), boulder densities, and degrees of protrusion. The geometry of each tracer clast (a-, b-, and c-axes) was quantified, and the location of each tracer clast was surveyed with a total station in the summers of 2017, 2018, and 2019. Both the morphologic setting (e.g., step, pool, riffle, backwater, directly above/below boulder) and constrainment class (e.g., unconstrained, shielded, imbricated, buried) were classified for each tracer at each survey occasion. There was a 80-90% recovery rate of tracer clasts; despite several reaches experiencing >Q50 snowmelt flood, the median transport distance for D10- to D50 clasts was ~0.1 m. Preliminary analyses showed large variation in particle-size thresholds for entrainment and relationships with transport distance within and among reaches. There was no clear relationship between local bed slope or calculated bankfull shear stress and transport distance. Differences in entrainment and transport distances among reaches was controlled by boulder density and protrusion, which likely increase grain resistance and thus critical shear stress, reducing sediment transport (as shown by previous studies in boulder-bed mountain streams). Factors negatively affecting sediment transport include shielding, proximity to boulders, and certain morphologic settings (e.g., backwaters and pools). Variability was too high to allow confident prediction of entrainment of individual grains; however, based on these results and observations, some general guidelines for stream restoration of spawning gravel in semi-alluvial boulder-bed channels are presented.      

How to cite: Polvi, L.: Controls on sediment transport in semi-alluvial boulder-bed streams—implications for restoration of spawning gravel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12935, https://doi.org/10.5194/egusphere-egu2020-12935, 2020

D1186 |
EGU2020-18431
Rocio Luz Fernandez, Daniel Parsons, Stuart McLelland, and Bas Bodewes

Sequential observations of channel adjustments in relation to short-term flow variability are required to evaluate the effects of temporal ordering of hydrologic events on channel form. With the increasing hydroclimate variability due to global climate change, fluvial morphology might also exhibit adjustments toward changing equilibria. By combining flume and numerical modelling we examine the mechanism of bed morphology changes of braided rivers to a sequence of low to moderate magnitude flood events. Over 60 runs were performed in a mobile bed flume (10 m x 2.5 m), with constant longitudinal slope (0.015) and mean grain size (0.45 mm) in the Total Environment Simulator at the University of Hull, UK. The outcomes of each run were characterized by a detailed digital elevation model, digital imagery and continuous monitoring of the sediment transported through the flume outlet. Sediment conditions included floods with equilibrium and deficit loads. Rivers were allowed to evolve from an initially flat-bed to a self-organized, steady state. The rate of change and rate of bed load movement against time were indicative of the gradual approach to equilibrium. The Delft3D code in depth-averaged (2-D) mode was used to reproduce different aspects of the braiding process over an up-scaling of the laboratory river. Data analysis allowed us to assess the effect of discharge variation on the braiding dynamics and on the width-to-depth ratio of channels, which although variable in time, fluctuated among defined values. Once in equilibrium, net changes in reach-averaged width and depth values were relatively minor. The adjustment of the river morphology through time was well fitted by an exponential decay expression, and we tested diffusive relationships held within our braided river system for both constant and varying discharge conditions. In long term process-response systems, climatic changes introduce sequences of disruption of equilibria such as those analysed in this study. The results might provide then a useful basis for analysing the similar but more complex long-term dynamics found in natural rivers.

 

How to cite: Fernandez, R. L., Parsons, D., McLelland, S., and Bodewes, B.: Changes in braided river morphology driven by flood sequencing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18431, https://doi.org/10.5194/egusphere-egu2020-18431, 2020

D1187 |
EGU2020-19653
| Highlight
Giulia Marchetti, Francesco Asaro, Simone Bizzi, Stefano Mariani, Barbara Lastoria, Francesco Comiti, and Claudio Prati

The identification and quantification of morphological changes occurring in the river channel over time are essential to understand rivers behaviour, assess sediment budgets, evaluate effectiveness of river management strategies and support the production of natural risk map. Recently, river science has made a breakthrough thanks to emerging remote sensing technologies and today we can rely on an unparalleled amount of data, at spatial and temporal resolution not available in the past. This has opened new perspectives for river monitoring and fluvial survey practices, allowing to cover areas up to the catchment scale and get information almost in continuum. This research aims to investigate the potential of radar satellite data collected from Sentinel 1 mission to infer information about rivers morphodynamics processes (such as erosion and deposition), that may occur on medium-large river (e.g., active channel width > 50 m) after a flood that caused significant morphological adjustments. Drone and satellite data were collected in September 2017 and September 2018 on a selected site along the Po river, in northern Italy, characterized by a large exposed sediment bar. In March 2018 a flood caused an avulsion and a new secondary channel was opened. We used the sequential drone acquisitions to generate a Dem of Difference, that revealed geomorphic changes of the monitored sediment bar up to 2 m erosion and 1.5 m deposition. We then exploited the radar data of Sentinel 1 and conducted a seasonal analysis using both the coherence data between image pairs and the backscattered radar signal, by investigating the variability of the radar signals through the year and the correspondent condition of the bar. Results show that there is a significant correlation between morphological changes occurred in the site and the associated values of both the amplitude and the coherence of the radar data pre and post the event that caused the morphological changes measured. Further studies are needed to better discriminate the different contributions to changes in amplitude and coherence driven by soil water content, vegetation, sediment size, atmospheric condition for the various time windows analysed. Despite that, these initial evidences are encouraging and new applications to other sites and flood events are planned because these results prove the sensitiveness of the radar signal to geomorphic events. Even simply the ability to detect where channel morphological processes are occurring and their expected intensity through Sentinel 1 data would allow to prioritize more detailed field campaigns by, for instance, UAV technology providing a notable advance compared to the current ability to monitor river morphological changes on large scale.

How to cite: Marchetti, G., Asaro, F., Bizzi, S., Mariani, S., Lastoria, B., Comiti, F., and Prati, C.: River morphological changes detection from drone and radar satellite data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19653, https://doi.org/10.5194/egusphere-egu2020-19653, 2020

D1188 |
EGU2020-19941
Hossein Amini, Federico Monegaglia, Simone Zen, Stefano Lanzoni, Guido Zolezzi, and Marco Tubino

The presence of lateral planimetric constraints preventing free migration and avulsion has a significant influence on the planform dynamics of river meanders. This particular confined kind of meandering rivers is definitely understudied, especially in comparison to freely migrating ones. Through a semi-analytical meander model, here we attempt to investigate the effect of the confined floodplain width through a morphodynamic modeling approach. The confined floodplain width is defined as the width between symmetric lateral confinement where the river is free to migrate, on the planform pattern and dynamics. Model results illustrate that weak confinement (i.e. loose floodplain boundaries) increases planform irregularity, with the river centerline preferentially lying close to floodplain boundaries, while strong confinement (tight floodplain boundaries) leads to a remarkable planform regularity, constituted by periodic sequences of sawtooth-shaped meanders. Bend orientation is reminiscent of the sub/super-resonant regime regardless of the confinement width. Model results are supported by good agreement with available field and remote sensing observation on selected case studies of confined meandering rivers in Canada previously studied by Nicoll and Hickin (2014).

 

How to cite: Amini, H., Monegaglia, F., Zen, S., Lanzoni, S., Zolezzi, G., and Tubino, M.: Modeling the planform evolution of confined meandering rivers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19941, https://doi.org/10.5194/egusphere-egu2020-19941, 2020

D1189 |
EGU2020-20909
Francesca Bassani, Matteo Bernard Bertagni, Luca Ridolfi, and Carlo Camporeale

The dynamics of a meandering river has been widely investigated by the scientific community. However, the effects of discharge variability on the meander evolution is still an open question. In this work, we present numerical simulations of the short-term evolution of a plane river morphology (the Ikeda, Parker and Sawai model is used to describe the stream hydrodynamics) forced by a stochastic flow discharge (simulated by a compound Poisson process). The comparison of the simulation outcomes with those obtained for the same river under a constant discharge (equal to the mean of the stochastic process) shows interesting results. The discharge variability slows down both the formation of the meanders and the occurrence of the cutoff events, and induces lower meander curvilinear wavelengths and excess bank velocities. A theoretical analysis of the relationship between the channel erosion rate and the river discharge for the Kinoshita curve confirms the obtained numerical results.

How to cite: Bassani, F., Bertagni, M. B., Ridolfi, L., and Camporeale, C.: The effect of flow variability on the river meandering dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20909, https://doi.org/10.5194/egusphere-egu2020-20909, 2020

D1190 |
EGU2020-21364
Colm M. Casserly, John J. O'Sullivan, Michael Bruen, Jonathan N. Turner, Craig Bullock, Jens Carlsson, Bernie Ball, Siobhan Atkinson, and Mary Kelly-Quinn

Sediment connectivity, though typically viewed as subsidiary to concerns surrounding fish passage, serves an important role in a functioning riverine ecosystem, with both substrate stability and particle size distribution acting as key determinants of benthic community structure and spawning habitat. However, despite more than a decade of pressure to restore stream continuity under the Water Framework Directive (WFD), there have been very few empirical studies on the impact that low-head dams (i.e. weirs) have on bed and suspended sediment conveyance, and little progress in the development of replicable quantitative methodologies for doing so. In this study we explore these knowledge gaps through field investigations of three gravel-cobble streams in southeast Ireland using RFID technology to investigate bedload connectivity, and integrated high-resolution monitoring of turbidity and discrete suspended sediment sampling to establish above dam vs, below dam patterns of suspended sediment conveyance.

Suspended sediment inputs and outputs over a range of flow conditions (above baseflow) reveal elevated sediment flux at the downstream station (below dam) compared to that coming into the reach (above dam). These observations are indicative of a local source of sediment between monitor­ing stations. Here we suggest that as sediment inputs became exhausted before peak discharge, the structure’s impounded zone (typically considered a depositional area) becomes the dominant source of sediment to the downstream reach. We argue that if sediment trapped behind the structure is available for transportation during high flow events, the system must be trapping sediment under lower flows, which is consistent with field observations.

Results for bedload connectivity and tracer transport over low-head dams demonstrate that particles exceeding the reach D90 can be carried through and over these structures, which is consistent with what has been reported from the US. This observation suggests that both structures may have reached a state of ‘transient storage’ as hypothesized by other authors. However, RFID tracer data when reinterpreted as fractional transport rates using a workflow based on existing empirical relations, indicate patterns consistent with supply-limited conditions downstream, demonstrating conflicting lines of evidence between the event-scale tracer movement and long-term sediment regime. Utilizing our empirical data and additional observations collected from a stationary RFID antenna mounted on a weir crest, we expand on existing models and mechanisms to show how a system may continue to exhibit supply-limited conditions downstream without the need for a net attenuation of sediment to occur indefinitely.

These results indicate that low-head dams may continue to alter the hydrosedimentary processes of fluvial systems long after dam construction and any hypothetical storage capacity has been reached. Though the impact low-head dams have on sediment disconnectivity to the downstream reach is likely to be variable and relatively localized, we hypothesize that the magnitude of any supply-limitation experienced downstream is predominantly a function of both dam height and the structure’s propensity to become drowned out under high flows.

How to cite: Casserly, C. M., O'Sullivan, J. J., Bruen, M., Turner, J. N., Bullock, C., Carlsson, J., Ball, B., Atkinson, S., and Kelly-Quinn, M.: Investigating the impact of low-head dams on sediment transport dynamics in gravel-cobble streams, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21364, https://doi.org/10.5194/egusphere-egu2020-21364, 2020

D1191 |
EGU2020-10589
Thomas Kolb, Markus Fuchs, and Ludwig Zöller

Revealing an amazing diversity of forms, river systems have always to be interpreted as products of their specific landscapes. Extremely sensitive to external and internal forcing, they reflect the particular characteristics of climatological and geological conditions as well as the changes of these conditions. These changes are regularly preserved in depositional series whose varying sedimentary characteristics can be attributed either to palaeoclimatic variations or to tectonic activities and their corresponding changes in fluvial discharge and sediment load. What applies to fluvial sediments in general, is particularly true for river terraces. Regularly, they are regarded as valuable palaeoenvironmental and archaeological archives and their particular importance is well documented by a huge and still growing number of studies spanning a wide range of climatic and regional settings.

However, the information gained from fluvial terraces and their significance for palaeoenvironmental and present-day fluvial research strongly depend on an accurate and precise dating of the terrace formation. Numerical ages are of fundamental importance for the interpretation of sedimentological, morphological and stratigraphical findings. They are essential for assessing the influence of various driving forces and for providing insights into the mechanisms and dynamics of river adjustments over variable temporal scales.

In this contribution, we present luminescence ages of fluvial deposits originating from an Upper Pleistocene river terrace in a small valley located in the headwater of the Main River, Germany. For this study, several samples from various locations throughout the river longitudinal course have been analysed. Surprisingly, the determined luminescence ages for material from the lowermost part of the valley are significantly older than those from the middle section, which in turn are older than those from the valley’s upper reaches. Based on the evaluation of a high-resolution digital elevation model (DEM) and on intensive fieldwork, we can be sure that all samples originate from the very same morphological unit, a well-preserved late Pleistocene fluvial terrace.

Our results suggest a diachronic alignment of sedimentation ages for fluvial deposits, starting with old ages close to the mouth of a river and getting progressively younger for locations approaching the upper reaches. If these findings are confirmed in other fluvial systems and are not only the result of very specific local conditions, they will be of great relevance for geomorphological research in fluvial landscapes. As a result, the widespread approach of deriving age estimates for fluvial terraces from numerical results merely determined for a single location appears to be inadequate and should be subjected to a critical review.

How to cite: Kolb, T., Fuchs, M., and Zöller, L.: Diachronism of river terrace formation? – Lessons to learn from luminescence dating results, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10589, https://doi.org/10.5194/egusphere-egu2020-10589, 2020