GM5.1

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

References:

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

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

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

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

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

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