Session GM11.2

[Programme]

GM11.2 | Rivers’ morphological response to extreme events and anthropogenic impacts

Rivers’ morphological response to extreme events and anthropogenic impacts

Convener: Vittoria Scorpio | Co-conveners: Ana Lucía, Virginia Ruiz-Villanueva, Adina Moraru, Andrea Gasparotto

Orals

| Tue, 16 Apr, 10:45–12:25 (CEST)

Room -2.20

Posters on site

| Attendance Wed, 17 Apr, 10:45–12:30 (CEST) | Display Wed, 17 Apr, 08:30–12:30

Hall X3

Posters virtual

| Attendance Wed, 17 Apr, 14:00–15:45 (CEST) | Display Wed, 17 Apr, 08:30–18:00

vHall X3

River morphology is inherently dynamic, shaped by a complex interplay of unsteady driving variables controlled by the water, sediment, and wood regimes. Natural events like floods and droughts, as well as human activities, can disrupt these fundamental factors. In response to these changes, rivers exhibit complex morphological variations that are very challenging to predict. Therefore, gaining a deeper understanding of how rivers respond to disturbances is fundamental for sustainable river management, evaluating flood risk, and achieving restoration goals.
This session welcomes contributions that explore the morphological response of rivers to human interventions and extreme events (i.e., floods and drought). We seek research that advances our understanding, modelling, and predicting capabilities about the recent past, present, and future trajectories of rivers. In light of these, we warmly encourage contributions focusing on river morphological changes driven by climate extremes or anthropogenic impacts, on river management and restoration projects, along with modelling and prediction of future channel evolution.

Orals: Tue, 16 Apr | Room -2.20

Chairpersons: Vittoria Scorpio, Ana Lucía, Adina Moraru

10:45–10:55

EGU24-19200

solicited

On-site presentation

Remotely sensed rivers to account for geomorphic processes in river basins threatened by anthropic pressures and climate changes

Simone Bizzi

Remote sensing since a few years are offering unprecedented opportunities to monitor the environment and its functioning. Fluvial geomorphology is affected by these advances. Nowadays we can address basin analysis of river geomorphic trajectories and simulate sediment connectivity and transport at the network scale thanks to available remotely sensed geomorphic datasets. However, these assessments and simulations present distinct limitations and must be integrated with field data and high-resolution datasets acquired in selected locations for validation purposes. Despite that, if properly managed the current ability to generate an understanding of river geomorphic functioning and sediment connectivity at the network scale is notable and can support modern river management in assessing scenarios of future strategic decisions such as, water resources issues, rehabilitation objectives and flood protection schemes. In this talk I will present two case studies where we have been applying such approaches. The first concerns the Mekong river, one of the largest and most threatened river system globally, and its strategic dam planning for the following 30-50 years. We have assessed sediment connectivity and transport for different scenarios of dam planning and estimate the risk of sediment starvation for the Mekong Delta at risk of drowning identifying more sustainable and less impacting dam locations compared to the existing ones. The second case is the Vjosa river, one of the last unpaired braided system in Europe, recently threatened by the possibility to build multiple dams along its course to produce hydroelectric energy. Here we have developed simulations able to link changes in sediment transport due to dam building with likely adjstments in river patterns and morphology with multiple consequences in terms of river equilibrium and ecosystem services provided. Finally, current capacity to understand river geomorphic functioning at the network scale will be discussed. Impacts of strategic management measures and climate changes can nowadays be predicted in terms of sediment transport and river geomorphic adjustment processes likely occurring in the coming years. Rarely modern management uses the full capacity of this discipline to address management decisions, and this lack of exploitation of available knowledge is a responsibility we need to correct as a community in the future.

How to cite: Bizzi, S.: Remotely sensed rivers to account for geomorphic processes in river basins threatened by anthropic pressures and climate changes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19200, https://doi.org/10.5194/egusphere-egu24-19200, 2024.

10:55–11:05

EGU24-16589

ECS

solicited

On-site presentation

Is stereo satellite imagery a reliable method to infer geomorphic dynamics in fluvial systems at high spatial resolution and precision?

Manel Llena, César Deschamps-Berger, Luca Demarchi, and Francesco Brardinoni

Remote sensing techniques (e.g., SfM photogrammetry, LiDAR) have been proved as a consistent method to reconstruct fluvial landscapes and processes at high temporal and spatial resolutions. Despite the advantages, these methods still presenting some limitations as, for example spatial covering, which usually is limited to <100 km length. In this context, stereo satellite imagery is presented as a valuable remote sensing technique which allow to reconstruct Earth’s relief at high spatial resolution covering extensive areas (>100 km) in one single satellite caption. Several studies have proved the accuracy and precision of this method in various geomorphological contexts, despite this, today there are very few works that study the reliability of these methods in fluvial environments with different degrees of complexity. The main objective of this study is to evaluate the use of stereo satellite imagery as a reliable method to reconstruct the topography of fluvial systems at high spatial resolution, precision, and accuracy. Additionally, we assess the reliability of this method to infer on geomorphic processes through the comparison of multi temporal high-resolution topography. To pursue these objectives two sets of stereo satellite imagery (i.e., DEIMOS and Pleiades) were compared with reference topographic datasets composed of SfM-photogrammetric (i.e., UAV platform) and GNSS-RTK surveys collected at the same time. These datasets were confronted in three different fluvial reaches located in the Marecchia River (Nortern Appennines). Study reaches presented contrasted characteristics in terms of fluvial pattern, active width, confinement, and channel-slope, which cover a wide range of fluvial types of mid-mountain rivers. Residuals between datasets were compared with different morphometric variables (e.g., slope, roughness) and channel characteristics (e.g., water depth).

How to cite: Llena, M., Deschamps-Berger, C., Demarchi, L., and Brardinoni, F.: Is stereo satellite imagery a reliable method to infer geomorphic dynamics in fluvial systems at high spatial resolution and precision?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16589, https://doi.org/10.5194/egusphere-egu24-16589, 2024.

11:05–11:15

EGU24-1587

On-site presentation

Transforming river morphology over 60 years of urban development

Peter Ashmore, Barlow Victoria, and McDonald John

Urban land development causes changes to river channel form and function and to community relationships and visions of rivers. The story of urbanization of Highland Creek, Toronto, Canada involves the transformation of the 100km²watershed from rural to almost 100% urban land-use over 6-7 decades. The unusually rich documentation of the watershed changes allows for understanding of the long-term trajectory of transformation of the channels based on geomorphic principles. It also shows the evolving institutional responses to managing and redesigning the river channels and valleys, and mitigating risks, which can be seen as part of the urbanization of rivers along with the physical response of the river channels. Urbanization has transformed the main channels to an extremely energetic state more like mountain streams despite the low relief terrain of the Toronto region with total stream power increasing by up to 10 times following urbanization. Measurements from historical air photos for multiple epochs over 60 years show that channels have widened progressively by a factor of 4 or 5, with accompanying changes in planform, triggered, in particular, by two or three large flood events, and consistent with hydraulic geometry predictions. The time trajectory and spatial pattern of changes to morphology of reaches that were free to adjust to the increased stream power are well-predicted by land-cover based predictions of stream power using the SPIN watershed analysis tool (Stream Power Index for Networks) which provides a means of predicting possible future changes as well as explaining historical change. Increases in specific stream power show potentially large increases in bed material particle mobility consistent with observed morphology changes. Freedom to evolve to a new state is partly the result of institutional decisions and vision to set aside valley land in the 1950s to reduce flood damage. In other places the process of urbanization has involved progressive engineering of the channel, reflecting approaches in 1960s and 70s to protect infrastructure and institute an engineering vision of river control which has eliminated fluvial processes of channel development and adjustment. Recent moves to channel restoration using geomorphic design approaches reflect create novel river morphology. Urbanization of channels does not end with land cover change and its physical hydro-geomorphic effects, nor is there a clear end-point and time-span of response that is often claimed in studies of urban fluvial change. Understanding urbanization as an ongoing process of ‘re-storying’ river channels within a socio-geomorphic system is essential to understanding urban river histories and futures, explaining urban river morphology, and recognising the entwined physical and socio-political power and processes that transform urban rivers.

How to cite: Ashmore, P., Victoria, B., and John, M.: Transforming river morphology over 60 years of urban development , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1587, https://doi.org/10.5194/egusphere-egu24-1587, 2024.

11:15–11:25

EGU24-1705

ECS

On-site presentation

Quantifying trajectories of geomorphic river recovery through analysis of assemblages of geomorphic units: Outcomes of nature-based river management post severe riparian clearance

Nuosha Zhang and Kirstie Fryirs

The removal of riparian vegetation and instream wood, as well as channelisation, river regulation and sediment extraction, has led to significant adjustments and metamorphosis in many rivers of southeastern Australia throughout the 19th and 20th century, post colonisation. With improvements in river management practice that saw a transition from an engineering approach to a passive nature-based river rehabilitation approach, coincident with a period of minimal flooding, signs of geomorphic and vegetative recovery have been detected since the 1980s across coastal rivers of NSW. Analysis of decadal trends in woody and non-woody vegetation coverage in 20 catchments in the coastal region, has shown on average, a 40% increase in vegetation coverage along riparian corridors since the 1950s. Some catchments now have a current day coverage of between 60% and 80%.

Coevally with vegetative recovery has been an improvement in the physical structure and function of these rivers, expressed as geomorphic river recovery. We have used ergodic reasoning to quantitatively analyse changes in the assemblage of geomorphic units that occur for rivers at different stages of geomorphic recovery. To track geomorphic river recovery we developed a semi-automating methodology that integrates Geomorphon tool and supervised classification, to map geomorphic units using Open Access LiDAR and Sentinel-2 images. We analyse the assemblages of geomorphic units for 78 river sections that span eight river types (River Styles), three valley settings and two bed material textures – sand and gravel. We find that geomorphic river recovery is not always linear and occurs in different patterns for different river types. As recovery progresses, adjustments tend to occur at the sub-unit scale by changing the form of individual units. For example, river recovery can be detected by changes in indicator geomorphic units. The presence of benches and islands indicates that recovery is underway across most river types. A statistically significant increase in abundance and area of benches and pools, and a decrease in abundance and area of floodplain steps can also be used to indicate that recovery is underway. Additionally, as recovery occurs, we observe that bank-attached bars tend to become more compound in structure. At the reach scale, confined and most laterally unconfined rivers exhibit linear and non-linear increases in richness, abundance, evenness, and diversity of geomorphic units during recovery. Partly confined rivers show more variable trends for these measures, and channelised fill rivers show decreased diversity.

Across this region, river resilience has been tested by severe fire and catastrophic flooding between 2019 – 2022. Remarkably, these rivers have shown high levels of resilience to these extreme events. For example, flood peak travel times have slowed dramatically since the 1980s, attesting to significant increases in geomorphic and vegetative roughness brought about by river recovery.

River recovery in these systems is a decadal process. Understanding recovery trajectories post colonisation land clearance has significant implications for the prediction of future channel evolution and provides invaluable insight for nature-based and recovery-enhancement approaches to river management.

How to cite: Zhang, N. and Fryirs, K.: Quantifying trajectories of geomorphic river recovery through analysis of assemblages of geomorphic units: Outcomes of nature-based river management post severe riparian clearance , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1705, https://doi.org/10.5194/egusphere-egu24-1705, 2024.

11:25–11:35

EGU24-12876

On-site presentation

High-resolution topographic surveys of five upland river realignment schemes show restoration increases and maintains geomorphic unit diversity at the decadal scale

Richard Williams, Helen Reid, Craig MacDonell, Fiona Caithness, Eric Gillies, and Hamish Moir

River restoration is key to realising ambitions to improve the biodiversity of rivers and to contribute to natural flood risk management. However, a dearth of detailed, accurate and consistently acquired, long-term topographic monitoring constrains the available evidence base to evaluate the efficacy of different river restoration approaches. Upland gravel-bed river realignment schemes are emblematic of this challenge. Here, the results from monitoring five contemporary upland river restoration sites in Scotland and the North-West of England are presented. The topography of 9 km of restored reaches at Whit Beck, the River Lyvennet, Swindale Beck, Allt Lorgy and the River Nairn was measured for a period of approximately one decade after each river realignment. The full extent of each scheme was surveyed every 1-3 years, with the frequency dependent on the geomorphic dynamism of the scheme. A variety of geomatics technologies were deployed to survey topography including, robotic total stations, RTK-GNSS, Structure-from-Motion photogrammetry, Terrestrial Laser Scanning and Unmanned Aerial Vehicle LiDAR. This unique dataset has enabled geomorphic change to be mapped and annual sediment fluxes to be quantified. Moreover, the high-resolution topographic datasets enable the geomorphic unit development of each scheme to be mapped using the Geomorphic Unit Toolbox (GUT). Together, this dataset enables three questions to be investigated: (i) what is the geomorphic unit composition of restored rivers?; (ii) how does geomorphic unit diversity develop post-restoration; and (iii) what geomorphic mechanisms are sustaining geomorphic unit diversity? We show that different restoration schemes have contrasting geomorphic unit assemblages, which are influenced by sediment supply, scheme constraints, in-channel and riparian wood and vegetation, and intervention through adaptive management approaches. The sediment budget for Swindale Beck exemplifies the trend in total volumetric topographic change through time; change is greater in the first few years following restoration and then declines once the river has adjusted the imposed boundary conditions. Topographic change initially increases the aerial extent of geomorphic units, the aerial extents of erosion and deposition between surveys then become similar and the extent of the active river channel remains approximately constant. Overall, across all schemes, there is declining geomorphic change with time but geomorphic unit, and thus physical habitat, diversity are maintained. These findings provide strong evidence for how physical habitat diversity and quantity have both increased and been maintained as a consequence of river realignment and should underpin efforts to scale up from demonstration sites to catchment-scale restoration efforts.

How to cite: Williams, R., Reid, H., MacDonell, C., Caithness, F., Gillies, E., and Moir, H.: High-resolution topographic surveys of five upland river realignment schemes show restoration increases and maintains geomorphic unit diversity at the decadal scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12876, https://doi.org/10.5194/egusphere-egu24-12876, 2024.

11:35–11:45

EGU24-11629

On-site presentation

Trajectories of River-Floodplain Adjustments Following Compounding Wildfire-Flood Disturbances

Ryan R. Morrison, Aleah Hahn, Daniel White, and Ellen Wohl

Wide, low-gradient segments within river networks (e.g., “beads”) play a critical role in absorbing and morphologically adapting to disturbances, such as wildfires and debris flow. However, the magnitude and rate of active channel morphological adjustment compared to pre-disturbance conditions and the post-fire response of in-stream restoration features and geomorphic units is not clearly understood. To better understand the impact of major disturbances on river beads, we analyzed trajectories of river morphology adjustments following the 2020 Cameron Peak wildfire and 2022 flood and debris flow at Little Beaver Creek, Colorado, USA. We used historical National Agriculture Imagery Program imagery (2009-2019) and post-fire drone-imagery surveys (2021-2023) to assess morphological change in a 500-m, low gradient bead of Little Beaver Creek. We analyzed remotely sensed imagery for pre- and post-geomorphic metrics in rates of floodplain destruction and formation, and changes in channel width and channel migration. Rates of floodplain destruction and formation, along with centerline migration greatly increased after the first post-fire runoff season and recovered to the historical range of metrics three years after the fire. The large flood in 2022 increased the rate of channel width reduction with immediate infilling of side channels, followed by the infilling of pools, and growth in bars and islands. The ability of the active channel of Little Beaver Creek to quickly adjust to fire and flood disturbances demonstrates the importance of river beads for enhancing river-floodplain resilience to large disturbance events, especially compounding hazards such as fires and floods. Our research also can inform river management and restoration about the importance of heterogeneous and dynamic river-floodplain systems to support resilient watersheds.

How to cite: Morrison, R. R., Hahn, A., White, D., and Wohl, E.: Trajectories of River-Floodplain Adjustments Following Compounding Wildfire-Flood Disturbances, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11629, https://doi.org/10.5194/egusphere-egu24-11629, 2024.

11:45–11:55

EGU24-10726

ECS

On-site presentation

Coarse deposits on river terraces: Evidence of suspended transport of gravels during high-magnitude floods

Andrea Brenna, Alvise Finotello, Vittoria Scorpio, Filippo Zarabara, and Nicola Surian

Mobilized coarse sediments in gravel-bed rivers are typically transported as bedload. In contrast, finer material (e.g., sands and silt) can be transported in suspension and, during overbank stage of floods, possibly deposited on topographically elevated surfaces such as floodplain or recent terraces.
We studied the rivers’ response induced by a high-magnitude hydrological event (recurrence interval of several hundred years) that affected several catchments in Central Italy in September 2022. Specifically, we focused on the Misa River, an Apennines gravel-bed stream that flows embedded 1.5 to 8 meters into the surrounding alluvial plain in its hilly sector. In the face of rather limited effects in terms of channel widening, we recognized the widespread presence of fresh gravel deposits (D₅₀ ranging from 6 to 19 mm) organized in lobes and fans placed on terraced surfaces at elevations 3-5 meters higher than the channel bed. The aim of this work was to investigate the morpho-sedimentary dynamics and transport mechanisms that may have led to the deposition of such anomalous coarse sedimentary bodies on river terraces.
The field work made it possible to characterize the grain sizes of the deposits, the topography of the river sections, the composition of the banks and the maximum hydrometric levels locally reached during the flood. The streambed slope was also calculated remotely based on available DTM data. The hypothesis we investigated was that the gravel particles may have moved in suspension during the paroxysmal phase of the flood event. We performed hydraulic calculations based on the classical Shields-Parker River Sedimentation Diagram, which considers dimensionless Shields stress and particle Reynolds number (a dimensionless surrogate for grain size) to determine the transport mechanism that affected the clasts (i.e., suspension, bedload, no motion). Despite the uncertainties related to water density and kinematic viscosity for which, in the absence of measured data, we assumed realistic values (e.g., 1078 kg/m³ and 4.6⋅10^-6 m²/s, respectively), the results obtained show that in most cases (10 out 12 anomalous deposits analyzed) the hydraulic conditions at the flood peak were consistent with movement in suspension of the medium and coarse gravels found on river terraces. Following the overflow, the hydrometric level dropped abruptly on terraces, inducing first bedload transport and then deposition of elongated gravelly lobes.
These results suggest that during intense floods, anomalous mobilization of fluvial gravels in suspension is possible, which can reach and reactivate external surfaces lying at considerably higher elevations compared to the active channel. This particular condition of coarse particle mobility is enabled by the enhanced unit stream power during flood events. Specifically, in the analyzed context, the stream power could not be dissipated via lateral erosion and increase of channel width due to the cohesiveness of the banks, the widespread presence of outcropping bedrock and bank protection structures. In light of the above, suspended transport of gravels and subsequent deposition on high surfaces outside the active channel should be considered as an additional morphodynamic process that can occur in gravel-bed rivers during intense flooding.

How to cite: Brenna, A., Finotello, A., Scorpio, V., Zarabara, F., and Surian, N.: Coarse deposits on river terraces: Evidence of suspended transport of gravels during high-magnitude floods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10726, https://doi.org/10.5194/egusphere-egu24-10726, 2024.

11:55–12:05

EGU24-12931

ECS

On-site presentation

Morphologic interaction of bridges during high-energy flood events by the example of the flood of mid-July 2021 in the Ahr Valley, Germany

Stefanie Wolf, Lisa Burghardt, Nina Stark, Anton Ahlswede, Michael Gardner, Anne Lemnitzer, and Holger Schüttrumpf

In mid-July 2021, heavy rainfall led to severe flash floods in the Eifel-mountain region in western Germany. The Ahr River, a tributary to the Rhine River in Rhineland-Palatinate, was most severely affected. Discharges accumulated rapidly in the narrow valley and formed a fast-moving flood wave, leading to record-breaking water levels. High hydraulic forces in combination with driftwood and debris accumulation led to local back-up, and eventually failure of many bridges [1].

The flood is proven to be a high-energy event that led to significant morphologic changes [2]. Besides lateral river course changes, severe local erosion was observed after the flood, and especially documented near bridges [3].

The Ahr Valley is well suited for a case study on the morphologic interaction of bridges with high-energy floods, as around 75% of the 114 bridges were damaged or destroyed in 2021 [1]. In April 2022, bathymetry data was collected at two affected bridges. Further, selected bridges were monitored by drone surveys every six months, allowing us to investigate the morphologic development over the last two years.

Besides bridge overtopping , pier scouring led to structural failure [1, 4, 5]. At a railway bridge between Reimerzhoven and Dernau (N: 50.532213, E: 7.062320) scouring occurred upstream on the pier in the middle of the riverbed, and bank erosion occurred on the right-hand in flow direction. Riverbed narrowing and sediment deposition at the remaining piers occurred on a small scale. Further upstream, at the town of Altenahr, massive debris accumulation, bridge overtopping, and pier scouring downstream of the middle pier led to structural failure. At this location, a railway bridge and a trafficable bridge were located next to each other (N: 50.514921, E 6.985825). The railway bridge experienced severe bank erosion on the left-hand side in flow direction. After removal, shoal formation led to the cut-off of a stagnant water pool [2]. Processes of pier scouring, bank erosion, and sediment deposition in the following two years differ in both examples. Further investigation and comparison help to gain an understanding of the complex morphologic interaction of bridges in high-energy flood events. Parameters, like the main flow direction and the local flow velocity as well as driftwood accumulation leading to bridge overtopping impact patterns of erosion and deposition. Results can support local water resources management as well as bridge construction authorities.

[1] Burghardt L, Schüttrumpf H, Wolf S et al. (2022) Analyse der Schäden an Brückenbauwerken in Folge des Hochwassers 2021 an der Ahr. Wasser Abfall 24:12–17

[2] Wolf S, Stark N, Holste I et al. (2023) Evaluation of the High-Energy-Flood of mid-July 2021 as a Morphologic Driver in the Ahr Valley [preprint]

[3] Lehmkuhl F, Keßels J, Schulte P et al. (2022) Beispiele für morphodynamische Prozesse und Verlagerungen in Folge des Hochflutereignisses 2021 im Ahrtal. Wasser Abfall 24:40–47. https://doi.org/10.1007/s35152-022-1349-7

[4] Pucci A, Eickmeier D, Sousa HS et al. (2023) Fragility Analysis Based on Damaged Bridges during the 2021 Flood in Germany. Applied Sciences 13:10454. https://doi.org/10.3390/app131810454

[5] Lemnitzer A, Stark N, Gardner M et al. (2022) Geotechnical Reconnaissance of the 2021 Western European Floods. GEER Association (Report - 76)

How to cite: Wolf, S., Burghardt, L., Stark, N., Ahlswede, A., Gardner, M., Lemnitzer, A., and Schüttrumpf, H.: Morphologic interaction of bridges during high-energy flood events by the example of the flood of mid-July 2021 in the Ahr Valley, Germany, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12931, https://doi.org/10.5194/egusphere-egu24-12931, 2024.

12:05–12:15

EGU24-6892

On-site presentation

A risk assessment of river levee breach in gravel-bed rivers under climate change: A case study of the Otofuke River, Japan

Toshiki Iwasaki, Kodai Takahashi, and Daichi Murakami

Climate change is a critical social concern in many research and engineering fields, and one of the vital issues in this regard is its significant impact on water and sediment-related disasters. Further increases in precipitation intensity and associated river discharge increases will change the flood characteristics and associated morphological changes in rivers, causing critical damage to river training structures and residential areas along rivers. Here, we assess the risk of the river levee breach caused by significant bank erosion due to in-channel morphodynamic processes using a large ensemble hydrological dataset and a physics-based morphodynamic model. The target river reach is the upper Otofuke River, Japan, a typical steep, gravel-bed river, and some river training structures along this river were damaged due to the significant morphological change of bed and bank during several huge flood events. Our observed dataset is highly limited, making it essentially difficult to assess risks related to river disasters, especially under future climate conditions. The use of a large ensemble climate calculation dataset, including precipitation, discharge, etc., provided in the context of climate change research will overcome the aforementioned difficulties and contribute to a comprehensive understanding of possible significant flood events in current and future climate conditions. We use a large dataset of river discharge hydrographs provided from the large rainfall calculation based on the dynamically downscaling climate calculation of the d4PDF. The thousands of hydrographs are categorized by the k-shape clustering method to evaluate the several important hydrograph shapes since the morphological change of rivers is highly affected by both peak discharge and overall shape characteristics of hydrographs. By using a physics-based morphodynanic model, iRIC-Nays2DH, and characterized hydrographs, we then simulate the possible river morphodynamic processes in the current and future climate conditions. Based on the river morphodynamic calculations, the risk of river embankment caused by morphological change of the river under current and future climate conditions is analyzed. The results show that the hydrograph shape has an important role in the morphological evolution of the river, such as sandbar and meandering development, so the damage intensity of river embankment caused by such morphological change is highly dependent on the hydrographs. More specifically, under the same peak discharge, a hydrograph with a single sharp peak causes less bank erosion; on the other hand, a relatively longer hydrograph drastically increases the risk of bank erosion and river embankment breach. Such a longer hydrograph that greatly impacts the river disaster has a small possibility of occurrence, but it does take place in the future climate conditions. It indicates that climate change does increase the risk of river levee breach caused by in-channel morphodynamic processes in steep, gravel-bed rivers.

How to cite: Iwasaki, T., Takahashi, K., and Murakami, D.: A risk assessment of river levee breach in gravel-bed rivers under climate change: A case study of the Otofuke River, Japan, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6892, https://doi.org/10.5194/egusphere-egu24-6892, 2024.

12:15–12:25

EGU24-370

On-site presentation

Fluvial and coastal sedimentary response to extreme fluctuations in rainfall, central California, USA

Amy East, Andrew Stevens, Alexander Snyder, Andrew Ritchie, Helen Dow, and Jonathan Warrick

Understanding landscape response to changing climate is essential to protecting lives, property, and infrastructure. In some regions, climate warming is associated with enhanced fluctuations between wet and dry hydrologic extremes; such hydroclimatic ‘whiplash’ has been evident in California, USA, over the past decade. We studied sedimentary responses to these changes on the central California coast, focusing on the 357-km² San Lorenzo River watershed and its nearshore zone. This study used fluvial suspended-sediment data together with biannual topographic and bathymetric coastal surveys to quantify the magnitudes and time scales of extreme-event signals. We find that in two extreme wet years (2017 and 2023), fluvial sediment loads were an order of magnitude greater than in years with average rainfall and nearly three orders of magnitude greater than during extreme drought. Extreme wet years are associated with substantial accretion in the nearshore zone around the river mouth, totaling several hundred thousand tonnes of new sediment in 2017 and 2023. The signal of aggradation can persist along the coast for 3–4 years. In wet years with multiple river floods, the floods occurring later in the wet season supply disproportionately coarse suspended sediment to the coast (60–75% sand), indicating their outsized importance on littoral sediment budgets as beach-building events. The dominance of coarse fluvial sediment after unusually large seasonal rainfall is attributable to landslides supplying coarse material to stream channels. In contrast, river floods occurring earlier in the wet season or in non-extreme-wet years supply finer-grained suspended sediment (20–30% sand, attributed to less landslide activity under drier antecedent soil conditions) and thus are less likely to influence coastal morphology. Understanding these process changes will be important for effective management of fluvial and coastal systems under a future, warmer climate with greater risk of both drought and extreme rain.

How to cite: East, A., Stevens, A., Snyder, A., Ritchie, A., Dow, H., and Warrick, J.: Fluvial and coastal sedimentary response to extreme fluctuations in rainfall, central California, USA, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-370, https://doi.org/10.5194/egusphere-egu24-370, 2024.

Posters on site: Wed, 17 Apr, 10:45–12:30 | Hall X3

Display time: Wed, 17 Apr, 08:30–Wed, 17 Apr, 12:30

Chairpersons: Virginia Ruiz-Villanueva, Andrea Gasparotto, Vittoria Scorpio

X3.111

EGU24-4413

ECS

Calibration of a diffusion model for aggrading channel after sediment overloading experiments with near-critical flow

Hasan Eslami, Alessio Radice, and Michele Iervolino

In alluvial streams shaped across geological times, the water and sediment discharges typically are in equilibrium with the corresponding supply conditions, preventing significant scouring or deposition over extended periods. However, various natural and human-induced actions may alter the balance between the river sediment transport capacity and the sediment supply. These disturbances may result in significant aggradation or degradation along specific reaches of the river. Degradation occurs when the sediment inflow discharge is smaller than the sediment load transported downstream of the reach; on the contrary, aggradation happens when the sediment entering the reach is higher than the sediment transport capacity of the channel. The present work focuses on the sediment aggradation problem, that in turn may lead to the increase of hydraulic hazard. Various mathematical approaches have been documented in the literature to predict the aggradation process in riverbeds. In this regard, several studies have focused on deriving an analytical solution for a parabolic diffusion equation. This equation is obtained by imposing several simplifying hypotheses (such as quasi-steady flow, quasi-uniform flow, etc.) on the Saint-Venant-Exner system of equations. In the present work, an analytical Fourier-series solution proposed by Gill in 1983 was applied to analyze 15 aggradation experiments, carried out at the Mountain Hydraulics Laboratory of the Politecnico di Milano (Lecco campus) using lightweight sediment material. The experimental conditions differed in terms of main control parameters such as the loading ratio (the ratio between sediment inflow discharge and initial sediment transport capacity of the channel) and the water discharge. Most of the investigated conditions corresponded to the near-critical flow regime. The same boundary conditions of the experiments were applied to the parabolic model to develop the corresponding analytical solution in terms of space/time evolution of the bed elevation. Upon comparing the analytical bed profiles with the experimental ones, acquired through a proprietary image processing technique, it was found that although for some experiments the theoretical and experimental results were consistent, the a-priori estimate of the diffusion coefficient of the model did not generally provide good agreement. Therefore, a further calibration of the diffusion coefficient of the parabolic model was performed. In this way, the space-time evolution of the bed profile in each experiment could be accurately represented, thus demonstrating the descriptive capability of the adopted simplified model for the investigated experimental conditions (that involved relatively high Froude number for which the applicability of a diffusion equation had not been ascertained). A constant value of the diffusion coefficient was enough to reproduce the evolution within an experiment, but the coefficient needed to be varied from an experiment to another, calling for further research on how the diffusion coefficient depends on the controlling variables.

How to cite: Eslami, H., Radice, A., and Iervolino, M.: Calibration of a diffusion model for aggrading channel after sediment overloading experiments with near-critical flow, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4413, https://doi.org/10.5194/egusphere-egu24-4413, 2024.

X3.112

EGU24-11424

Long-term Dynamics of Carbon Sequestration in Tropical Meandering Rivers through Remote Sensing, Numerical Modeling, and Stochastic Processes

Carlo Camporeale, Francesca Bassani, and Luca Salerno

This research explores the long-term impacts of intricate dynamics of fluvial corridors on the carbon cycle, focusing on the interaction with tropical meandering rivers during formative events and riparian vegetation, particularly large wood recruitment. A multidisciplinary approach – including satellite data analysis, deterministic modeling and stochastic processes – is employed to assess the magnitude of carbon exported by these rivers.

Meandering rivers exhibit lateral migration, with continuous erosion and deposition shaping their dynamics. Riparian vegetation, impacted by flooding and cutoff events, plays a pivotal role in carbon recruitment. A satellite-guided methodology across the Amazon basin correlates river-induced forest cover loss with eroded areas to estimate carbon recruitment.

To understand the long-term river dynamics, a stochastic toy model linking river evolution, sinuosity and carbon export is proposed. The model equation for sinuosity growth includes deterministic and noisy terms, accounting for river elongation and cutoff events, respectively. In particular, a compound Poisson process is used to describe the evolution of sinuosity over time, revealing the impact of cutoff events on long-term dynamics. The calibration of the parameters characterizing the Poisson process is performed through the results of numerical simulations for river planar evolution, based on Zolezzi and Seminara (2001) morphodynamic model.

This study indicates a close relationship between the carbon sequestration and the dynamics of tropical rivers, emphasizing the negative impact of alterations like damming and mining on river morphodynamics and carbon storage. This research provides valuable insights into the complex interactions within fluvial corridors, contributing to our understanding of carbon cycling in these critical ecosystems.

How to cite: Camporeale, C., Bassani, F., and Salerno, L.: Long-term Dynamics of Carbon Sequestration in Tropical Meandering Rivers through Remote Sensing, Numerical Modeling, and Stochastic Processes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11424, https://doi.org/10.5194/egusphere-egu24-11424, 2024.

X3.113

EGU24-10238

Assessing the relative importance of hydrological and geomorphological controls on river flood conveyance at a global scale

Ellie Vahidi, Andrew Nicholas, Philip Ashworth, Richard Boothroyd, Georgie Bennett, Hannah Cloke, Stephen Darby, Pauline Delorme, Helen Griffith, Solomon Gebrechorkos, Laurence Hawker, Julian Leyland, Yinxue Liu, Stuart McLelland, Jeffrey Neal, Daniel Parsons, Louise Slater, and Michel Wortmann

Climate variability is a significant driver of flood events. However, geomorphological changes in river channels, including variations in local and upstream sediment supply, play a crucial role in determining flood conveyance capacity and flood stage variations. The interplay between hydrology and geomorphology, and their relative impact on flood conveyance, can vary in different river systems depending on both the degree of internal channel dynamics and the nature and magnitude of external forcings. For example, rates of bank erosion, vegetation establishment on bar surfaces, and overbank sedimentation control the time required for floodplain reworking, the adjustment of channel morphology and the associated evolution of river flow conveyance capacity and stage-discharge relations.

To investigate the relative significance of hydrological and geomorphological controls on flood-stage variability, we employ a new computationally-efficient model of river and floodplain morphodynamics. This model simulates the evolution of river morphology and flow conveyance capacity by representing the interaction between processes of bank erosion, floodplain construction and river bed-level change over multiple centuries. The simple nature of the model enables its application at large spatial scales – e.g., to explore global variations in the controls on flood conveyance and its sensitivity to future environmental change. Simulated changes in conveyance capacity for a range of environmental settings were evaluated against trends in observed river gauging datasets. Convergent cross-mapping analysis was then applied to investigate the cause-and-effect relationships between controlling factors, including: (i) hydrologic regime; (ii) river sediment load; (iii) floodplain composition (e.g., fine versus coarse sediment); and (iv) lateral river dynamics (e.g., rates of erosion and accretion). Our analysis quantifies the causality between these factors and the resulting variability in river morphology (width and bed level), flood stage and channel conveyance capacity. Results indicate that in dynamic river systems, while the importance of climate-driven hydrological changes in driving conveyance capacity changes are acknowledged, geomorphological changes – specifically, variations in sediment supply and lateral sediment sources – may dominate over climate-driven trends.

How to cite: Vahidi, E., Nicholas, A., Ashworth, P., Boothroyd, R., Bennett, G., Cloke, H., Darby, S., Delorme, P., Griffith, H., Gebrechorkos, S., Hawker, L., Leyland, J., Liu, Y., McLelland, S., Neal, J., Parsons, D., Slater, L., and Wortmann, M.: Assessing the relative importance of hydrological and geomorphological controls on river flood conveyance at a global scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10238, https://doi.org/10.5194/egusphere-egu24-10238, 2024.

X3.114

EGU24-10883

ECS

Investigating river-restoration-effects on riverbed-stability by physical modelling

Silas Unrau, Sara Venuleo, Guido Derungs, and Henning Lebrenz

This study shows how experimental results provide fundamental insights in the challenge of river revitalisation, and thus represent a powerful tool to guide engineers’ actions. Results concerns a study case, namely the “Wiese Vital” project, a restoration project in Basel area (Switzerland), with the objectives of safeguarding Basel's drinking water supply while revitalizing its watercourse and providing flood protection.

The planned revitalisation measures involve the reconstruction of the Wiese riverbed, the introduction of structures to improve its morphological variability and the replenishment of fine sediment to improve the spawning habitat of native fishes.

The new Wiese riverbed will consist of a coarser sediment layer, about 1.2 meters deep, overlaying a layer of finer sediments, meant to protect the underlaying aquifer from undesirable water infiltrations and thus to ensure Basel's drinking water supply safety. The stability of the coarser layer was investigated using a physical model in scale 1 to 20, built in the hydraulic hall of the University of Applied Sciences and Arts Northwestern Switzerland.

Experiments investigated the stability of the coarse protective layer in presence and in absences of revitalization measures: with and without “ecological” structures and before and after the addition of fine sediments.

Results revealed that wrong placement of “ecological” structures can cause local erosion and threaten the stability of the riverbed. Moreover, they provided useful insights on the response of a coarser riverbed to the input of fine sediments.

How to cite: Unrau, S., Venuleo, S., Derungs, G., and Lebrenz, H.: Investigating river-restoration-effects on riverbed-stability by physical modelling, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10883, https://doi.org/10.5194/egusphere-egu24-10883, 2024.

X3.115

EGU24-17477

ECS

Lagrangian description for the drift of large floating debris in rivers during floods

Damien Sansen, Pierre Archambeau, Michel Pirotton, Sébastien Erpicum, and Benjamin Dewals

During floods, the drift of floating debris is a common phenomenon that may exacerbate the flood con-sequences. These objects often have the potential to induce clogging, particularly when they accumulate at bridge piers and obstruct river flow. Accurately predicting such clogging or other interaction with fixed structures is essential for enhancing flood risk assessment and management.
To effectively model their effect, the fundamental dynamics of floating objects must be investigated, as well as debris-structures interactions. The latter requires a model for obstacle generation, collision detec-tion and simulation of the influence of collisions on the debris dynamics. Besides, detailed validation against high quality laboratory data is a prerequisite before considering reliable model application to real-world rivers.
In this research, improvements have been brought to the Lagrangian modelling of large floating debris colliding with fixed obstacles. A promising approach involves discretizing obstacles into rectangles, for which the mathematical description of the collision process is known. Furthermore, the proposed method reproduces the effect of collisions by adjusting the debris dynamics rather than forcing its trajectory after collision. The debris motion modelled by the developed 2D Lagrangian model leads to plausible trajecto-ries, generally in agreement with experimental data. The model also succeeds in recreating clogging situa-tions. Precise consideration of collision dynamics makes it possible to distinguish between temporary and permanent clogging, depending on certain parameters such as the adopted geometry and the debris size. Extra developments are still necessary for extending these findings to the context of real-world rivers.

How to cite: Sansen, D., Archambeau, P., Pirotton, M., Erpicum, S., and Dewals, B.: Lagrangian description for the drift of large floating debris in rivers during floods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17477, https://doi.org/10.5194/egusphere-egu24-17477, 2024.

X3.116

EGU24-17425

ECS

Propagation of hydro-geomorphic disturbances through continental-scale river basins

Andrea Gasparotto, Andrew Nicholas, Rolf Aalto, Phil Ashworth, James Best, Muriel Brückner, and Renato Paes de Almeida

Hundreds of millions of people live close to, and depend upon, the world's large rivers for water, food, transport and the maintenance of a thriving ecosystem. However, these rivers are increasingly vulnerable to the effects of a wide range of natural and human-induced disturbances, including climate change, construction of large dams, river engineering works, deforestation, agricultural intensification, and mining activity. Over the past two decades, climate change and deforestation have impacted on the hydrology and sediment fluxes within the Amazon River Basin, and yet, the Amazon has remained one of the few large river systems that has been largely unaffected by dams. Nevertheless, because of extensive hydropower dam construction in Brazil, Bolivia, Peru and Ecuador now threatens the basin, with more 300 dams planned or under construction, this situation is changing rapidly. These dams are expected to trigger severe hydro-physical and ecological disturbances throughout the basin, including massive reductions in sediment and nutrient delivery to the lowland Amazon and its floodplains, substantial degradation of riverbeds and banks, significant changes in river water levels and flooding, and adverse impacts on river and floodplain ecosystems, on which the human population depends. There is a pressing need for action to assess and mitigate these impacts. However, our capacity to do this is severely restricted by an absence of quantitative models that can predict how environmental disturbances propagate through large rivers and floodplains, over continental distances, and decadal to centennial time periods. A key challenge in this respect is the need to develop models that are both physically-realistic and also computationally-efficient. The latter is critical for model application at the basin scale, and in order to derive large simulation ensembles that account for the substantial uncertainty in model parameters and environmental boundary conditions. We report here on the development and evaluation of such a model that operates at coarse spatial (10 km) and temporal (daily to annual) resolutions. Our new modelling approach simulates changes in river morphology (mean width, depth and slope), channel-belt topography (expressed as an elevation frequency distribution), and associated changes in flow conveyance, channel-floodplain connectivity and sediment delivery to downstream reaches. Model predictions are compared with, and evaluated against, simulations of river response to dam construction generated using a high-resolution physics-based modelling approach (with spatial and temporal resolutions of 50 m and <10 seconds). This comparison demonstrates that our new simplified model is able to reproduce the key trends in river evolution simulated by the physics-based model, and their dependence on the magnitude of the shift in hydrologic regime and sediment trapping efficiency for a range of environmental scenarios. Consequently, this new model may provide a suitable approach with which to evaluate the propagation of morphodynamic disturbances at the scale of very large basins, such as the Amazon.

How to cite: Gasparotto, A., Nicholas, A., Aalto, R., Ashworth, P., Best, J., Brückner, M., and Paes de Almeida, R.: Propagation of hydro-geomorphic disturbances through continental-scale river basins, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17425, https://doi.org/10.5194/egusphere-egu24-17425, 2024.

X3.117

EGU24-1296

ECS

Water Level Lowering and Increased Tidal Influence in the Mekong Delta driven by Human-Induced Riverbed Incision

Quan Le Quan, Grigorios Vasilopoulos, Christopher Hackney, Thomas Coulthard, Hung Nguyen Nghia, and Dan Parsons

River deltas provide ecosystem services that are vital to the world's population, supporting both lives and livelihoods. However, these low-lying areas face heightened vulnerability to the effects of climate change and increasing sea levels; a vulnerability further intensified by local resource exploitation. In recent decades, population growth, urbanization, and economic development have cause a surge in the demand for natural sand and hydropower. Sand mining across lowland rivers and deltas alongside river impoundment in upstream catchments is resulting in the rapid incision of riverbeds. These cumulative impacts, coupled with alterations in input hydrological conditions and rising sea levels at the delta front, have the potential to cause considerable disruptions in the flow hydraulics at the delta scale and alter related water-level dynamics many kilometres from the coastal zone. Despite numerous studies into anthropogenic influence in delta evolution, a significant knowledge gap persists regarding how the combination of stressors that drive river bed lowering influences alterations in water level across lowland rivers and deltas.

In this paper, we utilize long-term observation data to examine the relationships between water level and water discharge in the Vietnam Mekong Delta. Assessing these relationships across both spatial and temporal dimensions allows us to determine the effects of riverbed lowering from 1998 to 2018 while identifying the main hydrological and morphological drivers and impacts of these changes. In addition, we employ a 1D hydraulic modelling routine to assess the projected progression of riverbed degradation in the future, and assess the likely impacts of the water level regimes in the entire lower Mekong River and Delta, extending from Kratie to coastal Vietnam. Our results suggest that the delta's historical river bed lowering of approximately 3.06 m from 1998 to 2018 has led to simultaneous declines in mean water levels of approximately 0.65 m and an increase in the mean tidal range by approximately 0.19 m. The reduction in water level is more pronounced in the landward direction, whereas the increased tidal range is more prominent in the seaward direction. Under anticipated future scenarios, where the riverbed lowering is projected to average around 5.92 m compared to the conditions in 1998, there could be declines in mean water level of approximately 1.27 m, while, the maximum water level reduction upstream may reach 3.73 m. Simultaneously, the mean tidal range is expected to increase by approximately 0.46 m, with the maximum rise potentially reaching more than 1 m in the downstream delta region.

There are very significant implications of these trends which include a potential reduction in the level of flooding in landward parts of the delta but very significant consequences associated with tidal flood hazard seaward, as well as associated impacts such as the disconnection of channels from floodplains, decreased efficiency of infrastructure and irrigation works, an elevated risk of storm surge hazards, as well as the increased likelihood of water salinization.

How to cite: Le Quan, Q., Vasilopoulos, G., Hackney, C., Coulthard, T., Nguyen Nghia, H., and Parsons, D.: Water Level Lowering and Increased Tidal Influence in the Mekong Delta driven by Human-Induced Riverbed Incision , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1296, https://doi.org/10.5194/egusphere-egu24-1296, 2024.

X3.118

EGU24-20827

Modelling channel geomorphic change from landslide sediment delivery during Typhoon Mangkhut in the Philippines

Diego Panici, Georgina Bennett, Richard Boothroyd, Clàudia Abancó, Richard Williams, Fibor Tan, and Mark Matera

The Antamok River in the Philippines experienced a complex geomorphic response to Typhoon Mangkhut in September 2018, which triggered >500 landslides in the Ambalanga catchment. Landslides are known to influence channel geometry by delivering large amounts of sediment. However, the interaction between landslide sediment delivery and channel geomorphic change during extreme events is poorly understood and rarely examined in tropical settings. The study catchment also has a legacy of anthropogenic modifications, such as the presence of extensive small-scale mining and tailings storage facilities (TSFs) from large-scale mining activities.

The aim of this study was to use a mapping and modelling approach to test the hypothesis that landslide sediment delivery is a major control on channel geomorphic change. To accomplish this, we have applied the multi-phase model r.avaflow to a reach along the Antamok River encompassing the highest density of landslides and where it displayed a highly dynamic channel morphology. Landslide sediment delivery and TSFs are represented within r.avaflow by digitising areas of mapped landslides, allowing sediment to be delivered into the channel as well as being transported by the flow. To account for modelling uncertainty, we also tested the influence of several key-parameters by carrying out a sensitivity analysis.

Using the approach described, r.avaflow was used to simulate the effects of landslide delivery and TSFs on channel erosion and deposition during the typhoon event. Results showed a good agreement between observed and simulated channel width change. When compared with traditional methods (e.g., unit stream power), the model results were considerably more accurate and consistent with observations. Furthermore, sensitivity analysis suggested that simulations are dependent on the type of sediment and physical processes considered, whilst other parameters only had negligible effects.

Overall, the model simulations suggested that the impact of landslide and TSF sequences is highly dependent on the amount of sediment delivered by landslides, and a multi-phase model such as r.avaflow is possibly one of the most appropriate tools for simulating active channel width changes. Further research using these mapping and modelling tools is needed to better understand the contribution of sediment supply on channel geomorphic change during extreme events, that are otherwise difficult to observe and model.

How to cite: Panici, D., Bennett, G., Boothroyd, R., Abancó, C., Williams, R., Tan, F., and Matera, M.: Modelling channel geomorphic change from landslide sediment delivery during Typhoon Mangkhut in the Philippines, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20827, https://doi.org/10.5194/egusphere-egu24-20827, 2024.

X3.119

EGU24-9362

ECS

The trend of gravel bars' degradation in the last two decades is perceived positively by society

Adriana Holušová, Tomáš Galia, Zuzana Poledniková, and Lukáš Vaverka

River management and regulations can significantly impact the dynamic nature of gravel bars in rivers. The aim of this research is to identify changes in gravel bar morphodynamics including vegetation cover succession in three Czech rivers: the Odra and two of its tributaries. The spatiotemporal analysis focuses on the most recent years 2000-2020 and compares them with historical conditions (1937-1994 in the Odra and from the 1950s in the Olše and Ostravice). Additionally, we aimed to determine how society perceives gravel bars as features in river channels. We analysed gravel bar spatiotemporal morphodynamics using orthophotos and associated hydrological data. The survey of public perception included two sets of photographs depicting regulated and natural environments at different stages of vegetation growth, and assessed perceptions of naturalness, recreation, aesthetics, and vegetation.

The historical state of gravel bars showed naturally dynamic gravel bars with great variability of vegetation cover throughout the years. However, the recent period has demonstrated an increasing trend in vegetation cover with disruptions following the 2010 and 2014 floods and leading to temporary decreases of vegetation, notably in the two gravel-bed tributaries. The meandering Odra river conversely exhibited an uninterrupted upward trend. Additionally, the regulated sections of the Odra featured significantly less gravel bar area compared to natural sections. In the public survey, 239 respondents expressed a preference for gravel bars in natural river section. They particularly preferred those with abundant vegetation cover, across all assessed criteria. Unvegetated gravel bars were indicated to be preferable if they were either vegetated or removed from the river channel. Our research emphasizes the unfavourable viewpoint of society towards gravel bars with limited or no vegetation and preference of highly vegetated bars that is a direct reflection of recent trend toward a substantial increase in vegetation-covered gravel bars or loss of gravel bars. These findings highlight the loss or degradation of ecologically valuable habitats and society's misconception about gravel bars, which may affect future conservation efforts of these features.

How to cite: Holušová, A., Galia, T., Poledniková, Z., and Vaverka, L.: The trend of gravel bars' degradation in the last two decades is perceived positively by society, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9362, https://doi.org/10.5194/egusphere-egu24-9362, 2024.

X3.120

EGU24-19762

How does bed load transport influence the longitudinal profile of reservoir sedimentation?

Xin Zeng and Yuan Yuan

The construction of the reservoir has destroyed the equilibrium of the natural river and promoted the siltation of a large amount of sediment in the reservoir. The longitudinal profile after reservoir siltation will have a great impact on the amount of the reservoir siltation, the retention of effective reservoir capacity, and the raise of water elevation caused by the backwater effect. Therefore, the estimation of the longitudinal profile is of great significance to the calculation of the reservoir sedimentation. For mountain rivers, the amount of bed load usually accounts for a large proportion of the total sediment load, which affects the longitudinal bed profile after dam construction. Because the amount and gradation data of bed load are often not available, the effects of bed load transport on the longitudinal profile of reservoir sedimentation cannot be accurately simulated. Until now, how bed load transport influences the longitudinal profile of reservoir sedimentation has not been fully understood. The motivation of our study is to clarify the effects of bed load transport on the longitudinal bed profile of reservoir sedimentation.

We use a 1D mathematical model to study the evolution of the longitudinal bed profile after dam construction to answer the question. Various scenarios with different boundary conditions are considered to analyze the effects of bed load, such as the amount, gradation and distribution during the year, on the longitudinal bed profile after dam construction. Our results show the effect of the amount of bed load on the longitudinal bed profile of reservoir sedimentation.is greater than that of bed load gradation and distribution during the year. In the early stage of reservoir operation, the influence of the bed load transport on the longitudinal profile is negligible. With the increase of operation time, the effects of bed load becomes more and more significant. The larger the amount of bed load, the higher the bed elevation, and the larger the slope of the riverbed near the dam. For areas with large initial topographic slopes, the greater the bed load amount, the smaller the slope of the riverbed after siltation. In contrast, the effects of bed load gradation and distribution during the year.on the longitudinal bed profile of reservoir sedimentation is not significant.

How to cite: Zeng, X. and Yuan, Y.: How does bed load transport influence the longitudinal profile of reservoir sedimentation?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19762, https://doi.org/10.5194/egusphere-egu24-19762, 2024.

X3.121

EGU24-9610

ECS

Analyzing anthropogenic impact on river morphodynamics in the Upper Orcia Valley (central italy) through multitemporal NDVI assessment

Annalisa Sannino, Francesca Vergari, Giulia Iacobucci, and Maurizio Del Monte

Mediterranean environments are significantly shaped by human activities, and the morphology of rivers often reflects centuries of human intervention for land development. Previous studies, spanning decades, have highlighted the drastic river adjustments in the Italian landscape. These adjustments often reflect the man-induced land use changes within the drainage basin. The analysis of the rivers in Tuscany site (central Italy), has revealed profound changes, especially in terms of channel bed narrowing and incision. This work present the fluvial system scale analysis of the anthropogenic impact in the Upper Orcia Valley (southern Tuscany), that undergone a huge land reclamation, as evidenced by the archives of the Land Reclamation Authority, established in 1929. Accelerated erosion landforms, such as calanchi and biancane badlands, have been extensively reduced by this intervention. Consequently, a multitemporal analysis indicates a decreasing trend in erosion rates over the recent decades, accompanied by an increase in agricultural and forested areas and a narrowing of river channels. To comprehend the role of land cover and land use changes in river dynamics, the results of a multitemporal geomorphological survey and an analysis of land use changes were compared with spatio-temporal computation and analysis of the NDVI (Normalized Difference Vegetation Index), commonly used in ecology to gauge the impact of green biomass on soil erosion.
NDVI was computed using Landsat and Sentinel multispectral imagery (Landsat 1-2-4-5-8 and Sentinel-2), selecting images acquired every 5 years in May, corresponding to the Start of Seasons (SOS) month, within the 1975-2021 timeframe. The results revealed a general decrease in bare lands and a significant increase in dense vegetation cover. The overlap this data with multitemporal geomorphological mapping demonstrated a recolonization by forests along main riverbeds and in badland areas, indicating a reduction in sediment supply from hillslopes, possibly causing the observed channel narrowing and incision trends along main rivers. These findings can be attributed to increased land use for agriculture, artificial reforestation, and the gradual abandonment of rural areas, leading to the recent reconquest of broad-leaved forests.

How to cite: Sannino, A., Vergari, F., Iacobucci, G., and Del Monte, M.: Analyzing anthropogenic impact on river morphodynamics in the Upper Orcia Valley (central italy) through multitemporal NDVI assessment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9610, https://doi.org/10.5194/egusphere-egu24-9610, 2024.

X3.122

EGU24-6363

ECS

Geomorphological analysis of the Serio River aimed at its sustainable management

Sharon Pittau, Massimo Rinaldi, Tommaso Simonelli, and Vittoria Scorpio

The study of the historical evolution of rivers is a requirement for their sustainable management. This work aims to investigate the historical geomorphological evolution and the present evolutionary trends of the valley portion of the Serio River (Lombardia, Italy) in order to propose a sustainable management of its river corridor. In particular, we focus on the planimetric evolution of the active channel from 1954 until today through the analysis of: (i) channel morphological configuration, (ii) variation of channel width, and (iii) bank erosion rates.

Analyses were based on multi-temporal orthophoto mapping in GIS environment (with the highest temporal resolution over the last 25 years) along a valley segment of about 80 km.

Results show that in 1954 the Serio River was characterised by a braided pattern in the segment flowing through the high plain and by a sinuous-meandering in the segment flowing through the lower plain. In the sinuous and meandering segment, where the morphology configuration did not change over time, the active channel width decreased from a maximum of about 80 m in 1954 to a minimum of about 35 m until the 1970s-80s and then it remained stable due to the construction of anthropogenic works along the banks. The braided reach narrowed from a maximum of about 350 m in 1954 to a minimum of about 130 m in 1988, with decreases in the braiding index. By the 1990s, active channel width alternated phases of widening and narrowing (between about 10 m and 40 m). Correspondence between the widening and the occurrence of flood events was observed. Moreover, the high river dynamic, with a continuous lateral migration of the active channel, resulted in bank erosions that locally reaches up to 100 metres in a time interval of 1-5 years.

The final outcome of this study was to propose integrated management solution aiming at the mitigation of geomorphic hazards and at the improvement of the morphological status of the river, through the delineation of areas of potential future lateral erosion.

How to cite: Pittau, S., Rinaldi, M., Simonelli, T., and Scorpio, V.: Geomorphological analysis of the Serio River aimed at its sustainable management, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6363, https://doi.org/10.5194/egusphere-egu24-6363, 2024.

X3.123

EGU24-16078

The link between river planform confinement and an altered energy gradient: pathways to revitalisation for the Waikanae River, Aotearoa New Zealand

Jon Tunnicliffe, Steve Bielby, and Sue Clearwater

Imposed straightening of river course necessarily entails alteration of river gradient. As the amplitude of sinuosity is reduced, the bed slope over the shorter course is steepened. The Waikanae River on New Zealand's North Island was a meandering-to-wandering coastal river that has been subjected to a variety of disturbances, at a range of intensities over time, including gravel extraction and channel re-alignment. This has led to a change in longitudinal bed profile, and therefore a change in stream power profile. In its current configuration, the river experiences persistent aggradation in the lower reaches and there is reduced diversity of river form. Local Māori iwi and catchment community are looking at ways to re-establish sediment transport equilibrium, habitat complexity, and cultural connectivity while maintaining a suitable level of flood protection. In this presentation, we review the trajectory of the system over time and some potential options for restoration, leveraging 2D numerical sediment transport models to provide insights into a potential new morphodynamic and ecological equilibrium regime.

How to cite: Tunnicliffe, J., Bielby, S., and Clearwater, S.: The link between river planform confinement and an altered energy gradient: pathways to revitalisation for the Waikanae River, Aotearoa New Zealand, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16078, https://doi.org/10.5194/egusphere-egu24-16078, 2024.

Posters virtual: Wed, 17 Apr, 14:00–15:45 | vHall X3

Display time: Wed, 17 Apr, 08:30–Wed, 17 Apr, 18:00

Chairpersons: Andrea Gasparotto, Adina Moraru, Ana Lucía

vX3.17

EGU24-19149

ECS

Assessment and Prediction of River Bank Shifting Using Automated GIS and Remote Sensing Approaches: A Case Study of the Jamuna River in Bangladesh.

Jumana Akhter and Md Rayhan

Being the biggest delta in the world, Bangladesh is home to the mighty Ganges-Brahmaputra-Meghna (GBM) Basin. The dynamic morphological changes in the basin shape the geography of the country. Jamuna, one of the largest sand-bed braided rivers globally, is the most dynamically evolving river in Bangladesh. With its incessant morphological changes and perpetual rise and disappearance of bars and dunes in the channel, Jamuna takes a new form every few years. Hence, understanding the river bank shifts along with erosion-accretion patters of Jamuna is salient in assessing the river’s impact on its floodplain, surrounding landscape and population in the context of aggravating climate change scenarios. This study aims to assess the spatio-temporal changes of the Jamuna river banks for last four decades from December 1984 to January 2024 and predict the changes in 2034 and 2044. The research employed Google Earth Pro for manual digitization of bank lines with five-years intervals and the Digital Shoreline Analysis System (DSAS) in ArcGIS for trend analysis and future prediction. The study encompasses the whole length of Jamuna river situated in Bangladesh and divided the zonal changes of erosion and accretion in district levels for analyses. 2500 and 2700 transects were taken for left and right banks respectively with a spacing interval of 100m. The rate of changes was analyzed based on Linear Regression Rate (LRR), Weighted Linear Regression Rate (WLR) and End Point Rate (EPR) and, distance measurements were derived from Net Shoreline Movement (NSM) and Shoreline Change Envelope (SCE). Subsequent predictions were obtained using simple extrapolation of the analyses data using Kalman Filter Model. Root Mean Square Error (RMSE), t-test and R² were calculated to assess prediction accuracy for 2024 before utilizing the resultant data in forecasting analysis for next two decades. Additionally, net erosion and accretion area were determined for the study area relying on polygon-based analysis in ArcGIS. The results demonstrate that the rate of erosion and accretion fluctuates for the whole length of the river. Along the left bank, the average erosion rate is observed to be -42.93m/year and accretion rate is around 38.89m/year. For right bank, the average erosion rate is -59.20m/year with accretion rate being 55.61m/year. Both the banks have been shifting westward continuously and the right bank has been shifting more compared to the left bank. Bankline fluctuations are more prominent in the downstream closer to the conjunction of Jamuna and Ganges rivers in the central part of Bangladesh. The findings of the forecasting analysis showed that the alleviating rates of erosion will continue until 2041 for both banks. The quantitative nature and results of this study can be utilized to assess the extend of erosion control measures needed for the river. The historical trends and contemporary predictions of the study can be useful in further studies on the impacts that drastic river bank shifting can have on natural and anthropogenic factors surrounding the river channels.

How to cite: Akhter, J. and Rayhan, M.: Assessment and Prediction of River Bank Shifting Using Automated GIS and Remote Sensing Approaches: A Case Study of the Jamuna River in Bangladesh., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19149, https://doi.org/10.5194/egusphere-egu24-19149, 2024.

vX3.18

EGU24-37

ECS

Using a scaled model to assess the performance of different sediment augmentation strategies in a restored channel

Nisreen Alghorani, Elli Papangelakis, Kate Pearson, Marwan A. Hassan, and Lukas Mueller

Sediment augmentation is an increasingly popular strategy for restoring rivers, mitigating flooding, and improving fish habitat. However, it is still unclear where along a river sediment seeding produces effective results, or what the fate of sediment is once placed under different flow conditions. Using a set of flume experiments conducted on a scaled pool-riffle reach, we assess the evolution of the planned augmented sediment cover in the Penticton Creek restoration project in British Columbia, Canada. We investigated three sediment seeding patterns described based on the seeding locations through the pool as: Head-seed (HS), tail-seed (TS), and full-seed (FS). For each seed pattern, the reach response to flood events with magnitudes ranging from 2- to 100-year return discharges was assessed. Our results show that while the FS channels are superior at retaining alluvial materials during low floods (i.e., 2-yr), they rapidly lose this ability as the flood magnitude increases. Examining maps of bed erosion reveals that in the FS channels, nearly all the pool area is vulnerable to a high risk of bed scour during high flood events. However, the bed scour only occurred in the HS and TS channels at locations where sediment had been seeded, dispersing eroded materials throughout the pool area. Our findings suggest that for restoring fish habitat in channels with limited sediment supplies, HS and TS seeding patterns are more effective at mitigating the risk of bed erosion during extreme floods. From a practical perspective, an HS or TS needs less sediment to complete than an FS channel, providing a more economic strategy for restoring channels.

How to cite: Alghorani, N., Papangelakis, E., Pearson, K., Hassan, M. A., and Mueller, L.: Using a scaled model to assess the performance of different sediment augmentation strategies in a restored channel, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-37, https://doi.org/10.5194/egusphere-egu24-37, 2024.