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.

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.

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 100km² watershed 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.

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.

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.

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.

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.

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.

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.

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.