The Guiana coastline, stretching for 1500 km along the northeastern coastline of South America between the Amazon and Orinoco Rivers, is one of the most dynamic shorelines in the world. The frontage is characterised by a series of alongshore migratory, shore-attached mudbanks, with shoreline accretion and seaward expansion of mangrove vegetation during in-bank periods, followed by significant shoreline erosion during inter-bank phases. These coastal dynamics are of great concern to the nation states of Guyana and Suriname and the French overseas department of French Guyana where > 90% of the urban population live within the low elevation coastal zone.

Whilst considerable research has been undertaken along the Guiana coastline over the last four decades, the full determination of the dynamics of this long coastline remains challenging. Not all analyses have used sufficiently long temporal sequences of imagery to track at least one complete accretion-erosion cycle. Where high temporal resolution has been achieved, analysis has often been limited to one, and often only part, of the regional administrations.

This presentation provides the first ever analysis of rates of shoreline change across the entire Guiana coastline annually over a 35-year period (1987-2023). The seaward extent of mangrove forest or other coastal vegetation was selected as the shoreline proxy. This was extracted from Landsat multispectral 30 m resolution imagery using machine learning and image thresholding techniques. Annual shoreline change rates were measured at 200 m intervals over the 1500 km frontage, providing unprecedented insight into how the entire shoreline system has evolved.

This analysis discovered differences in the position, size, and speed of alongshore migration of nine mudbanks along the Guiana coastline, with mudbanks exhibiting either a graded or abrupt form of alongshore migration. Contrary to previous research, this analysis identified no evidence of a 30-year cycle in shoreline accretion – erosion across two extensive regions of the Guiana coastline: Saramacca, Suriname and Guyana. In both these locations, three other categories of landform were identified as affecting shoreline position: naturally migrating headlands, the presence of emplaced polders and sites of rapid accretion along anthropogenically modified coastlines. In addition, correlation analysis was conducted between shoreline change metrics, wave metrics derived from ERA5 reanalysis data, and climate indices including the North Atlantic Oscillation (NAO) and the El-Niño Southern Oscillation (ENSO). This analysis identified a statistically significant relationship between pan-Guiana shoreline position and the 18.6-year nodal cycle. However, at the landform scale, significant wave height and direction had the strongest statistical relationship with shoreline change. This analysis is supported with the release of a comprehensive pan-Guiana shoreline change dataset, facilitating future holistic research and management of the Guiana coastline.

How to cite: Rogers, M. and Spencer, T.: Holistic analysis of shoreline change and mudbank dynamics across the Guiana coastline, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7311, https://doi.org/10.5194/egusphere-egu25-7311, 2025.

Gravel barriers play an important role in protecting coastal communities and infrastructure along mid- to high-latitude shorelines. However, their ability to adapt and fulfil their protective role in the context of a global sea level rise and intensifying storm events remain uncertain. One the major obstacles to the creation of modelling tools for predicting the evolution of these coastal features is the diversity of their morpho-sedimentary character. Although gravel beaches are all characterized by a predominance of coarse-grained particles (> 2 mm), they are often mixed with varying amounts of finer sand particles, resulting in different beach sub-categories (e.g. pure gravel, composite, mixed sand-and-gravel). In addition to sediment variability (which links to sediment availability and supply), gravel beaches, like their sandy counterparts, organise themselves into various barrier landforms, such as spits, barrier beaches or beach ridge plains. It is commonly accepted that the morphodynamics of coastal barriers over several decades or centuries is closely tied to their geomorphological heritage that controls both accommodation space and sediment supply. The analysis of the environments surrounding the barrier is therefore just as important as the characterization of the barrier itself. Systemic approaches are usually considered at a local scale and rarely applied beyond the immediate sedimentary cell. To enhance consistency and gain a more comprehensive understanding of coastal barrier contexts and controls across the broader range of geomorphic contexts, a new approach of analysing these coastal features is needed.

An inventory of over 250 sites has identified gravelly shorelines around the UK, which have been subdivided according to beach and barrier types. Here, we demonstrate a framework for systematic morphometric analysis of gravel beach-barrier systems at the national scale. Barrier metrics (e.g. width, height, volume), inland topography, nearshore bathymetry and habitat mapping are extracted at a system scale that is divided into multiple segments to facilitate categorisation. The results represent a step forward towards a typology classification of gravel barrier systems. They also allow to highlight the importance of the various data sets when considering this approach, as well as identifying important gaps in data availability.  

How to cite: Pancrazzi, L. and Burningham, H.: Evaluation of the morpho-sedimentary diversity and multi-annual to multi-decadal dynamics of gravel barrier systems around the UK, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17923, https://doi.org/10.5194/egusphere-egu25-17923, 2025.

Low-lying coastal areas are vital hubs, hosting invaluable ecosystems and supporting large human populations for centuries. Nonetheless, these regions face growing threats from climate change, sea-level rise, and intensifying extreme weather events, negatively affecting the quality of life in coastal communities. In response, storm-surge barriers have been widely adopted as a global solution to mitigate coastal flooding risks, with numerous projects proposed and implemented over the past two decades, although questions arise on the long-term ecological response.

This study focuses on the flood-regulated Venice Lagoon (Italy), a pilot example of an artificially controlled estuarine system, to explore the future of urban coastal environments as they navigate the challenges of balancing wetland conservation with the resilience of coastal communities—two goals that are often interdependent yet conflicting. Using a custom-built two-dimensional numerical model, we investigate four years of floodgate operations (2020–2023) to compare different flood regulation scenarios and their effects on urban flooding risk and ecosystem health. Specifically, we simulated tidal and wind-wave-induced circulations across the Venice Lagoon and compared the results of the real-case flood-regulated condition with those of a hypothetically non-regulated scenario. Additionally, we examined a third, hypothetical, flood-regulated scenario in which floodgate closures are managed using an optimized approach to minimize their frequency and duration.

Our analysis shows that the current operational strategy, while effectively protecting Venice and surrounding urban settlements from flooding, significantly disrupts the submersion dynamics of salt marshes, thereby reducing sediment deposition and fostering ecosystem degradation. However, this study demonstrates the feasibility of adaptive, sustainable management strategies that balance the competing demands of mitigating flood risk while preserving valuable coastal ecosystems, enhancing their resilience to climate change as a whole.

How to cite: Michielotto, A., Finotello, A., Tognin, D., Mel, R. A., Carniello, L., and D'Alpaos, A.: A window to the future: balancing urban protection and ecosystem preservation in flood-regulated shallow coastal areas, insights from the Lagoon of Venice, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18006, https://doi.org/10.5194/egusphere-egu25-18006, 2025.

Estuaries and lagoons have historically served as sheltered areas for navigation and harbours, fostering settlement and trade. Over centuries, human interventions such as channel dredging and canal excavation have reshaped these environments to accommodate increasingly larger vessels and facilitate harbour access. While these modifications offer immediate benefits for navigation purposes, they alter the delicate hydro-morphodynamic balance of shallow tidal systems, potentially intensifying erosion and vulnerability to sea level rise. Therefore, understanding the side effects and long-term consequences of dredging and excavation is essential for developing informed management strategies for back-barrier lagoons.

Here we examine the effects of canal excavation and dredging on the hydrodynamics of two back-barrier lagoon systems in the northern Adriatic Sea: the Venice and the Marano-Grado Lagoons. In the Venice Lagoon, the Malamocco-Marghera canal, excavated in 1970, is periodically dredged to a minimum depth of -10 m along its 16-km path connecting the Marghera harbour to the open sea through the Malamocco inlet. In the Marano-Grado Lagoon, a 5-km canal completed in 1969, is dredged to -6 m to connect the industrial harbours on the Corno and Ausa rivers to the Porto Buso inlet. We constructed computational grids for the pre- and post-intervention scenarios, as well as for the present-day configurations, based on available bathymetric surveys for both lagoons. Using a 2-D finite element hydrodynamic model, we simulated tidal flows in the considered configurations, setting as boundary conditions a sinusoidal tidal wave with a 0.50 m amplitude and a 12-hour period, typical of the northern Adriatic Sea.

Despite differences in morphology and intervention scale between the two cases, consistent trends emerged. Comparisons of pre- and post-intervention scenarios reveal an increase in the water discharge through the inlet connected to the excavated channel. This increased water exchange leads also to a different subdivision of the sub-basin connected to each inlet. Moreover, the increase in the ebb-phase discharge is more pronounced than that in the flood phase, indicating that channel dredging promotes a shift toward ebb-dominant conditions, with implications for water and sediment dynamics.

These findings highlight the potential long-term consequences of excavation and dredging in shallow tidal systems and emphasize the need for management strategies that reconcile navigational needs with the preservation of the morphological integrity of back-barrier lagoon ecosystems.

How to cite: Tognin, D., Piazza, A., and Carniello, L.: Canal excavation impacts on the hydrodynamics of shallow back-barrier lagoons, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2568, https://doi.org/10.5194/egusphere-egu25-2568, 2025.

Wetlands are among the most unique and biologically diverse ecosystems on the planet, playing critical roles in water filtration, carbon sequestration, and supporting rich biodiversity. However, these ecosystems are increasingly under threat from a combination of climate change and anthropogenic pressures. To safeguard these vital systems and ensure their sustainable functioning, the Ramsar Convention, an international treaty for wetland conservation, was adopted by 172 countries. The Anzali Wetland, situated in northern Iran, is one such Ramsar site and represents a significant ecological and hydrological resource. This wetland receives freshwater inputs from over 11 rivers and maintains a dynamic connection with the Caspian Sea. However, a confluence of challenges, including climate change-induced flooding, agricultural and wastewater runoff, declining Caspian Sea levels, and accelerated sediment deposition, has severely threatened the wetland’s integrity and functionality.

Addressing these challenges requires comprehensive management strategies informed by robust scientific understanding. Decision-makers and stakeholders need accurate tools to predict the outcomes of various interventions and develop targeted restoration plans. Hydrological and hydraulic models have become essential tools in this context, providing insights into complex ecosystem dynamics and helping evaluate the effectiveness of proposed management measures before their implementation.

In this study, the HEC-RAS (Hydrologic Engineering Center's River Analysis System) model, a computational fluid dynamics (CFD)-based software, was employed to simulate the hydraulic behavior of the rivers flowing into the Anzali Wetland. This model is particularly well-suited for assessing open-channel hydraulics and has been tailored to represent the unique characteristics of the Anzali Wetland. Given that the wetland’s water levels are predominantly influenced by seasonal river inflows rather than Caspian Sea fluctuations, the model emphasizes the critical role of river hydrology in sustaining wetland productivity, including vegetation health and biodiversity.

The HEC-RAS model was calibrated and validated to cover the vast region and major inflows of the Anzali Wetland. It aimed to assess the effectiveness of existing flood control infrastructure, analyze contamination pathways and their impacts on water quality, and identify areas within the wetland prone to excessive sediment deposition. The model results also provide valuable insights into the interplay between hydrology, sediment transport, and water quality within the wetland. For example, the model highlights areas with severe sediment accumulation, threatening to disrupt aquatic habitats and navigation. Additionally, it identifies critical zones where agricultural and urban runoff introduce contaminants, adversely affecting water quality and wetland health.

The outcomes of this modeling effort serve as a vital decision-support tool for wetland managers and policymakers. By simulating different restoration scenarios, such as improved flood control measures, sediment management strategies, and contamination mitigation efforts, the model enables stakeholders to prioritize actions that will have the most significant impact on preserving and restoring the Anzali Wetland. This study underscores the importance of integrating advanced hydraulic modeling with ecosystem management to safeguard vulnerable wetlands like Anzali, ensuring their ecological, cultural, and economic functions for future generations.

How to cite: Fatheenia, A., Farhadi Cheshmeh Morvari, B., Hasanabadi, M., and Alizad, K.: Developing a Hydraulic Model for Sustainable Restoration and Management of Anzali Wetland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7216, https://doi.org/10.5194/egusphere-egu25-7216, 2025.

Coastal ecosystems are shaped by the dynamic interaction of freshwater and saltwater, governed by both oceanic and terrestrial hydrological processes. However, anthropogenic development and climate change are disrupting these processes, necessitating targeted conservation strategies to sustain ecosystem functions. This study examines the hydrogeological processes influencing water levels and salinities in a coastal wetland near Avoca Lagoon (NSW, Australia), an intermittently open and closed system that is manually breached when water levels exceed a threshold to prevent urban flooding. The wetland was specifically designed to support the breeding of the endangered Green and Golden Bell Frog (GGBF), whose eggs and tadpoles require a narrow range of low-salinity conditions for survival. We established two surface water and three groundwater piezometers at depths of 3.5 to 5.5 m to monitor water levels and salinity. Additionally, multiple electrical resistivity tomography transects were acquired near the wetland, and the lagoon's depth and salinity profile were surveyed using a kayak. The results reveal that lagoon levels rise rapidly after rainfall and decrease gradually through evapotranspiration and water loss to the ocean during dry periods. The wetland’s water levels closely follow those of the lagoon, indicating hydraulic connectivity through the subsurface. Manual breaching of the lagoon’s berm prevents flooding of low-lying areas but leaves the lagoon level too low to sustain wetland water, causing it to dry out. Salinity within the lagoon is stratified, with brackish water overlaying seawater. While these saline conditions are unsuitable for frog breeding, the wetland is surrounded by fresh groundwater, which can discharge into the wetland under lower lagoon levels to create favourable breeding conditions. High lagoon levels, however, breach the barrier between the lagoon and wetland, causing salinisation and compromising habitat suitability. Our investigation reveals the delicate balance of water level and salinity conditions required for GGBF breeding, requiring a critical "goldilocks zone". Effective habitat conservation strategies must address a complex interplay of hydrogeological processes to enable breeding conditions, including challenges posed by climate change-induced shifts in rainfall patterns and future sea level rise. These findings underscore the broader challenges coastal areas face under increasing anthropogenic and climatic pressures, highlighting the critical need for improved management approaches that integrate surface and groundwater processes to protect frog habitats and maintain broader ecosystem functionality.

How to cite: Rau, G. C., Palombi, B. R., Reinhard, P., Brown, W., Power, H., and Callen, A.: Hydrogeological controls on endangered frog breeding habitat in an urban coastal wetland: Insights for conservation strategies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5273, https://doi.org/10.5194/egusphere-egu25-5273, 2025.

The Pearl River Delta (PRD) is among the world’s most intricate delta systems, shaped by the dynamic interaction of upstream runoff and downstream tidal dynamics. However, the mechanisms underlying river-tide connectivity within such complex networks remains insufficiently understood, particularly the nonlinear feedback loops and spatiotemporal lag effects governing water level dynamics. This study employs an information-theoretic framework to investigate water level connectivity in the PRD, integrating relative mutual information (RMI) and relative transfer entropy (RTE) to quantify synchrony, causality, and directional information flow among hydrological variables. Results highlight the dominant role of upstream river discharge on water level synchrony in the Xijiang and Beijiang River systems, while downstream tidal dynamics exert greater causal effects in the Pearl River’s mainstream and coastal distributary regions. Since the 1990s, human activities, such as dam construction and channel dredging, have attenuated the influence of river discharge while leaving tidal impacts largely unchanged. Seasonal analysis reveals that that upstream river discharge predominantly governs water level connectivity during the flood season, whereas downstream tidal forcing becomes more prominent in the dry season, with spring tides amplifying these effects across both seasons. The study further shows spatiotemporal heterogeneity in connectivity, highlighting nonlinear feedback mechanisms and lag effects across subsystems. These insights underscore the adaptability and resilience of the PRD under both natural and anthropogenic pressures. By providing a novel perspective on deltaic process dynamics, this study contributes to the theoretical foundation for sustainable management and resilience planning in the PRD.

How to cite: Wang, Y. and Cai, H.: Information-theoretic insights into river-tide connectivity in the Pearl River Delta: Implications for complex network dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6115, https://doi.org/10.5194/egusphere-egu25-6115, 2025.

The Vietnamese Mekong Delta (VMD) is among the vastest low-lying areas in the world and particularly exposed to relative sea-level rise, pluvial, fluvial and coastal flooding. While new studies have shown how the impacts of climate change and land subsidence will further increase the vulnerability of the delta, current flooding characteristics are also shaped by land use and its changes over time, including the distribution of mangroves and urban sprawl. However, the implications of delta-wide land-use changes and the role of coastal habitats for driving flood dynamics in the VMD remain unknown. In addition, there is a lack of analyses that integrate all hydrometeorological forcings in a compound setting (pluvial, riverine and coastal) and use two-dimensional hydrodynamic modelling across the entire delta including the Ho-Chi-Minh-City province.

We address these shortcomings by applying a state-of-the-art two-dimensional hydrodynamic model (SFINCS) across the VMD, and incorporating latest digital elevation models and land-use data from 1985 and 2022. We touch upon difficulties in validating large-scale hydrodynamic models for vast low-lying delta regions and highlight the importance of high-quality digital elevation models (DEMs) for investigating the role of mangroves in nature-based coastal defense schemes by comparing the modelling results obtained for different DEMs (FABDEM vs DeltaDTM). Furthermore, we attribute characteristics of recent flood events to land-use land-cover change since 1985 and sea-level rise, and investigate the role of existing mangrove forests for flood risk mitigation. Preliminary results emphasize the contribution of land use change for compound flood dynamics and point towards the high value of mangroves as a natural surge buffer across the VMD, but specifically in the provinces Ca Mau and Ho-Chi-Minh-City.

How to cite: Kiesel, J., Seeger, K., Minderhoud, P., Couasnon, A., Nguyen, H. Q., Sadana, T., Van Loon, A., and Scussolini, P.: How mangroves and past land-use change have affected compound flood events in the Vietnamese Mekong Delta, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15430, https://doi.org/10.5194/egusphere-egu25-15430, 2025.

Delta plains are vulnerable environments, chiefly due to their low elevation, occurrence of highly dynamic depositional processes, high population density and anthropogenic pressure, and threats coming from climate change, more extreme weather events and accelerating rates of relative sea-level rise. Anthropic activities and interventions, typically aimed at reclaiming deltaic lands for agricultural, urban, and industrial purposes, have significantly modified the hydro-morphodynamic behavior of fluvio-deltaic environments, making the reclaimed land hydrologically disconnected from the river and starving natural wetlands of sediments.

A paradigm change in river-delta management plans is currently underway, from hard infrastructures to new approaches designed to “work with the river”, leading to a broad interest in so-called “nature-based” solutions to restore and create new deltaic lands.

The Po River Delta represents a prominent example of a strongly engineered deltaic system with compromised long-term sustainability. This work focuses on a crevasse delta recently formed in an abandoned and flooded embanked area in the Po Delta, which demonstrates that natural deltaic dynamics can occur also in strongly anthropogenically-modified deltaic plains and effectively build new emerged land. We used field analyses (collection of sediment cores with a hand auger corer) and remotely sensed data to characterize the sedimentary facies and morphosedimentary structure of the crevasse delta.

The study area was reclaimed and used through the 1950s and 1970s for agriculture. In the mid-1970s, levee breaching caused seawater inundation, after which the area was abandoned and partially colonized by reeds. The reclaimed land was hydrologically disconnected from the river and eventually sea level rise and subsidence caused the flooding of the entire area, evidenced in the stratigraphy by a laterally persistent serpulid-rich marker horizon. This situation, with only fine grained sediments deposited from suspension and bioturbation, persisted until 1999-2000, when a fluvial flood caused a natural breaching in the levees and re-establishment of natural deltaic processes and wetlands, with the formation of intertidal and vegetated crevasse delta lobes.

Through the sedimentological analysis of drilled cores, aerial and satellite images and their mutual correlation, this work aims to define and reconstruct the architecture and the morphosedimentary evolution of this crevasse delta, improving our knowledge of natural systems resilience: by reconnecting the river to its wetlands, we can reduce land loss and restore deltaic coasts by harnessing their land-building capacity.

This work is part of the research project “Ensuring resilience of the Po River Delta to rising relative sea levels using nature-based solutions for building land and mitigating subsidence (NatResPoNΔ)”, a PRIN 2022 PNRR project funded by the European Union – NextGenerationEU.

How to cite: Rossi, V., Finotello, A., Ghinassi, M., Irace, A., Zaggia, L., Mandal, A. R., Berton, A., Trifiró, S., Mantovani, M., and Cosma, M.: Nature-based solutions for resilient deltaic coasts: an example from a crevasse delta in the Po River Delta (Italy) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18993, https://doi.org/10.5194/egusphere-egu25-18993, 2025.

The acceleration of sea level rise caused by global warming increases the risk of coastal flooding for people living on river deltas, as well as the erosion of archaeological sites along the Mediterranean coasts. The inhabitants of deltaic areas from the northwestern Mediterranean were exposed to coastal changes during the Holocene, and especially to geomorphic evolution driven by regression and progradation dynamics. Coastal flooding related to transgression can lead to a reduction in terrestrial areas available for human activities, whereas the emergence of new lands during delta progradation can provide opportunities for the development of cities and agriculture, although it can also increase the vulnerability of coastal infrastructures and settlements to sediment accretion.

The archaeological site of Lattara is one of the oldest coastal cities of the northwestern Mediterranean and is particularly interesting to study the impact of flooding on human settlements. This ancient city was built on a delta lobe of the Lez River during the Iron Age in the late 6th century BCE. Already, Middle Neolithic settlements were present in the northern part of the city. However, the absence of human occupations between ca. 3000 and 800 BCE suggests an abandonment of the site over two millennia. Geoarchaeological and environmental studies showed that this period was characterized by high groundwater levels in the Lez delta plain and relatively deeper water in the lagoon south of Lattara. Coastal flooding could thus explain the absence of human settlements at Lattara in the Late Neolithic and most of the Bronze Age, but this hypothesis needs to be investigated further.

Here we present the relation between hydroclimatic changes, sedimentation, coastal flooding and human settlements in the Lez delta plain during the mid-to late Holocene using bioindicators (ostracods, molluscs), geomorphological features (accommodation space, sediment accumulation rates), hydrological parameters (sea level change, water depth, discharge rates), age models based on radiocarbon dating, and archaeological data. Our data points toward the evidence of low sediment accumulation rates in a humid climate from 6 to 3 kyr cal BCE. These low sedimentation rates in a context of continuous sea level rise led to increasing of an accommodation space in the lagoon.

Our new results are compared with multi-millennial environmental records in the northwestern Mediterranean to evaluate the role of hydroclimate changes on coastal flooding. Besides, hydroclimatic parameters from instrumental data were investigated to determine if the relationships between climate change and hydrological processes over the past millennia were similar to those of the last decades.

How to cite: Giaime, M., Degeai, J.-P., Joseph, C., Salel, T., and Piques, G.: Impact of mid-to late Holocene hydroclimatic variability and sediment dynamics on coastal flooding and human settlement in the Lez delta plain, southern France, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10579, https://doi.org/10.5194/egusphere-egu25-10579, 2025.

This study examines the impact of human-induced changes to the lower Brazos-San Bernard delta on sedimentation rates and sediment sources filling the estuarine lakes within its western inland area. The delta began forming in 1929 when the Brazos River mouth was relocated 10 km west of its original position, placing it 5 km from the San Bernard River's mouth. Over time, the wave-dominated Brazos Delta expanded westward, closing the San Bernard River mouth and extending beyond it. 

In 1949, the Gulf Coast Intercoastal Waterway (GCIWW) canal was completed, bisecting the Cedar Lakes, a series of five brackish lakes, and enabling sediment transport to these lakes. The San Bernard River drainage basin, situated between the larger Brazos and Colorado Rivers, occasionally receives floodwaters from the Colorado River during high-discharge events like Hurricane Harvey (2017). The geological differences between the Brazos and Colorado Rivers facilitated the development of distinct sediment "fingerprints" using X-ray fluorescence, color spectrometry, imaging, and grain size analysis. 

Eighteen vibracores collected across the study area, combined with 137Cs dating, enabled the identification of deposits formed before and after the GCIWW's creation. Findings revealed that prior to the GCIWW and the Brazos River mouth's relocation, Colorado River-derived deposits dominated the region. Following these alterations, the western Cedar Lakes recorded Gulf of Mexico overwash sands and occasional Brazos River flood deposits. The GCIWW acts as both a conduit for Brazos River sediment and a barrier to Colorado River sediment. 

North of the GCIWW, in lakes isolated from the canal, sediment records show a mixture of Brazos and Colorado River deposits. East of the San Bernard River, deposits include Colorado River material, Gulf of Mexico overwash, or layered deposits from both the Brazos and Colorado Rivers, reflecting simultaneous flooding events. Most Cedar Lake cores exhibit Pleistocene deposits at their base, overlaid by 50–90 cm of pre-GCIWW sediments and 90–130 cm of post-GCIWW deposits. This suggests a significant increase in sedimentation rates after the canal's construction. 

In summary, the GCIWW's creation and the Brazos River mouth relocation have significantly altered sediment sources and deposition rates in the Cedar Lakes and adjacent brackish lakes of the Brazos-San Bernard delta.  The results of this study are being used by various stakeholders to develop a better management plan for this region.

How to cite: Dellapenna, T., Robbins, C., Majzlik, E., Zhang, Y., and Ahmari, H.: Assessing mixed sediment sources of the brackish lakes of the Brazos-San Bernard River Delta of the northwest Gulf of Mexico and what it can tell us about how anthropogenic alterations have impacted sediment sources and sedimentation rates, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14528, https://doi.org/10.5194/egusphere-egu25-14528, 2025.

Managed realignment, the landward relocation of primary flood defences, is increasingly recognised as a sustainable approach to mitigating tidal flood risk in estuaries. However, the effectiveness of realignment relative to the size and location of intervention, and in relation to estuary size, remains poorly understood. This knowledge gap is critical, especially for urban estuaries where space for large-scale, nature-based interventions is limited. This study explores the scale-dependency of managed realignment using a 2D TUFLOW hydraulic flood model of the Clyde estuary, a large, meso-tidal urban estuary on Scotland’s west coast. Analytical solutions and existing flood models from eight other UK estuaries complement this analysis to facilitate comparisons between estuaries of a range of sizes. Our findings reveal that managed realignment exhibits scale-dependent behaviour: the effectiveness of managed realignment to reduce tidal flood risk is linearly proportional to the ratio of the size of the managed realignment to the estuary size. Larger estuaries, like the Clyde, require significantly more extensive realignment to achieve meaningful tidal flood risk reduction. Conversely, smaller estuaries achieve similar benefits with comparatively smaller interventions as they are more sensitive to geometric changes. Additionally for the Clyde, we also found that reconnecting a previously plugged palaeo-channel is more effective at reducing tidal flood risk than relocating primary flood defences. The results imply that a well-chosen location and size of realignment are needed to have a positive impact on reducing tidal flood risk in an estuary; this can be challenging due to existing land uses in highly urbanised estuaries. Hydrodynamic modelling will provide powerful tools to aid decision-makers and avoid risks of maladaptation, supported by long-term monitoring. Given the growing global adoption of managed realignment, this study offers critical insights into the scale-dependent behaviour of this strategy, helping to refine its implementation in diverse estuarine contexts.

How to cite: Prasojo, O. A., Williams, R. D., Naylor, L. A., Toney, J. L., and Hurst, M. D.: Right size, right place: scale-dependency of managed realignment in an urban estuary, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1524, https://doi.org/10.5194/egusphere-egu25-1524, 2025.

Estuarine dams, built between the estuary mouth and tidal limits, provide freshwater storage and storm surge protection but disrupt natural processes, altering hydrodynamics, sediment transport, and ecosystems. These changes affect freshwater discharge, tidal regimes, stratification, and sedimentation, often degrading water quality and obstructing fish migration. Globally, estuarine dams are found in 10% of 2,396 analyzed estuaries and, along with land reclamation, have caused nearly half of estuarine area loss over 30 years. Their construction peaked in mid-income countries during the 20th century, with limited development in low-income countries due to economic constraints and in high-income nations due to stricter environmental regulations. In a recent study of the Nakdong Estuary in Korea, the morphologic equilibrium following dam construction and subsequent restoration was investigated. Long-term numerical modeling revealed that the estuary achieved equilibrium approximately 15 years after restoration. In contrast, human-altered estuaries stabilized more quickly—within about 9 years—due to hydrodynamic adjustments and sediment redistribution that reduced energy dissipation. Model simulations effectively reproduced key morphological changes, including the transition from barrier island formation under wave-dominated conditions after dam construction to sand shoal development under tide-dominated conditions following restoration. Additionally, the model captured shifts in sediment texture: from sand-dominated under pristine conditions, to mud-dominated during the construction phase, and ultimately returning to sand-dominated post-restoration. This study highlights the value of realistic, long-term numerical simulations in understanding estuarine responses to human interventions and restoration efforts. The findings offer valuable insights for developing sustainable management strategies - conservation in low- and mid-income countries and restoration in high-income countries.

How to cite: Lee, G., Chang, J., and Harris, C.: Global altered estuaries with estuarine dams: Pathways for conservation and restoration, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2348, https://doi.org/10.5194/egusphere-egu25-2348, 2025.

The latest IPCC report projects that regardless of the climate scenario, the highest rates of future sea-level rise will occur along the west-central US Gulf Coast. Meanwhile, recent research has shown that coastal wetlands in the Mississippi Delta are unable to survive rates of sea-level rise higher than 3 mm/yr, a number that was exceeded two decades ago. Thus, the 1M+ inhabitants surrounded by marshland will have to adapt to rapidly changing environmental conditions. Here we adopt an interdisciplinary approach to assess this problem.

The archeologic record shows that indigenous people adapted quickly to changing conditions in the rapidly evolving Mississippi Delta, abandoning areas subject to transgression and settling on prograding delta lobes. Present-day populations are much less nimble, yet rapid coastal degradation (notably wetland loss) has been related to the population decline that has already commenced in this region. While catastrophic events (i.e., major hurricane strikes) are commonly thought of as driving population loss, we argue that socio-economic factors (notably a dwindling home insurance industry) may become equally important. One key question is how much continued sea-level rise this region will see.

The last interglacial (LIG, ~125,000 years ago) featured a global average temperature that reached about 0.5-1.5 °C above pre-industrial values. Remnants of a LIG shoreline have been identified in SE Louisiana more than 100 km landward of the present shoreline, with a reconstructed sea level of 3.1 ± 0.8 m higher than present (7.5 ± 1.1 m after correction for fault motion). Since anthropogenic climate change (~1.5 °C in 2024) has already brought us near the upper end of LIG warming, it is plausible that future sea-level rise to such an elevation is already locked in, although the timescale for this to play out remains uncertain. With respect to the LIG shoreline, the New Orleans metropolitan area is located on the “wrong” side.

If future warming is kept well below Paris Agreement levels (2 °C) the shoreline may eventually stabilize at a position comparable to that from the LIG. Conversely, if Paris goals are exceeded, sea level can be expected to rise to an extent that puts other metropolitan areas, farther inland and at slightly higher elevation, in jeopardy as well. The next few decades will be decisive as to whether Paris climate goals are met. As a consequence, the ultimate fate of several million inhabitants, along with trillions in economic and ecologic capital, will likely be determined by mid-century.

How to cite: Törnqvist, T., Keenan, J., Mehta, J., and Shen, Z.: Climate-driven depopulation of a low-elevation coastal zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3718, https://doi.org/10.5194/egusphere-egu25-3718, 2025.