Displays

Delta systems are growingly at risk from ongoing global changes, sediment dynamics, and face many pressures from both upstream land use change and downstream sea level rise. Historically, deltas have been key locations for human occupation, and currently hold over 340 million inhabitants globally. This continuing pressure and risks have led deltas to become locked-in, likely losing their ability to provide ecosystem services and resilience to global change. To assess whether global deltas ranging from living to locked-in have lost their resilience to changes, we (i) used historical HYDE data to reconstruct the development of population and land use in 48 major deltas over the last 310 years, (ii) determine whether deltas are in locked in states, and (iii) assess whether locked-in deltas are more at risk due to relative sea level rise (RSLR), hazards, anthropogenic condition, investment deficit, and provision of nature contributions to people. 46% of the analyzed deltas are living deltas (22 out of 48), i.e. yet to be locked-in. Of the locked-in deltas (26 out of 48), 21% (n=10) emerge due to engineered natural infrastructure as the development of cropland and irrigation and 33% (n=16) are locked-in due to institutional infrastructure, such as no development of population or agricultural land uses.  We find that average risk index is higher for locked-in deltas due to natural infrastructure changes (higher hazard and anthropogenic risks for cropland development and higher investment deficit risk for irrigation development). Surprisingly, the most at risk deltas to future RSLR are the ones locked-in due to institutional changes and living deltas. Many locked-in deltas may be even more at risk when considering their current ability to supply regulating and material ecosystem services/nature contributions to people. Our results suggest that locked-in deltas might be more at risk from current pressures, due to a reduction in the functioning of the natural processes that govern deltas, while deltas locked-in due to social and institutional infrastructure will be more at risk in the future. Further work is necessary to understand whether these trends can be reversed.

How to cite: Santos, M. J., Reader, M. O., and Dekker, S. C.: Are locked-in or living deltas more at risk?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7903, https://doi.org/10.5194/egusphere-egu2020-7903, 2020.

River deltas are low lying areas that will likely experience significant land loss because of relative sea-level rise. Most future projections of delta land loss, however, assume passive coastal inundation (using so-called “bath-tub” models) and as such they tend to be unvalidated and exclude morphodynamic processes such as sedimentation. To improve future projections of delta land area change, here we apply a morphodynamic model of delta response to RSLR to all 10,000 deltas globally. We use historic RSLR, sediment supply, and observed delta land area change from 1985-2015 to calibrate and validate this model for all these deltas. Applying our model using future RSLR scenarios, we find that by the end of this century deltas globally will have lost land under all RCP scenarios. Land loss is aggravated by river dams that have diminished sediment supply to many deltas. RSLR expected under RCP8.5 will force delta land loss at at rates exceeding 900 km²/yr by 2100. We predict cumulative land loss under RCP8.5 up to 2100 of ~35,000 km², or about 4% of total global delta area.

How to cite: Nienhuis, J. and van de Wal, R.: Global morphodynamic response of deltas to sea-level rise in the 21st century, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-917, https://doi.org/10.5194/egusphere-egu2020-917, 2020.

How to cite: Capuano, T. A., Araujo, M., Silva, M., and Varona, H.: Alterations in the thermo-haline structure and hydrodynamical circulation within the deltaic regions and continental platforms adjacent to the San Francisco and Parnaiba rivers (NE Brazil) due to the effects of global climate changes., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-356, https://doi.org/10.5194/egusphere-egu2020-356, 2020.

Climate change-related temperature increases and sea-level rises have a significant impact on coastal environment. The morphodynamic processes on tidal flats under this global change have been studied by many numerical and analytical models. Studies on morphodynamic processes requires accurate bed-level measurement data to reflect the complex intertidal morphodynamics. The newly-developed SED sensor (Surface Elevation Dynamics sensor) has been introduced to provide continuous long-term monitoring with relatively low cost of labor and acquisition. However, when in use, the instrument is inserted directly to the ground, inducing scour pits around measuring point. Thus, we introduce a new instrument which make use of laser ranging called LSED (Laster based Surface Elevation Dynamics) sensor. It could avoid touching the bed surface and obtain data with 1-millimeter vertical resolution. The developed sensors can be installed at both bare and vegetated tidal flats to monitoring short-term bed level changes under different settings. In light of this, we set up a group of Laser-SED sensors in National Mangroves Park in Hailing island, Yangjiang. Firstly, these new instruments were tested using data obtained from LSED sensors and traditional Sediment Erosion Bars. An excellent agreement in these measurement methods indicating that LSED sensors are reliable in bed-level measurements. The obtained LSED-sensor data was subsequently used to develop machine learning predictors, which revealed the main drivers of the accumulative and daily bed-level changes. We conclude that the LSED-sensors can further support machine learning applications to extract new knowledge on coastal biogeomorphic processes.

How to cite: Hu, Z., Zhou, J., and Peng, Y.: Monitoring bed level dynamics in mangrove wetlands by Laster ranging technique, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2872, https://doi.org/10.5194/egusphere-egu2020-2872, 2020.

Coastal wetlands provide multiple ecosystem services through carbon storage, rich biodiversity and provision of harvested goods. A key service is their provision of ‘free’ coastal defence by dissipating storm wave and tidal energy, and their ability to accrete vertically and provide a natural buffer against the impact of projected sea-level rise. However, under IPCC climate projections, extreme hydrodynamic events associated with storm surges are expected to increase in both frequency and magnitude, exposing the margins of salt-marshes to increased erosion stress. The resistance of coastal wetlands to erosion during these events is poorly understood, and lateral erosion rates vary dramatically between UK salt-marshes. The NERC-RESIST project is exploring why this resilience to erosion varies, with a focus on the effect of the structural properties of the marsh substrate, to develop rapid evaluation tools of salt-marsh resistance for coastal engineers and inform future conservation efforts.

The NERC-RESIST project explores how subsurface and surface structural characteristics of UK coastal wetlands affect their erodibility under tidal forcings, in order to provide coastal engineers with improved guidance for conservation schemes. In order to link internal sediment structure to erodibility, X-Ray CT scans were undertaken on large sediment cores recovered from two coastal wetlands (Tillingham, Essex; Warton, Lancashire) that are currently experiencing contrasting rates of lateral erosion. X-Ray CT scanning is a non-destructive imaging technique that allows a quantified analysis of 3D sediment properties, pore-space and root structure. After scanning, the cores were exposed to a variety of realistic wave energy conditions at the Grosser Wellen-Kanal (GWK) Large Flume Facility in Hannover, Germany, and high-resolution structure from motion imagery were collected to identify patterns of wave-induced erosion.

This talk presents a 3D characterisation and detailed mapping of the topology of both pore and root networks within cores from the two salt-marshes. Two basic hypotheses are tested: the first examines the contribution of root systems in binding saltmarsh sediments and thus strengthening them against lateral erosion, and the second examines the role of macropores in facilitating the penetration of storm-wave water and energy into the sediment, contributing to weakening and increased erosion. A distance-mapping method is applied based on these hypotheses to develop a simple index of sediment structural vulnerability to erosion. These predictions are then compared to observed rates and patterns of storm wave-induced erosion from the GWK experiments. This informs an evaluation of the relative importance of inherent sediment properties (sediment type, cohesion, strength) and sediment structural characteristics in determining the erodibility of salt-marsh sediments.

How to cite: Chirol, C., Brooks, H., Carr, S., Christie, E., Evans, B., Lynch, J., Möller, I., Royse, K., Spencer, K., and Spencer, T.: Effect of belowground structure on coastal wetland erosion resistance using X-Ray Computed Tomography, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10298, https://doi.org/10.5194/egusphere-egu2020-10298, 2020.

The Ganges-Brahmaputra-Meghna Delta (GBMD) is among the largest in the world, nourished by the ~1 Gt/yr sediment load of its titular rivers. Approximately 75% of this sediment load is debouched to the Bay of Bengal, with ~180 Mt subsequently reworked by tidal processes across the southwestern portion of the delta. This region includes this Sundarbans National Reserve Forest (SNRF), which is the words’ largest continuous mangrove stand. In addition to global sea level rise and the enhanced subsidence intrinsic to deltas, ongoing and proposed alterations to the upstream fluvial sediment supply threaten the future viability of this important ecological and cultural resource.

In this study, we use data collected in situ by acoustic and optical instrumentation to examine the physical processes controlling sedimentation in the mangrove forest along the southern coast during both the monsoon (October 2019) and dry seasons (March 2020).  These data are then compared with sedimentation rates measured using sediment elevation tables and marker horizons, as well as observations made 100 km further inland near the northern extent of the SNRF. At this inland site, sediment supply, inundation depth, and salinity have been identified as important factors controlling sediment deposition to the mangrove platform, which ranges from ~1 cm during the dry season (November – June), to > 2 cm during the monsoon (July-October). Data from the second location along the coast are vital for understanding the regional nature of the various threats to delta viability.

Preliminary analysis of the 2019 monsoon season data from the southern coast reveals the relative importance of water depth, water velocity, and mangrove pneumatophore density on modulating both water velocity and suspended sediment concentration. Previous studies have identified that while the inland location features a larger tidal range (~5 m vs. ~3 m), frequent cyclone activity likely impacts sedimentation at the coastal site. Data collected in March 2020 will address how these variables impact controls on sedimentation both seasonally and regionally. Results from this study demonstrate the importance of providing regional context to sedimentation studies, as delta communities adapt to dynamic forcing conditions.   

How to cite: Hale, R., Garnand, A., and Wilson, C.: Dynamics of sediment delivery to mangrove forests of the Ganges-Brahmaputra-Meghna Delta, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10619, https://doi.org/10.5194/egusphere-egu2020-10619, 2020.

Land subsidence is one of the slowest, natural processes faced by deltas throughout the world, yet it acts as an important catalyst which exacerbates all other threats associated with relative sea-level rise, such as increased flood vulnerability and salinization. This presentation summarizes the results of five years of research on land subsidence in the Mekong delta and highlights the major advances in approaches and insights gained in subsidence processes and rates of an entire mega-delta system.

The Mekong delta is heading towards an existential crisis as land subsidence rates are rapidly accelerating over the past decades up to ~5 cm/yr. As sediment starvation in the Mekong river greatly reduces the adaptive capacity to counterbalance subsidence, this results in wide-spread loss of delta elevation. With the Mekong delta having an average elevation of less than 1 meter above local mean sea level, these elevated rates of relative sea-level rise pose an imminent threat of land loss and permanent submersion in the coming decades.

Like in many densely populated and rapidly developing coastal-deltaic areas around the world, the main anthropogenic driver that causes accelerated subsidence is the overexploitation of groundwater. A range of future delta elevation projections, considering sea-level rise and simulated groundwater extraction-induced subsidence following extraction pathways, show the dire situation of the delta in spatial-temporal explicit maps of future elevation relative to local sea level.

Adequate (ground)water management aimed at strongly reducing current extractions is key in mitigating accelerating sinking rates and crucial to ensure the survival of the Mekong delta. The window of opportunity to act is swiftly closing as the delta is rapidly running out of elevation, and therefore time.

How to cite: Minderhoud, P. S. J., Erkens, G., Middelkoop, H., and Stouthamer, E.: The existential crisis of the Mekong delta: Impact of accelerating land subsidence , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9841, https://doi.org/10.5194/egusphere-egu2020-9841, 2020.

Salt marshes are widespread features of tidal landscapes and exert a primary control on the ecomorphodynamic evolution of these environments, delivering valuable ecosystem services. Among the latter, salt marshes furnish a shoreline buffer between the mainland and the sea, dissipating waves and mitigating erosion during storms, filter nutrients and pollutants, serve as an organic carbon sink, and provide diverse ecological habitats.

The sustainability of most of the modern salt-marsh systems worldwide is threatened by increasing anthropogenic pressures, as well as by changes in climate forcings. Particularly, the dramatic decrease in marsh extent, observed worldwide during the last centuries, has long been ascribed to the combined effects of rising relative sea level and sediment starvation. However, even though both those processes may cause the drowning of extensive salt-marsh areas, recent studies have demonstrated that the great majority of salt marshes worldwide are being lost due to the lateral erosion of their margins. If on the one hand the lateral retreat triggered by wind waves is recognized as a primary driver for salt-marsh lateral retreat, on the other hand it still remains questionable whether different local soil properties (e.g., water content, dry bulk density, organic matter content, inorganic grain size) and vegetation cover actively affect the resistance, and ultimately the erosion, of salt-marsh margins.

Here we investigate, by means of numerical modelling combined with field and laboratory analyses, how the interplays between incoming wave power, ecological features, and soil properties influence the erosion rates of salt-marsh margins in the Venice lagoon (Italy).

We show that lateral erosion rates of salt marshes are primarily controlled by the incoming wind-wave power, mediated by the presence of different halophytes, whereas significant influence of soil properties is observed.

Erosion rates are reduced in marsh edges colonized by particular associations of halophytic vegetation species, and along gently sloped and irregular margins facing very shallow tidal flats. Conversely, erosion rates are enhanced in cliffed margins exhibiting smooth planform morphologies, which are typically stricken by strong wind waves.

By clarifying the interactions between the dynamics and functional shapes of salt marsh edges, our observations might be valuable for the conservation and restoration of salt-marsh landscapes, especially in the face of a globally changing climate.

How to cite: D'Alpaos, A., Roner, M., Tommasini, L., Finotello, A., Ghinassi, M., and Marani, M.: Drivers and outcomes of salt marsh erosion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20331, https://doi.org/10.5194/egusphere-egu2020-20331, 2020.

Salt marshes attenuate waves and currents, thus protecting landward-lying constructed defences and the hinterland from incoming waves and extreme water levels. As such, understanding the stability of the marsh sedimentary platform is important, particularly as marsh edge erosion is common on many shores. To understand why marshes are losing material from their exposed fringes, we must better understand the relations between the marsh fabric and incoming hydrodynamic energy; this is likely to be strongly influenced by marsh biological, geochemical and sedimentological/geotechnical properties. Currently there is little systematic research into the within- and between-marsh variability in these properties and how they affect both marsh edge and marsh surface erosion processes.

We compare Tillingham marsh, eastern England, where the sediment is clay/silt-dominated and the marsh canopy is species-rich, to Warton marsh, Morecambe Bay, NW England, where the sediment is sand/silt-dominated and the vegetation species-poor. We determine soil shear strength by applying geotechnical methods which, to the best of our knowledge, have not previously been applied to salt marsh environments. Shear box and ring shear tests are used to determine the natural- and residual (i.e. post-failure) shear strength of the substrate, respectively. This is expressed as the cohesion of the sediment and the angle of internal friction. We demonstrate that the ring shear test consistently returns a lower angle of internal friction for the substrate, which is expected for the residual angle of internal friction. However, we are also able to link this reduction in the angle of internal friction to substrate composition (e.g. root content, organic matter and particle size distribution). This enhanced methodological understanding will improve our comprehension of marsh resistance to edge erosion and thus our ability to predict future erosion. Ultimately, accurate measurements of the shear strength of natural foreshores are essential for the informed implementation of nature-based coastal flood defences, including ‘de-embankment’/‘managed realignment’ schemes.

How to cite: Brooks, H., Moeller, I., Spencer, T., and Royse, K.: The influence of grain size and frictional/cohesional shear strength components on UK salt marsh substrate stability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17681, https://doi.org/10.5194/egusphere-egu2020-17681, 2020.

Adaptation of coastal areas facing climate change is a global challenge. Some of these low‐lying regions are commonly managed and engineered to reduce damage, loss of life, and environmental degradation caused by natural hazards originating from the sea. However, sea-level rise and changes in storm regimes are putting unprecedented pressure on these managed systems, forcing the adoption of “no active intervention” or “managed realignment” strategies in areas where “hold the line” options cannot be justified due to financial constraints. The aim of this research is to explore how disintegration of sea defences would affect creek topology under present day and future sea level rise scenarios, using the Hesketh marsh as a case study.  A reduced complexity numerical model is applied to produce ensemble predictions for analysis. Without the presence of vegetation, results suggest that creek geometry efficiency and density of tidal creeks are insensitive to sea level rise.

The model assumes the erodibility of the wetland is homogeneous and constant which leaves room for improvement because coastal environment is subject to changes as a result of global climate change and human activities. Changes in environmental stressors, such as sea level rise, elevated CO₂ concentration, changing storm patterns, etc. could adjust the resistance of the wetland to erosion in either way. Hence, the adequacy of current parameterizations of soil erodibility in numerical models requires further investigation.

How to cite: Li, X., Leonardi, N., and Plater, A.: Uncertainties of creek evolution in coastal wetlands facing sea-level rise, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9685, https://doi.org/10.5194/egusphere-egu2020-9685, 2020.

Salt marshes are valuable habitats, providing natural coastal protection. However, change in the extent of salt marsh habitats is occurring globally; regional hotspots include widespread losses in Northwest Europe. These lateral losses are occurring despite relative stability in the vertical dimension (i.e. surface elevation and its relation to rising sea levels). Whilst there are an increasing number of studies reporting and quantifying salt marsh losses, the understanding of what controls lateral marsh dynamics remains weak.

Numerical models and large-scale experimentation (e.g. in wave flumes) have, to a degree, improved understanding of the mechanisms by which salt marshes can change in the lateral dimension. However, empirical field evidence exploring the role of specific marsh properties and exposure characteristics is lacking. What biophysical factors (i.e. vegetation and sediment characteristics) control internal marsh substrate stability, and how do these factors influence the vulnerability of lateral marsh margins to external forcing?

The three-dimensional biophysical response of salt marsh substrates to external forcing representative of tidal flat conditions has been investigated. Intertidal sediment sections were extracted from two contrasting UK salt marsh sites: clay-silt rich Tillingham Marsh, Essex, Southeast England, and sand-dominated Warton Marsh, Morecambe Bay, Northwest England. Vertical sections of sediment were exposed to in-situ external forcing conditions on the fronting tidal flat at Tillingham Marsh. Structure-from-motion digital photogrammetry was used to quantify volumetric and structural changes on the vertical faces of the exposed sedimentary cores at approximately 14-day intervals. Three-dimensional structure-from-motion models were analysed alongside empirical water level measurements and meteorological data. Greater loss of material, typically around root structures, characterised the upper section of the sediment core from Warton Marsh. The Tillingham Marsh sediments were more resistant to erosion, including within the upper section. This indicates possible variability in the mechanical role of rooting structures (as also found in previous work (e.g. Feagin et al. 2009; Ford et al. 2016)), under a different marsh sedimentology.

Small-scale marsh stability is thus strongly influenced by physical sedimentology, biological root structures, hydrodynamic sequencing, and the interactions between these factors. A combination of inundation history, bulk sediment strength and belowground vegetation structure is likely to influence salt marsh lateral stability, at least at the cm to m scale. Understanding under which conditions (e.g. location, wave regime) these factors become more or less important, and how these small scale controls scale up to larger scales is crucial towards modelling and predicting future salt marsh change.

References:

• Feagin, R. A., Lozada-Bernard, S. M., Ravens, T. M., Möller, I., Yeager, K. M., & Baird, A. H. (2009). Does vegetation prevent wave erosion of salt marsh edges? Proceedings of the National Academy of Sciences of the United States of America, 106(25), 10109–10113. https://doi.org/10.1073/pnas.0901297106
• Ford, H., Garbutt, A., Ladd, C., Malarkey, J., & Skov, M. W. (2016). Soil stabilization linked to plant diversity and environmental context in coastal wetlands. Journal of Vegetation Science, 27(2), 259–268. https://doi.org/10.1111/jvs.12367

How to cite: Shears, O., Möller, I., Spencer, T., Royse, K., and Evans, B.: Erodibility of vertically exposed salt marsh sediments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-510, https://doi.org/10.5194/egusphere-egu2020-510, 2020.

We present the output of a research combining field based, expert knowledge and remote sensing identification of the rates of change, pathways and drivers of these changes, during the past 35 years and more where possible, in four Western Indian Ocean River Deltas: Tana River and Delta (Kenya), Rufiji River and Delta (Tanzania), Limpopo River and Delta (Mozambique) as well as Betsiboka River and Delta (Madagascar). These findings are a set of preliminary results of the collaborative and multidisciplinary effort produced during the WIODER project () that brings together the National Museum of Kenya, Kenweb Kenya, University of Dar Es Salaam in Tanzania, University Eduardo Mondlane in Mozambique, Centre National de Recherches sur l'Environnement in  Madagascar, University of Southampton in UK, IHE Delft in the Netherlands, Institut de Recherches pour le Développement in France, and International Development Research Center in Canada and Kenya.

We highlight the similarities in the physical environment and, to some degree, also in the socio-economic-political environments that are leading the actual changes, affecting resilience of the local population and their sustainable development.

We focused on the substantial changes in the following aspects: precipitation seasonality and intensity, flooding patterns and frequency, land cover, dry forest cover, mangrove cover, crop production, soil erosion, fish population, human population, human migration flow, frequency of human conflicts within the delta population.

The IPCC foreseen changes in climate towards an aridification of the Southern Africa river basins and a wetter condition in the Eastern Africa region. Some signals of these climatic forecast are already recorded in both regions.

Assuming that these trends will continue for the next 10 years or so, we created and here we present two main scenarios of what will happen in these deltas: one with mainly climate change drivers, and another one with climate change and dam drivers.

How to cite: Paron, P., Duvail, S., Hamerlynk, O., Hervé, D., Hutton, C., Juizo, D., Leone, M., Mwansasu, S., Nyingi, W., and Robison, L.: Preliminary identification of drivers and pathways of change in the Socio-Physical dynamics of the Western Indian Ocean Deltas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13797, https://doi.org/10.5194/egusphere-egu2020-13797, 2020.

Human modification of natural systems has typically enhanced provisioning ecosystem services (ES), such as agriculture, at the expense of biodiversity and other types of services. Long-term sustainability requires a balance of ES flows to both maintain human wellbeing, while preserving biodiversity. In river deltas this balance is critical – fertile flat land, well stocked fisheries, and water for use and navigation have promoted rapid population growth and development. Yet the same development degrades the key ES protecting these deltas from hazards and pollution. Many deltas face a critical juncture, at risk or already ‘locked-in’ to the need for engineered solutions to global change problems, unsustainable globally and in the long-term.

We created a global dataset of 237 deltas and collected indicators of the extent each was modified from its natural state, and its ES supply. Several types of modification to the most important aspects of delta systems were considered – overall human impact (human footprint), pressure and demand on ES (population density), modification of water systems (flow disruption) and modification of natural productivity (human appropriation of net primary productivity); grouping deltas by modification state. The impacts of this human modification on over 50 robust biodiversity and ES indicators were then analysed.

Firstly, we attempted to create bundles of commonly associated ES in delta areas. Hierarchical clustering highlighted several logical clusters related to crops, fisheries, water, species richness, biodiversity intactness and NPP. Secondly, we examined synergies and trade-offs between different ES. Provisioning services all showed clear correlations with one another, but clear trade-offs with supporting services or biodiversity. There were weaker synergies within and between regulating and supporting ES. Finally, the relationship between each type of modification and the ES was classified using six typologies fitted by applying a decision tree to their LOESS regression curves. Crop indicators typically had an inverted-U relationship, increasing in moderately modified deltas, but decreasing in the most modified, presumably pushed out by other land uses. While many other ES declined with modification, interestingly, species richness and intactness both began to increase again in the most modified deltas. In summary, this global analysis is the first to illustrate how ES vary along a gradient of development in deltas, and highlights the need to balance further modification against these critical services.

How to cite: Reader, M. O., Santos, M. J., Damm, A., Petchey, O., and de Boer, H.: Impacts of Global River Delta Modification on Ecosystem Services, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9009, https://doi.org/10.5194/egusphere-egu2020-9009, 2020.

There is growing recognition that new approaches, underpinned by more system-oriented decision support tools, will be required to facilitate development compatible with the Sustainable Development Goals (SDGs) and to prevent the risk of dangerous socio-environmental breakdown. We demonstrate the potential of Integrated Assessment Models (IAMs) to inform strategic policy decision making at a regional level, helping to understand key trade-offs as well as indirect or unintended impacts. The stakeholder co-produced Delta Dynamic Emulator Model (ΔDIEM) model is applied to the southwest coastal zone (pop. 14m) where high rates of extreme poverty prevail. The model integrates biophysical drivers, ecosystem services and community level household wellbeing, and in this work is applied an behalf of the Planning Commission of the Government of Bangladesh in order to assess strategic risk in coastal Bangladesh (2050) and particularly to support the Bangladesh Delta Plan 2100. The intervention we investigated included i) A proposed extensive polder network in the south-central region of coastal Bangladesh ii) Strategic development of a chronically waterlogged area of the delta. In both areas we highlight insights on implications of biophysical drivers on poverty, livelihoods and inequality as well as on risk transfer between regions and populations associated with implementation. In doing so we critically assess IAMs’ growing potential to ask and explore key questions and scenarios about the functioning of integrated biophysical and socioeconomic systems. Finally, we point to ongoing applications of the model in West Bengal

How to cite: Hutton, C., Nicholls, R., Chapman, A., Marcinko, C., Rahman, M., Haque, A., Harfoot, A., Hazra, S., and Salehin, M.: Application of an Integrated Assessment Model (IAM) to for strategic scale delta assessment of socio-ecological risk: Supporting Policy in Coastal Bangladesh, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12548, https://doi.org/10.5194/egusphere-egu2020-12548, 2020.

In the present context of climate change and rising sea level, sandy coasts are particularly vulnerable environments. Many studies around the world deal with the problem of coastal erosion of sandy beaches, but few still consider their evolution at the scale of the entire sediment coastal wedge (e.g. Certain et al., Mar. Petrol. Geol., 2005; Denny et al., Cont. Shelf. Res., 2013; Schwab et al., J. Coastal Res., 2013), i.e. taking into account the volume and dynamics of sand accumulated in the shoreface, which can potentially supply the foreshore. We are developing this global approach at the regional scale of the Normandy coastline in order to better understand the sandy beach behaviour since the 19th century and evaluate their stability.

The Normandy coasts show a high diversity in relation with the variability in the geological substratum, tidal range and currents, wave energy and incidence, and sediment nature. Our study aims at providing knowledge on the global behaviour of this composite coastal system at the scale of the whole coastal sedimentary wedge. The study is based on the quantification of sediment volumes accumulated in the subtidal (shoreface) domain. The mobility of the stocks as well as the evolution of the adjacent coastlines are also estimated over the last centuries. Longshore and cross-shore sediment transfers are studied. The purpose is to define the relationships between subtidal and intertidal domains and to discuss the synchronism/diachronism of observed evolutions at a regional scale.

A consistent database, processed and integrated under GIS, has been compiled, including old and recent maps, aerial photographs, geophysical data (seismic, sonar, multibeam echosounder, LiDAR) and sediments samples. We acquired new seismic and multibeam data in sectors that had not been already investigated.

The results obtained from the comparison of the numerous historical cartographic documents since the 18th century, allow illustrating the movements of the coasts including their progressive management. Due to the low accuracy of old charts, only high amplitude changes are identified. Seismic data enable to characterize the spatial distribution of the sediment cover thickness over the geological substratum. In the study area, the sedimentary cover is related to the last post-glacial transgression (Holocene) and may comprise several depositional units. The most basal unit corresponds to the infill of paleo-valleys. It is overlain by one to two units forming the sediment reliefs, such as banks or dunes fields or sand sheets, and representing the sediment stocks we quantify. The mobility and dynamics of the stocks are monitored over the two last centuries from historical bathymetric data. Significant differences in volumes are evidenced locally. Bedform morphology and size enable specifying sediment transport direction and intensities.

The first results show direct relationships between the behaviour of subtidal and intertidal domains, allowing to better understand the distribution of stability, erosion or accretion areas.

How to cite: Grenard-Grand, E., Tessier, B., Le Bot, S., and Ponsolle, J.: Secular evolution of sandy coasts of Normandy (NW France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3494, https://doi.org/10.5194/egusphere-egu2020-3494, 2020.

Coastal erosion and flooding are an increasing issue in Great Britain and pose a significant threat to people living and working in coastal environments, as well as the associated threats to infrastructure and assets. Recent storms, including Storm Callum in 2018, Storm Frank in 2014 and the east coast tidal surge in 2013, have highlighted these issues and caused widespread flooding, power outages and travel disruption. Repairs to homes, buildings, infrastructure and coastal defences cost tens of millions of pounds and took several months to complete with disruption to life, livelihoods and the national economy continuing long after the events.  

The geomorphological variability of Great Britain’s ca. 11,000 mile long coastline, from steep, hard cliffs to weak, easily erodible cliffs and wide flat estuaries, is challenging to represent and therefore consider in a modelling environment. Consequently, the variability, particularly in cliff geology, lithology and rock properties, is often under-represented in coastal modelling and coastal management planning. This results in potentially critical factors such as cliff complexity (e.g. multiple lithologies, jointing and bedding structures, permeability), cliff morphology, and the coastal buffer, being overlooked, all of which can influence the way coastal landforms respond to changing climatic drivers. Finding an accessible, objective and multi-scaled way of communicating this variability to a wide range of coastal practitioners is important in helping to address coastal vulnerability and increase resilience regionally and nationally.

Using a novel partitional clustering approach, we have developed a new classification system for the coastline of Great Britain, which divides the coastline into specific domains based on a range of physical variables. This method combines data available from the existing BGS Coastal Vulnerability Dataset which includes geology type, cliff strength, foreshore environment and inundation potential. In addition, open source datasets, including wave power and height, tide height and tidal current speed, have been incorporated. The datasets have been attributed to ca. 4 million transects at 5 m intervals along the coastline. Effective multivariate clustering data driven techniques, with expert assessment, have been used to cluster the dataset in an iterative way. This approach enables the capture of the thoughts and processes that we as geomorphologists consider when comparing one coastal area with another and will provide a tool for communicating variability in the coast and its resilience to erosion and flooding.

This is the first time such a method has been applied nationally in Great Britain and will provide a potential new benchmark for describing the GB coastline and the changes that it may be subject to. The resulting coastal domains dataset will soon be made available to practitioners in the UK and will assist in making more informed decisions when considering coastal management.

How to cite: Lee, K., Vernon, R., Williams, C., Payo Garcia, A., and Lee, J.: Coastal domain analysis for geo-coastal assessment in Great Britain, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5001, https://doi.org/10.5194/egusphere-egu2020-5001, 2020.

Global development for at least the next decade is to be guided by the internationally-agreed Sustainable Development Goals (SDGs) of the United Nations, which aim to improve societal well-being while limiting the impact of development on the environment and climate. Research on the SDGs is booming and awareness is increasing of how socio-economic context will affect local implementation and global achievement of the goals. Yet, the implications of context for the SDGs still receive only a small fraction of SDG research attention, and research explicitly on feedbacks affecting the SDGs in specific environmental contexts is rare.

Using 49 coastal river deltas as a case study, we show that the environmental context of places will affect implementation and achievement of the SDGs. We quantitatively compare pathways to the SDGs between deltas and non-delta areas using three plausible future development scenarios until 2100. The scenarios represent three Shared Socio-economic Pathways (SSPs): SSP1--sustainable development with low challenges for climate change mitigation and adaptation; SSP2--a 'middle of the road' scenario with intermediate challenges; and SSP3--a future with high mitigation and adaptation challenges due to rapid population growth, slow technological change, high inequalities, and weak institutions. We use the Integrated Assessment Model IMAGE to project global gridded outputs related to key SDGs in deltas for each scenario.

Our analysis reiterates the importance of deltas for achieving the SDGs. Population densities in deltas globally could rise as much as ten times that of non-delta areas under the worst case scenario (SSP3). This would place immense pressure on implementing and achieving most SDGs in these places. Similarly, urbanisation of deltas is expected to increase more rapidly than non-deltas, creating challenges for sustainable cities (SDG 11). Many deltas are also saturated with cropland, the demand for which is expected to continue under all scenarios, with implications for achieving zero hunger (SDG 2) in these places and globally, as well as for delta biodiversity (SDG 15). Moreover, urban expansion and intensified agriculture may result in major groundwater extraction, which has dramatic effects on delta subsidence and therefore sustainability in the longer term.

We describe how environmental processes and feedbacks pose serious risk to achieving sustainability goals in deltas, in particular due to their potentially profound delta impacts beyond the SDG time horizon of 2030. These environmental processes are often not captured in socio-economic or integrated assessment modelling, nor in much SDG research to date, which potentially limits large-scale SDG research, planning, and assessment. We argue that the importance of environmental context for achieving the SDGs extends beyond deltas into other environments (e.g., mountains, semi-arid regions). We conclude that greater attention to the biophysical and geomorphological setting of places should be paid in research, planning, and governance for the SDGs.

How to cite: Scown, M., Dunn, F., Dekker, S., van Vuuren, D., and Middelkoop, H.: Environmental context will affect achieving long-term Sustainable Development Goals: The case of coastal deltas, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5534, https://doi.org/10.5194/egusphere-egu2020-5534, 2020.

Delta deposits show large spatial heterogeneity in terms of depositional rate and age, which is critical to the study of delta erosion in response to the declining fluvial sediment load observed at many river mouths in the world. In this study, we demonstrate the magnetic susceptibility (χ) as a rough indicator to reveal age variations and stratigraphic heterogeneity in the Yangtze River subaqueous delta. Ages of three short sediment cores (<2 m) collected at 20-32 m water depth from the Yangtze River subaqueous delta were determined using ²¹⁰Pb, ¹³⁷Cs, and optically stimulated luminescence (OSL) dating. In addition, depth variation of χ, which is influenced by post-depositional diagenesis and hence age, was used to roughly estimate sediment ages among the cores in a quick way. The profiles of ²¹⁰Pb, ¹³⁷Cs, and OSL results indicate the spatial variability of ages, ranging from the last 100 years to more than 2000 years. Cores at shallow water depths are younger than those from deeper sites. Modern deposits (i.e., <100 years old) occur primarily at water depths shallower than ca. 30 m, which can be explained by the trapping depth of bottom plumes. The Core in the northern part of the subaqueous delta shows much older ages than the core at the southern site with similar water depth, which is caused by their distance relative to the mouth of active sediment discharge distributary. Profile of χ confirms such spatial variation of ages in terms of depth distribution pattern and χ value. Older sediments show lower and uniform χ values due to the reductive dissolution of ferrimagnetic minerals, while younger sediments show higher χ values in the top layer but they decline with increasing depth. Considering the quick way of magnetic measurement, stratigraphic correlation based on χ can be used first to screen for cores before they are subjected to more detailed dating. This study shows that the methodological approach of combining sediment dating with magnetic measurement has great potential in revealing heterogeneous deltaic deposits, which could be easily neglected in morphodynamical and biogeochemical study.

How to cite: Cheng, Q., Wang, F., Chen, J., Ge, C., Chen, Y., Zhao, X., Nian, X., Zhang, W., Liu, K., Xu, Y., and Lam, N.: Stratigraphic heterogeneity in the Yangtze River subaqueous delta revealed by chronological and mineral magnetic approaches, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6263, https://doi.org/10.5194/egusphere-egu2020-6263, 2020.

The Lena Delta is the largest arctic delta in the world (about 29000 km²). Unlike other deltas, its formation was the result of both erosion and accumulation during Late Pleistocene and Holocene. It was caused by combination of continuous sea-level fluctuations and neotectonic movements. The last ones have different speed and direction. From previous studies it is known that western part of delta has uprising tectonic movements while the eastern one is sinking. This asymmetry develops along the fracture extended submeridially across the delta. The aim of this research is to measure the amplitude and speed of these movements by using geomorphologic methods. For that purpose results of German-Russian expedition “Lena” were used. In 2013, 2014, 2015 surface morphology of the biggest delta’s islands Sobo-Sise, Kurungnakh, Jangylakh-Sis and Khardang-Sise located in both eastern and western parts was investigated with high-quality sattelite instruments. These islands consist of the Late Pleistocene Ice Complex (IC) remnants with altitude 20-66 m above sea-level (a.s.l.), eroded by river and sea, and the first accumulative terrace of the delta with 2-15 m a.s.l. IC remnants accumulated in the Late Pleistocene 50-17 ka cal BP. The first terrace was forming in Holocene from 8 ka cal BP to 2 ka cal BP. So, there were made a number of geomorphologic profiles with use of high-quality satellite instruments across river terrace and IC remnants during the expeditions. In this study, we equated them to one level and compared. With use of radiocarbon age and digital elevation models (DEM) data we compared heights and age of islands in eastern and western parts and estimated neotectonic movements’ speed difference. Since 2000 years BP tectonic asymmetry represented in terrace surfaces has been increasing with rate about 2 mm per year. Before 2000 cal BP speed difference approximately values 1 mm per year. Our data correlates with water-flow measurements in the delta, modern water-level observations in Laptev Sea and geophysical investigations.

How to cite: Aksenov, A., Bolshiyanov, D., Makarov, A., Pravkin, S., Lazareva, E., Cherezova, A., and Grigoriev, M.: Late Pleistocene-Holocene tectonic asymmetry of the Lena Delta, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2749, https://doi.org/10.5194/egusphere-egu2020-2749, 2020.

Coastal landforms such as barriers are crucial in protecting coastlines and reducing the rate of erosion and retreat. Sea-level rise threatens to change the baseline in which such landforms exist, therefore changing sediment fluxes and hydrodynamics at coastlines. Understanding the stability of landforms under changing conditions is crucial to protect and mitigate against the influence of future sea-level rise on coastal infrastructure, ecology and populations. By studying past periods of sea-level rise with rates similar to those projected for the future, we can begin to understand how coastlines may evolve over the next few centuries.

Dogger Bank, in the southern North Sea, experienced marine transgression during the Early Holocene. Over a period of 800 years, sea level rose by 7-8 m. This rate of ~10 mm/yr is similar to that projected within the next century. Our study area is located on the southeastern side of the former Dogger Bank island. Between 9.5 and 8.7 ka BP, two phases of coastal barriers were present, retreating with different mechanisms at different time periods due to antecedent topographic changes and evolving hydrodynamics. Barrier phase A was drowned in place due to a low-angle topography and little reworking of the barrier. Barrier phase B retreated by continuous overstepping, which occurred due to a higher-angle topography and an increase in wave energy. Complete inundation of the study area occurred by 8.7 ka, with the barrier phase B first becoming an isolated barrier, then breaking down completely. The subsequent wave ravinement transitioned the landform from barrier to offshore sand bar. At this time, the rate of sea-level rise had increased to as much as 20 mm/yr during the pre-8.2 ka sea-level jump, causing the final barrier breakdown and inundation of Dogger Bank. The coastal morphology in the study area is now buried beneath up to 20 m of shallow marine sand, deposited as the dominant tidal current transported sediment from west to east.

The unique landform preservation at Dogger Bank allows unprecedented spatial and temporal resolution into the investigation of coastal response to sea-level rise. This study adds evidence to the growing body of work that sea-level rise is the driver of, but not necessarily the controlling factor in, barrier retreat mechanism. Furthermore, a rarely-preserved landform, the isolated barrier, is presented. The results of the study provide valuable insights into the transition from coastal to fully marine during transgression of low-relief coastal areas, which provides an analogue for future sea-level rise scenarios.

How to cite: Emery, A., Hodgson, D., Barlow, N., and Cotterill, C.: Sedimentary and geomorphic responses to changes in rate of sea-level rise: Holocene marine transgression of Dogger Bank, North Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16524, https://doi.org/10.5194/egusphere-egu2020-16524, 2020.

The continental shelves submerged during the last marine transgression could constitute a unique laboratory to analyse how coastal landforms developed and evolved within the framework of a rising sea level. Such features therefore represent precious witnesses in the light of the high rates of sea-level rise predicted for the end of the century. Unfortunately, the majority of the coastal landforms have been wiped away during and soon after their submersion as a consequence of the pervasive wave and tidal action. Therefore, only few examples of well-preserved submerged coastal landforms are available.

In this study we focused our attention on the Italian side of northern Adriatic Sea, where a wide, low-gradient continental shelf, coupled to a very rapid marine ingression, allowed the partial conservation of the transgressive coastal landforms. Such study was carried out through the analysis of almost 10,000 km of high-resolution geophysical surveys (CHIRP-sonar profiles) and tens of stratigraphic cores carried out in the area during the last 30 years.

We recognized a series of almost 100 remnants of paleo tidal inlets which formed during the post-LGM transgression that led to the submersion of the Adriatic shelf. Despite paleo tidal inlets are often almost completely erased by the wave ravinement processes, when preserved they represent ideal markers for reconstructing the timing and impact of sea-level rise on the transgressed coastal plain. A wealth of information can be obtained by their analysis, such as the paleo coastlines locations, the dimensions of the paleo lagoon systems and, in particular conditions, the relative paleo sea-level. Such features therefore represent valid means to reconstruct the impact of the transgressive sea on the coastal area.

In particular, the paleo tidal inlets recognized in the northern Adriatic Sea suggest the recurrent formation followed by rapid overstepping of large lagoon systems during the early Holocene. Moreover, these features can be subdivided into clusters based on the depth of their top, thus allowing to infer the position of a series of paleo coastlines and suggesting the occurrence of periods of stasis of the relative sea-level rise, which allowed the formation of such inlets.

Although remnants of paleo tidal inlets are common on the northern Adriatic Shelf, they are almost absent in the northernmost portion of the basin (i.e. the Gulf of Trieste), where a series of paleo fluvial systems have been identified, thus providing a direct witness on the evolution of the coastal plain during a transgressive phase and right before its rapid submersion.

This research provides new insights on two main topics: i) it improves our knowledge on the post-LGM marine transgression, therefore contributing to reconstruct the history of sea-level rise and to constrain the modelling of future behaviour; ii) it contributes to understand the evolution of tidal inlets and lagoon-barrier island systems under the forcing of high rates of sea-level rise.

How to cite: Ronchi, L., Fontana, A., and Correggiari, A.: Coastal landforms evolution during the Holocene marine transgression: a witness from the past to understand the future, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7625, https://doi.org/10.5194/egusphere-egu2020-7625, 2020.

The Seine-Maritime coastline (France) is a macro-tidal environment (8 m tidal range), developing along an epicontinental sea, the English Channel. The SW-NE coast is opened to westerly atmospheric flows, generating occasionally wind sea with energetic waves (Hs: 4.65 m decennial return). High chalk cliffs and a wide marine erosion platform partially hidden on its upper part by a flint pebble beach, characterise this 130 km long coast.

Observations since the end of the 1990’s show a recent and massive sanding up of the marine erosion platform. This raises the question of the origin of the sandy fraction and the sedimentary dynamics on the intertidal area.

We present herein an innovative method that combine grain-size and geochemical analysis in order to highlight sand sources and transport direction along these rocky coast.

Sixteen beaches were sampled during low tide and fair-weather conditions. At each site, 3 samples were collected along the cross-shore beach profile (from the pebbly upper beach to the low tide limit).

Grain-size results show that for all sites, medium to coarse-grained sands dominate in the upper beach (mode 315-400µm) while fine sands dominate in the middle and low foreshore (mode 160-250µm). A decrease in grain-size is thus evidenced from the upper beach to the low foreshore.

The geographical variability of the sand composition and consequently sources was determined on the basis of geochemical data. In order to avoid the granulometric effect on the data, X-Ray fluorescence analysis (xSORT, SPECTRO AMETEK) were performed on the two major grain-size modes of each sample. Eighteen calibrated chemical elements (Si, S, K, Ca, Ti, V, Mn, Fe, Ni, Ga, As, Br, Rb, Sr, Y, Pb, Th and U) were measured at each station. Statistical processing performed step by step on the data allows to gradually reduce the number of significant geochemical parameters. Finally, 4 major elements (Si, Ca, Sr, K) as well as the ratio Sr/Ca have been considered as the best proxies of sample discrimination and potential source.

The first results indicate a longshore gradient of Si and Ca, especially for the finest sands (160-200µm). From SW to NE, i.e. in the direction of the littoral drift, and whatever the position across the beach profile, there are an enrichment in Si (sands are more siliciclastic) and an impoverishment in Ca.

This gradient highlights differentiated longshore sediment transport and sorting, in relation probably with sediment sources (siliclastic sands vs bioclastics sands).

How to cite: Peuziat, B., Costa, S., Tessier, B., Murat, A., and Gregoire, G.: The intertidal sanding up of the Seine-Maritime coast (Normandy, France): Sedimentological and geochemical approaches., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9751, https://doi.org/10.5194/egusphere-egu2020-9751, 2020.

Sea-level rise will lead to substantial changes to coastal geomorphology over the coming century and it is imperative to understand the implications. This includes the underlying stratigraphic influences on seabed morphology and the historical context with which they have formed. On the densely populated coastline of Eastern Australia, coastal erosion is a significant concern for residents and stakeholders. In South East Queensland, and particularly the coastal zone surrounding Bribie Island spit in Northern Moreton Bay, the accelerated erosion of the spit and discovery of indurated sand horizons in nearshore regions both above and below the seabed create a convergence of the past influencing the present.

Indurated sand horizons are predominantly considered to be the relict B horizon of the pedogenic processes that formed a podosol soil profile. Whilst not ubiquitous under present sea level, their presence presents a unique opportunity to study an accessible palaeosol unaltered by further pedogenesis and carbon input (as opposed to terrestrial indurated sand formations). This allows for an analysis of a time in Northern Moreton Bay during lower sea levels and how these horizons affect present day morphology. Data acquisition consisted of high and low frequency acoustics, coupled with core samples for geological analysis.

Our results show the indurated sands buried under 1-2 m of marine sands sloping downwards to the east. This suggests the present-day seabed follows the contours of the sub-surface indurated sand. High-resolution bathymetry of exposed indurated sand outcrops near Bribie Island spit indicate a dune-like shape suggesting a formation from coastal sand dunes into active terrestrial soil during lower sea levels. The dune troughs having accumulated greater mineral and organic material than the peaks, which can be attributed to the former surviving inundation from rising sea levels and the latter having undergone a weaker pedogenesis and subsequently erosion. Exposed indurated sand outcrops with a vertical face or ‘scour step’ are elevated to the surrounding marine sand seabed. Similar elevated structures were found to be a barrier to onshore sediment transport from offshore deposits and limiting beach replenishment whilst also offering protection from dampening long period waves and large storm swells. Core samples taken through the indurated layer from behind the spit to the shipping channel offshore showed elevated levels of aluminium and iron compared to surrounding marine sands, and consistent with podosol soil formation.

The techniques used here suggest that historical terrestrial geomorphology has determined the shape, mineralogy and strength of indurated sand layers. As these indurated sand layers were submerged and further modified by present day sea level, they may play an important role in coastal geomorphology and protection as sea levels rise further in the coming century.

How to cite: Heatherington, C., Albert, S., Cossu, R., Kemp, J., and Grinham, A.: Indurated sand horizon influences present day coastal geomorphology of nearshore Northern Moreton Bay, South-East Queensland, Australia , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12122, https://doi.org/10.5194/egusphere-egu2020-12122, 2020.

Beachrocks are coastal sediments that are lithified through the precipitation of carbonate cements. It is widely acknowledged that lithofacies in beachrocks are variable and their interpretation is useful when using beachrock as a sea level indicator or when studying shoreline evolution over the centurial to millennial scales. Surprisingly however, the facies variability of beachrocks remains understudied as they are almost exclusively described as seaward dipping, slab-shaped outcrop forming in low energy dissipative beach environments. The Mission Rocks coastline of north-eastern South Africa is in stark contrast. Here the coast comprises an up to 3 m thick raised shore platform of beachrock, where a variety of sedimentological facies are observed. These comprise seaward-dipping planar bedded sandstones and conglomeratic units, often interbedded with bimodally-orientated trough cross bedded sandstones. In our study we aim to use sedimentological facies analysis, petrography and cathodoluminescence to unravel the deposition- and cementation processes of this beachrock facies.

In particular, an unusual beachrock breccia interposed amongst the breakdown remnants of the platform forms the basis of this paper. The breccia documents a cycle of simultaneous erosional breakdown and depositional buildup of the beachrock platform, a yet undescribed process for the development of beachrock.  Since it forms as a thin veneer (< 0.10 m), with a slightly thicker infill (≤ 0.5 m) amidst erosional hollows and gullies of the + 2 m high rocky platform, it raises into question the necessity of a thick sedimentary overburden, that is typically considered the requirement for beachrock cementation in the mixing zone.  Timing of beachrock formation is constrained by recent anthropogenic activities, as the underlaying platform was mined for building purposes during WWII and it is in these quarry slots and crack that the beachrock is found. While it is generally suspected that beachrocks may form at the centennial scale, evidence for this remains weak. Not only can the interpretation of this facies contribute to our understanding of the long term processes that form and break down beachrocks on high energetic coastlines, it provides insight into rapid beachrock formation and as such its utility as a sea level index point.

How to cite: Falkenroth, M., Green, A. N., Cooper, J. A. G., and Hoffmann, G.: The phoenix of beachrocks: Simultaneous breakdown and formation of an unusual facies on a high energy coastline (Mission Rocks Beach, South Africa), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21990, https://doi.org/10.5194/egusphere-egu2020-21990, 2020.

The Tagus River ebb-delta is located near an important city center off Lisbon, Portugal. The Tagus delta hosts various kilometer scale landslides, the most important of which has been mapped and described with a presumable age of ~11 ky and 10 km in length, 4 km wide and 20 m of maximum thickness. An equivalent area of gas trapped in the sediments has also been reported (Terrinha et al., 2019).

The TAGUSGAS project aims at characterizing the nature and source of the gas. A multibeam and backscatter survey was carried out recently covering an area of 44 km². Several morphologic artifacts were found. The magnetic survey carried out simultaneously allows at discriminating the anthropogenic origin of some of these artifacts. It also allows at distinguishing gas and igneous rock sources of acoustic blanking in the seismic reflection record.

The multibeam and backscatter basemap also serves as a tool to decide targets for seafloor sites for sample collection.

The authors would like to acknowledge the FCT financial support through project UIDB/50019/2020 – IDL and TAGUSGAS project (PTDC/CTA-GEO/31885/2017).

How to cite: Ribeiro, C., Terrinha, P., Rosa, M., Neres, M., Noiva, J., Brito, P., and Magalhães, V.: Nature and origin of gas trapped in sediments in the Tagus River ebb-delta, off Lisbon, Portugal, the TAGUSGAS project, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20761, https://doi.org/10.5194/egusphere-egu2020-20761, 2020.

The morphological and hydrological equilibrium of many deltas worldwide is changing due to anthropogenic activities. A key example of such a delta is the Rhine-Meuse Delta (RMD) in the Netherlands. It is home to an important shipping and economic centre (Rotterdam) and thus has been strongly affected by anthropogenic activities. Changes include embanking, narrowing and deepening of channels, major dredging and sediment relocation, the building of ports and harbours, and dam building upstream. There is currently a net annual loss of sediment from the delta. Considering current and future sea level rise it is crucial that the RMD receives sufficient sediment or it risks drowning, increased flood risk, decreased ecological area and channel bed degradation.

Here, we estimate the future delivery of suspended sediment from upstream using BQART, and the volume and sediment flux from the sea using a 1D morphological model. We ignore bedload fluxes as they make up a small proportion of the annual supply. We use these estimates to investigate sediment redistribution between channels in the RMD based on suspended sediment-discharge relations. Projections for 2050 and 2100 are presented based on region-specific climate scenarios for discharge and sea level and incorporate projected future upstream reservoir construction. The sediment concentration in the branches is compared with discharge-area relations and current bed level trends to demonstrate potential sedimentation-erosion trends for individual branches.

Projections for the 21^st century indicate that sediment delivery to the RMD from upstream is likely to decrease slightly, while sea level rise will cause tidally driven suspended sediment delivery to move further inland. It is estimated that the already negative sediment budget of the delta will be exacerbated by dredging, which removes all incoming sediment at the coastal boundary. The severity of sediment starvation depends on the climate change scenario. Our work indicates that certain channels will be at risk of erosion due to this sediment starvation, whilst other branches will experience net sedimentation. Sediment input from the coast could also reach further inland, assuming current dredging practice remain unaltered, which could provide an opportunity for the system to regain equilibrium. We recommend that a sustainable sediment management strategy is undertaken in the region to counteract the negative effects of sediment starvation.  

How to cite: Cox, J., Dunn, F., and Nienhuis, J.: The effect of climate change on sediment distribution and delivery within the Rhine-Meuse Delta, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5710, https://doi.org/10.5194/egusphere-egu2020-5710, 2020.

The Vietnamese Mekong Delta (VMD) is home of 18 million people, provides enough food to cover 50% of the country’s nutritional needs and underpins the welfare of the rapidly growing population of the wider region. The longer-term future sustainability of this great delta, formed over millennia, is uncertain. The region is threatened by climate change induced eustatic sea-level rise (SLR), and by severe land loss. The latter is the result of a number of factors that are, in their majority, driven by human activities. They include dam impoundment that reduces the amount of sediment reaching and slowly building up the delta, sand mining which rapidly depletes the delta from its slowly accumulated sediment reserves and ground water extraction which enhances sediment compaction and accelerates delta subsidence.

In May 2018 we undertook a delta-scale survey to map the bathymetry of all of the main distributary channels of the VMD. Comparisons of these survey data with existing datasets from 1998 and 2018 reveal major increases of channel depth. They show that between 1998 and 2008 the VMD lost in excess of 370 million cubic meters of sediment, while the respective value for the period between 2008 and 2018 is 635 million cubic meters, suggesting an accelerating trend of sediment loss from the system.

We assume a ‘business as usual’ scenario for delta management practices and propagate delta degradation into the future, generating delta analogues for years 2028 and 2038. We combine these delta analogues with projections of SLR for the region for up to year 2098 and a number of boundary condition scenarios into a delta-scale hydraulic model. The fluvial-tidal interactions resolved in our numerical modelling simulations reveal that channel deepening is the key driver of tidal ingress into the delta plain for the next few decades. For the longer-term future (2098), the combined effects of predicted SLR and channel incision can lead to an increase of tidal ingress by 20%. This may destabilise delta bifurcations, is likely to increase bank erosion and flood risk into the future and can have sever implications for saline intrusion into the delta plains.

How to cite: Vasilopoulos, G., Le Quan, Q., R. Parsons, D., E. Darby, S., N. Hung, N., P. D. Tri, V., Haigh, I., Voepel, H., Aalto, R., and Nicholas, A.: Sediment starvation is the primary factor of tidal ingress in the Mekong delta, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15155, https://doi.org/10.5194/egusphere-egu2020-15155, 2020.

Deltas are fascinating landforms subject to riverine (input of water and sediments) and marine processes (waves, tides) where bifurcations are the building block controlling the distribution of water, nutrient and sediment fluxes among the distributary channels of the network. In this work we focus on the role of tides as a key factor in controlling bifurcation behaviour. Recently it has been suggested and observed that tidal deltas (i.e. delta influenced or totally dominated by the tides) have the tendency to less numerous but more stable branches in comparison to fluvial-dominated deltas [Hoitink et al., 2017]. River bifurcations subject to unidirectional flow have been widely studied in the last decades. However, in the case of tidal bifurcations, the acting physical mechanisms and controlling factors are still not well understood, and a theoretical framework is still lacking. In order to fill this gap and understand how the stability and evolution of a delta could be affected by the tides, we investigate through an analytical model, the equilibrium configurations and stability conditions of a tidal bifurcation under the hypothesis of small tidal oscillations. In particular, we extend to the tidal case the previous works of Bolla Pittaluga et al. [2015] and Seminara et al. [2012] relative to the equilibrium and stability of a single bifurcation, and to the equilibrium of a single river dominated estuary, respectively. Results show that higher tidal amplitude and a closer position of the junction node to the sea, tends to hamper the development of unstable solutions, reducing the asymmetries in water and sediment fluxes between branches obtained when the upstream width-to-depth ratio falls above a critical value. Field observations of natural deltas seems to corroborate our findings.

How to cite: Ragno, N., Bolla Pittaluga, M., and Tambroni, N.: Morphodynamic equilibrium of tidal bifurcations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2690, https://doi.org/10.5194/egusphere-egu2020-2690, 2020.

Deltas are home to hundreds of millions of people worldwide and form a key part of many coastal environments. Due to their low elevation, many deltas are threatened by sea level rise as well as direct human influences on flow and subsidence. Added to this, the volume of sediment exported by rivers to the coast has been reduced by around 1.4 billion tons per year, starving deltas of the building material needed to construct and maintain their valuable subaerial land in the face of these challenges. The calibre and cohesivity of sediment have both been shown to be important factors in determining the erosion, deposition and stability regimes within a delta system. However, it has not yet been shown how the qualities of river and substrate sediment affect how resilient deltas are to sediment reduction.

This study uses numerical modelling to investigate how the cohesivity of incoming river sediment and the erosion resistance of the delta’s substrate affect how deltas respond to a reduction in supplied sediment. Delft3D was used to create a series of stable deltas with varying fluvial and basal sediments, that where then exposed to sediment reduction. The loss of land area, change in channel geometry and other metrics where extracted from model output using Matlab to assess the effects of this sediment reduction, and how these effects varies between deltas.

How to cite: Johnson, J., Parsons, D., Hackney, C., Edmonds, D., and Best, J.: The effects of fluvial and basal sediment properties on the morphodynamics of deltas undergoing sediment supply reduction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19617, https://doi.org/10.5194/egusphere-egu2020-19617, 2020.

The interaction of marine (tides and waves) and fluvial processes determines the sedimentary fill of coastal systems in the fluvial-to-marine (FTM) transition zone. Despite frequent recognition of tidal and wave influence in modern and ancient systems, our understanding of the relative importance of marine processes and their impact on mud deposition, coastal system stability and sedimentary architecture is limited. This study combined subsurface field observations and numerical simulations to investigate the relative importance of river flow, tides, waves, and mud input in governing the sedimentary fill in funnel-shaped basins along the FTM transition. Model simulations show a self-forming bar-built estuary with dynamic channels and sandy bars flanked by mud flats, which is in agreement with trends observed in nature. From three-dimensional virtual sedimentary successions, statistical tendencies for mud distribution and thickness were derived for the spectrum of marine and fluvial processes, and these values provide quantitative information on the net-to-gross ratio and mud architecture. The relative influence of marine and fluvial processes leads to a predictable facies organization and architecture, with muddier and more heterogeneous sediments toward the flanks. For the first time, our simulations allow the sedimentary fill in basins along the FTM transition to be related explicitly to hydrodynamic conditions, providing new insights into the morphosedimentary evolution of coastal systems, with implications for system stability in the modern and sequence stratigraphy preserved in the ancient.

How to cite: Parsons, D., Van de Lageweg, W., Braat, L., and Kleinhans, M.: Controls on mud distribution and architecture along the fluvial-to-marine transition , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22631, https://doi.org/10.5194/egusphere-egu2020-22631, 2020.