The ability of saltmarshes to accrete sediments and keep pace with sea-level rise is key to their multifaceted role as nature-based solutions to current environmental challenges, including their capacity to accumulate and store ‘blue’ carbon. While saltmarshes can gain elevation through in-situ organic production and trapping of organic and minerogenic sediments, thresholds exist above which rates of sea-level rise outstrip saltmarshes’ vertical accretion capability. Current and future anthropogenically enhanced rates of sea-level rise may therefore pose a significant threat to saltmarsh resilience. A negative accretionary balance (i.e. sea-level rates exceeding sediment accumulation) may result in transgression and potentially erosion, impacting on a range of ecosystem services and threatening stored carbon. Consequently, understanding the relationship between sea-level rise and saltmarsh accretion is critical for projecting future changes to saltmarsh ecosystems. Here, we use age-depth models based on Bayesian analysis of ²¹⁰Pb, ¹³⁷Cs and ²⁴¹Am activities to quantify sediment accumulation rates for 34 cores from 21 saltmarshes distributed around the coastline of England, Scotland, and Wales. These sites were selected to encompass the range of different marsh types found in Great Britain, including large open-coast systems, back barrier, estuarine-fringing, and loch-head marshes. Site average sedimentation rates vary between 0.12 and 1.28 cm yr^-1, with a mean of 0.41 ± 0.16 cm yr^-1. We compare sedimentation rates at 1 cm depth increments with corresponding site- and time-specific rates of sea-level rise, modelled using estimates of barystatic, sterodynamic and inverse barometric contributions that we benchmark against long tide-gauge records. This comparison enables us to determine the accretionary balance and its development since the start of the 20^th century at each core location. We discuss these results in the context of spatially explicit projections of accelerated future sea-level rise around the coast of Great Britain.

How to cite: Garrett, E., Dangendorf, S., Hibbert, F., Smeaton, C., Austin, W. E. N., Barlow, N. L. M., Koot, M. B., Blake, W., and Gehrels, W. R.: The accretionary balance of saltmarshes in Great Britain since 1900, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12648, https://doi.org/10.5194/egusphere-egu24-12648, 2024.

The number of salt marsh restoration schemes is set to increase substantially this decade, driven by three key factors: a) legislative requirements to compensate for losses; b) a transition to a natural approach for coastal management; and c) growing interest in ecosystem services salt marshes provide. Monitoring is key to evaluating the success of restoration projects and can inform future projects. However, many schemes have been found to use ineffective monitoring strategies. A major barrier to developing effective monitoring programmes has been the resources required to carry out frequent, spatially explicit surveys using traditional survey methods, which will become an even greater problem as more schemes are created. Consequently, improvements to survey methods are required.

This project aims to improve our understanding of the role that remote sensing combined with field data can play in assessing the development of salt marsh restoration programmes. To address this aim, monitoring has been carried out at a new restoration site in the Adur estuary, on the south coast of the UK, since its creation in 2019. Field data has been collected, including biannual changes in species cover along transects. These surveys have found positive development at the Adur site, with clear successional changes in vegetation cover. Drone flights have also been carried out at the site to accompany the transect surveys, with machine learning algorithms used to develop models of the observed changes. Preliminary testing has found the machine learning models produce accurate results, demonstrating that remote sensing can be a valuable asset to existing monitoring practices.

How to cite: Agate, J., Ward, R., Joyce, C., and Burnside, N.: Monitoring and Assessment of Salt Marsh Restoration Using Field and Remotely Sensed Data, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6298, https://doi.org/10.5194/egusphere-egu24-6298, 2024.

Soil temperature has a marked effect on ecological processes in salt marshes and is significantly affected by tidal fluctuations. Periodic tidal inundation modifies the heat exchange between marsh sediments and the atmosphere, and also induces additional heat exchange between the sediments and tidal water. Tide-driven porewater flow further modulates heat transfer within the sediment, a process complicated by seasonal variations in atmospheric and tidal temperatures. In addition, macropores such as crab burrows are common in salt marsh sediments, and they are expected to regulate heat exchange and temperature distribution within the salt marshes by altering water movements. This study aims to explore how spring-neap tides and atmospheric conditions collectively influence heat exchange and temperature dynamics in salt marshes. We developed a marsh-creek model incorporating sediment-atmosphere/water heat exchange and validated it against laboratory experiments. Extending the model to the field scale, we analyzed temperature dynamics across the marsh and heat fluxes at the sediment-atmosphere/water interfaces over an annual cycle. We also compared simulation outcomes of water and heat dynamics in scenarios with and without macropores. Finally, we discussed the implications of our findings for accurately assessing tidal and atmospheric heat exchange and the associated temperature variations.

How to cite: Xu, X., Yu, X., and Xin, P.: Tidal influence on heat exchange and temperature dynamics in salt marshes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7257, https://doi.org/10.5194/egusphere-egu24-7257, 2024.

Salt marshes are among the most efficient blue carbon (C) sinks in the world, partly due to the slow decomposition of their plant-derived organic matter (OM) in the soil. The fate of this C sink under sea-level rise is still uncertain due to limited knowledge about the processes controlling OM decomposition under different inundation levels. In an in-situ manipulative experiment, we compared salt marsh OM decomposition and quality across simulated sea-level scenarios and litter types (absorptive root, fine transportive root, leave, and rhizome of the shrubby C3 halophyte Halimione Portulacoide) for 170 days. The OM decomposition rate varied only between the longest and shortest inundation treatments, that was lower than the mean inundation of our site. The OM decomposition and C loss rates varied strongly across litter types. Fine absorptive was the slowest to decay, releasing up to 40% less C than the other litter types. Changes in lignin composition varied across litter types, but were unaffected by sea-level rise scenarios. Our study suggests that 1) the assessment of soil C dynamics in salt marshes based on aboveground litter or bulk belowground litter patterns is inadequate because of a marked difference in OM decomposition across litter types; 2) belowground litter lignin quality could be a good proxy for OM decomposition in salt marshes; and 3) sea-level rise is unlikely to decrease OM decomposition under current sea-level rise projections.

How to cite: Arnaud, M., Bakhos, M., Rumpel, C., Dignac, M.-F., Norby, R. J., Bottinelli, N., Deborde, J., Geairon, P., Kostyrka, P., Gernigon, J., Lemesle, J.-C., and Polsenaere, P.: Salt marsh organic matter quality and decomposition under sea-level rise scenarios: from leaves to fine absorptive roots , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5879, https://doi.org/10.5194/egusphere-egu24-5879, 2024.

Coastal wetlands are important transitional environments, providing several ecosystem services such as carbon sequestration, water filtration and habitat provision for diverse plant and animal species. At the same time, various human activities, such as fishing, aquaculture, tourism and industrial operations, are centred around coastal wetlands. Therefore, the morphological evolution of many coastal transitional systems is not only influenced by natural processes, namely tidal currents and wind waves, but also anthropogenic interventions have been playing a pivotal role in adapting the morphological features to meet the needs of human activities. For instance, extensive areas are usually confined by artificial levees for aquacultural activities, large channels are dredged and inlets are stabilized for navigation purposes. However, the long-term effects of these modifications on the morphological evolution of shallow tidal systems are still unclear and can potentially affect the ecosystem as a whole.

The aim of this work is to investigate the consequences of natural and anthropogenic drivers on two back-barrier lagoons in the northern Adriatic Sea: the Venice Lagoon and Marano-Grado Lagoon. Despite sharing a similar microtidal regime and meteorological conditions, the morphology of these systems exhibits distinct characteristics. The Venice Lagoon is about 550 km² and it is connected to the Adriatic Sea with three inlets. Its evolution has been strongly affected by human interventions, such as the construction of jetties at the inlets at the beginning of the 20^th century, the excavation of navigable channels between 1930 and 1970 and, more recently, the installation of a storm-surge barrier system, named Mo.S.E., to prevent flooding of the city of Venice. Instead, the Marano-Grado Lagoon is smaller, covering an area of 160 km², and it is relatively more pristine than the Venice Lagoon. It is connected to the Adriatic Sea with six inlets, of which only two were provided with jetties in the 20^th century, and it experienced fewer modifications of channels for navigation purposes.

We applied a 2D, finite-element model to describe the hydrodynamic flow field and the wind-wave generation and propagation in these two environments. The computational grids were built upon the most recent bathymetric survey available for each system, that is the 2017 survey for the Venice Lagoon and the 2011 survey for the Marano-Grado Lagoon. We then performed an extensive calibration of the model with water level and discharge data available. Results allowed us to investigate similarities and differences in the hydrodynamics of the two tidal systems, highlighting the effects of specific anthropogenic interventions on the ecosystems.

How to cite: Piazza, A., Tognin, D., Canesso, D., and Carniello, L.: Interplay between natural drivers and human activities in two Northern Adriatic lagoons, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-961, https://doi.org/10.5194/egusphere-egu24-961, 2024.

The sinuous channels that wind through tidal coastal wetlands resemble meandering rivers. However, features indicative of active meandering over time, such as oxbow lakes and meander cutoffs, are challenging to find in tidal realms. Specifically, while alluvial plains shaped by meandering rivers are filled with scars of meander cutoffs, tidal coastal settings have been perceived by geomorphologists for much of the past century as lacking morphological evidence of cutoff events, even though both environments exhibit similar meander-planform dynamics and width-adjusted migration rates. This led to the broad interpretation that tidal and fluvial meanders differ morphodynamically.
We re-examined this conclusion by identifying, measuring, and compiling examples of meander cutoffs from various tidal coastal wetlands and fluvial floodplains worldwide. We suggest that cutoffs in tidal meanders are far more widespread than previously thought, and the shapes and geometric properties of tidal and river cutoffs are indeed remarkably similar. This indicates that while tidal and fluvial environments differ in many ways, they nevertheless share the same physical mechanism affecting meander morphodynamical evolution.
The perceived scarcity of tidal cutoffs is likely a result of pronounced channel density and hydrological connectivity in coastal wetlands, coupled with the reduced size of most tidal channels and dense vegetation cover. Moreover, despite allegedly similar forming mechanisms, morphodynamic differences arise after meanders have cut off. We observe that tidal meanders remain preferentially connected to the channel from which they originated, preventing the formation of crescent-shaped oxbow lakes and thus making tidal cutoffs more difficult to detect.
While these factors do not erase tidal meander cutoffs, they collectively inhibit oxbow-lake formation and render tidal cutoffs ephemeral, hardly detectable geomorphic features. We thus argue that similar morphodynamic processes drive cutoff formation in tidal and fluvial landscapes, with differences arising only during post-cutoff evolution. This bears important implications for understanding the ecomorphodynamics of coastal wetlands and predicting their long-term evolution.

How to cite: Finotello, A., Gao, C., Lazarus, E. D., D'Alpaos, A., Ghinassi, M., Ielpi, A., Rinaldo, A., Parker, G., Gao, P., Wang, Y. P., and Tognin, D.: Stream Meandering in Coastal Wetlands: Patterns, Processes, and Ecomorphodynamics Implications, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16633, https://doi.org/10.5194/egusphere-egu24-16633, 2024.

Biogeomorphic interactions between tidal channels and marsh plants play a crucial role in enhancing coastal resilience to climate change. Previous studies linking the channel formation with vegetation dynamics predominantly focused on the early initiation, characterized by local-scale plant-flow feedbacks. However, the influence of rapid changes in landscape-scale vegetation pattern on the channel initiation remains poorly understood, especially in micro-tidal system. In this study, we investigated this relationship through biogeomorphic modeling combined with the analysis of satellite images in a rapidly expanding marsh in China under Spartina alterniflora invasion. The satellite images demonstrated the increase in drainage density and the decrease in unchanneled path length following plant encroachment. Our modeling results showed that local flow acceleration between vegetation patches was insufficient to initiate channels rapidly before the merging of isolated patches under micro-tidal conditions. With plant expansion, the continuous marsh caused landscape flow diversion from homogenous platform flow to concentrated channel flow, which promoted evident tributary channel initiation in the landward marsh zone. The vegetation removal scenarios further highlighted that the flow divergence from adjacent platforms due to the spatial heterogeneity in plant configuration amplified the magnitude of local hydrodynamics and further channel incision. Our findings emphasize that the initiation of tidal channels not only depends on local plant-flow interaction but is largely driven by landscape vegetation configuration under micro-tidal conditions.

How to cite: Wang, D., Bai, J., Gu, C., Gourgue, O., Belliard, J.-P., Cui, L., Ke, Y., Xue, L., Wen, L., and Temmerman, S.: How does landscape vegetation configuration regulate local channel initiation in rapidly expanding marsh?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17274, https://doi.org/10.5194/egusphere-egu24-17274, 2024.

Flocculated particles, formed by the aggregation of clay particles, are common in rivers. These flocs exhibit a different behaviour than primary particles: they can deform and break apart, and they have greater settling velocities than the particles of which they are composed. The latter allows flocs, unlike primary clay particles, to deposit on the river bed in mildly turbulent conditions, potentially leading to interactions with the bed. Particularly in sand-bedded rivers, where bedforms shape the riverbed, there exists a potential interaction between flocs and the riverbed.

Physical experiments were carried out in an annular flume, using a flocculant to induce flocculation. Different amounts of flocculant and various shear stress conditions were applied, and the resulting floc characteristics and bedform geometry were measured.

Under lower shear conditions, the flocs were larger and transport rates were lower than under high shear conditions. However, under both shear conditions, flocs were transported via saltation and in suspension, and they became integrated within the sediment bed either as individual flocs, clusters, or sheets. Deposition occurred predominantly on the leeward side of the dune, revealing distinct stratigraphy patterns. The presence of flocs had a negligible impact on the actual geometry of the bedforms.

This investigation highlights the active role of flocculated clay particles in sediment transport in riverine systems, contrary to the general assumption that clay particles behave passively as wash load. This finding has the potential to affect sediment transport rates of fines and contaminants and could have far-reaching impacts on the interpretation of mud deposits in the sedimentary rock record. For modelling and predicting the sediment dynamics in river systems a comprehensive understanding of the transport mechanisms of clay flocs is essential and should be taken into account.

How to cite: de Lange, S., van der Wilk, A., Chassagne, C., Ali, W., Hoitink, T., Borne, M., Brodersen, K., and Waldschläger, K.: The capturing of flocs by migrating subaqueous dunes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5150, https://doi.org/10.5194/egusphere-egu24-5150, 2024.

Sedimentary cores from the Ravenglass Estuary, NW England, lack some of the sedimentary structures which can be seen in other estuarine sands due to their unconsolidated nature, making it difficult to meaningfully interpret depositional environments using standard sedimentological facies analysis. Here we explore how sediment texture, obtained from laser particle size analysis, and bulk geochemistry, obtained from portable X-ray fluorescence, can be used independently, or in combination, within a machine learning model to automatically classify sub-depositional environment and estuarine zone at the estuary surface to create a model which can be used to classify subsurface sediment samples. Using an adapted an established machine learning workflow we select the most informative geochemical elements to be included in the training set for the classification model.

The most important geochemical elements for modelling represent major elements of the most abundant minerals in the estuary, and minor elements representing trace signals of sulfide mineral deposits present in the hinterland. Models that are trained exclusively on textural data significantly outperform those that use geochemical data when classifying sub-depositional environment but are comparable when classifying estuarine zone. However, the combination of textural and geochemical data in training sets improves model performance in all but one class when compared to separate textural and geochemical models.

Ultimately, we have applied the surface-calibrated combined textural and geochemical model to classify paleo-sub-depositional environment in geotechnical cores obtained from the Ravenglass Estuary. This allows us to interpret their environmental evolution and build scaled correlation panels spanning key areas of the estuary to suggest how the estuary has changed since its formation.

How to cite: Nichols, T. E., Houghton, J. E., Worden, R. H., Duller, R. A., Griffiths, J., and Utley, J. E. P.: Using sediment texture and geochemistry as predictors to automatically classify sub-depositional environments in a modern estuary, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10523, https://doi.org/10.5194/egusphere-egu24-10523, 2024.

Deltas are among Earth’s most important depositional landforms. They host megacities, serve as ecological hotspots, and control fluxes of water, sediment, and nutrients between terrestrial and marine domains. Nearly all river deltas are subject to some degree of wave-influence, and most river mouths are wave-dominated. These systems exhibit strong spatiotemporal variability across a range of scales and form under hydrodynamic conditions that are highly nonlinear. As a result, the processes involved in the formation and evolution of wave-influenced deltas are poorly understood, limiting our ability to predict how they will respond to future changes.

Here we explore the factors governing wave-influenced delta evolution using a coupled flow-wave-transport model (Delft3D-SWAN). We present the first physics-based simulations to correctly reproduce the morphological attributes commonly observed in wave-influenced deltas (smooth cuspate shorelines, simple distributary networks, systems of barriers and lagoons) and capture the emergent processes that govern their evolution.

We show that wave-influenced deltas grow through a combination of shoreface accretion, crevasse splays, and distributary channel avulsions. Shoreface accretion occurs when fluvial sediment is deposited in the nearshore faster than it can be removed by wave-driven currents, and manifests as spit / barrier growth and migrating sand waves. Splays and avulsions lead to aggradation on the delta top, but also increase the area over which fluvial sediment is distributed, effectively decreasing the deposition rate at each channel mouth and in turn limiting shoreface accretion. We show that for a given wave climate, the relative importance of these processes determines a delta’s morphology and is controlled by the fluvial sediment composition and the receiving basin depth via the distributary channel network. In systems with shallow receiving basins or high sand loads, distributary channels are shallow, wide, and unstable. Network reorganization is frequent, creating deltas with smooth shorelines, many distributaries, and few barrier features. By contrast, a deep receiving basin or an abundance of cohesive fluvial sediment leads to enhanced levee formation, stabilizing distributary channels and reducing network complexity. Simpler networks deliver proportionally more water and sediment to individual distributary mouths, favoring shoreface accretion and leading to deltas with high protrusion angles and an abundance of barrier features.

Our results provide insight into the processes involved in wave-influenced delta growth and the factors governing those processes. This information can be used to improve stratigraphic interpretation of delta deposits and helps inform sediment management practices for improving delta resilience in the face of anthropogenic pressures, such as land use and climate change.

How to cite: Broaddus, C. and Foufoula-Georgiou, E.: Controls on the growth of wave-influenced river deltas: The roles of fluvial sediment composition and basin depth, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21037, https://doi.org/10.5194/egusphere-egu24-21037, 2024.

Estuaries worldwide are expected to suffer increasing tidal flood risk due to climate change. Climate change is causing sea-level to rise and storm frequency and severity to increase, inducing more frequent tidal floods in estuaries, where most of the world’s largest cities are located. The key to mitigating this flood risk is by reducing tidal propagation from offshore to onshore. Flood alleviation in an estuary requires slowing down the passage of water coming into an estuary, rather than creating space for flood water. There is a nearly infinite supply of water flooding an estuary and thus, in contrast to terrestrial rivers, making space is not an effective solution for flood risk mitigation. To test this hypothesis, we investigate the effectiveness of natural tidal flood interventions for reducing water surface elevations along estuaries by performing sensitivity analyses on a 1D analytical model of tide propagation for estuaries worldwide, considering the impacts of future sea level rise. We find that increasing estuary bed roughness is the most efficient tidal flood intervention, whereas adding space for flood water has minimal impact on water surface elevations. A more focused  2D numerical model experiment simulating the hydrodynamics of the Clyde Estuary, Scotland, also reveals that roughening the estuary bed and banks significantly slows down the passage of water by absorbing tidal wave energy, delaying the tidal wave peak arriving from offshore and consequently reducing water surface elevation and tidal flood extent. However, the 2100 SLR projection consistently reduces the effectiveness of all such interventions, highlighting the challenges of implementing the most effective solutions for alleviating future tidal flood risk.

How to cite: Prasojo, O. A., Hurst, M. D., Williams, R. D., Naylor, L. A., and Toney, J.: Slowing down the tidal flood wave is the key to reducing tidal flood risk in estuaries worldwide, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17737, https://doi.org/10.5194/egusphere-egu24-17737, 2024.

Soft sediment coasts are shaped by interactions between sedimentological and biological processes. Intertidal shellfish beds of the World Heritage site ‘Wadden Sea’ are an important habitat with a variety of sedimentological and ecological functions. They are dominated by two ecosystem engineers, the native Blue mussel (Mytilus edulis) and the invasive Pacific oyster (Magallana gigas). Their extents and biomasses are highly variable and are influenced by many environmental factors as well as human interventions. Among the latter, continuous dredging and dumping of sediments for the maintenance of shipping channels and harbours in the large German estuaries and rivers have led to a change in natural sediment dynamics and are suspected to influence the occurrence of mussel beds and seagrass areas in the Wadden Sea National Parks. The outer Ems estuary is a prime example in this context. River deepening and maintenance dredging have been taking place for decades and have led to heavy siltation and lack of oxygen in the lower Ems and an increased water turbidity can also be observed in the outer Ems. In addition, coastal construction measures have caused morphological changes in ​​the Ems estuary.

Within an interdisciplinary project, changes in the morphodynamics, sediment transport and currents are being investigated and methods are developed that allow an assessment of resulting ecological effects on mussel beds, as well as their sedimentological feedbacks in the outer Ems estuary. Investigations on the extent to which sediment shift and deposition affect the occurrence of mussel beds are lacking in the Lower Saxon Wadden Sea. For this purpose, detailed measurements of sedimentation and erosion on selected mussel beds and analyses of the sediment composition were conducted with high spatial and temporal resolution between 2019 and 2023 using sedimentation-erosion-bars and analysis of grain size composition. Mussel beds with different distance to the sediment deposition sites and areas with dense mussel and oyster colonization, gaps in the mussel bank and the tidal flats around the banks were compared. In addition, the extent and biomass of the mussel beds and the condition of Blue mussels were examined and related to sedimentation rates. The results will be presented and discussed against the background of sea level rise in the Wadden Sea and will be provided as a basis for the development of an ecological sediment management for the outer Ems estuary. Within the interdisciplinary project it will be considered how sediments can be sensibly directed to prevent siltation of mussel and seagrass areas on the one hand and to support the natural growth of the seabed where sediments are needed on the other hand, safeguarding the outstanding universal value of the World Heritage site.

How to cite: Nordhaus, I., Merk, V., and Scheiffarth, G.: Influence of river sediment management on the sedimentology and ecology of shellfish beds within the World Heritage site ‘Wadden Sea’, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19676, https://doi.org/10.5194/egusphere-egu24-19676, 2024.