Inorganic sediment supply is a critical component of a salt marsh’s ability to vertically aggrade in response to relative sea level rise, yet there remains significant uncertainty on the primary sources, timing, and rates of sediment delivery to marshes. This is particularly true for the Northeastern, U.S. Atlantic coastline where the magnitude and sourcing of sediment varies widely due to post-glaciated landscapes. Here we present results from a 3-year study between 2020 and 2023 designed to inform management and restoration decisions related to northeast marshes through the development of a scalable method for assessing the availability and distribution of inorganic sediment to and within marshes, including the identification of thresholds of inorganic sediment delivery required to maintain a stable marsh platform under various rates of sea level rise for the region. Field investigations involved instrumental observations, deployment and recovery of seasonal sediment traps, and the collection and analysis of marsh core samples. The study targets 12 marsh systems spanning environmental gradients for the region that allowed us to examine different sources and delivery mechanisms of sediment. Our compilation of existing data reveals spatial variability in marsh accretion rates, but also highlights regional trends and the general agreement among rates determined through a variety of different methodologies and time spans. Our instrumental observations and sediment trap deployments confirm differences in sediment delivery among marshes. Back barrier marshes with relatively small watersheds predominantly accumulated inorganic sediment during the fall in response to large storms and wave activity suspending coastal and offshore sediment deposits (marine sources) that are carried into marshes through tidal advection. In contrast, marshes proximal to large rivers (>10,000 km² watersheds) have higher accumulation rates and receive the bulk of their inorganic sediment in response to fluvial delivery of terrestrial sediment during spring freshet events. Among our 12 study marshes, only one experienced its highest rate of sediment accumulation during summer months, which we attribute to substantially greater crab herbivory promoting internal recycling of sediment. Overall, we have measured sediment accumulation in over 450 individual traps across spring, summer and fall seasons in twelve marshes. The results from the analysis of these samples represents the largest dataset of its kind for the region and enable defining regionally appropriate input variables for modeling the spatial variations of sedimentation across marsh surfaces as a function of tidal inundation and distance to the nearest channel, as well as providing defined sedimentation limits needed to sustain healthy marsh growth under future sea level rise and various potential restoration pathways.

How to cite: Woodruff, J., Autery, M., Baranes, H., Cook, T., Griswold, F., Hansen, L., and Yellen, B.: Controls on Sediment Delivery to New England Salt Marshes and Resulting Limits on Future Resilience, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10238, https://doi.org/10.5194/egusphere-egu23-10238, 2023.

Coastal marshes represent a highly valuable ecosystem that provides a wide array of ecosystem services. Unfortunately, marshes survival might be compromised by sea level rise, limited sediment supply, and subsidence. Storm surges represent a fundamental source of sediment for starving marshes because of their ability to resuspend bottom material in channels and tidal flats and transport it to the marsh surface. However, their intermittent nature makes the quantification of their effect not trivial. In this study, we selected 11 storm surges with different intensity in Terrebonne Bay, Louisiana, USA and simulated them with the Delft3D-FLOW model coupled with the Simulating Waves Nearshore (SWAN) module. Simulations revealed that the deposition on the marsh platform is correlated with storm intensity and duration. However, when the storm return period is considered, the surge with 1.7 years return period was found to have the highest geomorphological work. This indicates that the most impactful storms are those that balance intensity with frequency. We introduce a new approach to derive long-term vertical inorganic accretion rates based on simulations of real storm surges. We evaluated every possible combination of 11 storms and selected the one that is most closely related to in situ field measurements. Using a linear model, we derived a spatially distributed inorganic accretion rates with values consistent to field measurements. This method has the advantage of considering real scenarios and can be applied in any marsh-bay system. Overall, this study stresses out the central role storm surges have into feeding sediment starving coastal marshes.

How to cite: Cortese, L., Zhang, X., Simard, M., and Fagherazzi, S.: Quantifying the impact of storm surges on mineral accretion rates of coastal marshes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4280, https://doi.org/10.5194/egusphere-egu23-4280, 2023.

Coastal defences such as dikes are increasingly pressured by climate change. Increasing storm surge, extreme rainfall and periods of draught requires evermore strengthening of dikes to maintain flood risk standards. Conventional dike strengthening (i.e., heightening and/or widening) will be either structurally or financially unfeasible. Therefor, engineers are exploring other, more sustainable, methods to ensure future flood safety. A promising method is incorporating tidal marshes in the coastal defence system. Tidal marshes reduce dike loads by wave attenuation, increase bio diversity and ecology and under the right circumstances are able to grow with sea level rise. Moreover, in case of dike failure, resulting in a dike breach and inundation of the hinterland, tidal marshes have been shown to reduce breach erosion rates. This reduction positively affects flood risk. However, in order to quantitatively estimate the effect, dike breach models need to also model tidal marsh erosion. In this study we tested a mature tidal marsh, in-situ, in winter conditions under high flow velocities (up to 2.5 m/s) to measure the erosion and estimate erodibility. We measured little erosion, order millimeters after a cumulative 2-2.5 hours. Small-scale experiments, such as the Jet Erosion Test, showed high resistance to erosion (85-140 Pa) and large varying erodibility (6.5-45 cm³/N·s). By estimating the shear stresses acting on the soil during the experiment we compare the data with the small-scale results. The comparison gives insight in whether the small-scale experiment results can be accurately translated to full-scale erosion. Also, the experiment showed which (erosion) mechanisms are important for tidal marshes during a dike breach.

How to cite: van den Berg, M., Stoorvogel, M., Schoutens, K., van den Hoven, K., Rikkert, S. J. H., Herman, P. M. J., and Aarninkhof, S. G. J.: In-situ tidal marsh erodibility under high flow velocities, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6563, https://doi.org/10.5194/egusphere-egu23-6563, 2023.

Coastal wetlands are providing a variety of ecosystem services, among others valuable carbon stores, collectively referred to as blue carbon. The carbon sequestration potential is constrained by the difference between organic matter burial and its decomposition. Soil conditions are influencing organic matter decomposition rates and must be accounted for in blue carbon budgets. The differential soil to atmosphere CO₂ efflux between salt marsh sites experiencing differing geomorphic conditions (eroding vs. prograding) was measured in this study performed in Scotland, UK. Further, potential processes responsible for soil to atmospheric CO₂ flux were determined, including groundwater level, soil temperature and soil characteristics (i.e., grain size, carbon content and carbon stable isotopes). Eroding salt marsh sites had a 26.48% higher CO₂ efflux than expanding sites. Generalised linear mixed effects model (GLMM) and Linear mixed effects model (LMM) showed the relationship between CO₂ efflux and tidal cycle, erosion status, and the distance to the seaward vegetation edge. The efflux of CO₂ from the salt marsh is influenced by the underlying geomorphological conditions. These results highlight that salt marshes should be regard as heterogenous systems, especially considering analyses of future carbon storage budgets.

How to cite: Stolpmann, L., Balke, T., and Bass, A.: Increased CO2 Efflux from Retreating Salt Marshes Occurs Before Active Erosion, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16381, https://doi.org/10.5194/egusphere-egu23-16381, 2023.

Salt marshes are valuable ecosystems in coastal wetlands as they provide many functions. However, salt marshes are increasingly threatened by the sea-level rise induced by climate change. Whether there is enough sediment availability is the key to the survival of salt marshes. Storm events can provide a large amount of sediment from mud flats to salt marshes and are the main driver of marsh accretion in the long term. Sediment can be transported into the marsh via both marsh edges and marsh creeks. How important the role of marsh creeks is in delivering sediment into marshes during calm weather and storm events needs to be further investigated. Therefore, one field campaign in Paulina Saltmarsh (the Netherlands) has been conducted in the summer. As storm events frequently occur in winter in the Netherlands, the other field campaign in Paulina Saltmarsh is ongoing in January. Water depth, velocity, SSC, and bed level change have been measured simultaneously at three locations: the mud flat, the marsh creek, and the marsh edge. According to the results from the summer field campaign, we found that the marsh creek generally functions as a conduit for exporting sediment during calm weather. Sediment import can only be observed when the mud flat was eroded and provided more sediment to the creek. Due to the lack of sediment availability, mud flats cannot be recovered easily after erosion. During storm events, marsh creeks are expected to reverse the role in delivering sediment. In addition, the contribution of creeks and marsh edges to the transport of sediment will be explored. This work reveals different sediment transport regimes under different conditions, and highlights the role of creeks in the expansion of marshes during storm events.

How to cite: Sun, J., van Prooijen, B., Wang, X., and He, Q.: The role of marsh creeks in the development of salt marshes during storm events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3181, https://doi.org/10.5194/egusphere-egu23-3181, 2023.

Nature-based  strategies,  such  as  wave  attenuation  by  tidal  marshes,  are  increasingly  proposed  as  a  complement  to  mitigate  the  risks  of  failure  of  engineered  flood  defense  structures such as levees. However, recent analysis of historic coastal storms revealed smaller  dike  breach  dimensions  if  there  were  natural,  high  tidal  marshes  in  front  of  the  dikes.  Since  tidal  marshes  naturally  only  experience  weak  flow  velocities  (~0-0.3  ms^-1 during  normal  spring  tides),  we  lack  direct  observations  on  the  stability  of  tidal  marsh  sediments  and  vegetation  under  extreme  flow  velocities  (order  of  several  ms^-1)  as  may  occur  when  a  dike  behind  a  marsh  breaches.  As  a  first  approximation,  the  stability  of  a tidal marsh sediment bed and winter-state vegetation under high flow velocities were tested in a flume. Marsh monoliths were excavated from Phragmites australis marshes in front of a dike along the Scheldt estuary (Dutch-Belgian border area) and installed in a 10 m long flume test section. Both sediment bed and vegetation responses were quantified over 6 experimental runs under high flow velocities up to 1.75 ms^-1 and water depth up to 0.35 m for 2 hours. These tests showed that even after a cumulative 12 hours exposure to high flow velocities, erosion was limited to as little as a few millimeters. Manual removal of the aboveground vegetation did not enhance the erosion either. Present findings may be related to the strongly consolidated, clay- and silt-rich sediment and P. australis root system in this experiment. During the flow exposure, the P. australis stems were strongly bent by the water flow, but the majority of all shoots recovered rapidly when the flow had stopped.  Although  present  results  may  not  be  blindly  extrapolated  to  all  other  marsh  types, they do provide a strong first indication that marshes can remain stable under high flow conditions, and confirm the potential of well-developed tidal marshes as a valuable extra  natural  barrier  reducing  flood  discharges  towards  the  hinterland,  following  a  dike  breach. These outcomes promote the consideration to implement tidal marshes as part of the overall flood defense and to rethink dike strengthening in the future.

How to cite: Schoutens, K., Stoorvogel, M., van den Berg, M., van den Hoven, K., Bouma, T. J., Aarninkhof, S. G. J., Herman, P. M. J., van Loon-Steensma, J. M., Meire, P., Schoelynck, J., Peeters, P., and Temmerman, S.: Stability of a Tidal Marsh Under Very High Flow Velocities and Implications for Nature-Based Flood Defense, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1874, https://doi.org/10.5194/egusphere-egu23-1874, 2023.

Tidal marshes are valuable coastal ecosystems that are threatened by global climate warming and the resulting sea level rise. Whether they drown or continue to exist, depends on the trapping of sediments that builds up the land surface. Tidal channel networks, which typically occur within tidal marshes, are the major supply routes for sediments towards the marshes and hence are expected to affect the capacity of marshes to keep up with sea level rise by sediment trapping. The development and evolution of tidal channel networks and the sediment trapping are locally determined by so-called bio-geomorphic interactions between plants, water flow and sediment transport. However, the effect of different environmental variables on channel network formation remains poorly understood. In this research, we investigated the impact of spatio-temporal plant colonization patterns by means of flume experiments. Four scaled landscape scale experiments were conducted in the Metronome tidal facility, a unique flume that tilts periodically to generate tidal currents. Two control experiments without vegetation, a third experiment with a channel-fringing vegetation colonization pattern, and a fourth with patchy vegetation colonization pattern. Seeds were distributed by water in the channel-fringing experiment, while a manual sowing method was used to obtain laterally expanding circular patches in the patchy experiment. Our results show that vegetation and their respective colonization pattern affect channel network formation both on a landscape scale and local scale. More extensive and effective channel networks are found in vegetation experiments. These results indicate that channel-fringing or patchy recruitment strategies might produce landscapes that are more resilient to sea level rise.

How to cite: Hautekiet, S., Rossius, J.-E., Gourgue, O., Kleinhans, M., and Temmerman, S.: Coastal marsh resilience: a study on the role of bio-geomorphic self-organization, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14280, https://doi.org/10.5194/egusphere-egu23-14280, 2023.

The resilience of saltmarshes mainly depends on their ability to gain elevation by sediment accretion to keep pace with sea level rise. While vegetation is known to facilitate sediment accretion at the plant scale by trapping mineral sediments and producing organic matter, the long-term impact at the landscape scale is still poorly understood. Here we use the biogeomorphic model Demeter to reveal contrasting vegetation impacts on spatial patterns of sediment accretion under different sea level rise regimes. Under contemporary sea level rise rates (2-10 mm/yr), vegetation inhibits sediment transport from tidal channels to platform interiors and creates levee-depression patterns. Hence, intertidal platforms accrete slower with vegetation than without, but this trend attenuates with increasing sea level rise rate, as water depth increases, and vegetation drag decreases. Under extreme sea level rise rate (20 mm/yr), platform interiors don’t keep up and turn into open water, while vegetation allows to preserve intertidal levees. Our results help to better understand some basic biophysical mechanisms that will control the fate of coastal wetlands under global climate change.

How to cite: Gourgue, O., Xu, Y., Belliard, J.-P., van Belzen, J., van de Koppel, J., Kleinhans, M. G., Fagherazzi, S., and Temmerman, S.: Contrasting saltmarsh vegetation impacts under increasing sea level rise rates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16448, https://doi.org/10.5194/egusphere-egu23-16448, 2023.

Many studies have been carried out in the last decade to assess the rates of sediment transport and deposition on tidal flats and salt marshes, however, a need to characterize the transport fluxes between the various habitats as a function of tidal range, their position relative to mean sea level, and flow-asymmetries in the vegetation effect remain. This study uses fieldwork data to characterize the sediment fluxes and deposition from the tidal flats towards the marsh platform, in a channel margin of the Ria Formosa coastal lagoon (south Portugal). Sediment fluxes were measured in a cross-shore transect, during neap and spring tide conditions. The dominant intertidal species are Spartina maritima and the seagrass Zostera noltei. Current measurements were used to assess bottom shear stress conditions. Deposition rates, instantaneous suspended sediment, and near-bed velocities were linked through theoretical formulas and used to characterize time-averaged conditions for sediment delivery and deposition to the site.

The results showed that suspended sediment concentrations and sediment deposition varied across-shore with no specific relation to elevation. Maximum current velocities were recorded in the vegetated tidal flat, in the order of 0.20 m/s, and in the low marsh due to flow-plant interactions and an increase in turbulence. Deposition rates ranged between 20 to 45 g/m²/hr, after a complete tidal cycle, and were higher in the mid-upper marsh. The hydroperiod was not the main contributor to sediment deposition in the study area. Measured sediment transport was tidally driven, with shifting current angles during the cycle and major alongshore components during peak flood velocities. Flow-spartina interference in the low marsh significantly affected local sediment resuspension. The obtained results provide insights into the dynamics and variability of flow and mass transfer along a transition from the vegetated tidal flat to the upper marsh and can be used in sediment transport models for mesotidal marsh systems.

Acknowledgments: A. Rita Carrasco was supported by the contract DL57/2016/CP1361/CT0002, and Katerina Kombiadou was supported by the institutional contract CEECINST/00146/2018, both funded by Fundação para a Ciência e Tecnologia (FCT). This study had also the support of FCT under the project LA/P/0069/2020 granted to the Associate Laboratory ARNET and CIMA BASE UID/00350/2020.

How to cite: Carrasco, A. R., Kombiadou, K., and Matias, A.: Vegetation effects and sediment deposition in a mesotidal wetland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2023, https://doi.org/10.5194/egusphere-egu23-2023, 2023.

Assessment of coastal wetland resilience under rising sea levels using models is challenging due to uncertainties in processes and external drivers. In addition, a number of assumptions and simplifications are required in order to be able to carry out long-term complex simulations that include processes over a wide range of time and spatial scales. Some of those simplifications can have important implications for the assessment of wetland resilience. In this contribution we look at a number of simplifications typically used in coastal wetland evolution models, and we try to quantify their effects on the results. We include simplifications related to hydrodynamics, sediment transport and vegetation dynamics focusing on issues of process description, process interactions and spatial and temporal discretisation. We pay special attention to the identification of methods that include a level of simplification that allows for efficient computation with acceptable margins of error. We apply our model to a number of coastal wetlands worldwide with a variety of settings in terms of vegetation, tidal conditions, sediment load and find that accelerated sea-level rise towards the end of the century will greatly compromise wetland resilience.

How to cite: Saco, P., Rodriguez, J., Breda, A., and Sandi, S.: Modelling biophysical Interactions in coastal wetlands to assess resilience to sea-level rise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15132, https://doi.org/10.5194/egusphere-egu23-15132, 2023.

With climate change and increased coastal land alteration, salt marshes globally are becoming increasingly degraded. Salt marshes of the Northeastern United States (Maine to Virginia) are particularly vulnerable given the history of intensive alteration such as ditching and tidal restrictions since European colonization. Such alterations reduce the accretionary potential of salt marshes in this region, in turn reducing their ability to keep up with accelerating relative sea level rise. This ultimately leads to reductions in marsh area and loss of ecosystem function, including flood protection, carbon burial, habitat provision, and nutrient filtration. Through collaboration between multiple government, academic, and non-profit organizations, we investigate the following questions: (1) What are the spatial patterns of salt marsh vulnerability to relative sea level rise across the Northeast United States? (2) Additionally, how is this vulnerability linked to specific salt marsh modifications (e.g., ditching, and tidal restrictions)? To address these questions, we combine the Unvegetated to Vegetated Ratio (UVVR) salt marsh vulnerability metric, computed from 2014-2018 using Landsat imagery, with mapped tidal restrictions (e.g., culverts, bridges, tide gates, dikes) and ditches for the Northeastern coast of the United States. We hypothesize that estimated salt marsh lifespans, a mass balance between relative sea level rise and sediment budget (estimated using UVVR), will be shortened where salt marsh modifications are most intense. Results will be used to drive science-based decision making through prioritization of salt marsh restoration.

How to cite: Peck, E., Walker, J., Ackerman, K., Besterman, A., Carr, J., Cook, T., Correll, M., Deegan, L., Defne, Z., Ganju, N., Hartley, M., Jakuba, R., Staudinger, M., Wilson, B., Woodruff, J., and Yellen, B.: An assessment of salt marsh vulnerability & restoration potential in the Northeastern United States using physical and ecological indicators, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8653, https://doi.org/10.5194/egusphere-egu23-8653, 2023.

Managed dyke realignment is increasing being implemented in the Upper Bay of Fundy Canada in response to increased vulnerability of dyke infrastructure to erosion and overtopping.  The Making Room for Wetlands project was initiated in 2017 to develop an evidence-based framework for implementing managed realignment (MR) and tidal wetland restoration in this hypertidal estuary.  MR was conducted at two sites: Converse (near mouth of Missiguash River) and Belcher St. (near head of Cornwallis River) and included both pre and post restoration monitoring of hydrology, soils & sediments, vegetation and morphology.  Earthworks included removal of an aboiteau structure, channel excavation, inner dyke construction and levelling of old dyke infrastructure.  Detailed hydrodynamics, sediment transport and deposition data were collected seasonally at the Converse site since first tidal waters were introduced in Dec 2018.  Ecomorphodynamic changes were quantified using repeat high resolution RPAS surveys, field measurements and RSET stations at both sites.  The rate of evolution of tidal wetland landscape differed between the two sites.  High sedimentation rates (~10 cm/yr) associated with the turbidity maximum at Belcher provided a disturbance surface rapidly colonized by annual halophytes and was fully vegetated by Year 3 with a mix of tidal brackish species (equivalent to reference site).   Sedimentation also played a role at Converse, filling in the borrow pit used to construct the inner dyke within the first two years, facilitating the development of a shallow tidal creek network.  Additional sedimentation over the former agricultural surface aided in slower development of a hybrid creek network incorporating the relict ditches.  Large spring tides play an important role in sediment supply to the marsh platform.  Positive sediment flux values into the site were recorded in association with erosion of the inlet channel which typically occurred during higher spring tide events.  A preliminary model linking sediment flux at the inlet with deposition on the marsh surface was developed.  While some halophytic vegetation was established within the first two years at Converse, vegetation established increased markedly in Year 4.   Erosion of the tidal inlet during high spring tides provided important subsidies of sediment in addition to baseline concentrations within tidal waters to the marsh platform that facilitated the evolution of the tidal wetland landscape.  While the establishment of tidal marsh vegetation was slower at Converse than Belcher both are providing ecosystem services.  Implications for modelling the trajectory of tidal wetlands after managed realignment including MR design, timing of earthworks and position within the estuary are discussed.   

How to cite: van Proosdij, D., Elliott, M., Lewis, S., Graham, J., Nichols, K., and Bowron, T.: Influence of hydrodynamic and sedimentary processes on tidal wetland landscape evolution for Making Room for Wetlands in the Bay of Fundy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15798, https://doi.org/10.5194/egusphere-egu23-15798, 2023.

Managed realignment, the process of breaching flood defence structures for habitat restoration and flood defence purposes, is becoming an increasingly popular form of coastal management across Europe and North America. Typically managed realignment has been implemented at former intertidal locations that have been embanked and reclaimed, on the assumption these areas should be able to support intertidal habitats again. Despite this assumption, during the construction of managed realignment sites extensive engineering and landscaping works are often carried out, including the construction of drainage channels rather than utilising remnant pre-reclamation intertidal drainage channels. These engineering works are intended to encourage a range of habitat types and support the intended post site breaching land use, such as grazing. However, it has been demonstrated that managed realignment sites have more simplified creek and drainage networks, and lower topographic variability, than natural saltmarshes, which might restrict drainage, impact the plant communities that can colonise, and prevent widespread sedimentation and seed dispersal.

In contrast to managed realignment, unmanaged realignment is the natural breaching of flood defences without any costly engineering or landscaping works. Unmanaged realignment sites provide an opportunity to assess the “natural” morphological evolution of restored saltmarsh sites without the influence of extensive site design, engineering, or landscaping features. However, there remains no analysis of the evolution of ‘recent’ unmanaged realignment sites, with most studies focusing on historic breaches. This study provides an assessment of the occurrences of unmanaged realignment on the coast of the United Kingdom, which has the most realignment sites in Europe, since from 1996. The subsequent morphological evolution of these sites is then evaluated through an assessment of the change and development of the creek and drainage networks. Results indicate differences in post-breach morphology in relation to the history and former land use management of the sites. Findings are discussed in terms of the benefits of unmanaged realignment, and considered in context of long-term shoreline management planning, including habitat creation, flood defence and carbon storage. It is recommended that further data are collected on unmanaged realignment sites to understand their development and to enable comparison with managed realignment sites, with examples of such comparisons included in the discussion.

How to cite: Dale, J.: Is unmanaged realignment an appropriate coastal management strategy?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-761, https://doi.org/10.5194/egusphere-egu23-761, 2023.

        Coastal flooding hazard has intensified in past few decades, induced by the effect of climate change. The presence of mangroves can mitigate wave effect on levees and seawalls thus reducing flood risk at the inshore regions. Meanwhile, mangroves can facilitate the tidal flat that they have encroached with having a relative accretion by trapping sediment, which enable these regions to counteract the drowning associated with sea level rise. Previous studies have provided a solid theorical base for understanding the hydrodynamic process within mangroves through numerical modeling and prototype experiments, which allow for an in-depth comprehension also on the geomorphological changes associated with different mangrove settings by field observation. In this study, a transect characterized by changing mangrove condition (plant size and density) was set up at the Nanliu delta, the largest delta in the southwest part of China, colonized by Aegiceras Corniculatum (AC). A series of hydrodynamic, turbidity and bio-morphodynamic data were acquired during both normal and storm weather conditions, which revealed the capability of native AC in both attenuating wave height and capturing suspended sediment in relation to vegetation dimension and densities. The results showed that the wave damping coefficient of AC was three times larger during the storm period than during normal weather conditions. Moreover, wave height was linearly attenuated with landward wave propagation. Our work further indicated that the slopes and intercepts of the linear fits between wave height and landward wave propagation distance under storm and normal conditions are closely related to incident wave height, water level and submerged vegetation volume. A vegetation parameter used to evaluate the vegetation occupied volume in a water column was calculated as an indicator of different settings, which proved to be highly correlated with the sediment transport process. These findings highlight how mangrove forests can positively act in reducing coastal flooding hazards suggesting the possibility of designing naturally based interventions exploiting the mitigation capacity of this vegetation type.

How to cite: Zhou, X., Dai, Z., and Carniello, L.: Hydrodynamic effects of mangrove in different vegetation settings, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1456, https://doi.org/10.5194/egusphere-egu23-1456, 2023.

Mangroves are increasingly recognized as an effective nature-based coastal defence strategy. Mangrove trees are proven to reduce the height of propagating long-period waves such as storm tides and extreme sea levels. Existing empirical studies, however, are limited to small scales (~10²-10³ m) or only cover continuous belts of mangroves. Here we present water level measurements along a 20 km channel and in the surrounding mangrove forests for regular neap- and spring tides in a natural mangrove forest in the Guayas Delta, Ecuador. For tides with peak water levels which are high enough to flood the surrounding mangroves, inundation levels reached 45 cm with attenuation rates up to 40 cm/km. Along the entire 20 km channel, however, no attenuation occurred. Instead, we measured amplification with rates varying between 4.3 and 4.6 cm/km. Amplification rates increased with peak water level until water levels were high enough to flood the surrounding mangroves, upon which amplification rates decreased with peak water level. The latter implies that with higher peak levels, such as during an extreme sea level event, the capacity of mangroves to dampen amplification or even attenuate increases.

How to cite: Pelckmans, I., Vermeulen, B., Alex Ramos-Veliz, J., Mishell Rosado-Moncayo, A., E. Dominguez-Granda, L., Belliard, J.-P., Gourgue, O., and Temmerman, S.: Observations of tidal attenuation and amplification in a mangrove forest: channels as conduits, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12734, https://doi.org/10.5194/egusphere-egu23-12734, 2023.

How will mangrove forests cope with fast-changing tropical coastal environments under global change? How will coastal ocean processes impact thousands of kilometres of mangrove shorelines where millions of people live? So far, these questions have been insufficiently addressed due probably to the complexity and diversity of interwoven sedimentary, hydrological and ecological processes involved on any mangrove coast. Evidence of changes on several open mangrove coasts is, however, highlighted by decades of aerial and satellite images. Besides, time series of ocean data are now provided worldwide by different international services such as the E.U. Copernicus Marine Service.

The French Guiana (FG) coast of South America offers foundations for exploratory semi-empirical models of the impacts of hydro-sedimentary processes on mangrove coasts. The 320-km long mangrove shoreline is submitted to marked, often extremely rapid, alternating erosional or accretional phases generated by the alongshore northwestward migration of giant mud banks. Experimental studies have been carried out for decades to show and explain processes involved in the permanent transformation of the FG coast and its consequences for biodiversity and socio-economic activities. Studies suggest a leading forcing role of ocean wave and current regimes on mud bank migration, erosion and accretion, the combination inducing exceptional rates of mangrove landward retreat or seaward expansion that can attain up to 500 m per year.

Here, we present a modelling approach, named MANG@COAST, designed to simulate mangrove shoreline landward retreat and seaward advance, as close as possible to observations of those mangrove shorelines visually and annually delineated in satellite images acquired since 2013. Time series of wave and current data associated with data on the extents, shapes and locations of mud banks constitute input data. Our modelling approach is based on interaction graphs describing relations between three entities (mangrove, ocean, mud bank) and implemented using Ocelet, a domain-specific language. Two equations were proposed to consider: (1) the role of the subtidal part of the mud bank in the dissipation of ocean waves and currents, and (2) the ability of mangroves to expand over new consolidated mud substrates. Five coefficients are used to weigh the respective influences of waves, currents, mudbank extent and mangrove expansion. Their final values are adjusted from an iterative optimization process searching for convergence of the simulated mangrove shoreline with the observed ones.

This model was run on different sectors of the FG coast for the 2013-2022 period to deliver a set of coefficients for each sector. First, the geographical variability of coefficient values is discussed as a function of local coastal geomorphology and orientation. Second, the particular seasonal effect of waves and currents on mangrove shoreline fluctuations is tested by embedding final coefficient values in the equations and by applying daily-computed trends of wave and current data to different mud bank configurations. We put particular emphasis on examining the role of the ocean wave regime during the high wave-energy season from December to April. Third, we explain how indices of coastal vulnerability can be built and could contribute to the Guyana Coastal Observatory (https://observatoire-littoral-guyane.fr/).

How to cite: Augusseau, P.-E., Proisy, C., Gardel, A., Staquet, A., Santos, V. F., Brunier, G., and Anthony, E. J.: Simulating the impacts of hydro-sedimentary processes on open mangrove coasts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7335, https://doi.org/10.5194/egusphere-egu23-7335, 2023.

Mangroves, a coastal wetland ecosystem with complex above-ground root systems, are known to modulate flow and sediment transport and promote sedimentation – processes that could drive the long-term geomorphic evolution of mangrove forests. However, insights on how and to what extent mangrove forests impact the sedimentary processes and evolution of landscapes and their adjacent areas are limited. This study aims to address these challenges by using a hydrodynamic-sediment transport model and contribute to understanding the effective restoration and management of mangrove forests under the impact of climate change effects such as sea-level rise. A new model was developed to represent the impacts of species-specific three-dimensional root structures (e.g., “prop roots” of Rhizophora species and “pencil roots” of Avicennia and Sonneratia species) on flow and sediment transport and implemented in a hydrodynamic-sediment transport model. This model was applied to a restored estuarine mangrove forest in the Philippines influenced by tidal and fluvial processes. The results show the significant impacts of mangroves on the sedimentation of fluvially-transported sediments in the mangrove forest and nearby areas, which contributed to the areal expansion of the mangrove forest. In addition, due to the increased hydraulic resistance of the mangrove forest following restoration, significant amounts of river flow and sediment discharge are diverted to the other tributary, decreasing the sediment supply downstream of the mangrove forest, a phenomenon that could possibly explain the trend in sediment loss in the area. These results suggest the significance of mangrove forests in driving landscape evolution, not only within the mangrove forest itself but also in adjacent areas, highlighting the importance of considering these areas as a connected system for the management and restoration of mangrove forests.

How to cite: Yoshikai, M., Nakamura, T., Blanco, A., Hernandez, B., Herrera, E., Ferrera, C., Almarza, F., Dimalanta, W., Basina, R., Albano, G., Rollon, R., Suwa, R., Ray, R., Primavera-Tirol, Y., San Diego-McGlone, M. L., and Nadaoka, K.: Modeling flow and sediment dynamics in an estuarine mangrove forest and adjacent areas: the impact of mangrove forest dynamics on landscape evolution, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13835, https://doi.org/10.5194/egusphere-egu23-13835, 2023.

The European flat oyster (Ostrea edulis) provided relevant ecosystem functions and services as a keystone filter feeder and habitat forming species across the North Sea. At the turn of the last century, the species suffered a severe decline following a brief period of massive economic exploitation. Today, a few local populations still exist in e.g. the UK, the Netherlands, and Denmark. In the German North Sea, O. edulis is classified as functionally extinct.

The Sylt-Rømø Bight is one of the largest tidal back-barrier environments of the European Wadden Sea and is located on the border between Germany and Denmark. The environment underwent significant changes over the last millennia driven by natural landscape dynamics and, more recently, by anthropogenic claims to land and coastal resources. Despite its demise as a living organism, O. edulis shells are still widely found in the area and remain a part of the sedimentary environment that potentially bears evidence of past ecological and geomorphological change. Written and graphic accounts on the past state, composition and distribution of Ostrea reefs are rare and unreliable.

In this case study, we explore shell assemblage counts, geospatial data, and radiocarbon dating as tools to better understand ecological change over the past few centuries. More than a hundred dredge profiles were collected in the German part of the Sylt-Rømø Bight during two campaigns in summer 2021 and 2022. The results show marked spatial differences in the shell material regarding total dredged volume, overall species composition, size distribution, and taphonomic state. These differences can partly be explained by landscape dynamics and the economic history of the area (blue mussel cultivation). Radiocarbon dating of Ostrea shells (n=42) from three locations suggests that Ostrea banks at more exposed sites were short-lived and dynamic features, while ages from a less exposed site suggest the continuous presence of Ostrea over (at least) three thousand years.

This example showcases that methods from geoarchaeological and geoscientific contexts can help to shed light on the more recent paleoecology of coastal environments over historical timescales, where sufficient depth of information is lacking from other sources. Beyond pure curiosity, the information has a clear value as benchmark to define aims for marine conservation and restoration efforts.

How to cite: Sander, L., Pogoda, B., Reise, K., and Waser, A.: Ecological change in the early Anthropocene: Insight from death assemblages of the locally extinct European flat oyster (Ostrea edulis) in the Sylt-Rømø Bight, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8957, https://doi.org/10.5194/egusphere-egu23-8957, 2023.

The ongoing threat to coastal areas is well documented worldwide. The dynamic equilibrium of these landscapes is governed by the interaction of erosional and depositional processes and it is therefore critically affected by the intertwined effects of increasing anthropogenic pressure and climate changes, such as the increasing rates of sea level rise (SLR) and the intensification of extreme events.
The various human pressures which affect coastal areas can have several impacts on their evolution. As an example, the Venice Lagoon, the largest brackish water body in the Mediterranean Sea, has been extensively affected by human interventions, such as the diversion of the major rivers in the Renaissance period, the excavation of navigable channels, and the construction of jetties at the inlets in the 20^th century. In addition, after the November 1966 flood event, when water levels reached the maximum value ever registered in Venice (194 cm above the Punta della Salute reference datum) and heavy rainfalls caused the surrounding rivers (Piave, Brenta and Sile) to overflow, some defensive structures were designed and later adopted to reduce the flooding risk of the city of Venice and the surrounding floodplain. As an example, the levee which separates the Sile River and the Venice Lagoon was modified by building a spillway allowing the river flood to debouch into the lagoon. More recently, the mobile barrier system, known as Mo.S.E., designed to protect the city of Venice and the surrounding urban settlements from flooding has been activated, regulating the high water levels due to storm surges. However, the impacts of such defensive structures on lagoon hydrodynamics and morphodynamics remain poorly understood.
In this work, we applied two numerical models to investigate the potential effects of increasing anthropogenic pressures combined with a changing climate, on the hydrodynamics of the Venice Lagoon. The two-dimensional wind wave tidal model, coupling a hydrodynamic module with a wind wave module, allowed us to evaluate the effects of SRL (based on IPCC projections) on the lagoon hydrodynamics. In addition, the model allowed us also to analyse the possible effects of the repeated activations of the Mo.S.E. system on lagoon hydrodynamics and on the related ecosystem services, comparing regulated and non-regulated present and future scenarios. Finally, an integrated three-dimensional hydrodynamic model, was used to account for changes in water density (e.g., changes in water salinity) in the above described scenarios, thus monitoring the dynamics of salinity gradients under extreme conditions, driven by freshwater inflow through the Sile River spillover.

How to cite: Michielotto, A., Tognin, D., Matticchio, B., Carniello, L., and D'Alpaos, A.: Investigating natural and anthropogenic impacts on the hydrodynamics of the Venice Lagoon (Italy), a numerical approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8582, https://doi.org/10.5194/egusphere-egu23-8582, 2023.

Salt marshes are intertidal coastal ecosystems characterized by mostly herbaceous halophytic vegetation and shaped by complex feedbacks between hydrodynamic, morphological, and biological processes. These crucial yet endangered environments provide a diverse range of ecosystem services but are severely exposed to climate change and human pressure. The importance of salt marshes as ‘blue carbon’ (C) sinks, deriving from their primary production coupled with rapid surface accretion, has been increasingly recognized within the framework of climate mitigation strategies. However, uncertainties remain in the estimation of salt-marsh C stock and sequestration at the basin scale and large knowledge gaps still linger in the response of marsh C pools under increasing anthropogenic interventions, such as storm-surge regulation. In order to provide further knowledge in salt-marsh C assessment and investigate marsh C pool response to management actions under different scenarios, we analysed organic matter content in salt-marsh soils in 720 samples from 60 sediment cores to the depth of 1 m, and we estimated C stocks and accumulation rates in different areas of the Venice Lagoon (Italy), which has recently become regulated by a storm-surge barrier system. OC stocks in the surface 1 m were highly variable in different marshes averaging 17,108 ± 5,757 ton OC km^-2 (range 9,800 − 24,700 ton OC km^-2). The estimated OC accumulation rate was 85 ± 25 ton OC km^-2 yr^-1, confirming the CO₂ sequestration potential of tidal environments, which, however, resulted to be crucially affected by marsh accretion rates and their human-induced variations. By hindering sediment supply provided by storm surges which are largely responsible for marsh accretion, flood regulation can dramatically reduce the CO₂ sequestration potential of salt marshes. We estimate that storm-surge barrier operations in the Venice Lagoon may reduce the annual marsh CO₂ sequestration potential by about 33%, with high costs in terms of ecosystem service loss. Our results highlight the need for integrated coastal management policies to enhance the resilience of anthropic and natural environments and to preserve the ecosystem services delivered by coastal wetlands.

How to cite: Puppin, A., Tognin, D., Paccagnella, M., Zancato, M., Ghinassi, M., Marani, M., and D'Alpaos, A.: Blue carbon stock in marsh soil and impacts of flood regulation in the Venice Lagoon (Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-504, https://doi.org/10.5194/egusphere-egu23-504, 2023.