There is a growing interest in adoption of Engineering with Nature or Nature Based Solutions for coastal protection including large mega-nourishment interventions. However, there are still many unknowns on the variables and design features influencing their functionalities. There are also challenges in the optimization of coastal modelling outputs or information usage in support of decision-making. Artificial Intelligence, especially deep learning, is a powerful technology that has been rapidly evolving over the last couple of decades and can offer new means of analysis for the coastal science field. Yet, the potential of these technologies for coastal geomorphology remains relatively unexplored with respect to other scientific fields. In the current study, application of Artificial Neural Network is tested in combination with fully coupled hydrodynamics and morphological model (Delft3D) for predicting morphological changes and understanding the behaviour of large mega-nourishment intervention (Sand Engine).

For prediction of morphological change, two sets of deep learning models were tested, one set relying on localized modelling outputs or localized data sources and one set having reduced dependency from modeling outputs and, once trained, solely relying on boundary conditions and coastline geometry. The first set of models provides regression values greater than 0.95 and 0.86 for training and testing. The second set of reduced-dependency models provides regression values greater than 0.84 and 0.76 for training and testing.

For understanding the behavior of sand engines, a holistic framework is proposed which supports the choice of coastal protection schemes through the synthesis of numerical modelling outputs into an Artificial Neural Networking model whose computational efficiency allows the creation of a standalone computer application (Sand Engine App) illustrating the effectiveness of different users’ defined sand engines. In support of this app, twelve Artificial Neural Networking ensemble models structures were trained to predict the influence of different sand engines on water depth, wave height and sediment transports in its vicinity. The ensemble models were trained on simulated data obtained from more than five hundred numerical simulations with different sand-engine designs and different locations along Morecambe Bay conducted in Delft3D. These ensemble models provided good performance with majority of the models having testing regression greater than 0.90. These ensemble models were then packed into a Sand Engine App developed in MATLAB and designed to calculate the impact of different sand engine features on the above variables based on users’ inputs of sand engine designs.

How to cite: Kumar, P. and Leonardi, N.: A holistic framework for coastal forecast and evaluation of coastal protection schemes through Integration of hydro-morphodynamic modelling and Artificial Intelligence into the Sand Engine App, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-183, https://doi.org/10.5194/egusphere-egu23-183, 2023.

Coastal flooding and erosion threaten coastal communities and climate change boosts their effects to unprecedented levels. Even if coastal erosion has a direct effect on coastal flooding, their effects are normally studied in isolation due to the complex interplays among them, leading to potentially erroneous flood risk estimates (Toimil et al., 2022). To handle this challenge, an advanced shoreline evolution model enriched with observations (Álvarez-Cuesta et al., 2022) is coupled with a profile translation model to provide the boundary condition over which to run a 2D flood model. This suite of process and physics-based models enriched with observations is applied to a 40-km coastal stretch in the Spanish Mediterranean to develop erosion-enhanced flooding projections. In this study, the effects of neglecting erosion are studied at the storm scale and in the long term on two key flood magnitudes: the total water level and the flooded area. Additionally, the most relevant climate-related uncertainty sources are identified and their relative importance evaluated. At this particular coastal site where the suite is illustrated, results reveal that neglecting erosion has a more important effect on the flooded extent than the choice of the climate model and the sea level rise trajectory by the end of the century.  This application sets the basis for establishing general behavioural rules that may come to question the conclusions of erosion-uncoupled flooding studies.

Toimil, A., Álvarez-Cuesta, M., & Losada, I. J. (2023). Neglecting the effect of long-and short-term erosion can lead to spurious coastal flood risk projections and maladaptation. Coastal Engineering, 179, 104248.

Álvarez-Cuesta, M., Toimil, A., Losada, I.J. (2022). Modelling long-term shoreline evolution in highly anthropized coastal areas. Part 1: Model description and validation. Coastal Engineering, 169, 103960.

How to cite: Álvarez-Cuesta, M., Toimil, A., and Losada, I.: Quantifying the effects of coastal erosion on flooding projections at the climate change scale, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8522, https://doi.org/10.5194/egusphere-egu23-8522, 2023.

A major challenge facing climate change and sea level rise is to predict coastal evolution and in particular coastal erosion, which seems likely to increase in the next century. Coastal erosion affects many regions of the world, often highly populated. In addition to natural processes, coastal erosion is closely interrelated with anthropic activities such as sea defence building or the modification of river sediment supply (e.g. river dam sediment trapping and land use changes). Several studies have already focused on predicting coastal change using more or less complex models on local scales. However, the majority of these models don't take into account one of the main components: the input of sediment from rivers. There are also difficulties in adequately representing the shoreline evolution by a partial understanding of processes and the difficulty of obtaining detailed and long-term data. We present simplified dynamic model on a global scale fed with new satellite observations of coastal ocean hydrology and morphology. The main objective of this study is to explore, thanks to this new model, future scenarios of shoreline change with the identification of potentially vulnerable hotspots. It will help in improving sustainable coastal management strategies.

How to cite: Amélie, A., Rafael, A., Vincent, R., Sebastien, C., Julien, B., Fabrice, P., and Marcan, G.: Predicting coastline changes under human and climate drivers : GLOB-COAST MODEL, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5531, https://doi.org/10.5194/egusphere-egu23-5531, 2023.

Urban flooding continues to be one of the most damaging natural hazards globally. Despite efforts to mitigate flood risk, it continues to increasingly cause damage and disruption; in the US, tropical cyclones, severe storms, and inland flooding events are the first, third, and fourth most costly billion-dollar disaster events, respectively (NOAA National Centers for Environmental Information, 2022). Flood events in the US, including those caused by tropical cyclones and severe storms, cost an average of 40.6 billion USD each year and 1.7 trillion USD since 1980 (NOAA National Centers for Environmental Information, 2022). Beyond physical damages, flooding impacts local transportation networks, disrupting mobility necessary for day-to-day functions as well as for recovery.

In response, the US Department of Defense has increased efforts to evaluate and improve the resilience of the missions that are carried out at coastal military installations. Under a changing climate, many of the factors that contribute to flooding at installation locations are nonstationary in nature, such as sea level rise, river discharge trends, land subsidence, and storm severity and frequency. Hazard modeling and subsequent resilience analytics should take nonstationarity trends into account, which in turn may lead to plans and designs that deviate from historical status quo to better cope with new realities.

In this study, Adaptive Hydraulics 2D Shallow Water (AdH-SW2D), a high fidelity, finite-element model, is used to simulate hydrodynamic conditions under nonstationary conditions in sea level and river discharge over one hundred years. Two locations are identified as case studies – Camp Lejeune, NC, USA and NAS Gulfport, MS, USA. Simulations are carried out under low, intermediate, and high sea level rise for nine annual exceedance probabilities in river discharge. Maps of flood depth and extent are generated to examine the effects of nonstationarity on urban flood conditions in each location over time. Inundation conditions form the basis for various inquiries including road network connectivity, installation mission resilience, and the compounding impact of multiple threats.

Reference: NOAA National Centers for Environmental Information (NCEI) U.S. Billion-Dollar Weather and Climate Disasters (2022). https://www.ncei.noaa.gov/access/billions/, DOI: 10.25921/stkw-7w73

How to cite: Everett, C., Zimmerman, J., Maze, G., Chung, S., Savant, G., and Kurth, M.: Effects of Nonstationarity on Flooding of Coastal Infrastructure, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8686, https://doi.org/10.5194/egusphere-egu23-8686, 2023.

Across Europe, the North- and the Baltic Sea coasts are expected to experience the highest increase in extreme sea levels until the end of the century, mostly due to rising sea levels. This increase will have serious implications for coastal flooding and adaptation planning, which is particularly true for Germany. Without adjusted and/or upgraded adaptation, Germany is projected to be among those European countries that suffer largest absolute flood damages in 2100.

Here we use a fully validated modelling framework to explore the effectiveness of raised dike crest elevations and managed realignment as a nature-based adaptation option in reducing flood extent and exposed population during a 200-year surge event, also accounting for two sea-level rise scenarios (1 m and 1.5 m). We explore the potential for managed realignment in the study region by introducing a fully automated modelling approach that considers elevation, land use and infrastructure.

We find that managed realignment is more effective in reducing the population exposed to coastal flooding compared to increasing dike crest elevations. However, maximum reduction in population exposure amounts to 26 %, suggesting that redesigning existing dikes by managed realignment is not enough for maintaining flood risk at today´s levels. Our results show that the greatest potential for protecting people and property from coastal flooding in the future lies in developing adaptation strategies for those coastal sections, where currently no dikes are present. Here we argue that more landward dike lines, as created by managed realignment, allow for a wider buffer zone between land and sea, and constitute a promising option to complement conventional coastal defense schemes.

How to cite: Kiesel, J., Honsel, L., Lorenz, M., Gräwe, U., and Vafeidis, A.: Raising dikes and the large-scale implementation of managed realignment are not sufficient to mitigate increasing flood risk along the German Baltic Sea coast, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7510, https://doi.org/10.5194/egusphere-egu23-7510, 2023.

Coastal cities and settlements play a key role in moving toward higher climate resilient development. But as urbanization trends in exposed areas continue, this will exacerbate the associated flood risk affecting climate change, including sea level rise. Understanding future sea level rise and future urban growth is essential for urban adaptation to flood risk. Here, we combined a future land use simulation (GeoSOS) model and floodplain inundation model (LISFLOOD-FP) to simulate and evaluate the impacts of future socioeconomic development scenarios under different representative concentration pathways and proposed possible adaptations measures for the coastal city Mumbai, India. The results showed that flood inundation under different scenarios showed differences. Our results confirm that sea level rise will significantly affect the coastal Mumbai urban area.

How to cite: Fang, Y.: Coastal exposure and adaptability under the sea-level rise in coastal cities: a case study in Mumbai, India, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16022, https://doi.org/10.5194/egusphere-egu23-16022, 2023.

The ability to anticipate the changes in water levels, waves and current velocities associated with storms is critical to determining the storm damages including morphological changes and coastal structure interaction. Typically, storm surge forecasts are generated using complex modeling systems such as ADCIRC, COAWST or Delft3D which solve the Navier-Stokes equations driven by the wind and wave forcing. These models often take time and effort to set up and needs significant computational resource to produce results at the required resolutions. The advantages are obvious – all the relevant physics are represented with a high degree of accuracy in these models. However, the accuracy of the models is often overshadowed by the uncertainties in the forecast of the storm itself. To capture the effects of these uncertainties, we need to resort to ensemble simulations, which brings us to the main disadvantage of these systems – they require significant computational effort to execute even one of the scenarios. Thus, to get timely information about the surge, it is necessary to either reduce the number of members in the ensemble or reduce the resolution at which the model simulates the event, thereby reducing the confidence in the model results. Here we investigate an alternative approach to storm surge predictions – use a reduced complexity model to compute the surge and compare the results to a full model as well as to data to assess the effectiveness of the models. As a case study, we will compare the two approaches using the forecasts from Hurricane Ida (2021) which impacted Louisiana. We will use the Delft3D FM system as representative of the full physics model and compare the results to that produced by SFINCS, which is a reduced complexity model. Comparisons of water levels at available NOAA tide stations are used to validate the model and quantify errors in the system. Wave statistics are compared against available buoys.

How to cite: Veeramony, J., van Ormondt, M., Kacey, E., and Allison, P.: Comparison of a reduced-complexity model to a full model for storm surge predictions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3668, https://doi.org/10.5194/egusphere-egu23-3668, 2023.

Many flat coastal plains are already subject to recurrent high-tide flooding events under calm meteorological conditions. As a consequence of sea level rise (SLR), the frequency of such harmful flooding events is expected to increase. Although not catastrophic, the coastal flooding may become a major adaptation challenge as they could cause substantial transportation and economic disruption. In many low-lying coastal zones in the world, an increasing occurrence of high-tide flooding has indeed been recorded. This is often accompanied by land subsidence. The French Guiana coast, located in northern South America, is a coastal plain where nuisance flooding has been recently observed (Cayenne city). In this study, we first performed a hindcast analysis of this recently observed chronic flooding to better understand the associated drivers. Then, we developed projections over the 21^st century for three cities of this French territory: Cayenne, Kourou and Mana.

Hindcasts and projections of relative SLR (i.e., accounting for vertical land motion) relied on CMIP5 climate model results and IPCC/SROCC estimates for the region, together with records from permanent GNSS stations in French Guiana. The daily maximum tide level was determined from hourly tide gauge observations and tide predictions. SLR and tide water levels combined with digital terrain models have allowed mapping of low-lying areas exposed to coastal flooding, and the associated flooding frequencies.

The hindcast analysis revealed the absence of significant land subsidence/uplift and suggested that the recently observed high-tide flooding in Cayenne have been fostered by climate change-induced SLR. Over the 21^st century, our results showed that urban zones exposed to high-tide flooding will progressively expand in the three cities, although at different rates. The sectors most exposed to flooding are located along urban channels and within swamps. Conversely, sandy coastlines are less exposed to chronic flooding due to their fast and dynamic evolution in response to changing hydrodynamic forcing factors. The results of this study provide the first scientific basis in French Guiana available for decision-makers and stakeholders in order to develop coastal management and adaptation strategies.

How to cite: Longueville, F., Thieblemont, R., Idier, D., D'anna, M., Bel madani, A., and Le Cozannet, G.: High-tide flooding in French Guiana due to sea-level rise, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9204, https://doi.org/10.5194/egusphere-egu23-9204, 2023.

Accelerated relative sea-level rise (SLR) rate is driven by by changing climate and vertical land motion (VLM) at the world's coastlines. Today, many coastal environments and communities, such as river deltas, wetlands, and cities experience accelerated land subsidence rates due to human-induced processes such as groundwater extraction and coastal populations ( >500M people worldwide) experience, on average, a four times higher relative sea-level rate than the global mean average.

Although subsidence is the dominant force driving relative SLR worldwide, its effect is often still overlooked and/or not fully integrated into global and regional sea-level rise projections. As a result, SLR impact assessments underlying coastal adaptation plans for many governments around the world underestimate future relative SLR rates and consequent flood risks. It is more important than ever for the global scientific community on coastal land subsidence to unite, in a similar fashion as the IPCC has come into existence in the 80's, to combat the growing global challenge of land subsidence.

For this purpose the International Panel on Land Subsidence (IPLS) (www.IPLSubsidence.org) was recently launched. The IPLS initiative welcomes all experts from disciplines related to coastal land subsidence, elevation dynamics and relative sea-level rise. The IPLS is envisioned to grow in the coming years connecting the different research communities working on coastal VLM, to become a global focal point of scientific knowledge on coastal land subsidence and create consistent contemporary rate and projections of coastal VLM for the world’s coastlines.

The IPLS aims to connect the different research communities working on VLM and elevation change in the coastal zone, consolidate knowledge and identify gaps. The IPLS’s first milestones are to 1) present the First Global Assessment Report on Land Subsidence, including 21^st-century projections of land elevation, to inform governments, scientific communities and the public worldwide and 2) propose a consistent framework to combine VLM and sedimentation dynamics across scales and disciplines, 3) properly integrate contemporary and projected VLM and coastal elevation change into IPCC's AR7.

The IPLS welcomes experts from all related disciplines to join the initiative and become part of this global effort.

How to cite: Minderhoud, P. S. J., Shirzaei, M., and Teatini, P.: Presenting the International Panel on Land Subsidence (IPLS) - Combat relative sea-level rise at global scale, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13847, https://doi.org/10.5194/egusphere-egu23-13847, 2023.

The coastline of the African Gulf of Guinea hosts large river deltas, coastal wetlands and mangrove ecosystems. It is in places densely populated, with many capital megacities with millions of inhabitants such as Lagos, Abidjan and Accra situated at the coast. And population projections for these cities suggest a continued staggering increase in the coming decades. Also, many of the economic activities are located along the coastline. For example, Nigerian coastal areas are home to 85% of the nation industry and to more than 100 million people. On the other hand, the coastland retains pristine transitional environments formed during the Holocene, well recognized for the great biological richness and the high level of endemism. These pressures and foreseen growth make these lowly elevated coastal areas particularly vulnerable to relative sea-level rise (SLR).

Regional or local impact studies of climate-change induced global sea-level rise are scarce and, when available, do not account for vertical land motion, i.e. land subsidence. Coastal land subsidence is critically under-quantified at global scale and the Gulf of Guinea region is no exception to this. Meanwhile, examples elsewhere in the World show coastal economic development and population growth can accelerate land subsidence to rates that are magnitudes larger than global SLR. With the fast-paced developments taking place along the Gulf of Guinea’s coastline, land subsidence poses an unknown but potentially large hazard to this region.

Here, we present the first findings in our work to assess coastal subsidence in this fast-changing region of the World, through regional and local case studies and satellite assessments. By building on the recent advancements in global elevation data, we aim to update coastal elevation estimates and identify low lying hotspots with high vulnerability to relative SLR. Ultimately, we aim to project coastal elevation change for this important region and, in case of human-induced land subsidence, show how much relative SLR can potentially be avoided by changing to sustainable use of natural resources. Hence, we may be able to alter the fate of deltas, wetlands and coastal cities by quantifying and addressing land subsidence as early as in this fast growing region of the world.

How to cite: Teatini, P., Minderhoud, P., Hauser, L., Bonì, R., Avornyo, S. Y., Ikuemonisan, F. E., Ohenhen, L., Wade, C. T., Zoccarato, C., Addo, K. A., Ozebo, V. C., Shirzaei, M., Ngom, F. D., and Woillez, M.-N.: Into uncharted territories – subsidence along Africa’s Gulf of Guinea coast?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9845, https://doi.org/10.5194/egusphere-egu23-9845, 2023.

Coastal hazards and extreme events are increasing in frequency and severity due to climate change, making the littoral zone even more vulnerable and requiring continuous monitoring for its optimized management. The Ebro Delta ecosystem, located in the NW Mediterranean, was subject to storm “Gloria” in the winter of 2020, the most severe coastal storm registered in the area in decades. In this study, Landsat-8 and Sentinel-2 satellites were used to monitor flooding impact and water quality status, including chlorophyll-a, suspended particulate matter, and turbidity in different time slots to evaluate pre-, syn-, and post-storm scenarios. Image processing was carried out using the ACOLITE software and the on-the-cloud Google Earth Engine platform for water quality and flood mapping, respectively, showing a consistent performance for both satellites. This cost-effective methodology allowed us to characterize the main water quality variation in the coastal environment during the storm, as well as to detect a higher flooding impact (7311 ha) compared to the one registered 3 days later by the Copernicus Emergency Service (3672 ha) regarding the same area. Moreover, the time series revealed how the detrimental impact on water quality and turbidity conditions was restored two weeks after the extreme weather event, and no phytoplankton blooms appeared during the study period neither in the Ebro Delta nor in adjacent regions. These results, obtained within the EuroSea project (H2020 grant agreement No 862626), demonstrate that the used workflow is suitable for monitoring extreme coastal events in the Ebro Delta using open satellite imagery at 10-30 m spatial resolution and four-day revisit time, thus providing valuable information for early-warning to facilitate timely assistance and hazard impact evaluation. The integration of high-resolution remote sensing tools into ecological disaster management can significantly improve current monitoring strategies, supporting decision-makers from the local to national level especially in prevention, adaptation measures, and damage compensation.

Keywords: Remote Sensing; Water Quality; Flooding; Coastal Management; Sentinel-2; Landsat-8.

How to cite: Roca Mora, M., Navarro Almendros, G., Bonnet Dunbar, M., and Caballero de Frutos, I.: Monitoring the 2020 storm Gloria in the Ebro Delta (Western Mediterranean) using Earth Observation data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14074, https://doi.org/10.5194/egusphere-egu23-14074, 2023.

Water level at the coast integrates several components from different genesis. This study focuses on the climate-related component of extreme coastal water levels, i.e. the variations in water level over time due to the atmospheric and oceanic circulation variability and their interaction. The extreme climatic component of coastal water levels mostly integrates the storm surge, which represents the sea level variations due to atmospheric wind and pressure drivers, and the wave set-up, which represents the coastal water level variations induced by wind waves. Extreme events of the climatic component of coastal water levels under storm conditions are a major hazard in coastal impacts. These extreme events significantly increase coastal total water levels, which may result in coastal damage through catastrophic flooding episodes.

This study assesses the atmospheric storm conditions in the North Atlantic Ocean linked to extreme climate-induced water levels along the Atlantic coast of Europe. A coastal water level indicator that integrates the storm surge and wind wave set-up components is computed first. North Atlantic atmospheric wind-pressure synoptic patterns are classified, identifying the storm-related patterns most probable to cause extreme water level events along the Western European coast. In addition, the track of the storms causing coastal water level extremes are identified and its main characteristics are analyzed (e.g., track orientation, storm intensity, landfall latitude). Results allow to unravel the differences between coastal regions regarding the characteristics of the storms potentially responsible for coastal impacts. For completeness, the projected changes in the storm conditions causing coastal climate-induced water level extremes by the end of the century due to climate change are explored.

How to cite: Lobeto, H. and Menendez, M.: On the understanding of the North Atlantic storm conditions responsible for coastal flooding along the Western European coast, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14474, https://doi.org/10.5194/egusphere-egu23-14474, 2023.

External surges are a key component of extreme water levels in the North Sea. Caused by low pressure cells over the North Atlantic and amplified at the continental shelf, they can drive water level changes of more than one metre at the British, Dutch and German North Sea coast. To extend the knowledge on external surges, an automated approach to detect external surges was developed, using high resolution tide gauge data. The newly collected dataset of 101 external surges spanning from 1995 to 2020 revealed the phenomenon of series of external surges, whereat two or more external surges follow each other less than 72 h apart. This phenomenon accounts for 33% of the detected external surges. Serial events tend to occur more often during wind–induced storm surges. While the inclusion of external surges in design water levels is already applied in a few design level approaches, the effects of serial external surges on storm surges have not been studied. This presentation highlights past events of combined storm surges and serial external surges and analyses the corresponding storm tracks. Based on these events, modifications for further study are proposed, creating potential high impact events, which consist of storm surges combined with serial external surges.

How to cite: Müller, A., Gerkensmeier, B., and Gönnert, G.: A new dataset of external surges in the German Bight and its application to potential high impact scenarios, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11758, https://doi.org/10.5194/egusphere-egu23-11758, 2023.

Compound floods, particularly in estuaries and coastal areas, are gaining increasing attention among the recent extreme climatic events. Understanding which driver dominates inundation depth (ID) is still an open question. In this study, a detailed and extended assessment of flood damages from 2009 to 2022 is conducted across the USA coast, based on the National Flood Insurance Program (NFIP) insurance claims records and historical storm events that occurred during the corresponding period.

To identify the relative importance of the driving mechanisms (inland vs. coastal flows) for a particular location, we propose an index [hereafter named D-Index] that identifies the topology of the local draining potentials to either the closest river, or to the coast. The D-Index captures the topographic control over ID, and it considers the vertical hydrologic distance between a location and its nearest water body, either a river stream, or the coastline.

The D-index was initially developed and validated considering 1051 simulations of historical flood events covering a time span of 40 years in Connecticut (CT), USA, and several simulated storms associated with future climate scenarios. For the analysis, we simulated river discharge time series for each event using a physically based distributed hydrological model and retrieved the storm surge from tidal stations. These time series are used as upstream and downstream boundary conditions for 2D hydrodynamic simulations. We focused the analysis on seven locations along the coast of CT for which we had available LIDAR-derived 1m DEM. To capture the variability of inundation characteristics over the full-scale gradient from river to coast, we highlight the correlation of ID to different drivers in distinct categories of the D-Index. We identified thresholds of standard deviation of the D-Index to identify areas where ID strongly correlates with either of the flood drivers. For validation, we demonstrated that it is possible to use the results obtained from the 1 m analysis to generalize the findings using coarser (still high quality) resolution DEM for the entire CT coast to derive zones dominated by surge, river flow, or the compound effect of both. The areas mapped as surge dominated based on the D-index overlap well with the SLOSH ranking.

We demonstrated the actual impacts of major events, e.g., Irene (2011) and Sandy (2012), to analyze the differences in the corresponding claims data by detecting the underlying flood drivers. To date, the claim records have been investigated based on individual drivers, for example flood caused either by excessive river flow or by coastal flooding. Hence, it is crucial to assess how compound flooding reflect on insurance flood claim records. The results obtained in this study demonstrate the potential of integrating a flood type-specific mapping system into a compound flood impact estimation. The outcome of this study will be helpful for the coastal communities to better understand their risk to the compounding impacts of various environmental forcings (heavy precipitation, surge, and the effect of sea level rise), which is important for increasing their resilience to future compound flooding events.

How to cite: Mitu, M. F., Sofia, G., Shen, X., and Anagnostou, E. N.: Assessing the Compound Flooding Risk and Impacts across the Coastal Areas of the United States, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10790, https://doi.org/10.5194/egusphere-egu23-10790, 2023.

With a total coast length of approximately 30,000 km, UK coastlines experience significant coastal erosion annually and therefore substantial resources are allocated for their maintenance and rehabilitation. According to various sources, approximately 18% of UK coastlines are protected with defense works. This shows the enormous size of the sector and highlights the importance of introducing innovations to reduce the costs of coastal defense. Traditionally, various ways were applied for coastal erosion and flooding control including rock/concrete/wood revetments, sea walls, dykes, breakwaters, groynes, beach nourishment and recycling, and sand dunes/bags. Among these methods, revetment is possibly the most popular solution that can be constructed by various materials such as rock, concrete, gabions, and wood. While concrete revetments offer convenient access to the coast and require minimum maintenance relative to rock revetments, they are more expensive and are less effective in controlling wave runup and overtopping. On the other hand, rock revetments are a more cost-effective option and provide significant wave disputation, but they occupy large spaces at the coast, require continuous maintenance and undermine the aesthetic value of beaches.

An alternate middle approach is to apply steel-wire mesh pebble/rock cells that carry the advantages of both concrete and rock revetments and minimize their drawbacks. The diamond mesh has an 8.3 cm unit width and 14.3 cm unit height, and are composed of relatively uniform rock/pebble units with diameters in the range of 20 – 25 cm. The cells are normally made to the thickness of 75 cm, but can be altered to withstand the different coastal conditions. Unlike gabions, the cells are more porous resulting in higher wave energy absorption and minimizing wave runup and overtopping. The inclusion of strong tension cables between each cell compartment when tensioned do not allow any movement in the rocks within the cells, enforcing the structure, providing stability and structural integrity making it robust for the most critical conditions.

The purpose of this research is to test the hydrodynamic performance of the system through physical modelling. In this study, we focus on a particular type of cell called the TECCO CELL^®manufactured by Geobrugg Inc. which offers marine grade stainless wire with high tensile strength steel. Such wire materials would ensure long-term durability of the system. This innovative coastal defense system was applied in Beesands (southwest England) in 2016 and yielded satisfactory performance in the past few years by the environmental bodies and the townsfolks for stopping the coastal erosion. We make 1/10 scaled models of the steel-wire mesh pebble/rock cells in the laboratory and study the interaction between the cells and incident waves by measuring various hydrodynamic performance criteria such as wave runup, transmission, overtopping and reflection. Based on our findings, we make recommendation on maximizing the hydrodynamic performance of steel-wire mesh pebble/rock cell revetments.  

How to cite: Heidarzadeh, M. and Sheibani, M.: Performance of steel-wire mesh pebble/rock cells in coastal erosion and flooding control: a physical modelling study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14732, https://doi.org/10.5194/egusphere-egu23-14732, 2023.

The Mediterranean Sea hosts two subduction systems along the convergent Africa-Eurasia plate boundary that have produced strong ground shaking and generated tsunamis.  Based on historical descriptions and sedimentary records, one of these events, in 365 CE, impacted a broad geographical area qualifying it as a ‘megatsunami’. Understanding how megatsunamis are produced, and where they are more likely, requires a better understanding of the different secondary processes linked to these events such as massive slope failures, multiple turbidity current generation, and basin seiching.

An extensive collection of cores located in distal and disconnected deep basins, identified turbidites which were analyzed using granulometry, elemental (XRF), micropaleontological, and geochemical data in order to define sedimentary processes during the propagation of the CE 365 Crete megatsunami. The deposit contains a volume of detrital siliciclastic and biogenic components as large as 800 km³. The sediment from the European and African margins was remobilized and transported by tsunamis to abyssal depositional sites and isolated basins during the catastrophic event. The resedimented deposits, when viewed across multiple geomorphological locations of the marine depositional sites (canyon mouth, abyssal, isolated higher basin), demonstrate a complex sequence of processes triggered by the megatsunami wave propagation.

The tsunamis produced multiple far-field slope failures that resulted in stacked basal turbidites. It also caused transport of continent-derived organic carbon and deposition over basal turbidites and into isolated basins of the deep ocean. The composition of sediment in isolated basins suggests their deposition by large-scale sheet like flows similar to what has been caused by the Tohoku earthquake associated tsunamis. A local turbidite bed at the base of the deposit in isolated basins on the accretionary wedge indicates that sediment remobilization from basin walls was possibly related to the passage of the tsunami in deep water. When the tsunami wave hit the continental margins in Italy and Africa, it triggered turbidity currents on the slopes, which resulted in the stacked basal sand and silt units of the resedimented deposit. The analyses of geophysical data shows the presence of additional deposits of a similar nature from older layers with an age of about 14 and 40-50 Ka. This is significant for rectifying and resolving where risk is greatest and how cross-basin tsunamis are generated.

How to cite: Polonia, A., Nelson, H. C., Vaiani, S. C., Colizza, E., Gasparotto, G., Giorgetti, G., Bonetti, C., and Gasperini, L.: Geological record of megatsunamis in Mediterranean deep sea sediments, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7827, https://doi.org/10.5194/egusphere-egu23-7827, 2023.

Coastal boulder deposits of western Ireland include over 1200 boulders and megagravel documented to have moved during storms in the winter of 2013 - 2014. The data come from seven areas on the mainland and outlying islands. The mass spectrum for the transported clasts ranges from 0.1 tonnes for the lightest boulder to 620 tonnes for the heaviest megagravel. This is a unique dataset that provides the opportunity for detailed quantitative analysis of dislodgement criteria.

We employ the Boulder Dislodgement and Sliding Model of Weiss et al (2022) to calculate dislodgement flow velocities for each boulder. In each of the seven areas, we derive the dislodgement flow velocity to mass (DV-M) relationships for each area and perform hypothesis tests to decern if the DV-M relationships in the different areas are governed by (a) the characteristics of the geological formation and topography from which the boulders are quarried, (b) the characteristics of the storm events, such as significant wave heights and period, or (c) a interactsions/combined influence of (a) and (b).

Interrogating the boulder data and model results objectively, with statistical and other quantitative methods, will help to shed light on the question of whether coastal boulder deposits are useful event records in the context of non-linear wave-wave interactions nearshore that may result in significant under- and overestimations of the causative event if nonlinear wave-wave interactions are ignored. For example, constructive wave-wave interactions can result in a wave condition that is much larger and dislodges a significantly heavier boulder than the individual waves could. Similarly, destructive wave-wave interaction can also cause much smaller wave conditions than the individual waves indicate and cause only the dislodgment of much lighter boulders than the individual waves would.

The wide range in boulder masses and topographic settings n the dataset can help to establish a better understanding of potential uncertainties for dislodgement velocities in general; and this approach should be valuable for application in other situations, for example, for isolated boulders in coastal settings or for cases in the stratigraphic record. 

How to cite: Weiss, R. and Cox, R.: Boulder Dislodgement By Storms: What can we learn from the boulder fields Central-Western Ireland?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10562, https://doi.org/10.5194/egusphere-egu23-10562, 2023.

Large boulders lying at abnormally high levels are common proxies to determine the occurrence of extreme past geophysical events such as storm or tsunami. Empirical formulas are derived to reconstruct the event conditions associated to the boulder displacement (e.g., Nott, 2003). Up to now, to the best of our knowledge, no one directly identified the individual wave responsible for the movement of a large boulder. In this paper, we report such an observation and analyze the associated hydrodynamic conditions.

On the 2/28/2017, a large concrete block of about 50T, protecting the Artha breakwater in Saint Jean de Luz (south west of France) was displaced from the block armor unit to the crest of the structure during a large storm event. The vertical displacement of the block is assessed to a minimum of 5.3m. This type of event is quite rare (last known date is 1951), nevertheless, this time, the scene was by chance captured by a photographer taking pictures of the storm at the same moment. This allowed assessing the approximate time at which the event occurred and analyze the corresponding data available locally at this time namely : waves (integrated parameters and free surface signal 1 km off-shore), water level and the associated photographs.

First, the water level is found to be close to its maximum (high tide with high tidal coefficients). During the storm, the significant wave height evolved between 5.61 to 7.47m, and the maximum wave height from 8.53 to 11.11m. Surprisingly, at the time of the block displacement, these parameters were not maximum (i.e., 6.8m and 8.53m, respectively). Nevertheless, the investigation of the free surface signal shows the existence of a large wave of more than 14m high and about 25s long approximately 1min30 before the block displacement, not detected by the zero-crossing wave buoy algorithm. This individual wave satisfies the definition criterion of a rogue wave (i.e., H_wave/H_s>2.2).

With 1D phase resolving wave simulations, we first validated the wave propagation time from buoy to breakwater (i.e., 1min30) and assessed the reflection component at the buoy. The latter was considered as sufficiently moderate at the time of observation to consider the 14m recorded wave as an incident one. Next, we used a combination of two models nested : SWASH (1D) and the Navier-Stokes OLA-FLOW (2DV), to simulate the event considering the free surface signal as input. The hydrodynamical conditions obtained with the computations were finally compared to the existing empirical formulas and discussed.

The simulations show that the long rogue wave keeps its coherence up to the breakwater and generates a strong and abnormally long duration flow above and within the blocks (i.e. the porous medium). This result strengthens the idea that the force duration (along with force magnitude), is an important factor to consider to predict large boulder or concrete block displacement. This study also shows a possible original effect of rogue waves in the nearshore area.

How to cite: Abadie, S., Burkhart, E. P., and Parvin, A.: Large concrete block displaced by a rogue wave : analysis of an event incidentally captured by a photographer, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3336, https://doi.org/10.5194/egusphere-egu23-3336, 2023.