• Introduction

Sand aprons are ubiquitous depositional sedimentary features that offer insights into the sediment dynamics of coral reef environments. Global studies found that the extent of sand aprons are not related to reef platform size and their widths are a function of environmental factors such as swell period and height, tidal amplitude, latitude, and exposure to wind and waves (Rankey and Garza-Perez 2012).  Recent studies using numerical modelling have found that sand aprons in reef flats attain a critical water depth resulting in constant depth (Ortiz and Ashton 2019), and that lagoon infilling through sand apron progradation is a self-limiting process (Rankey 2021). Sand apron progradation is an eco-morphodynamic process and climate change, including intensification and increased frequency of marine heatwaves, ocean and coastal acidification, and changes in wave and tropical storm climates are triggering changes that need to be understood to inform sound management of coral reefs.

This paper presents data on the Holocene evolution of the sand aprons on 21 offshore platform reefs located on the southern Great Barrier Reef and how it can be used to infer past wave climates. We then present the recent wave climate for the study area (Smith et al. 2022) and analyse sand apron evolution accordingly.

• Methods

The sand aprons on 21 reefs located on the Capricorn Bunker Group (Southern Great Barrier Reef) were assessed from high-resolution satellite imagery, obtaining digital bathymetric models and digitizing the reef and lagoon contours. We then measured reef area and lagoon area to calculate the percentage of lagoon infilling. The wave climate for the study area was obtained from satellite altimetry using an open-source Python tool (Smith et al. 2020).

• Preliminary findings

Our results showed that the most important factor for lagoon infilling was the size of the reef, with larger reefs typically appearing less infilled than smaller reefs. Wave incidence seemed to be unimportant: the three reefs with less than 50% infill were all medium-sized and exposed to incident waves while all six protected reefs had infilling above 50%. While some authors had pointed out at relative sea-level changes to explain current sand apron stability (Harris et al. 2015), our results show that the self-limiting nature of the sand apron progradation, combined with relative sea-level changes, is a better explanation for sand apron stability. In any case, the extent of the sand apron can be used to infer wave climate at the time of sand apron progradation. For example, for One Tree Reef, one can argue the sand apron could had stopped prograding 4 ka BP because of the self-limiting sediment transport and remained stable until the sea-level fell.

The wave climate on the Southern Great Barrier Reef is characterized by significant wave heights (Hs) of 1.7 m and has been stable for the past 33 years (Smith et al. 2022). Future changes in the wave climate, storm frequency, increases in sea level and changes to sediment availability caused by anthropogenic climate change will modify the eco-morphodynamics of the sand aprons and the percentage infilling of the lagoons.

How to cite: Vila-Concejo, A., Hamylton, S., Fellowes, T., and Salles, T.: Wave climate and sand apron development on the southern Great Barrier Reef, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4585, https://doi.org/10.5194/egusphere-egu23-4585, 2023.

Coastal zones are low-lying areas that support highly dynamic and productive ecosystems of great ecological and economic value. Anthropic and natural processes coexist and interact between them mediated by environmental fluctuations. The nature of coastal areas makes them susceptible to climate change effects. For instance, sea level rise and the increased frequency and intensity of storms and surges have a great impact on the morphodynamics of sandy beaches. Understanding how these environments behave under today's changing conditions is key to proposing efficient adaptation measures and management strategies.  However, the wide range of modulators involved in beach morphodynamics, and their high dynamism, make the integrated monitoring of these areas costly (time, human, and  economic resources) and challenging. Despite technological improvements and the increased availability of low-cost instrumentation and data (video monitoring systems, satellites’ observations, near-real-time oceanographic instruments/data), long-term and high-frequency data-sets, including morphological and wave data, remain scarce. 

Since 2011, the ICTS SOCIB (Balearic Islands Coastal Observing and forecasting System) has been monitoring three beaches of the Balearic Islands through the deployment of Modular Beach Integral Monitoring Systems (MOBIMS). MOBIMS aims to fill the gap of high-resolution and continuous beach monitoring by combining data from hybrid field surveys-remote sensing systems. MOBIMS is composed of low-cost open-source video monitoring imagery (SIRENA), Acoustic Wave and Current Profilers (AWAC), meteorological stations, and bi-annual high-resolution bathymetries and topographic surveys, as well as sediment granulometry. 

In this study, we present the analysis of the Son Bou Beach (Menorca, Spain) MOBIMS dataset generated over the last 12 years (2011-2022). The analysis focuses on characterizing the response of Son Bou beach to extreme events (i.e., storms), by means of shoreline position-change detection. Over 170 shorelines were derived from the SIRENA video-monitoring system, and meteorological and oceanographic data corresponding to 150 coastal storms were collected. The most energetic events eroded the beach, moving the shoreline landward significantly; but, accretive storms were also found, increasing the width of the beach. The presence of a coastal lagoon and the well-preserved dunes was crucial to understanding the beach response and its behaviour under different wave conditions. 

How to cite: Criado Sudau, F. F., Fernandez-Mora, À., Soriano-González, J., Gallo, M., Gómez-Pujol, L., Orfila, A., and Tintoré, J.: Erosive and accretive response of a natural beach to storm events, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16017, https://doi.org/10.5194/egusphere-egu23-16017, 2023.

Long-term modeling (decades) of shoreline changes cannot be easily challenged with physics based models. The best alternative is to use simple behavioral template models (Davidson & Turner, 2009), all the complex cross-shore erosion/accretion processes being encapsulated in a few parameters. Most of these cross-shore models draw on the phenomenological idea that a beach relaxes towards equilibrium (Wright & Short, 1984). Calibrated against reliable data series of cross-shore changes, this type of model reaches good predictive skills (Splinter et al., 2014; Castelle et al., 2014). However these shoreline models need to be improved by taking into account long-shore process (Robinet et al., 2018).

This paper addresses the feasibility of a combined model that includes longshore sediment transport effects in a relaxation type cross-shore shoreline evolution model. Longshore transport produces long-term changes of the beach morphology and shoreline position. The longshore contribution is worked out on the basis of the one-line approach in which the shoreline position depends on the alongshore gradient of the volumetric sediment transport rate change. The analysis, which decomposes time dependent variables into averages and fluctuations (Reeve et al., 2014), provides (i) a relationship between the equilibrium beach angle and the wave forcing angle and (ii) a shoreline evolution equation for longshore transport only. This model is merged with the Splinter et al. (2014) behavioral model. This combined model is calibrated an tested on the Narrabeen (Australia) semi-embayed beach data (Turner et al., 2016). The combined model reproduces with good agreement the shoreline trends and variability. We show that the longshore component clearly contributes to the seasonal shoreline fluctuations. The model is also applied to low energetic beaches of the Vietnam coast (Nha Trang and Da Nang).

Davidson, M., Turner, I., 2009. A behavioral template beach profile model for predicting seasonal to interannual shoreline evolution. Journal of Geophysical Research: Earth Surface 114.

Wright, L., Short, A., 1984. Morphodynamic variability of surf zones and beaches: a synthesis. Marine geology 56, 93–118.

Splinter, K., Turner, I., Davidson, M., Barnard, P., Castelle, B., Oltman-Shay, J., 2014. A generalized equilibrium model for predicting daily to interannual shoreline response. Journal of Geophysical Research: Earth Surface 119, 1936–1958.

Castelle, B., Marieu, V., Bujan, S., Ferreira, S., Parisot, J., Capo, S., Sénéchal, N., Chouzenoux, T., 2014. Equilibrium shoreline modelling of a high-energy meso-macrotidal multiple-barred beach. Marine Geology 347, 85–94

Robinet, A., Idier, D., Castelle, B., Marieu, V., 2018. A reduced complexity shoreline change model combining longshore and cross-shore processes: The LX-Shore model. Environmental Modelling & Software 109, 1–16.

Reeve, D., Pedrozo-Acuña, A., Spivack, M., 2014. Beach memory and ensemble prediction of shoreline evolution near a groyne. Coastal Engineering 86, 77–87.

Turner, I., Harley, M., Short, A., Simmons, J., Bracs, M., Phillips, M., Splinter, K., 2016. A multi-decade dataset of monthly beach profile surveys and inshore wave forcing at Narrabeen, Australia. Scientific Data 3.

How to cite: Barthelemy, E., Tran Hai, Y., Almar, R., and Marchesiello, P.: Long-term shoreline evolution.  A combined cross-shore and long-shore model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12380, https://doi.org/10.5194/egusphere-egu23-12380, 2023.

Ocean-facing shorelines experience morphological changes on many temporal and spatial scales in response to various processes such as wave breaking, overwash, as well as wave- and wind-driven currents. The nature of coastline response can vary due to several factors, including the underlying sub-bottom stratigraphic structure, surficial sediment type, and local vegetation cover, among others. These eco-geomorphic changes are significant for both understanding coastal community hazards and infrastructure planning for short- and long-term shoreline stability.

Cape Cod Bay, MA, is a semi-enclosed embayment in the northeastern United States, open on the north to the Gulf of Maine. Typically, the coastline experiences the largest impacts from strong Nor’easter storms that occur in the late fall or winter months. Some sections of this coastline are affected more severely than others. We investigate the processes that cause spatial variability of coastal response to storm impacts by using geophysical surveys, shoreline-change analysis, and numerical modeling.

We simulated the Gulf of Maine and Cape Cod Bay from Jan – April, 2021, using the COAWST modeling system, including ocean, wave, infragravity wave (InWave), and sediment transport models, with an initial focus using a uniform seafloor sediment distribution. This time period included several strong Nor’easter events. The modeling used several grids to simulate bay-scale (order 100’s meters for Cape Cod Bay) down to nearshore-scale (order several meters for an 18km section of coast) processes. Bay-scale results produce storm-driven circulation of landward surface flows and seaward near-bottom currents, alongshore sediment fluxes, and sediment convergences at regional shoals. Nearshore modeling identified the largest impact from storm events when the surge, tide, and strongest waves all coincided. Analysis of the InWave model results reveal localized zones of increased wave heights, spaced along the 18km section of coast, due to bathymetrically induced alongshore convergence of wave energy flux. These locations correlate with observed regions of increased erosion and severe coastal impacts. Modeled and observed shoreline-change demonstrate locations of high correlation but the model does not capture all the variability. Computed net sediment fluxes for the modeled time period along the coast agree with regional sediment flux observations.

How to cite: Warner, J. C., Brothers, L., Himmelstoss, E., Sherwood, C., Foster, D., and Farris, A.: Coastal Erosion Processes along Cape Cod Bay, MA, USA, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10008, https://doi.org/10.5194/egusphere-egu23-10008, 2023.

Modelling the response of sandy beaches to sea level rise is a major scientific challenge and several types of models can be applied. Process-based models are useful tools for understanding beach responses on short time scales, but they are computationally expensive and tend to accumulate errors when resolving the many short-term processes they contain. Alternatively, reduced-complexity models can be an interesting option for long-term modelling. Furthermore, to simulate the long-term response of beaches, it is necessary to combine different model forcing sources (wave and sea-level, e.g., from buoys or hindcast models). The aim of this contribution is to quantify the effect of using different forcing sources in morphodynamic modelling with these model types.

Two numerical models are applied to the embayed microtidal beach of El Castell (Palamós, Catalunya, NW Mediterranean). The XBeach process-based model, which solves the full 2DH nearshore hydrodynamics and the corresponding bed evolution, and the Q2Dmorfo reduced-complexity model, which simulates the bed level variations by calculating the sediment fluxes parametrically directly from the wave field without resolving the currents. Both models are first calibrated using two topobathymetric surveys conducted in January and July 2020 and wave and sea-level data measured from an AWAC deployed at 14.5 m depth during those 6 months. Calibration is performed using the most sensitive parameters, i.e., those related to cross-shore transport. In XBeach, the surfbeat mode must be used to obtain realistic results. It generates an aleatory spectral wave time series at the boundary that includes groupiness. To handle the effect of this randomness, a total of 5 realizations are made for each set of parameter values and a mean bathymetry is computed out of these realizations. To assess the model performance, the Brier Skill Score (BSS) is calculated both for the modelled bathymetry and its coastline during the 6-month period. Moreover, the Standard deviation (STD) of the 5 realizations is also computed. The chosen optimum parameter setting is the one maximizing the BSS (0.36 and 0.79 for the bathymetry and shoreline respectively) and minimizing the STD so that the result is robust and reproducible. In Q2Dmorfo, the BSS of the simulated final coastline is calculated for each set of parameter values. The optimum parameter setting also produces a maximum BSS of 0.79.

Once both models are calibrated, the other potential forcing sources are applied. They include wave and sea-level datasets from a sea-level and wind-waves 72-years hindcast generated with the hydrodynamic-wave coupled SCHISM model, a wave dataset from an offshore buoy propagated to the AWAC position using SWAN model and a sea-level dataset measured by the tidal gauge at Barcelona harbour. The results show a very significant sensitivity to the wave forcing source and much less sensitivity to the sea-level source. By using the dataset propagated from the buoy by SWAN, both models represent well the observed beach rotation, whereas using the dataset obtained with SCHISM, the beach rotation is systematically under-predicted by both models, giving negative BSS values. This is because SCHISM predicts wave angles biased to the west.

How to cite: Carrion, N., Falqués, A., Ribas, F., Calvete, D., de Swart, R., Durán, R., Marco-Peretó, C., Marcos, M., Amores, A., Toomey, T., Fernández-Mora, À., and Guillén, J.: Morphodynamic modelling of an embayed Mediterranean beach: effect of the forcing sources, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5349, https://doi.org/10.5194/egusphere-egu23-5349, 2023.

Wind-induced airflow acceleration over irregular foredunes promotes the genesis of sandy depressions named blowouts. During their development, they transfer sediment to the back-dune helping to maintain the available barrier sediment budget while promoting eco-geomorphological feedbacks that regulate the plant community and biodiversity. However, the mechanisms controlling their dynamics and evolution are still not well understood and demand further research, as these are key landforms for the future management of coastal dunes under rising seas and climate change. This work aims to identify the internal (e.g. blowout morphometric characteristics) and/or external factors (e.g. metocean conditions, human interventions) influencing the migration and spatiotemporal evolution of a series of blowouts present in the foredune of a coastal stretch of 1.3 km situated in Ancão Peninsula, South Portugal. To achieve this, their morphometric characteristics (area, orientation, width, length, width-length ratio and centroid position) were mapped and their changes analysed over time, together with the storm frequency, wave power, dune toe location and anthropogenic interventions in the area. The previous was done using a 49-year set of historical aerial photos, orthophotos, and Google Earth images as well as time series of metocean conditions (from 1972 to 2021). The estimated Kendall’s bivariate correlation coefficients showed that, with 0.1 significance level, blowout migration rates were positively dependent on blowout area, width, length, orientation and dune toe retreat. Besides, fastest migration rates occurred in narrower and lower dune crest areas as these offer less resistance to erosion. It was not possible to obtain certainties on the statistical dependencies with the metocean conditions due to the low temporal image resolution at the beginning of the study period. Nevertheless, two main phases of significant dune toe retreat (1996-2001 and 2008-2011) were concomitant with the impact of several extreme storm clusters reported in the literature (1998-2000 and 2008-2009). The role of extreme events in the study area is threefold: (1) shoreline erosion and dune scarping, which further debilitates the foredune, (2) increase in the total blowout area as the widths and lengths of the medium and large blowouts increase, and (3) disappearance of small blowouts as well as blowout genesis. Lastly, the jetty construction updrift (finished by 1972) seemed a very likely trigger of the initial dune instability in the area while foredune fencing promoted the artificial sealing of blowouts, which showed approximate sealing times of 4 years after fencing implementation.

How to cite: Talavera, L., Costas, S., and Ferreira, Ó.: Factors controlling blowout morphodynamics and evolution in southern Portugal, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8084, https://doi.org/10.5194/egusphere-egu23-8084, 2023.

Onshore wind is the primary driver of sediment transport allowing the construction of coastal dunes. In contrast, offshore winds result in the seaward export of sediment inducing a loss of the terrestrial beach sedimentary budget. Many parameters limit aeolian sand transport such as moisture, beach slope, beach length, vegetation and sediment characteristics. Although the impact of grain size on wind transport is well known, few studies have focused on its temporal variability. The temporal evolution of grain size characteristics is particularly important in microtidal environments where the relatively small tidal range minimises the mixing of the beach sand and winds have a strong impact on grain size sorting, resulting in the coarsening of the beach grain size. Leucate beach (SE, France) is a wave-dominated microtidal environment, subject to a strong offshore wind (72 % of the annual time and 17.5 % over 10m/s) which made this site suitable to this study. During the 19 months of meteorological surveys, 5 field measurements campaigns of 1 to 3 days were conducted. For this purpose, wind processes (intensity and direction); aeolian sand transport (24 runs), morphological variations of the beach-dune system and also many sub-surface sediment samples were collected.

The results show a large temporal variability in beach grain size ranging from medium to very coarse sand in relation to wind and wave conditions. Aeolian processes produce a coarser beach grain size at days/months scale, whereas short marine storms (day) induce a mixing and finer  beach grain size, resulting  in very different  aeolian sediment transport values for  similar incident wind conditions. For example, with a wind speed of 10 to 14 m/s the measured sediment flux ranged from10 kg/m/h (coarse beach grain size) to 50 to 150 kg/m/h (medium beach grain size). Morphological variations of the upper beach surface are not significant when the sand is coarse but can cause lower by the upper beach surface by 0.5 m when the sand is composed of medium-sized. The time scale of the temporal beach grain size variations is closely related to the frequency and intensity of marine and wind storms. This study quantifies the effect of the beach grain size variability on the aeolian sand transport and thus the morphological changes of the beach. We conclude that because of their importance in the temporal variability of sediment size and the inherited sedimentological framework of the beach, it is crucial to take into account marine and aeolian processes to refine the predictions of wind transport rates. This confirms the need in a microtidal environment, to obtain beach grain size temporal data to better understand the aeolian sediment transport rates affecting a study site and not underestimated its impact when calculating transport rates with empirical formulas and numeric models.

How to cite: Lamy, A., Robin, N., Hesp, P., Smyth, T. A. G., René, C., Feyssat, P., and Hebert, B.: Aeolian sand transport and beach morphology influenced by temporal beach grain size variability in a microtidal environment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12389, https://doi.org/10.5194/egusphere-egu23-12389, 2023.

Coastal dunes are important habitats that provide a variety of ecosystem services (ecological, economic, coastal protection, etc.) and, as such, their monitoring is a priority for environmental protection (i.e., EU Directives). Eco-geomorphologic feedbacks between dune plants and coastal topography are fundamental to the self-organisation capacity of coastal dunes and a shift in community structure and composition (i.e., expansion of invasive species) can cause a domino effect, potentially crippling previously established system adaption mechanisms. It follows that monitoring dune vegetation is crucial, especially in protected areas and in fragmented and stressed dune environments. Even though recent improvements in spectral and spatial resolution of satellite imagery open new and exciting prospects for large-scale environmental monitoring, this potential is largely unused in dune ecogeomorphology, due to the challenges related with the small size and density of dune plants and the complexity and heterogeneity of the existing species. Machine learning techniques and subpixel classification methodologies, like the Random Forest Soft Classification (RFSC), have shown promising results in similarly challenging environments in terms of plant size and heterogeneity, with high accuracies in subpixel fractional abundance of marsh-vegetation species. Even though subpixel classification could improve monitoring biodiversity from satellite imagery, similar approaches have never been tested for dune environments. These challenges and gaps inspired the present work, built around the idea of testing subpixel classification methods for dune plant species identification using high-resolution satellite imagery. Here we present preliminary results from the application of RFSC to the western barrier of the Ria Formosa system (S. Portugal) using WorldView2 (2017) imagery and training/validation samples from UAV, along with the next steps planned to test the hypothesis that RFSC methods can be successfully used to identify dune plant species and to assess their predictive capacity and identify potential limitations.

Acknowledgements: The work was implemented in the framework of the DEVISE project (2022.06615.PTDC), funded by FCT (Fundação para a Ciência e a Tecnologia) Portugal. The authors acknowledge the project DUNES (52334), funded by ESA (European Space Agency), for the acquisition of the WorldView2 imagery used.

How to cite: Kombiadou, K., Costas, S., Yang, Z., and Silvestri, S.: Preliminary results for dune vegetation identification from high-resolution satellite imagery, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15730, https://doi.org/10.5194/egusphere-egu23-15730, 2023.

Abstract

Coastal dunes play an essential role in defence against the sea in many countries, including the Netherlands. Sandy Anthropogenic Shores (SAS) is the recent nature-based human intervention for dune reinforcement. SAS are coastal zones (including shores, dunes, lagoons, etc.) that are created or heavily modified by moving large amounts of dredged sand from offshore to near the coast. This allows natural processes (waves, winds, and currents) to spread the sand and reinforce the foredune for longer-term coastal safety. Not only the natural processes but also vegetation cover near the dune foot and on the foredune play essential roles in trapping windblown sand from the coast, steering embryo dune development and dune growth. The Sand Motor and Hondsbossche Dunes are two examples of SAS in the Netherlands. Previous studies of vegetation on SAS have mainly assessed the influence of natural conditions such as climate change, nutrient availability, sand burial, beach shape (morphology and width), hydrodynamic wave characteristics, etc., on vegetation propagation. The impacts of short-term human management of the coast and recreational activities still need to be addressed. We chose an agent-based model (ABM) approach to simulate human activities, including management and use of SAS and coupled it with a biophysical model similar to DUBEVEG. DUBEVEG is a biophysical model simulating the interactions between natural processes caused by wind, wave and tide, vegetation cover, and beach-dune sediment dynamics. We conducted several interviews and workshops with stakeholders from the management sectors. We also surveyed the beach users to elicit knowledge about social dynamics and their interaction with the biophysical system (morphology and vegetation). Using NetLogo, we developed an ABM, translating the elicited knowledge into a quantitative model. The developed ABM was used to analyse how various landscape designs of the modified coast (e.g., types and locations of recreational facilities such as restaurants, beach houses, artificial lagoons, entrances, car parking, etc.) influence human activities. Then, we used the model to explore the impact of human dynamics on vegetation growth and embryo dune development in the long term. The developed ABM model improves the sustainable design and management of the SAS by exploring the influence of SAS's initial design and short-term decisions about recreational activities and flood safety measures on the long-term evolution of the landscape (vegetation and morphology).

How to cite: Bakhshianlamouki, E., Augustijn, E.-W., Wijnberg, K., Voinov, A., and Brugnach, M.: Building an agent-based model to explore the interactions between human activities and vegetation cover on sandy anthropogenic shores (SAS) in the Netherlands, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3177, https://doi.org/10.5194/egusphere-egu23-3177, 2023.