GM6.1 | Coastal morphodynamics: nearshore, beach and dunes

Examining the morphodynamics of coasts from the nearshore through to inland dune systems, is a fundamental requirement in understanding their short- to long-term behaviour. Operating across large spatial and temporal scales, examination of their resulting landforms is both difficult and complex. Recent methodological advances, however, now enable traditionally isolated coastal disciplines to be examined across various zones, promoting integration along multiple time and space scales, helping to couple processes with landform responses.

At the coast, dunes provide a physical barrier to flooding during high energy storms, while beaches and nearshore areas help dissipate storm impact through a series of dynamic interactions involving sediment transfers and sometimes rapid morphological changes. Examination of complex interactions between these three interconnected systems has become essential for the understanding, analysis and ultimately, the management of our coasts.

This session welcomes contributions from coastal scientists interested in the measurement and modelling of physical processes and responses within the three sub-units over various spatial and temporal scales. It will highlight the latest scientific developments in our understanding of this part of the planet's geomorphic system and will facilitate knowledge exchange between the submerged (e.g., nearshore waves, currents, and sediment transport) and sub-aerial (e.g., beach and aeolian dune dynamics) zones.

Convener: Emilia Guisado-Pintado | Co-conveners: Derek Jackson, Irene Delgado-Fernandez, Susana Costas, Melanie Biausque, Edoardo Grottoli
Orals
| Wed, 26 Apr, 08:30–10:15 (CEST)
 
Room G1
Posters on site
| Attendance Wed, 26 Apr, 10:45–12:30 (CEST)
 
Hall X3
Posters virtual
| Attendance Wed, 26 Apr, 10:45–12:30 (CEST)
 
vHall SSP/GM
Orals |
Wed, 08:30
Wed, 10:45
Wed, 10:45

Orals: Wed, 26 Apr | Room G1

Chairpersons: Emilia Guisado-Pintado, Susana Costas
08:30–08:35
Coral reef dynamics
08:35–08:45
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EGU23-4585
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GM6.1
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solicited
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Highlight
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On-site presentation
Ana Vila-Concejo, Sarah Hamylton, Thomas Fellowes, and Tristan Salles
  • 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.

08:45–08:55
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EGU23-10588
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GM6.1
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ECS
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On-site presentation
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Lachlan Perris, Ana Vila-Concejo, Tristan Salles, and Thomas Fellowes

Coral reefs are the most biodiverse and productive ecosystems on Earth. Changes to global climate will alter the conditions required for coral reef survival. In atoll reefs, coral species distribution and survival is determined by waves and tidally induced flows. The outermost part of atoll reefs, the forereef, presents a key morphological control over wave attenuation and consequently nutrient distribution at the scale of entire reefs. These high-energy environments provide protection from the effects of wave inundation to over 200 million people globally. Despite this, the mechanisms of wave transformation over the complex bathymetries of forereefs have received little attention. Here we focus on processes of wave transformation and attenuation by the elongated troughs and depressions typical of forereef slopes known as spurs and grooves (SaGs) and provide a better understanding of the morphodynamics of SaGs. We combine a quantitative morphometric analysis with wave transformation modelling and hydrodynamic field data from One Tree Reef (OTR) of the southern Great Barrier Reef, Australia. We used a 50 cm resolution LiDAR bathymetry dataset and novel methods of morphometric spectral analysis at swell wave exposed, semi-exposed, and protected locations. The wave transformation model, XBeach, was calibrated and validated with published SaG field data and wave data obtained from a 33-year analysis of satellite altimeters. The effects of SaGs on wave energy dissipation were examined under various forecasted climate change scenarios (RCP 2.6, RCP 8.5, and a disaster scenario) to elucidate their role in preventing future coastal hazards. Finally, field data collected from a fourth site on at OTR over a three-day period in October 2022 used an array of two acoustic doppler velocimeters and an 8 Hz pressure transducer. Current and wave measurements were taken in a long (140 m) and deep (max spur height = 5.3 m) groove under typical wave conditions (Hs = 0.8 m, Tp = 7 s). Findings from the numerical models suggest SaGs play a critical role in dissipating wave energy, increasing wave dissipation by bed friction by 75%. Our results demonstrate that a decrease in wave energy dissipation results in an exponential increase in wave overtopping with storm waves producing overtopping > 3 m under worst-case scenarios. Additionally, we found that high bathymetric gradients in SaGs increase dissipation by wave breaking by up to 52% under RCP 8.5, leading to a 71% increase in mean wave energy dissipation despite an 89% reduction in bed friction factor and a 1 m increase in relative sea level. Field observations demonstrate the wave and tide-dominated flow regimes through forereef grooves. Future work aims to investigate the form and process feedbacks in forereef hydrodynamics and consider global to regional scale climate forecasts. To facilitate this, we provide field data and analytical codes open source. Understanding the morphodynamics of forereefs is critical to forecasting reef health, wave transformation, and coastal hazard reductions under climate change conditions.

How to cite: Perris, L., Vila-Concejo, A., Salles, T., and Fellowes, T.: The Influence of Coral Reef Spur and Groove Morphology on Wave Transformation and Attenuation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10588, https://doi.org/10.5194/egusphere-egu23-10588, 2023.

Beach and Nearshore processes
08:55–09:05
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EGU23-16017
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GM6.1
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ECS
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On-site presentation
Francisco Fabián Criado Sudau, Àngels Fernandez-Mora, Jesús Soriano-González, Marcos Gallo, Lluís Gómez-Pujol, Alejandro Orfila, and Joaquín Tintoré

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.

09:05–09:15
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EGU23-12380
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GM6.1
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Highlight
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On-site presentation
Eric Barthelemy, Yen Tran Hai, Rafael Almar, and Patrick Marchesiello

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.

09:15–09:25
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EGU23-10008
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GM6.1
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Highlight
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On-site presentation
John C. Warner, Laura Brothers, Emily Himmelstoss, Chris Sherwood, Dave Foster, and Amy Farris

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.

09:25–09:35
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EGU23-5349
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GM6.1
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ECS
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On-site presentation
Nil Carrion, Albert Falqués, Francesca Ribas, Daniel Calvete, Rinse de Swart, Ruth Durán, Candela Marco-Peretó, Marta Marcos, Angel Amores, Tim Toomey, Àngels Fernández-Mora, and Jorge Guillén

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.

Dune processes
09:35–09:45
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EGU23-8084
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GM6.1
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ECS
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Highlight
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On-site presentation
Lara Talavera, Susana Costas, and Óscar Ferreira

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.

09:45–09:55
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EGU23-12389
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GM6.1
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ECS
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On-site presentation
Antoine Lamy, Nicolas Robin, Patrick Hesp, Thomas A.G. Smyth, Camille René, Pierre Feyssat, and Berthil Hebert

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.

09:55–10:05
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EGU23-15730
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GM6.1
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On-site presentation
Katerina Kombiadou, Susana Costas, Zhicheng Yang, and Sonia Silvestri

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.

10:05–10:15
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EGU23-3177
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GM6.1
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On-site presentation
Elham Bakhshianlamouki, Ellen-Wien Augustijn, Kathelijne Wijnberg, Alexey Voinov, and Marcela Brugnach

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.

Posters on site: Wed, 26 Apr, 10:45–12:30 | Hall X3

Chairpersons: Susana Costas, Emilia Guisado-Pintado
Beach and nearshore processes
X3.11
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EGU23-4602
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GM6.1
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ECS
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Highlight
Thomas Fellowes, Ana Vila-Concejo, Eleanor Bruce, Maria Byrne, and Elaine Baker

Coral reef islands are under threat from warming and rising seas, ocean acidification and increased storminess. These islands are low-lying accumulations of sediment derived from a continual supply of shells and skeletons from calcifying reef organisms. Over 200 million people rely on reefs and their islands for their livelihoods, including Small Island Developing States (SIDS). Coastal States and SIDS commonly use coral islands to support and extend their maritime jurisdictions. Coral islands are morphologically active (e.g., erode, accrete, migrate) across broad spatial-temporal scales and have been extensively studied using historical aerial imagery, satellite imagery and satellite derived bathymetry (SDB). The future of coral islands and the reefs that support them is not certain, and it is unclear what eco-morphological tipping points may cause constructive and destructive impacts. A better understanding of coral island stability and evolution to changing conditions is urgently needed. Here we focus on 31 coral islands on 10 offshore coral reefs that extend Australia’s maritime jurisdictions in the Coral Sea (SW Pacific) and NW continental shelf (Timor Sea). We digitised island morphology (e.g., shoreline positions, island area and shape) from imagery and SDB (1976-2022) and compared this to local ocean and climate data (e.g., cyclone tracks, sea surface temperatures (SST), sea-level rise) to identify potential tipping points and processes. Initial results show that since the 1970s a third of the coral islands were stable (n=9; <3% change in area), half increased in size (n=15) and a fifth decreased in size (n=7). As expected, small (<10 Ha) and unvegetated islands were more active when compared to large and vegetated islands. Meanwhile, we suggest observed increases in island size may be a short-lived response to increasing SST and marine heatwaves that can degrade reefs and produce additional sediment, combined with tropical cyclones that have the capacity to transport additional sediments to islands. The question remains whether islands will survive if reefs continue to degrade, and sediment supply is reduced. Any island loss will have serious social, environmental and geo-political consequences.

How to cite: Fellowes, T., Vila-Concejo, A., Bruce, E., Byrne, M., and Baker, E.: Eco-morphodynamic stability of low-lying coral reef islands in a changing climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4602, https://doi.org/10.5194/egusphere-egu23-4602, 2023.

X3.12
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EGU23-15658
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GM6.1
Emilia Guisado-Pintado and Manuel Fermin Isla

Sandy coastal systems are very dynamic systems in which morphological changes occur over different time scales that ranges from hours to decades. However, it has been widely reported that major storms are the main responsible of the most significant changes in short to medium time scales. Major storms have been defined using a variety of environmental variables, but they are normally associated with high values of four main parameters: 1) Wave heights (Hs); 2) Duration (Du); or 3) Frequency (Fq); 4) Orientation (Or).

In this study we aim to characterise types of major storms and to categorize morphological impacts over a hybrid coastal system. The study site, known as Punta Rasa, is located in the Samborombón bay in the outer part of the Río de La Plata estuary (Argentina) and corresponds to a zone of interaction between a large sandy spit and a backwash tidal flat system. Methods used combine statistical analysis of wave climate time-series, analysis of wave energy patterns through nearshore numerical modelling (SWAN) and comparison of pre- and post-storm morphological changes using satellite images derived indexes (e.g. NDWI).

Results allowed to characterise four types of major storms impacting the study area: High-Energy Storms (HES), defined by an average storm Hs below the 1% exceedance (> 2.6 m), Long-Lived Storms (LLS) represented by an exceedance of the 1% of Du (> 60 hours), Storm Groups (SG) in which storm frequency is less than 6 days and Northeastern moderate storms (NMS) defined by their eastern, onshore oriented direction. Under HES and NSM storms erosional areas are dominant over depositional (62.34%), which most of the system showing shoreline retreat and a growth of the end spit area. For LLS and SG storms the morphological impact varies alongshore with a general flattens of the end spit (showing a ‘rounded-shape’ morphology) and erosional hotspots over the southeastern coastal section.

How to cite: Guisado-Pintado, E. and Isla, M. F.: Major storms influence on the morphological evolution of a hybrid spit-tidal flat system in Argentina, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15658, https://doi.org/10.5194/egusphere-egu23-15658, 2023.

X3.13
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EGU23-8572
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GM6.1
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ECS
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Dominique Townsend, Julian Leyland, Hachem Kassem, Charlie Thompson, and Ian Townend

Few studies focus on the changing morphology of the nearshore zone of mixed sediment beaches, despite the fact that these beaches are found across the world. In the UK, these beaches make up ~25% of the coastline, and are often utilised as a first line of defence against coastal flooding. In Pevensey Bay, East Sussex, active beach management (sediment recycling and recharge) maintains the mixed gravel barrier beach to protect around 10,000 properties, culturally significant landmarks and internationally important wildlife sites. During the past 25 years, this management approach has successfully maintained the volume of the upper shingle part of the beach. However, the sandy foreshore area is experiencing a continuing loss of 8000 m3 of sediment per annum.

This study seeks to understand the drivers behind the sustained loss of volume. Examination of multibeam bathymetry data revealed the presence of transverse finger bars with a wavelength of approximately 80 – 120 m, orientated at 45 degrees from the shoreline in the subtidal zone extending between the -3.0 to -6.0 mOD contours. Sediment grab samples taken perpendicular to the crests and troughs, revealed the surface sediments to be comprised of very well sorted fine sand, with D50 ranging between 150 – 169mm. Strong tidal currents flowing over these bed features modulate the sea surface roughness which can be detected in the X-band radar reflectance imagery. Using weekly averages of X-band radar reflectance imagery we show that the bars were a permanent feature over the 18-month period of observation and provide an indicative migration rate of approximately one wavelength a year to the east, which was validated against monthly bathymetric data. This novel approach of studying mobile sea bed features revealed a steady migration rate during the winter months, and virtually no movement during the summer period, suggests that the movement of the bars is driven by relatively higher energy south westerly waves. It is thought that the movement of these bars may be linked to erosive and accretive pulses which move easterly across the bay on the upper beach face. Understanding the process dynamics and broader role within the bay-wide sediment budget of these features is essential in comprehending the loss of sediment from the bay and will contribute to the future sustainable management of the site, where the management strategy for the next 100 years is currently under review.

How to cite: Townsend, D., Leyland, J., Kassem, H., Thompson, C., and Townend, I.: Linking nearshore morphological change to long term observed sand loss from a mixed sediment beach., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8572, https://doi.org/10.5194/egusphere-egu23-8572, 2023.

X3.14
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EGU23-8844
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GM6.1
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ECS
Alexandre Paris, Julien Chauchat, Éric Barthélemy, and Cyrille Bonamy

Coasts are hosting most of the human population worldwide and hosts a large part of the economic activities. Among the various types of coastal environments, sandy beaches represent one third of the global shoreline of which a large proportion is eroding (Luijendijk et al., 2018). This phenomenon is accelerating under the effect of climate change and the understanding and mitigation of the shoreline erosion is a fundamental issue in coastal engineering.

In this contribution we analyse survey data from two well-documented Atlantic beaches: Duck (North Carolina, USA), a microtidal East-exposed beach and Truc Vert (Aquitaine, France) a meso/macrotidal West-exposed beach. A statistical analysis of the waves data over 2 to 3 decades provides useful information to evaluate the various possible morphodynamic beach states following Masselink & Short (1993) classification. This classification is based on the Dean number and the relative tidal range. Using the measured bathymetries, it is possible to verify the Masselink and Short classification. For example, using Duck data, a morphological analysis is performed on the 18 available bathymetries from the year 2019. These data illustrate the up-state and down-state sequences between reflective (summer) and dissipative (winter) states. In particular, the variability of the beach morphology increases significantly during intermediate beach states.

Applied to the two datasets, a modeling approach combining a one-line model, ShoreFor (Splinter et al., 2014), and 2D depth-averaged process-based model, XBeach (Roelvink et al., 2009), is envisaged. ShoreFor is run to predict shoreline and bar location (Splinter et al., 2018) and XBeach simulations are used on specific subsets of the entire computational window for intermediate 2D morphological state predictions.

  

 

 

Luijendijk, A., Hagenaars, G., Ranasinghe, R., Baart, F., Donchyts, G. and Aarninkhof, S. (2018), The State of the World’s Beaches, Scientific Reports, 8(6641).

Masselink, G. and Short, A. (1993), The Effect of Tide Range on Beach Morphodynamics and Morphology: A Conceptual Beach Model, Journal of Coastal Research, 9(3), 785–800.

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

Splinter, K.D., Gonzalez, M.V.G., Oltman-Shay, J., Rutten, J., Holman, R. (2018), Observations and modelling of shoreline and multiple sandbar behaviour on a high-energy meso-tidal beach, Continental Shelf Research, 159, 33—45.

Roelvink, D., Reniers, A., van Dongeren, A., van Thiel de Vries, J., McCall, R., and Lescinski, J. (2009), Modelling storm impacts on beaches, dunes and barrier islands, Coastal Engineering, 56(11–12), 1133–1152.

How to cite: Paris, A., Chauchat, J., Barthélemy, É., and Bonamy, C.: Characterisation of sandy beach through morphological indicators and long-term modelling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8844, https://doi.org/10.5194/egusphere-egu23-8844, 2023.

X3.15
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EGU23-1117
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GM6.1
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ECS
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Eva Pavo-Fernández, Vicente Gracia, Manel Grifoll, and Gorka Solana

The coast of Mozambique is the result of a complex geologic setting coupled with oceanographic and hydrological processes. Its geomorphological characterization linked to the meteo-oceanographic processes has not been yet investigated in detail, despite their relevance in land use planning and coastal risk prevention. This is vital to identify present and future conflicts being a fundamental tool for its management. This work pursues a first analysis of the combined description of geomorphological features in Mozambique jointly with a simplified hydrodynamic characterization, leading to a first assessment of the longshore sediment transport. The work aims to identify the present littoral cells following the methodology proposed by Bray, Carter and Hooke (1995) and the comprehensive approach followed by the DOORS EU project. The analysis is supported by an extensive literature review, a detailed description of the wave climate taken from the ERA5 data set, along with data obtained using low-cost and do-it-yourself equipment, and the resulting longshore sediment transport in different scenarios, present and future. The identified units cover a wide range of coastal archetypes from exposed sandy beaches to barrier systems or coral reefs. From an energetic point of view, Mozambique's coast showed significant changes depending on the region. Likewise, sediment transport also differed in direction and magnitude between regions. This work is an important base for the management and risk assessment of the Mozambican coast, which remains highly unexplored. This study is funded by the PITACORA project (TED2021-129776B-C21) and the FI AGAUR grant (2022 FI_B 00897).

How to cite: Pavo-Fernández, E., Gracia, V., Grifoll, M., and Solana, G.: Longshore Sediment Transport Patterns In Mozambique: A Tool For Coastal Planning, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1117, https://doi.org/10.5194/egusphere-egu23-1117, 2023.

X3.16
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EGU23-4707
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GM6.1
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Md. Yousuf Gazi, Ana Vila-Concejo, and Thomas Fellowes

Beaches in estuaries and bays (BEBs) are usually distinct from open coast beaches in terms of environment, process, and morphology, and their morphodynamics remain unresolved. The present study aims to establish a relationship between profile morphology, sediment characteristics, and hydrodynamic forces. We investigated four beaches of Gamay (Botany Bay) located in the south of Sydney, SE Australia. Profiles of these beaches were measured regularly from 2016 to 2022 and sediment samples were collected from each beach. Wave data were recorded from the Sydney Waverider buoy. We used principal component analysis (PCA) to ascertain the temporal and spatial scales of the data variability, and spectral analyses to investigate the waves. Sediment samples were analyzed by Laser Diffraction Particle Size Analyzer (Mastersizer 3000).

Our preliminary results indicate that different profile morphotypes are associated with different sediment characteristics and wave climates on BEBs.  BEBs with the least exposure to ocean swell waves have the narrowest and steepest profiles, with BEBs more exposed to ocean swells having moderately steep profiles. BEBs containing coarser sediments show steep profiles compared to beaches with fine to very fine-grained sediments. In general, the shape of profiles changes in order from straight-through concave to convex-concave with increasing wave energy. Current research aims to parameterize these relationships with offshore and locally generated wave parameters. The insights obtained from this study will assist to understand fundamental mechanisms governing the morphological evolution of BEBs.

How to cite: Gazi, Md. Y., Vila-Concejo, A., and Fellowes, T.: Assessment of empirical relationship among cross-shore beach profile morphotypes, sediment characteristics, and wave signature in beaches in estuaries and bays (BEBs), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4707, https://doi.org/10.5194/egusphere-egu23-4707, 2023.

X3.17
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EGU23-3480
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GM6.1
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Highlight
Bjoern Lund, Milan Curcic, Hans Graber, Brian Haus, Jochen Horstmann, and Neil Williams

This presentation studies coastal marine X-band radar (MR) bathymetry and current measurements made in Monterey Bay, California, during the Coastal Land–Air–Sea Interaction (CLASI) experiment from Jul-Aug 2021. Numerous past studies have shown coastal MR bathymetry and, to a lesser extent, current measurements, but only a few of them have investigated their environmental limitations. Widely cited thresholds for MR ocean measurements are a minimum wind speed of 3 m/s and significant wave height of 0.5 m. Monterey Bay has a strong year-long diurnal sea breeze which also influences the wave height. As a result, wind and wave conditions fluctuated from favorable to unfavorable for MR ocean measurements on a quasi-daily basis. Here, we examine the limitations of MR bathymetry and current measurements as a function of wind and wave conditions, which were measured by an Air–Sea Interaction Spar (ASIS) buoy within the radar field of view. Unlike the popular cBathy algorithm, which adjusts its bathymetry results to the mean water level using ancillary tide gauge measurements and applies a Kalman filter that ingests several days of data per bathymetry estimate, we measure the bathymetry with a temporal resolution of 4 min albeit at a relatively low spatial resolution of 360 m. This allows MR-based measurements of tidal elevation, which we validate using data from the CO-OPS tide gauge in Monterey. The MR current measurements are validated using an ADCP that was mounted on the ASIS buoy. The novelty of this study lies in the comprehensive evaluation of the MR bathymetry and current measurements' environmental limitations. We show how their maximum range decreases for wind speeds under 4 m/s with a success rate of <20% for winds <2 m/s and <10% for winds <1 m/s. These findings can guide investigators who plan to include MR ocean measurements in their coastal field studies.

How to cite: Lund, B., Curcic, M., Graber, H., Haus, B., Horstmann, J., and Williams, N.: Coastal marine X-band radar bathymetry and current mapping, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3480, https://doi.org/10.5194/egusphere-egu23-3480, 2023.

X3.18
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EGU23-13513
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GM6.1
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Albert Falqués, Nil Carrion, Francesca Ribas, Daniel Calvete, Candela Marco-Pereto, Ruth Duran, and Angels Fernandez-Mora

The response of sandy beaches to the expected sea level rise during the XXI century is a major scientific concern. Process-based 2DH morphodynamic models are unable of making projections to 2100 due to the high computational coast and to the accumulation of errors in resolving short-term processes. Reduced complexity models are then an alternative to make such projections. However, these models need to be validated and this can be performed with data and/or with process-based models. With the final aim of making long term projections of an embayed beach with the Q2Dmorfo reduced-complexity model, we do such validation with the XBeach process-based model.


Cala Castell is an embayed sandy beach at the Costa Brava (Catalonia, Spain) about 300 m wide, bounded by rocky headlands and facing to the South. Bathymetric surveys were conducted on 28 January and 8 July 2020. During this period, an AWAC was deployed in front of the beach, measuring mean sea level and wave height, period and direction. Both models were calibrated to reproduce the observed coastline behaviour and the best Brier skill score was high for both, BSS=0.79, with final shorelines being similar among them and to the observed one. However, since intermediate bathymetric surveys are not available, it is not possible to compare how models perform during particular wave events. To shed some light into this, a number of synthetic events are here investigated on a synthetic beach based on the geometry of Cala Castell. The wave conditions are selected so as to mimic typical wave conditions at the site, where the dominant wave direction is quite oblique, from the East. The model parameter values are the optimum ones after the calibration.


The cross-shore transport is parameterized in a completely different way in both models and it is hardly comparable but the longshore transport for oblique wave incidence should be consistent. We focus the comparison in the latter by using slightly different initial bathymetries, so that each model starts from its “equilibrium” bathymetry. For Q2Dmorfo, the bathymetry is constructed from a parabolic curve approximating the initial observed shoreline and the optimum equilibrium beach profile of the calibration. For XBeach, we first run the model during 10-30 days with constant shore-normal wave conditions over the initial Q2Dmorfo bathymetry to obtain an equilibrium configuration. Then, when applying oblique wave incidence conditions there is in both models a similar tendency to beach rotation, with shoreline retreat in the central part of the beach and progradation in the downwave part, although important quantitative differences may arise. The best comparison is done by analyzing the volume accumulated in each cross-shore profile per alongshore distance unit (m3/m) and it is found that the volume of sand eroded or accumulated in each profile for both models compare quite well. Therefore, on the basis that XBeach is close to reality when simulating a single event, this confirms the capability of Q2Dmorfo to describe longshore processes reasonably well even at the event time scale.

How to cite: Falqués, A., Carrion, N., Ribas, F., Calvete, D., Marco-Pereto, C., Duran, R., and Fernandez-Mora, A.: Long term modelling of a mediterranean embayed beach: reduced-complexity model vs XBeach model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13513, https://doi.org/10.5194/egusphere-egu23-13513, 2023.

Dune processes
X3.19
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EGU23-14812
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GM6.1
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ECS
Diego Lopez-de-la-Nieta, Emilia Guisado-Pintado, and Víctor F. Rodríguez-Galiano

Coastal dune systems, and its vegetation, serve as a natural buffer against erosion and flooding caused by wave storms and rising sea levels. However, these systems are normally under strong pressures mainly caused by changes in land uses (e.g. tourism or urban development) which in turns could led to a perturbation of the vegetation coverage or the total replacement by hard structures such as promenades. In other coastal areas, however, the vegetation has been proved to undergo an increase in extension and coverage. This effect, known as "greening" (e.g. Jackson et al., 2019), seems to be caused by the combination of changes in climate and atmospheric composition and a reduction in windiness, among others factors. Further, the strong conservation measures carried out in these coastal areas, such as the use of fences and the restoration with autochthonous species, have contributed to this process.

This contribution focuses on testing the temporal and spatial change in dune vegetation, coverage and density, in the southeastern Mediterranean coast of Andalusia (Spain). The study site, known as Cabopino (Marbella, Málaga), presents one of the best-preserved Mediterranean coastal dunes (Artola dunes). The coastal-dune system, protected as a Red Natura 2000 site, has undergone little anthropogenic pressures in the past few years. Methodology approach includes a temporal analysis from 2017 to 2022 by calculating NDVI and EVI2 vegetation indexes using the Sentinel-2 MSI (MultiSpectral Instrument) level 2 sensor and Google Earth Engine platform. The dune area was delineated using photointerpretation techniques on the basis of the PNOA orthoimage of 2016 and considering land uses coverage maps and the European Union Habitats - EUR28 (Habitats Directive) of the same year. The NDVI and EVI2 indexes were calculated on composites of images by season (spring, summer, autumn, and winter), using the maximum values of each pixel.

Results show that vegetation coverage of the Artola dunes have remained stable during the study period (2017-2022), with small variations in the foredune sector. In terms of vegetation density, NDVI and EVI2 indexes show values of around 0.5-0.6 at the peak of vegetation development (spring and winter) which remains constant from 2017 to 2021. On the contrary, an important decrease in vegetation density is found during the 2022 spring season, with NDVI/EVI2 values of 0.1-0.25. This decrease is coincident with a negative anomaly in winter precipitation in this Mediterranean coastal area.

These preliminary results seem to be in line with previous work in the field that support the idea of a global “greening” of coastal dunes because of global (and climatic) change. Nevertheless, vegetation dynamics in Mediterranean coastal dunes seem to be highly controlled by local meteorological conditions, specially to winter precipitation, which ultimate determines vegetation growth and stabilisation.

How to cite: Lopez-de-la-Nieta, D., Guisado-Pintado, E., and Rodríguez-Galiano, V. F.: Changes in dune vegetation trends in the southeastern coast of the Iberian Peninsula, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14812, https://doi.org/10.5194/egusphere-egu23-14812, 2023.

X3.20
|
EGU23-3510
|
GM6.1
Gerben Ruessink

Blowouts on coastal foredunes provide efficient pathways for wind-driven sand transport inland. In this way blowouts affect beach-dune sand budgets, facilitate landward dune migration under sea-level rise and stimulate the dunes’ biodiversity by maintaining a heterogeneous landscape.  While our understanding of the dynamics of blowouts on time scales of months to decades is advancing due to the increasing availability of remote sensing data sets, we have limited empirical data and hence understanding of the short-term (hours) aeolian activity in blowouts and how this activity builds up to longer term blowout dynamics.

This contribution documents the first results of a study designed to cover multiple scales of aeolian activity (hours to months) in the foredune trough-blowout system of the Dutch National Park Zuid-Kennemerland. The data used here consist of (i) photographs of one of the deflation basins taken by a time-lapse trap camera every two hours during the day, (ii) wind speeds and directions (at 1-m height) measured by 4 continuously operating ultrasonic anemometers positioned from the seaward side of the same basin to the adjoining depositional lobe, and (iii) wind data (at 10-m height) from a nearby offshore meteorological station. The 120-m long instrumented basin has an approximately linearly upward sloping floor with steep lateral walls and is about 100 m wide at its mouth, reducing to about 20 m at the start of the depositional lobe. Good-quality images were manually classified into five categories with increasing aeolian activity from no transport (class 0), isolated streamers on a small portion of the deflation floor (class 2) to intense streamer activity everywhere (class 4). The first results, based on images collected in fall 2022, indicate that streamer activity (class 2 or higher) is restricted to moments when the wind speed at the blowout mouth exceeds about 5 to 6 m/s and the offshore wind approach direction is within 40 to 50 degrees of the blowout axis. For these directions the wind is steered into the blowout and, depending on the approach angle, accelerates over the deflation floor by up to 50%. Aeolian activity on the deflation floor is absent or very small (class 0 or 1) for larger approach angles (even at offshore wind speeds of 10 – 15 m/s) because the wind is no longer steered into the blowout. These first results thus indicate the importance of the wind speed on the deflation floor and the offshore wind approach angle (relative to the blowout axis) for short-term aeolian activity in the blowout.

How to cite: Ruessink, G.: Observations of aeolian activity on the deflation floor of a foredune trough blowout, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3510, https://doi.org/10.5194/egusphere-egu23-3510, 2023.

X3.21
|
EGU23-14767
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GM6.1
Susana Costas, Juan B. Gallego-Fernández, Luisa Bon de Sousa, and Katerina Kombiadou

Coastal dunes result from multiple interactions between biotic and abiotic factors. The complexity of the resultant dune ecogeomorphology will therefore be determined by the spatial and temporal variability of the involved factors and their interactions. This work explores the longshore variability of morphological features, plant community distribution and accumulation patterns of a dune segment (1.4 km-long) located at the downdrift end of a sandy peninsula in the Ria Formosa, Portugal. To understand the main drivers of the observed variability and the implications for dune morphological response, this information was combined with recent multidecadal shoreline evolution data (i.e., 60 years). The integrated results document significant differences in dune morphology, sedimentation patterns and plant zonation, with two distinct dune configurations or states identified in close proximity. One (western sector) shows a narrower dune system, vegetation cover characterised by pioneer species with low densities, and squeezed plant zonation. Conversely, the other (eastern sector) presents a wider dune system with a new foredune, a more developed plant zonation and relatively high vegetation density. Both states could be partially explained by the recent shoreline trends and inlet shifts, with stable to retreating trends in the western sector and shoreline progradation in the eastern one. Plant zonation and accumulation patterns suggest that the dune along the retreating sector is in a cycle of inland migration, encouraged by the reduced accommodation space and the low retention capacity of the vegetation across the dune stoss. Alternatively, observations along the prograding sector suggest that the greater accommodation space and the stabilising feedback between vegetation and topography promoted the seaward progradation of the system and the development of an incipient foredune. Outcomes support the importance of biogeomorphic feedbacks for the dune configuration. However, they also evidence that the role of vegetation within this feedback is rather passive and primarily regulated by physical factors, including regional (low precipitation and sediment transport potentials) and local conditions (e.g., variations in the sediment supply alongshore). Therefore, despite the undeniable role of vegetation in reinforcing dune topography, it is worth highlighting that local external forces may dominate dune response, inhibiting, allowing or reinforcing ecogeomorphic interactions in the long-term.

This work is supported by projects 2022.05392.PTDC and PTDC/CTA-GFI/28949/2017, funded by the Portuguese Foundation for Science and Technology.

How to cite: Costas, S., Gallego-Fernández, J. B., Bon de Sousa, L., and Kombiadou, K.: Coastal dune ecogeomorphic states regulated by extrinsic factors, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14767, https://doi.org/10.5194/egusphere-egu23-14767, 2023.

Posters virtual: Wed, 26 Apr, 10:45–12:30 | vHall SSP/GM

Chairpersons: Derek Jackson, Irene Delgado-Fernandez, Edoardo Grottoli
Coastal processes
vSG.2
|
EGU23-8519
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GM6.1
|
ECS
Andrew Thrower, John Barlow, and Roger Moore

In Northern Europe, chalk is an important material affecting many coastal communities. This has led to a great deal of research into coastline recession to quantify rates of coastal erosion and to understand the processes and mechanisms that control coastal rock mass instability of chalk cliffs. Along the UK and French chalk coastlines, estimates for rates of coastal retreat vary, but are generally understood to be between approximately 0.2 my-1 and 0.6 my-1.  Along the Sussex coastline rockfalls from the coastal chalk cliffs tend to be small in nature, typically producing less than 1,000 m3 of material. Larger rockfalls, however, do occasionally occur and can exceed 20,000 m3. Understanding the relationship between rock strength and wave energy is of great importance in predicting cliff recession. Although the importance of understanding the contribution of rock control on coastal erosion is widely recognised, establishing a direct link between rock material properties and rates of erosion has proven to be difficult.                                                               

This study presents the findings from five years of coastal monitoring at a site in Sussex (United Kingdom) using UAV photogrammetry. Rates of coastal erosion have been calculated and are found to be in general agreement with those in the published literature. Sequential monitoring undertaken at the site enabled the identification of many rockfalls with calculated volumes ranging between <100 m3 and >4,000 m3. Sampling of rock materials in the field for laboratory testing was undertaken. Extracted dried core sub-samples were tested with a Leeb hardness tester (Equotip) to derive a surface hardness at regular intervals along the cliff-line. Previous research (Thrower et al, 2022) has already demonstrated that the Leeb hardness of chalk has a good relationship with intact dry density and can therefore be a useful tool for estimating rock strength. The results of this study show a good correlation between Leeb hardness and back-wear along the cliff-line at the study site, indicating a relationship between rock strength and rates of coastal erosion. The data show that sections of the study area characterised by softer relatively low-density chalk have been eroded at faster rate than areas characterised by harder relatively high-density chalk. This work demonstrates the potential that Leeb hardness testers have in the characterisation of rock mass properties for quantifying rock control in coastal studies. Furthermore, such testing could provide useful data in predicting future cliff recession behaviour at coastal sites.

How to cite: Thrower, A., Barlow, J., and Moore, R.: Establishing a relationship between rock control and rates of coastal erosion using high precision sequential UAV photogrammetry data and Leeb rebound impact hardness testing, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8519, https://doi.org/10.5194/egusphere-egu23-8519, 2023.

vSG.3
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EGU23-7789
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GM6.1
Edoardo Grottoli, Melanie Biausque, Derek W. T. Jackson, and J. Andrew G. Cooper

Shoreline change analysis of the entire coast of Northern Ireland was conducted for the period from 1830 to 2021. Even with its 790Km length (including the main offshore islands) the Northern Ireland coastline is remarkably diverse, comprising several notable coastal typologies (cliffs, rocky coasts, sandy beaches, sand dunes, etc.) as well as a number of commercial and industrial activities. Human pressure co-exists with the natural coast and each impacts the other in many ways.

Shorelines were digitised using multiple temporal and spatial datasets (historical maps, aerial photos and orthophotos). The average temporal interval of shorelines ranges from 7.8 to 37 years, depending on location and data availability. The seaward vegetation line was selected as the primary shoreline proxy, whereas cliff edge, rock-water line or anthropogenic structures were chosen subordinately or if deemed more appropriate. Shoreline uncertainty was assessed using various errors inherent from each dataset from which the shoreline was digitised. The ArcMap® tool DSAS 5.0 was used to calculate shoreline rates and distances along 25 m spaced transects covering the entire coastline.

Over the last two centuries the maximum retreat value was highlighted in Lough Foyle between the Roe River mouth and Magilligan Point where the shoreline progressively retreated up to -250 m. The largest shoreline advancement (+3.500 m) was recorded in Belfast following its port expansion.

Sandy coastlines were shown to have the larger natural changes. Magilligan point, after an advancement phase toward its northern extremity between 1830 and 1919, underwent a constant retreat phase that has now extended to its eastern side up. Benone Beach, Castlerock and Portstewart Strand are the only vegetated coastal dune sites along Northern Ireland’s north coast that are advancing in the last 190 years(190-270m) Significant shoreline retreat was recorded at White Rocks (-85 m), Ballycastle (-65 m), Runkerry (-25 m) and White Park Bay (-65 m). In Dundrum Bay, Co. Down, the largest retreat values (-75 m) occurred at Murlough and maximum advancements greater than 200 m at Ballykinler.

The largest shoreline advancements were all attributed to human activities such as land reclamation in the loughs (mainly during the 19th century), seaward expansions of ports (e.g., Belfast, Bangor, Carrickfergus, Portavogie and Warren Point) or construction of power stations and wastewater treatment areas. Shoreline advancements recorded for the salt marshes of Larne and Foyle Loughs and White Water River mouth also followed human interventions. High rocky coasts, apart from limited rockfalls, were less subject to shoreline changes. The work will contribute to better define coastal cells along the Northern Ireland coastline and inform coastal managers for future development plans of the coast.

How to cite: Grottoli, E., Biausque, M., Jackson, D. W. T., and Cooper, J. A. G.: Two centuries of shoreline change in Northern Ireland., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7789, https://doi.org/10.5194/egusphere-egu23-7789, 2023.

vSG.4
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EGU23-7961
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GM6.1
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ECS
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Highlight
Melanie Biausque, Edoardo Grottoli, Derek W. T. Jackson, J. Andrew G. Cooper, and Emilia Guisado-Pintado

Multiple intertidal barred systems (MITB) are complex coastal features observed under specific conditions including low to moderate energy waves and macrotidal forcing. Although primarily tide-dominated, MITBs can undergo significant morphological changes under storms and extreme events. In December 2021, storm Barra crossed the U.K. and was the 2nd most energetic event in the past 25 years, to occur on Northern Ireland’s east coast. At the MITB site in Dundrum Bay, offshore waves reached a maximum significant wave height of 5.5 m on December 7th, 2021 and were associated with a peak period and wave direction of 10s and 179°N-oriented respectively. Pre- and post-Barra intertidal DGPS surveys were conducted at Dundrum Bay on the 6th and 9th of December to identify beach morphological changes on MITBs. Despite direct onshore waves reaching the bay, a strong alongshore variability was recorded in the response to Murlough and Ballykinler beaches to Storm Barra. Indeed, according to preliminary results, the western end of the bay shows an elevation of the beach profile, the central area presents onshore migrations of the bars to no significant changes, while the eastern side of the bay (Ballykinler) displays bar crests flattening and linear post-storm profiles.

Although storm Barra was the most energetic event recorded during the winter 2021/2022, smaller storm events had modified the morphology of Murlough and Ballykinler beaches throughout the preceding season, leading to some already low pre-Barra beach profiles. Secondly, at the peak of Barra’s energy, the waves were southeast oriented. But the direction of the waves rapidly shifted from onshore to offshore, possibly modifying the impact of Barra on the system. Finally, the shape of the bay, the location of the different profiles and the complex nearshore bathymetry and local geology must have also played a key role dictating the alongshore pattern shown. Previous studies of Dundrum Bay have shown that physical processes driven by waves/tide and geomorphology interactions can undergo significant local modification, leading to a strong alongshore variability in the profiles’ response to seasonal events. Nearshore SWAN simulations will help highlight the role of nearshore hydrodynamics including wave dissipation and re-orientation, wave-driven sediment transport and the impact of storm surge on MITB during an extreme event.

How to cite: Biausque, M., Grottoli, E., Jackson, D. W. T., Cooper, J. A. G., and Guisado-Pintado, E.: Impact of a high-energy storm event (Storm Barra) on a multiple intertidal barred system., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7961, https://doi.org/10.5194/egusphere-egu23-7961, 2023.