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GM6.9

EDI
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.

This session is supported by the Commission on Coastal Systems (CCS) of the International Geographical Union (www.igu-ccs.org) and by the Spanish working group of the UNESCO IGCP 725 ‘Forecasting coastal change’.

Public information:

This session explores coastal morphodynamics from the nearshore through to inland dune systems. Contributions include a range of studies focused on the measurement and modelling of physical processes and responses within nearshore-beach-dune systems over various spatial and temporal scales. The session 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: Irene Delgado-Fernandez | Co-conveners: Emilia Guisado-PintadoECSECS, Derek Jackson
Presentations
| Thu, 26 May, 08:30–10:00 (CEST)
 
Room D2
Public information:

This session explores coastal morphodynamics from the nearshore through to inland dune systems. Contributions include a range of studies focused on the measurement and modelling of physical processes and responses within nearshore-beach-dune systems over various spatial and temporal scales. The session 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.

Thu, 26 May, 08:30–10:00

Chairpersons: Derek Jackson, Irene Delgado-Fernandez

08:30–08:40
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EGU22-13280
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solicited
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Highlight
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Virtual presentation
Gerben Ruessink et al.

The prevention of marine flooding is one of the most important functions of foredunes along developed coasts. Consequently, many foredunes have been managed into densely vegetated, uniform and stable ridges of sand. While such foredunes reduce the risk of coastal flooding under present-day conditions, it is increasingly feared that they are less resilient to persistent erosion under climate change (e.g., rising sea levels). The dense vegetation blocks the sand exchange between the beach and the backdunes and accordingly, prevents the backdunes from growing with sea-level rise. In various countries around the world, dune management is therefore now adopting a more dynamic approach. The excavation of gaps through the foredune, termed notches, is an increasingly adopted management measure (e.g., United Kingdom, France, New Zealand, the Netherlands) to restore the natural sand pathway from the beach into the backdunes without simultaneously increasing the risk of flooding. In addition, it is hoped that the renewed sand influx improves the natural values of the backdunes by creating more diverse habitats, including bare sand areas for flora and fauna depending on open conditions. The geomorphic dynamics of notches is, however, not well understood, especially on the time scale of years. This also prevents understanding what factors contribute to success (long-term mobility) or failure (rapid stabilization) of dynamic dune management.

Here we analyse the geomorphic evolution of five notches in the foredune of the Dutch National Park Zuid-Kennemerland since their excavation in the 2012/2013 winter (up to October 2021; 8.5 years) using 24 digital elevation models (DEMs) derived from airborne lidar and drone surveys. DEMs of Difference illustrate that the notches have persistently eroded since their excavation, mostly through alongshore widening and steepening of the lateral walls, at an approximately constant volumetric rate of about 17,500 m3/y. Landward of the notches, depositional lobes have formed that in October 2021 extended up to 280 m into the backdunes and were locally up to 10 m thick. The total deposition volume increased approximately linearly with time by about 33,350 m3/y, thus surpassing the annual erosion volume by almost a factor of 2. This reflects substantial aeolian transport of beach sand through the notches. Under the assumption that the excess deposited sand all originates from the 850-m alongshore section of beach fronting the five notches, the annual input of beach sand equals approximately 19 m3/m/y. These results highlight that the notches facilitated highly efficient onshore sand pathways during the entire 8.5-y study period and showed no sign of rapid stabilization. In more detail, the data also suggest that the notches oriented with the dominant wind direction are more efficient in facilitating this onshore transport than notches with other orientations. Future work will focus on the fusion of the elevation data with high-resolution satellite imagery to better understand the impact of sand deposition on vegetation dynamics and on the role of vegetation in determining the future evolution of the depositional lobes.

How to cite: Ruessink, G., Arens, B., and Kuipers, M.: Multi-annual geomorphic evolution of excavated foredune notches, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13280, https://doi.org/10.5194/egusphere-egu22-13280, 2022.

08:40–08:45
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EGU22-3685
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ECS
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Highlight
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On-site presentation
Lara Talavera et al.

Blowouts are sandy depressions of different shapes formed by wind-induced erosion of foredunes or dune fields. Their initiation has been linked to air flow acceleration due to the occurrence of irregularities in the topography caused by natural or human causes, or by the lack of vegetation, and they contribute to maintain the available sediment budget in barrier islands migrating inland. In this study, we investigated the morphometric characteristics (area, orientation, and shape/structure) 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) that has been retreating for the last 60 years. For that, a set of historical aerial photos, orthophotos, and Google Earth images, covering a 45-year period from 1972 to 2017, was analysed. In addition to the blowout mapping, the dune foot, trampling paths and human occupation (e.g. restaurants, walkways, umbrellas) observable in the previous imagery were mapped in order to find possible causes that could help explaining the observed foredune fragmentation and blowout development. Finally, we characterised the present-day plant species distribution along and across the study area in order to understand the impact of these landforms in the plant community and possible eco-morphological feedbacks. The findings showed that during the analysed period: (1) blowout dimensions ranged from 1.2 m2 to of 2200 m2, with 50 to 80% of the blowouts displaying sizes below 100 m2; (2) the orientations of the smallest blowouts (below 100 m2) showed high variability (from SSE-NNW to W-E orientations), whereas the bigger ones (above 400 m2) were mostly SW-NE oriented, coinciding with the dominant winds in the area; (3) most of the blowouts had mixed shapes and branched structures, likely enhanced by human trampling; (4) the number  of blowouts and their morphometric parameters were not clearly related with the shoreline retreat/progradation; and (5) the transition between the blowout lobes and the stablished foredune suffered a change in the plant community dominated by species such as Artemisia campestris subsp. maritima, suggesting a shift to burial tolerant species.

Besides wind conditions and shoreline changes, human pressure seems a very likely trigger of blowout morphology reinforcement and even blowout initiation in the area, although the origin of some of these features seems related also to already-existing irregularities in the topography, suggesting the fragility of this sector and supporting its tendency to migrate inland.

How to cite: Talavera, L., Costas, S., and Ferreira, Ó.: Blowout morphodynamics in southern Portugal , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3685, https://doi.org/10.5194/egusphere-egu22-3685, 2022.

08:45–08:50
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EGU22-8561
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Highlight
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On-site presentation
Susana Costas et al.

Coastal dunes result from the accumulation of wind-blown sand transferred inland from the beach and trapped by physical barriers such as vegetation. Vegetation also plays an active role on dune growth through the onset of feedbacks with the dune topography and the air flow. All these complexities have been tentatively captured by recent numerical models, which also may help to better understand dune response to disturbances as well as possible evolutionary patterns. Duna is a simplified process-based model, which integrates air flow, sediment transport and vegetation dynamics to reproduce the morphodynamic response along the dune profile. The present work focuses on extending Duna to include the influence of fetch-limited conditions and to accommodate different wind incidence angles by using projected wind-parallel dune profiles.

The performance of the model was assessed by comparing model outputs with wind profile and sedimentation observations from contrasting dune morphologies and environments, showing a good agreement and promising results. Duna was further used to test several hypotheses related to air flow dynamics and topography and to explore sediment transport and accumulation patterns across a series of different dune morphologies, including basic biogeomorphic feedbacks.

How to cite: Costas, S., Kombiadou, K., and Roelvink, D.: Exploring coastal dune adaptation through a simplified process-based model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8561, https://doi.org/10.5194/egusphere-egu22-8561, 2022.

08:50–08:55
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EGU22-9982
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On-site presentation
Christy Swann et al.

Existing aeolian transport models often fail in field environments. The discrepancy between models and prediction has been attributed to inadequate field measurements and uncertainty in the general knowledge of the fundamental physical processes (turbulence) driving sand transport. The main challenges of measured and modeled aeolian transport include (1) coarse resolution measurements (relative to fluid-grain scale physics) made with obtrusive instrumentation that disrupt natural fluid and sediment flow, (2) an inadequate understanding of the fundamental physical relationships between turbulence and sediment transport, and (3) the inability of aeolian transport models, derived from wind-tunnel observations, to simulate natural boundary layer processes at the appropriate field scales (mm - cm).

Here, we introduce F-PTV, a Field-based Particle Tracking Velocimetry system. The field-based system is capable of providing the first unobtrusive measurements of turbulence and the resulting sand transport by wind in a field environment and consists of 3 integral components: (1) an illuminated volume, (2) neutrally-buoyant seeding material in the form of helium bubbles, and (3) 4 high speed cameras. The 527 nm laser and 4, high-resolution, high-frame rate cameras are mounted on rigid frames to be deployed on the subaerial beach. The laser beam is directed to the surface through a fiber-to-volume optics collimator at a height of 2.23 m and directs a defocused beam vertically down to create an ellipsoidal cone of light over the sampling area. At 1200 frames per second, the cameras capture the scattered light from helium bubbles and sand particles passing through the illuminated volume, enabling us to track individual helium bubbles and sand grains. The F-PTV system has the capability to provide the first unobtrusive observations of turbulence and transport in a field environment.

How to cite: Swann, C., Gray, C., Braithwaite, E., Key, C., Trimble, S., and Kelley, M.: Measuring turbulence in a natural boundary layer using a field-based particle tracking velocimetry system, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9982, https://doi.org/10.5194/egusphere-egu22-9982, 2022.

08:55–09:00
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EGU22-12029
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ECS
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On-site presentation
Luisa Bon de Sousa et al.

Coastal dunes host priority habitats that provide different ecosystem services, such as biodiversity or socio-economical resources, and they are the first line of defence against the impact of storms, providing protection to adjacent coastal communities. However, the capacity of these systems to provide such services depends on their morphological and ecological status, which may change spatially and over time. Despite the relevance of assessing to dune state, a simplified and universal approach for this purpose is lacking. The present work explores the temporal and spatial variability of a series of morphological parameters and their best combination to inform about the state and resilience of coastal dunes. For that, the morphology of one sandy peninsula and one barrier island, with contrasting exposure to meteocean conditions and anthropogenic pressure, located within the Ria Formosa in the south coast of Portugal, were analysed. The available dataset covers the period between 2008 and 2018 and consists of Digital Terrain Models (DTMs) and orthophoto mosaics. Shoreline indicators (e.g., wet/dry line, debris line, vegetation seaward limit and dune heel line) were mapped in all orthophoto mosaics and extracted each 10 m alongshore. Parallelly, cross-shore profiles were defined at each 10 m from the DTMs to automatically extract the position and the elevation of the dune crest, dune toe and berm. The extracted parameters and indicators allowed estimating the width and slope of the different segments within the beach and the dune. Results show significant differences in the dune crest height alongshore each barrier while other parameters such as the dune toe presented a rather intra-barrier homogeneous distribution intra-system. The latter parameters are significantly different from barrier to barrier while the dune crest height values partially overlapped when both barriers were compared. Observed temporal and spatial variability of the extracted parameters was tentatively compared with the historical evolution of the evaluated barrier and the incident wind and wave conditions suggesting that the parameters might be regulated by different drivers. For instance, while the dune crest height appears strongly regulated by the historical evolution of the coast, the rest of the parameters, homogeneous within each barrier, appear to be controlled by external factors associated to the variable orientation of the coast relative to the main wave and wind climate. These results might be highly relevant when assessing the adaptation capacity of these type of systems and thus to their resilience with implications on the definition of coastal barrier states and tipping points in barrier response to disturbances.

How to cite: Bon de Sousa, L., Costas, S., and Ferreira, Ó.: Morphological parameters to assess the state of coastal dunes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12029, https://doi.org/10.5194/egusphere-egu22-12029, 2022.

09:00–09:05
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EGU22-13449
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ECS
Nicholas O Keeffe et al.

Coastal dunes are effective natural buffers against climate change-induced sea-level rise and storminess. Coastlines characterised by the presence of blowouts at the beach-dune interface may be more susceptible to coastline retreat through the enhanced landward transport of beach and foredune sediment. Blowouts are highly effective transport pathways, but the dynamics of aeolian sediment transport governing their evolution are poorly understood. Their morphological form is indicative of aeolian transport, and the propensity of their topography to modify airflow sufficiently to support transport has been extensively researched. Although there is a growing number of studies detailing blowout sediment flux, those involving synchronous measurement of flow and sediment movement from the beach into the dune field are rare.

This study examined airflow and sediment transport dynamics at the beach-dune interface of a trough blowout at Sefton Dunes, northwest England. A dense array of 3D sonic anemometers were co-located with transport sensors and deployed during an oblique onshore wind event. Instantaneous flow and transport dynamics were measured on the back beach, the adjacent upwind foredune, and within the throat of the blowout. Strong alongshore deflected airflow across the upwind foredune led to high-intensity sediment transport into the blowout throat. Inside the blowout throat, airflow and transport displayed extremely high spatial and temporal variability across the relatively confined throat area. Airflow speeded up close to the upwind blowout wall but sped ‘down’ close to the exposed (downwind) blowout wall. Transport (expressed in counts min-1 and Activity Parameter) showed low correlations with a range of wind variables such as wind speeds and TKE. Transport intensity followed a general pattern opposite wind speeds, with lower transport intensities close to the upwind blowout wall and higher transport intensities close to the downwind blowout wall area. Multiple topographically-forced flow modifications were observed (particularly at the blowout throat), and relatively minor 10-20° directional shifts led to large variations in flux intensity within the blowout. Results have provided detailed, high temporal, and spatial insights into how beach sediment is delivered to the blowout throat area and then driven landwards and reveals how foredune blowouts help facilitate beach sediment bypassing through foredunes, contributing to medium-scale coastal dune evolution behaviour.

How to cite: O Keeffe, N., Delgado-Fernandez, I., Jackson, D., Costas, S., Farrell, E., O’Neil, A., and Smyth, T.: Airflow dynamics and sediment transport through foredune blowouts , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13449, https://doi.org/10.5194/egusphere-egu22-13449, 2022.

09:05–09:10
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EGU22-9987
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ECS
Sarah Trimble et al.

Ripples are the result of wind blowing across sandy surfaces on Earth and other planetary bodies, and therefore ripple presence is evidence that aeolian transport has occurred in that environment. Consequently, ripples are a useful indicator when predicting volumetric transport of windblown material, estimating surface roughness to calculate shear velocity, and interpreting sedimentary deposits across our solar system. Improving prediction of the fluid flow conditions that produce ripples, and the volume of material transported in the form of ripples, are both critical to interpreting landscape evolution across planetary bodies. Here, we present a set of field observations aimed at quantifying the volume and migration rate of aeolian ripples under various flow and transport conditions.


These experiments were conducted during the Aeolian Turbulence and Transport EXperiment (ATTEX) in October 2021 at NASA Wallops Island Flight Facility on the eastern shore of Virginia, USA. Ripple height, wavelength and volume were measured over a 4 x 10 m area using a Riegl VZ-1000 terrestrial laser scanner (TLS). Vertical arrays of sonic anemometers and cup anemometers were used to record 3-dimensional velocity fluctuations and mean wind profile to estimate shear velocity over the rippled surface. A vertical array of saltation traps deployed from the surface to 30 cm was used to estimate vertical flux and saltation rates. Grain size distributions of captured saltation and migrating ripples are compared.

How to cite: Trimble, S., Kelley, M., Swann, C., Hode, A., and Braithwaite, E.: Capturing volumetric ripple migration with a terrestrial laser scanner  , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9987, https://doi.org/10.5194/egusphere-egu22-9987, 2022.

09:10–09:15
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EGU22-12141
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Virtual presentation
Mouncef Sedrati et al.

Studies on the dynamics of convex beaches "Cuspate foreland" have shown that these formations are directly influenced by the joint action of climatic-oceanic and meteorological-marine forcing conditions such as wind, swell, tide and associated currents. They are very dynamic beaches that can migrate longitudinally over a few to hundreds of meters, as well as flatten or lengthen in response to variable weather and sea forcing. These cuspate foreland beaches may also manifest seasonal cycles of beach erosion and recovery driven by the bi-directional approaches of wave climates or by the seasonal changes in swell and wave patterns. 

The "Grands Sables" beach on the island of Groix in the Morbihan - France is one of the most famous convex beaches in Europe. This beach is located to the north of the Pointe de la Croix, the eastern tip of the island of Groix. This beach, extending over nearly 800m, is located on a coastline formed by low cliffs extended to the south by the wide rocky plain. This work describes the seasonal morphodynamics of the Grands Sables cuspate foreland over a period of three years. and three-dimensional beach changes were measured and coupled to wave energy and wind conditions. Thus, the winter seasons, dominated by a strong westerly swell component, favour the movement of the beach towards the south. The summer seasons, on the other hand, allow the beach to find a state of stability between the north-east quarter winds and the south-west/south-east quarter winds, which compensate each other. These north-east quarter winds, which are frontal to the coastline, are also the driving force behind the morphological changes in the beach profile. They contribute to the cross-shore remobilisation of sediments in the intertidal domain, directly at the top of the beach and indirectly below the MHWL level through the action of the swell. In addition to seasonal variations, exceptional storm events also contribute to the southward or northward migration of the beach. During these events, the direction of beach movement is based on the incidence of wave and wind climate in relation to the orientation of the coastline. Results indicate that energetic waves play a significant role in shoreline dynamics and Grands Sables landform shape. Seasonal or high-energy event-driven morphological changes of the beach have occurred without a significant loss of local sedimentary stock.

The findings of this study have improved the understanding of seasonal and multiannual cuspate foreland morphodynamics, setting the groundwork for a potential long-term evolution model of Les Grands Sables beach. 

How to cite: Sedrati, M., Drean, L., and Bulot, G.: Annual and seasonal shore morphodynamics of a Cuspate Foreland: Les Grands Sables (Groix Island, France) , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12141, https://doi.org/10.5194/egusphere-egu22-12141, 2022.

09:15–09:20
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EGU22-13069
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ECS
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On-site presentation
Eunju Lee et al.

For the last few decades, coastal erosion has arisen as a global issue. Coastal erosion is accelerated mainly due to climate change (e.g., global sea-level rise and extreme waves) and artificial coastal structures. It results in a net recession of the shoreline and losses of coastal properties such as coastal infrastructure and beach material. As a result, the coastal communities become more vulnerable to coastal hazards and extreme waves. To protect coastal communities from extreme waves, coastal sand dunes were introduced as a natural and nature-based feature (USACE, 2013). For instance, the South Korean government placed an artificial sand dune on a beach on the west coast of South Korea where severe coastal erosion had occurred in an attempt to restore it. To achieve an adequate design of a coastal sand dune, it is essential to predict precise wave-induced sediment transport. However, the accuracy in wave-induced sediment transport prediction has not been sufficiently improved. Therefore, a laboratory experiment should be conducted to investigate the mechanism of wave-induced sediment mobilization and improve the accuracy of its prediction.

This study carried out a large-scale two-dimensional movable-bed experiment to analyze the erosion and accretion mechanisms of the dune-beach system during a storm and a post-storm. An entire storm event was reproduced in the flume, which was 100 m long, 2 m wide, and 3 m deep. The dune and beach profile was simplified by considering a representative natural dune on the west coast of South Korea on a 1:4 scale. An Acoustic Doppler Velocimeter (hereinafter ADV), ADV profiler, wave gage, and echo-logger were mounted on a movable cart to obtain wave and morphological characteristics over a wide range of flume. Also, CCTV and stereo cameras were installed to observe the erosion process of the dune and the entire wave transformation even in the very shallow water region (swash zone). So, through stereo imagery, wave transformation and runup were successfully measured. In addition, the Echo-logger measured the acoustic backscattering strength of the water column at a specific location near the sandbar crest to invert the backscattering strength measurements into suspended sediment concentration.

The tested wave conditions, representing a typical storm in South Korea for the past two decades, were divided into seven sea states consisting of one normal case, five erosive cases, and one accretive case sequentially. The dune face collapsed during the erosive case, and two sand bars were generated underwater. After the erosive cases, the accretive case caused the onshore sandbar to move landward and decay. This study shows the relationship between nearshore hydrodynamics and morphological evolution through data obtained by the experiment. Moreover, with the sedimentation aspect, the sediment core data on the sand bar, sampled after the experiment, successfully captured the storm history.

How to cite: Lee, E., Yoo, J., Choi, J. W., and Shin, S.: Large-scale movable-bed experiment on hydrodynamics and morphological evolution of the dune-beach system during a single storm event, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13069, https://doi.org/10.5194/egusphere-egu22-13069, 2022.

09:20–09:25
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EGU22-13108
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On-site presentation
Derek Jackson et al.

Hurricane Irma, after building to a category 5, struck the island of Barbuda in 2017, causing widespread devastation and dramatic coastal and nearshore alteration. Pre-and post-event airborne (terrestrial) LiDAR, along with satellite-derived bathymetry for the site provided detailed topographical quantification of the seabed and landform response to this extraordinary event, and presented a unique set of forcing conditions with which to observe Category 5 impacts on low lying island environments.

Using pre-hurricane bathymetry, Hurricane-generated waves were simulated across the nearshore using in situ and far field measurements of initial wave conditions. An initial SWAN simulation was conducted from the offshore using the WW3 wave climate as boundary conditions to develop the wave spectrum for nested, nearshore high-resolution (10m) grids focussed on the island. Water levels from the local tidal cycle were also accounted for using a set of non-stationary runs and local tide gauge information with an input grid of every 6 hours to simulate tides during the passage of the storm.

Significant bathymetric changes were noted throughout the nearshore zone as a result of the Hurricane event with distinctive erosion and accumulation patterns observed. We highlight direct wave forcing (bed shear stress) and its coincidence with these patterns of sediment dispersal. Terrestrial dune ridge topography was also dramatically altered with severe flattening of relief (and vegetation) during the Category 5 event.

Our study helps demonstrate the heterogeneous nature of the impact that hurricanes of this magnitude have on low lying island environments and shows the dramatic before and after changes they can have on the local coastal landscape.

How to cite: Jackson, D., Guisado-Pintado, E., Dolphin, T., and Heal, R.: Nearshore and coastal impact of Hurricane Irma (2017) on Barbuda, eastern Caribbean, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13108, https://doi.org/10.5194/egusphere-egu22-13108, 2022.

09:25–09:30
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EGU22-13109
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ECS
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Virtual presentation
Melanie Biausque et al.

Multiple Intertidal bars (MITB) are complex features described as a succession of sandbars located within the intertidal area of meso- to macrotidal beaches. Despite being found in many locations where conditions allow, the dynamics of these coastal bedforms remain unclear. They appear to be long-term features, relatively stable in form, but at the same time their movements can be prone to strong, short-term dynamics. MITB can play a significant role in wave energy dissipation as well as helping protect the beach/dune system to energetic events. In the context of sea level rise, our understanding of their behaviour over short to longer timescales, is important for coastal management and adaptation planning.

Three years of monthly DGPS surveys, conducted in Dundrum Bay (Northern Ireland) from May 2019 to April 2022, along with analysis of offshore wave forcing, were investigated to characterise both cross-shore and longshore dynamics. Additional cross-shore profiles were recorded every 10m to gather more detailed topographical understanding of changes in the intertidal. From interpolated topographic surveys, we identify complex sediment exchanges between the dune, the upper and intertidal beach areas. In addition, survey-to-survey difference maps as well as cross-shore profiles, were used to track cross-shore migrations of sandbars across the intertidal areas.

Preliminary results suggest that both incident wave energy and its direction are key hydrodynamic forcing variables that drive cross-shore migrations. Understanding sediment exchange between the different cross-shore sections of the beach is however, more complex and still under investigation.

Cross-shore drainage channels, essential in evacuating water-excess during tidal ebb, were observed intersecting bars at several locations. The evolution of these channels was closely associated with the alongshore migrations of bars. Cross-shore channels show a migration towards the inlet, that separates two main sections of the site, suggesting a migration of MITB bars towards the inlet. Consequently, a longshore sediment transport takes place from each sides of the bay toward the inlet, highlighting the significant role the inlet has in sediment circulation cells within Dundrum Bay.

How to cite: Biausque, M., Grottoli, E., Jackson, D., and Cooper, A.: Multiple intertidal bars: Three years of cross-shore and longshore dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13109, https://doi.org/10.5194/egusphere-egu22-13109, 2022.

09:30–09:35
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EGU22-13113
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Highlight
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On-site presentation
Edoardo Grottoli et al.

Shoreline change and storm forcing are analysed for Murlough and Ballykinler beaches (Dundrum Bay, Northern Ireland) over the last two centuries. The two beaches are divided by a pronounced ebb tidal delta and an inlet channel connecting the outer Dundrum Bay with the inner bay. Twenty-four shorelines were digitised from multiple datasets (historical maps, aerial photos, orthophotos and GNSS surveys) covering from 1833 to 2020. The seaward dune vegetation line was selected as shoreline proxy. Shoreline uncertainty was assessed considering various errors inherent from each dataset from which the shoreline was digitised. A coeval storm dataset since 1825 and an extreme water levels (EWLs) dataset from 1901 to 2020 were built using hindcasted wave parameters, historical news and recorded water levels from two local tide gauges. Volume changes from 1963 to 2014 were calculated applying the Structure-from-Motion technique to historical aerial photos.

Over the entire study period, Murlough displayed a retreat trend along 90% of its shoreline, whereas Ballykinler experienced an accretional pattern along 86% of its length. Three foredune blowouts characterise Murlough beach with an increasing landward extent toward the inlet. Murlough’s blowouts were reactivated and underwent recovery multiple times throughout the analysed period. In Ballykinler, a large blowout generated in 1951 is now replaced by an advancement trend particularly over the last 20 years.

On both sites, the largest blowouts were evident in 1951 and a clear erosive signature was also left by the 2013-2014 winter storm season. Three consecutive EWLs were recorded in 1946 and at the start of 2014, indicating that prolonged EWL events combined with a cluster of storms, were a significant driver of episodic coastal retreat phases. Volume analyses from 1963 to 2014 confirm that the sand moves from Murlough toward Ballykinler whose foredune gained more than twice the sand volume lost from the foredune in Murlough. Comparisons of recent and historical beach profiles of Ballykinler confirmed a beach growth of about one meter in elevation since 1963 throughout the entire beach profile. The role of the ebb tidal delta linked to wave energy dissipation and wave direction requires further investigation to explain the entire sediment dynamics of the study site.

How to cite: Grottoli, E., Biausque, M., Jackson, D., and Cooper, A.: Shoreline change and storm forcing over the last two centuries in Dundrum Bay (Northern Ireland), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13113, https://doi.org/10.5194/egusphere-egu22-13113, 2022.

09:35–09:40
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EGU22-10217
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Highlight
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On-site presentation
Florin Tatui et al.

Rip currents are powerful, shore-normal jet-like flows of water through the surf-zone, which are variable in space and time due to changes in incident wave conditions and nearshore morphology. Despite the low to moderate wave energy environment of the Black Sea, rip currents represent an important hazard on many of its beaches. Furthermore, there is a very low level of public’s awareness related to rip currents associated dangers. Regardless of this context, to our knowledge, no field experiments aiming at measuring rip currents dynamics and behaviour were conducted so far on the Black Sea coasts.

We present the first results following the RORIP1 field experiment, which took place for 10 days (11 – 21 October 2021) on Eforie Nord beach, Romanian Black Sea coast. We monitored 3 individual rip currents from a total of 10, which pose a great hazard for a 1-km long beach, the most dangerous on the entire Romanian Black Sea coast (135 and 104 drowning rescues in 2019 and 2020; 22 and 4 casualties in 2017 and 2018). We employed a complex methodology for rip currents monitoring comprising video techniques (video camera and UAV), topographic and bathymetric surveys, sediment sampling, drifters and ecological dye deployments, offshore wind and waves (Spotter buoy) and nearshore hydrodynamic measurements (3D current meter, pressure sensor).

Rips form in the troughs of multiple crescentic bars, often keeping their same position for a longer time period, and are fed by water influx through wave motion and breaking on the bar crests. Drifters and ecological dye deployments, complemented by UAV surveys and video camera footage, highlighted both ‘circulatory flow’ and ‘exit flow’ circulation regimes. In some cases, alongshore feeding channels between adjacent rips developed in the proximity of the shoreline, enhancing the offshore flows.

A number of 7 individual surf-zone drifters were released in the vicinity of shoreline during 3 deployments. Despite the low-energy wave conditions (offshore average Hs between 0.35 and 0.55 m; average periods between 3.6 and 4.5 seconds; propagating from ENE), all drifters experienced exit behaviour from the surf zone. Drifters registered average surface velocities between 0.34 and 0.43 m/s, with maximum instantaneous values exceeding 2 m/s. They travelled between 175 and 325 m during the periods of deployment ranging from 15 to 26 minutes, reaching cross-shore distances of 150 m from the shoreline. The above depicted surface dynamics is in good agreement with the preliminary modelling (Delft3D) employed for this area, which showed similar circulation patterns and surface flows for comparable hydrodynamic conditions.   

Our results, backed by a suite of complex analysis, demonstrate the high potential of rip currents to generate strong offshore flows even during low-energy wave conditions along Eforie Nord beach (Black Sea). This poses a great danger for beach safety and awareness of their related hazards is an urgent task for beach managers in the near future.

How to cite: Tatui, F., Zainescu, F., Miron, F., Vespremeanu-Stroe, A., and Mateescu, R.: RORIP1 – First field experiment of rip currents dynamics on the Black Sea coast, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10217, https://doi.org/10.5194/egusphere-egu22-10217, 2022.

09:40–09:45
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EGU22-9363
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ECS
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Highlight
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On-site presentation
Jorn Bosma et al.

In the context of coastal climate adaptation, coastal protection interventions involving soft and dynamic structures have noticeably gained popularity over the past decades. Huge sand nourishments of the order of Mm3 to tens of Mm3 have already been implemented along several stretches of vulnerable coastline. In most cases the added sediment concerned well-sorted sand with a D50 often very similar to that of the host area and was deposited in an open-coast system, where the morphodynamics are dominated by waves. Consequently, most of our understanding of nourishment dynamics stems from well-sorted sediment in wave-dominated environments. However, sand nourishments are increasingly applied to more sheltered systems, where tidal currents become dominant over wave-driven processes. Here, sand deposits are usually retrofitted to inadequate, hard flood defences to act as a buffer against erosion of such infrastructure. Frequently, coarser sediment than that of the host area is used in parts of the design to enhance this buffering capacity. Introducing different grain sizes into a system is expected to result in more complex, differentiated sediment transport, as coarser fractions are mobilised at different moments and places than finer fractions. By focussing on a nourishment on the leeward side of a barrier island, we aim to find quantitative answers to the questions when and how the mixed-sediment composition changes and what implications that has for the morphologic evolution of the area.

This research is based on an extensive 6-week field campaign in the early fall of 2021 at the 3-km long Prins Hendrikzanddijk, a retrofit nourishment on the island of Texel. We deployed long- and cross-shore arrays of instruments that measured a range of parameters such as pressure, flow velocity, suspended sediment concentration and bedforms. These measurements were further complemented by almost daily DGPS-measurements of the bed levels and a spatially extensive set of sediment samples which had been collected from the foreshore and upper shoreface at various time intervals. The conditions captured during the study period ranged from very calm (no waves) to stormy (Hm0 up to 0.6 m), while tidal currents reached velocities up to 0.6 m/s. Throughout these varying conditions, we encountered wide grain-size distributions in the top 5-6 cm of the bed almost everywhere, which sometimes also revealed multimodality. From a longshore perspective, D50 was generally largest in the centre, decreasing in either direction away from it. The beach surface was further characterised by transient bands/patterns of surfacing coarse and shell-rich material, which could disappear altogether overnight –often hand in hand with a smoothening of the cross-shore profile and a shift in beach-step position– after a period of increased wave activity. We will further elaborate on our obtained results and findings at the conference.

How to cite: Bosma, J., Woerdman, J., klein Obbink, M., Price, T., and van der Lugt, M.: Sediment transport and sorting processes at a back-barrier beach nourishment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9363, https://doi.org/10.5194/egusphere-egu22-9363, 2022.

09:45–09:50
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EGU22-9380
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ECS
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On-site presentation
Clayton Soares et al.

Sankt Peter-Ording is the only mainland beach system of the Wadden Sea between Den Helder in the Netherlands and Blåvandshuk in Denmark. The initiation and morphological evolution of this barrier system was reconstructed based on historical sea charts and literature. It was documented how a narrow Sankt Peter-Ording sand bar first appeared between 1920 and 1925. In the 1960’s a second bar attached to the north end and fully merged around 1994. Since its arrival the shoreline has been moving landward, permanent foredunes (today as high as 10m) have formed since the 1970’s and a salt marsh has formed behind the barrier. Early mapping campaigns show that the beach has grown over three times in area from approximately 4sq. km in 1943 to 12 sq. km in 2016. The retreat of the central beach is accompanied by accumulation along the Northern and Southern heads of the beach. The landward movement of the central beach is ~7m/a, while the north head has been growing at ~25m/a in the northerly direction. Extensive accumulation was identified in the South head of the beach, which moved ~2km in the SE direction since 1949 and roughly 400m in width. Coast parallel bedforms in the foreshore and at the south head of the beach ranged from 50-300m in length. Analysis of foreshore bathymetry reveals a prevalent SE direction of sediment movement. A foreshore bar system and channel have been moving at an average celerity of ~30m/a in the S-SE direction. Schematic spatial impression on the sediment transport pathways of the system is evaluated from the morphological development.

How to cite: Soares, C., Herrling, G., and Winter, C.: Tracing 100 years of morphological development of the Sankt Peter-Ording sand, a barrier beach system in the North Frisian Wadden Sea., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9380, https://doi.org/10.5194/egusphere-egu22-9380, 2022.

09:50–09:55
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EGU22-10549
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ECS
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On-site presentation
Salma Ezzahzi et al.

 

Abstract

The coastline of Essaouira is located on the Atlantic coast of Morocco. The bay of this city is a large sandy system strongly marked by coastal and fluvial dynamics which makes it a perfect example to understand the process of sedimentology in a bay area.

The use of satellite imagery allows us to determine the morphological evolution of the bay of Essaouira and the mouth of Oued Ksob. The results of our study show the evolution of the coastline of the bay of Essaouira with areas in erosion and others in accumulation. The analysis of satellite images allows us to distinguish three very different sectors: the first which is north of the estuary of Wadi Ksob because of its closed nature (bay), this area receives most of the flood contributions of Wadi Ksob, which helps to maintain its morpho-sedimentary balance. The second sector, which is composed of the estuary and the mouth of the Wadi Ksob, is characterized by a hydrodynamic both fluvial and maritime. The third sector is located south of the estuary of Wadi Ksob, this sector is an open beach that is not affected by the presence of an obstacle which is the island of Mogador.

The sedimentological study is carried out on four different areas which are: the dunes, the mouth of Wadi Ksob, the area south of the estuary and the area north of the mouth. Several analyses were carried out namely: granulometry, morphoscopy, calcimetry and X-ray diffraction.

The granulometric, morphoscopic, calcimetric analyses and the results obtained by X-ray diffraction as well as the presence of certain minerals confirm that the origin of the sediments studied is both continental and marine. In addition to the natural factors that condition the morphological evolution of the whole coastline of Essaouira, the island of Mogador presents an obstacle and plays an important role on the morphogenesis as well as the sedimentation along the coastline

Keywords: coastline, coastal dynamic, fluvial dynamic, bay, satellite images, oued Ksob, mouth, sedimentologicalstudy, granulometry, morphoscopy, calcimetry, X-ray diffraction

How to cite: Ezzahzi, S., Algouti, A., and Algouti, A.: Morphological evolution (1985-2021) and sedimentological study of the Essaouira bay and the mouth of Oued Ksob (Morocco), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10549, https://doi.org/10.5194/egusphere-egu22-10549, 2022.

09:55–10:00
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EGU22-10759
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ECS
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Virtual presentation
Serena L Teasdale et al.

The Holderness coastline of Eastern England is the fastest eroding coastline in Europe. The coast is characterised by ‘soft sediment’ tills, which make it distinctly susceptible to cliff retreat, in turn, these pose a socio-economic threat to local communities. The controls and future projections of the rates and patterns of retreat rely upon robust monitoring and process-based understanding of the geomorphological processes. Herein, we report on a 12-month monitoring study (June 2019 to May 2020) along a 220 m stretch of the Holderness coastline (Withernsea), whereby the spatial and temporal patterns of failure were captured using terrestrial LiDAR. Failure footprint, volumetric change and total eroded volume of the cliffs were estimated and compared against local hydrodynamic and meteorological records. The results reveal that >36% of individual failure events occurred solely in the upper portions (upper 75% vertical height) of the cliff, with a further >38% over the central section of the cliff face, with <26% occurring solely at the cliff toe (lower 25% cliff height). These findings disprove the widely accepted assumption that failure is primarily driven by wave attack, and we instead propose that instability in soft cliffs occurs as a result of moisture-driven ‘structural weakening’ with the influence of wave action primarily acting to remove failed material.

How to cite: Teasdale, S. L., Hackney, C. R., Milan, D. J., Bennett, G. L., and Parsons, D. R.: Insidious Retreat of the Holderness Coastline: Capturing Spatial and Temporal Patterns of Failure using Terrestrial Laser Scanning (TLS) , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10759, https://doi.org/10.5194/egusphere-egu22-10759, 2022.