Displays

Predicting coastal system response and evolution requires an accurate delineation and understanding of coastal cell boundaries and sediment transport pathways. Recent studies along highly embayed sandy coastlines show that important sediment transport into and out of the embayments may occur under particular conditions; however, key processes (e.g., mega-rips, headland bypassing), driving forces, flux rates and local factors (e.g., headland/embayment morphometric parameters) influencing these sediment fluxes are still poorly resolved. Here, we investigate the nearshore sediment transport dynamics along a 15-km stretch of the embayed coastline of SW England using the process-based numerical model Delf3D. 

Numerical simulations (coupled wave and tide) are conducted to compute major circulation modes and sediment fluxes for a wide range of modal and extreme conditions. Based on the hindcast wave data, predictions of sediment fluxes over multi-annual timescales are then produced allowing for resolution of potential sediment budgets. 

Results indicate that extreme events (H_s  > 7 m) involve multi-embayment circulation and mega-rip formation (0.7 m s^-1 at > 20 m depth) in the down-wave sectors of the embayments with subsequent significant sediment flushed beyond the base of the headlands (c. 10⁴ m³ day^-1 cross-shore and 10³ m³ day^-1 bypassing). Accretionary phases over moderate-high swell periods (up to H_s = 4 m) are characterized by the presence of clockwise intra-embayment circulation with predicted currents (0.4 – 0.5 m s^-1 flow below 10 m depth) inducing a slow transport of sand from the updrift to the downdrift part of all the embayments (c. -10² – -10³ m³ day^-1). This circulation mode is combined with weaker bypassing rates around the shallower and wider headlands (10² – 10³m³ day^-1) that is partially conditioned by the direction of the waves.

 Our study suggests that major mechanisms for redistributing material to and along the lower shoreface (up to 25 m depth) for embayed coastlines are the longshore residual flow around headlands, the presence of mega-rips and the embayment-scale circulation, with the latter being a function of embayment length and headland configuration. Hindcasted yearly bypassing rates around the headlands are episodic, occur mainly during high-energy events and range between 10³ and 10⁵ m³ y^-1. Hence, the magnitude of this bypass suggests that lower shoreface sediment fluctuations should be considered a critical mechanism that will inevitably affect coastal evolution over longer temporal scales (> 10 years), specifically along high energy and sediment starved coastlines.

How to cite: Valiente, N., Masselink, G., McCarroll, R. J., Saulter, A., Scott, T., Conley, D., and King, E.: Modelling nearshore sediment fluxes in embayed settings over a multi-annual timescale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7875, https://doi.org/10.5194/egusphere-egu2020-7875, 2020.

Modelling changes in nearshore bathymetry (<10m depth) is complicated by the nonlinear interactions between sediment, waves, and currents that can cause complex flow and transport patterns such as rip currents. Rip currents are of particular interest because of their implications for both sediment transport and beach-goer safety. An active area of research is using remote sensing (e.g., radar, video imagery) to estimate the existence and location of rip currents. Radar actively measures surface flow directions at high resolutions, however, the equipment can be expensive and difficult to set up. In contrast, video cameras are less expensive and more accessible, but can only provide passive observations that estimate derived surface quantities such as current speed and direction, and wave runup. Time exposure (timex) images from video cameras also provide information about the location of bright pixels (indications of breaking waves). Previous research has relied on the appearance of elongated, shore-normal regions of dark pixels (intersecting bright white regions) as a clear indicator of rip current presence, making timex images a prime candidate for automated detection of rip currents on beaches with video cameras installed. However, it is also known that rip currents vary widely in appearance, and that a better understanding of these parameters is necessary for automated rip current detection.

In this study, radar data and Argus camera imagery from the United States Army Corps of Engineers Field Research Facility at Duck, NC, USA were evaluated to determine how often radar measured offshore flow indicative of a rip current spatially correlates with dark, shore-normal features in the camera imagery. Radar data for two different times were processed to obtain surface current directions. Timex imagery from the video cameras on the same dates were evaluated with a machine learning algorithm   (Maryan et al. 2019) to objectively define the dark shore-normal features previously assumed to indicate rip currents’ existence within the imagery. A confusion matrix between these two datasets (surface flow direction and machine-identified rip current regions) confirms that dark, shore-normal features in the timex images are not always rip currents, and that offshore directed surface currents are not always visible as dark features in timex images. These results provide the first quantitative evaluation of how often rip current detections are missed and show that additional information is required for accurate automated rip current detection from camera imagery.

Further analysis will include using wind and wave data from field instruments at the site to reveal which conditions produce (1) offshore flow that is correlated with dark, shore-normal features in the timex imagery, (2) offshore flow that is not correlated with dark, shore-normal features in the timex imagery, and (3) dark, shore-normal features without focused offshore flow. This ongoing study could lead to the clarification of specific conditions under which the existence of rip currents can be correlated with a particular feature that machine learning techniques can be trained to recognize in camera imagery, thereby improving the accuracy of automated rip current detection. 

How to cite: Trimble, S. and Penko, A.: A quantitative evaluation of rip current appearance in Argus timex imagery: when and where does offshore flow correspond to visible features?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11261, https://doi.org/10.5194/egusphere-egu2020-11261, 2020.

Predicting changes to global shorelines presents a challenge that will become increasingly urgent over coming years as sea-level rise (SLR) accelerates. Current shoreline prediction models typically estimate the impact of SLR using variations of the ‘Bruun Rule’, which fails to account for many relevant processes, potentially producing erroneous results. To address this shortcoming, we introduce a simple rule-based model that predicts change across a wide variety of sand, gravel, rock and engineered (anthropogenic) coastal environments, at the scale of years to centuries, accounting for trend rates of change as well as natural short-term variability. Applying recent findings of laboratory and field-based research, the model translates 2D cross-sections of the shoreface, then integrates these changes across multiple alongshore profiles (into pseudo-3D). Uncertainty is accounted for using a probability distribution for inputs (e.g., rate of SLR, depth of closure, depth to bedrock). The model accounts for: (1) dune erosion and slumping [for large dunes]; (2) barrier rollback and overwash [for low barriers]; (3) aeolian dune accretion; (4) non-erodible bedrock layers, including those below ‘perched’ dunes; (5) seawall and revetment backed profiles; (6) onshore transport from the lower shoreface; (7) cross-shore variability due to storm erosion; (8) alongshore variability due to beach rotation; (9) alongshore re-distribution of dune erosion across the shoreface of a closed embayment; and (10) other sources and sinks (e.g., estuary infill, longshore flux, headland bypassing, biogenic production). We apply the model to two extensively monitored macrotidal embayments in the UK: Perranporth (sandy, dissipative, cross-shore dominant transport) and Start Bay (gravel, reflective, bi-directional alongshore dominant). For the dissipative sandy site, the primary modes of coastal change are predicted to be: (1) sea-level rise profile translation; and (2) extreme event cross-shore fluctuations. By contrast, for the reflective gravel site, the primary modes are: (1) short-term fluctuations in alongshore rotation; and (2) multi-decadal trends in longshore flux. For the steep gravel barrier, sea-level rise profile translation is important but secondary. Relative to the new model, the Bruun Rule underpredicts shoreline recession in front of cliffs and seawalls, and overpredicts where large erodible dunes are present. This new shoreface translation model is easily transferable to many coastal environments and will provide a useful tool for coastal practitioners to make rapid assessments of future coastal change.

How to cite: McCarroll, J., Masselink, G., Valiente, N., Wiggins, M., Kirby, J.-A., Scott, T., and Davidson, M.: A novel shoreface translation model for predicting future coastal change, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11236, https://doi.org/10.5194/egusphere-egu2020-11236, 2020.

The shoreline evolution and the occurrence of storm events are analysed for the last two centuries in Dundrum Bay, Co. Down in the SE coast of Northern Ireland (U.K.) as part of the INTERREG MarPAMM project. The study site is a macrotidal beach (5.5 m max spring tidal range) predominantly sandy and characterised by multiple intertidal bars (‘ridge and runnels’). The study site is characterised by a slightly embayed coastline, 8 km long and SW-NE oriented, interrupted by a tidal inlet that links the inner bay with the ebb tidal delta. The shoreline, described here as the dune vegetation line, has been digitised using a dataset of historical maps, aerial photographs and RTK-DGPS surveys from 1833 to 2019 (186 years). Sixteen shorelines have been digitised and a quantitative assessment of the uncertainty associated with each shoreline position has been performed. Shoreline changes statistics have been computed by means of ArcGIS extension DSAS 5.0 using a confidence interval of 99.7% on 325 cross-shore transects 25 m spaced. Storm events were identified across 194 years using historical news from local newspaper articles (1825-2019) and hind-casted wave data (1948-2019). The total change in shoreline movement, with no reference to the period, ranged between 7 to 253 m, with both these values located in the inlet and related to the generation and growth of a sand spit. An erosional trend affected the SW part of the study site (Newcastle-Murlough beach) with peak values of -55 m between the oldest and the most recent shoreline available (1833-2017): negative values increased towards the inlet and 90% of transects showed an erosive trend in this area. Accretion characterised the NW part of the bay (Ballykinler) with maximum values up to +209 m, again in proximity to the inlet: 87% of the computed transects showed an accretional trend in this area. In the Newcastle-Murlough area, the erosional trend lasted from 1859 to 1962, transited through a stable situation between the 1962 and 2012 and restarted erosion after 2012 up to a stabilisation in most recent years. Considering the entire analysed period, the maximum shoreline loss per year at Newcastle-Murlough was 0.30 m/year. In Ballykinler, the accretional trend lasted from 1859 to 2012 and except from a slight decrease in 2014, it is still ongoing. Considering the entire period, at Ballykinler the maximum gained was 1.3 m/year. The shoreline experienced the highest variations around the inlet area, driven by the generation and growth of a sand spit. Considering the high rate of changes in the inlet area, a further counter-clockwise movement is expected for the seaward part of the inlet channel. Since the accretion rate in Ballykinler beach is, in some places, four times that of the erosional rate of Newcastle-Murlough beach, differences in nearshore bathymetry, storm exposure and ridge and runnel dynamics between the two sites require further investigation. The study also aims to highlight the importance of combining multi-temporal geographic data with historical information in documenting long-term coastal changes within Marine Protected Areas of the UK.

How to cite: Grottoli, E., Biausque, M., Jackson, D. W. T., and Cooper, A. J. G.: Two centuries of shoreline evolution and storm events in Dundrum Bay, Northern Ireland., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4882, https://doi.org/10.5194/egusphere-egu2020-4882, 2020.

Crescentic sand bars have attracted significant attention from coastal scientists during the last decades, which has lead to comparatively good understanding of their formation mechanism, as well as their characteristics and dynamics (e.g. Van Enckevort et al., 2004; Price and Ruessink, 2011). However, the effect of wave obliquity on crescentic bar formation is not yet clear, and processes like coupling of crescentic bars with megacusps deserve further attention. Furthermore, the mechanisms leading to crescentic bar straightening are not well understood. Previously, this was mainly linked to high-energetic wave conditions, but more recent studies (e.g. Price and Ruessink, 2011; Garnier et al., 2013) indicate that this is not always the case. Instead, those studies have found that bar straightening predominantly occurs when the waves are obliquely incident. Finally, there are not many studies of crescentic bars in fetch-limited environments with insignificant tides (such as Mediterranean beaches). Therefore, the objective of the present work is to increase our knowledge on the dynamics of crescentic bars (including bar straightening) using data from an open, Mediterranean beach (Castelldefels beach, 20 km southwest of Barcelona) with hardly any tides and limited fetch.

Crescentic bar dynamics have been analysed using a nearly 8-year dataset of time-exposure video images (October 2010 to August 2018). The crescentic bar events, including formation and destruction moments, have been detected using visual analysis. Wave conditions in front of the study site have been collected by propagating 2D spectra (measured by a permanent wave buoy in front of Barcelona harbour) using the SWAN spectral wave model. The first results indicate that there is a lot of morphodynamic variability at the study site, even for low-energetic wave conditions (H_m0 < 0.5 m). Tens of crescentic bar events, including formation, evolution and destruction, can be observed. The bars show a large variation in wavelength (ranging from 100 to 500 m), which is often related to splitting and merging of individual crescents. Furthermore, the results reveal a strong relation between crescentic bar formation and the initial configuration of the bathymetry. Crescentic bars develop often when the sandbar is located some distance from the shoreline, whilst they are hardly observed when the sandbar is located close to the shoreline. Further work (which will be presented at the conference) consists of a detailed analysis of bar characteristics, including their alongshore migration, and the quantification of the role of wave conditions (especially wave direction) on crescentic bar dynamics.

References
Garnier, R., Falqués, A., Calvete, D., Thiebot, J., & Ribas, F. (2013). A mechanism for sandbar straightening by oblique wave incidence. Geophysical Research Letters, 40(11), 2726-2730.
Price, T. D., & Ruessink, B. G. (2011). State dynamics of a double sandbar system. Continental Shelf Research, 31(6), 659-674.
Van Enckevort, I. M. J., Ruessink, B. G., Coco, G., Suzuki, K., Turner, I. L., Plant, N. G., & Holman, R. A. (2004). Observations of nearshore crescentic sandbars. Journal of Geophysical Research: Oceans, 109(C6).

How to cite: de Swart, R., Ribas, F., Calvete, D., Simarro, G., and Guillén, J.: Characteristics and dynamics of crescentic bar events at an open, Mediterranean beach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18730, https://doi.org/10.5194/egusphere-egu2020-18730, 2020.

Quantifying aeolian transport within the swash zone is critical to understanding feedbacks between aeolian and nearshore processes in coastal environments. In the swash zone, high moisture contents are thought to significantly limit the amount of sediment available for transport by wind. These assertions are supported by empirical relationships between the threshold for aeolian transport and moisture content that show gravimetric moisture contents greater than ~5% severely restrict the transport of windblown sand. Yet, during strong wind events aeolian transport can occur in the swash zone where moisture content is significantly higher. Here, we present field observations of fully-saturated aeolian transport on a wet beach and highlight the proficiency of winds to sustain aeolian transport in the swash zone.  

Field observations were collected during the passing of Tropical Storm Nester on a dissipative beach north of Corolla, North Carolina, USA in the early morning hours of October 19. 2019. Beach width ranged between ~50 and 100 meters and observations were made during a falling tide. Alignment of predominate winds and beach orientation provided a nearly unlimited fetch with an abundant sediment supply from the drier upper beach. Mean grain sizes of surface grab samples in the swash zone were 0.17 to 0.19 mm and moisture content in the swash zone ranged from 8 to 13% during the observational period.

Videos of fully developed, saturated transport in the form of nested streamers, approximately 5-20 cm wide, were recorded. A vertical array of cup and sonic anemometers measured near surface fluid flow. Cup anemometers were sampled at 1 Hz and observed wind velocities at 7, 18, 44, 68 and 93 cm above the surface. Ultrasonic anemometers sampled 3 dimensional velocity components at 32 Hz via at 53 and ~100 cm.  Sustained wind velocities were 9.5 m/s at 93 cm above the surface with gusts reaching 14 m/s. A series of vertically-segregating saltation traps captured particles in transport and showed minimal size-segregation with height. Gravimetric moisture content of captured saltation ranged from 0 to 4%.

Pulses of abundant aeolian transport during the storm were largely driven by largescale coherent eddies initiating transport from the drier upper beach. These upper beach sediments sustained transport on the lower, wet beach. The spatial and temporal variability of the exceedance of both fluid and impact thresholds strongly controls transport. These field observations demonstrate the proficiency of wind to transport of large volumes sand in the swash zone during strong alongshore wind events.

How to cite: Swann, C. and trimble, S.: Aeolian transport on a wet beach: Field observations from the swash zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11977, https://doi.org/10.5194/egusphere-egu2020-11977, 2020.

Integrated modelling approaches for the evolution of the entire dune-beach system have become increasingly sought-after, not only for management purposes, but also to allow better understanding of the feedbacks between processes and scales and a closer approximation of where critical system thresholds may lie. The effective reproduction of both destructive and constructive processes over a broad spectrum of temporal scales is crucial to any, such, integrated approach. Recent improvements of the XBeach-Duna model regarding approximation of nearshore processes were tested using in-situ data from the Emma storm impacts on a reflective beach (Praia de Faro, in S. Portugal). The model results compare well with measured post-storm and recovered profiles, showing high model skill under both erosive and constructive regimes. Building from this event-scale analysis, a gradual increase of temporal windows in simulated forcing conditions, through wave schematisation, is presented and discussed in terms of optimisation between gains in simulation time and losses in geomorphic change information. This methodological approach and findings are the basis that will allow passing on to dependable, long-term simulations of the beach-dune system evolution.

Acknowledgements: The work was implemented in the framework of the ENLACE project (ref. 28949 FEDER), funded by FCT (Fundação para a Ciência e a Tecnologia)

How to cite: Kombiadou, K., Costas, S., Roelvink, D., and McCall, R.: Post-storm recuperation as a stepping-stone towards long-term integrated modelling in steep beaches, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17470, https://doi.org/10.5194/egusphere-egu2020-17470, 2020.

With a view to assessing morphodynamic responses of the southern Swedish coast to predicted future sea level rise, the Geological Survey of Sweden has conducted detailed onshore and offshore sediment mapping and commissioned modelling of nearshore wave and current dynamics. Seamless, full coverage land and seabed mapping from approximately 3 m above sea level to 1000 m offshore has been completed along 500 km of coastline. On land, mapping of surficial sediments was done using conventional field-based methods and a high-resolution LIDAR-based digital elevation model. For the seabed, sediment and bathymetric mapping was based on ship-borne hydroacoustic surveying data, as shallow and close to shore as permitted by the ship draught, involving multibeam, swath-sonar, side-scanning sonar, sediment profiling and reflection seismics. For the white ribbon zone, i.e., the nearshore zone that is too shallow for the ships to enter, airplane-borne LIDAR and orthophoto-data were acquired. Ground-truthing in the form of sediment-sampling and visual observations was also done to verify sediment interpretations in the hydro-acoustical data. The exposure of the coast to waves and currents was modelled from several decades of historical wind data.  The sediment-, bathymetric- and topographic data were then combined with the modelled data of exposure to wind, waves, and currents to analyze spatial patterns of sediment erosion, transport and deposition. The results have been compiled into maps showing the location and distribution of mobile sediments, their transport pathways and storage compartments in the nearshore and deeper offshore zones, whether these compartments are closed or leaky, and their onshore-offshore exchange, including long-term trends in coastline accretion and erosion. The results show the coastline adapting to sea level rise that is associated both with the cessation of postglacial isostatic uplift and global warming, and to other climatic factors such as long-term changes in dominant wind direction.  At present, erosion causing long-term shoreline recession is localized. However, there is high potential for this to become a much more general and high magnitude problem in coming decades along this heavily populated, low lying, sedimentary coastline.

How to cite: Nyberg, J., Goodfellow, B., Ising, J., and Hedenström, A.: Nearshore morphodynamics along the coastline of southern Sweden from detailed surficial mapping and hydrodynamic modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-406, https://doi.org/10.5194/egusphere-egu2020-406, 2020.

Embayed beaches separated by irregular rocky headlands represent around 50% of the world’s shoreline and are important zones ecologically and commercially. Accurate determination of sediment budgets is necessary for prediction of coastal change over long timescales in these zones. Some headlands have been shown to permit sediment bypassing under particular forcing conditions, therefore knowledge of sediment inputs and outflows via headland bypassing are important for sediment budget closure. Recent modelling work demonstrates bypassing rates are predictable for an isolated headland, however, it remains to test this predictability using a range of real headland morphologies, and to examine the influence of embayment morphology, sediment availability and tidal effects.

We show that bypassing rates are strongly influenced by the relative proximity between adjacent headlands, and the degree of embaymentisation. Tidal currents are secondary to wave forcing, mildly moderating bypass rates, whereas tidal elevation strongly influences bypassing rates largely through variations in apparent headland and embayment morphology.

A fully coupled (3D hydrodynamics and waves) numerical model was used to simulate sand transport along a 75 km long macrotidal, embayed coast in the north of Cornwall, UK. Twenty-five embayments were included in the analysis. Nine wave conditions were simulated and bypass rates were analysed for three tidal elevations. Simulations were performed with both uniform sediment availability and a realistic spatial distribution of sediment, and both including and excluding tidal currents. It is shown that many of the embayments along this stretch of coast exhibit headland bypassing under energetic wave forcing, highlighting the need for accurate bypass rate predictions for sediment budget determination on embayed coasts.

Headland extent relative to surf-zone width was a critical control on sand bypass rates in line with previous work. Predictive expressions were accurate to within a factor of 4 for beaches exhibiting a ‘normal’ circulation pattern (embayment length long relative to surf zone width), however, they did not predict well cases where embayment cellular circulation was dominant (embayment length short relative to surf zone width).  Tidal currents exhibited a secondary control relative to wave forcing, moderating bypass rates by up to 20% in this macrotidal environment. Large differences in the apparent morphology of the embayments between high and low tide strongly impact bypassing rates, with greatest bypassing occurring at low-tide when headland cross-shore length is smallest. Bypass rates were reduced for realistic sediment distributions versus uniform sediment availability, due to larger transport magnitudes when sediment is available off the headland toe.

This work highlights the extent to which headland bypassing occurs along this embayed coast with implications for similar coasts worldwide. It also emphasises the need for accurate predictions of headland bypassing in these regions and suggests areas for further efforts to focus to refine future predictive parameterisations.

How to cite: King, E., Conley, D., Masselink, G., Leonardi, N., McCarroll, R., Scott, T., and Valiente, N.: Wave, Tide and Morphological Controls on Embayment Circulation and Headland Sand Bypassing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-781, https://doi.org/10.5194/egusphere-egu2020-781, 2020.

The choice of the object of study is due to the alarming problem of the stability of its shores and, above all, the sandy beaches of the recreational and tourist complex of international importance Varadero, Ikakos peninsula. Its relief is represented by a low (on average 10 m) abrasion-accumulating plain with several remnants of indigenous carbonate rocks of calcarenites, the maximum elevation of which is 27 m. From the west, the peninsula is washed by the waters of the Strait of Florida, and from the east by the waters of a small shallow-water Cardenas Bay. In tectonic terms, the Ikakos Peninsula is represented by a fault-block structure complicating the centricline of the neotectonic trough of Remedios, bordering about Cuba island. Holocene deposits of the peninsula on the western side are represented by marine organogenic sand of beaches, limited by organogenic conglore breccia of the Seboruko terrace and cliffs of Miocene calcareous sandstones, and on the eastern side, where mangrove vegetation is widely developed, mainly by sediments of marshes and small shallow lagoons.

Based on the results of comparative interpretation of the Ikakos Peninsula satellite images from 2003 to 2013, a map of the types of its shores was compiled . Comparison of images of different times showed that most part of the western coastline is stable. For the accumulative part of it, this is obviously a consequence of artificial sanding, and for the abrasive part, it is a consequence of the expansion of benches with a boulder block. The beaches are most stable in the middle part of the peninsula, probably because migrating alongshore sediment fluxes from both the southern and northern sides of the peninsula rush here. The most mobile were the basal and distal parts of the peninsula.

In the short term, the morpholithodynamics of the coastal geosystem of the Ikakos Peninsula will be determined mainly by its latest tectonics and sea level kinematics. The western margin will be determined by sand reserves in the coastal shelf zone. If the peninsula maintains a tendency toward a weak and moderate uplift, the abrasion of the coasts formed by calcarenite will slow down. On sandy coastal areas with increased flotation of beach-forming material, the amount of material will be reduced. Therefore, to maintain the beaches, it will be necessary to carry out competent and timely sanding and provide measures to extinguish the energy of storm waves at a submerged slope. The distal part of the peninsula will increase. On low-lying mangrove shores, lagoons and bogging will shrink, and halophilic, mainly mangrove, vegetation will advance into the Gulf of Cardenas waters.

This work was supported  by the Russian Foundation for Basic Research, projects no. 18-05-34002, 20-05-00009 and by the Russian Science Foundation, project no. 20-17-00060.

How to cite: Kosyan, R., Dunaev, N., Repkina, T., and Juanes Marti, J.: Forecast of development of sea coasts on their morphodynamicstate according to the results of space images descryption, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1407, https://doi.org/10.5194/egusphere-egu2020-1407, 2020.

The change of barrier islands could be a precursor of coastal landscape evolution. The barrier islands on the coast of southwest Taiwan are continuing narrowing and landward moving in the past decades. The government has tried to install eight detached embankments to protect Dingtoue barrier island in 2001. In this study, we try to monitor the landform change by using UAV photogrammetry. Dingtoue barrier island is 1.3 km in length and with area of 30.5 ha. We have already conducted 4 campaigns of UAV photogrammetry between March 2018 and September 2019, and they can reveal the landscape of the end of summer and winter monsoon. We use Agisoft Metashape to process the aerial photos for acquiring the DEM and ortho-rectified image with the spatial resolution of 0.5 m and precision level of 0.04 m in both horizontal and vertical direction. We sub-divide Dingtoue barrier island into beach and sand dune zones for further analysis by using Arc GIS. The DEM of difference and areas will be obtained in beach and sand dune as well.

The results show that area of Dingtoue barrier island is increasing 5101.2 sq.m, while volume of Dingtoue barrier island is decreasing 26722.1 cu.m at the end of the 2018 summer monsoon. The beach part is increasing in both area and volume, while the sand dune part is decreasing in both area and volume. The northern part of the beach is extending to east and the sand dune zone is retreating to further east. The southern part of the beach is extending to west part, which is the sea in the past. Area of Dingtoue barrier island is increasing 719.4 sq.m, while volume of Dingtoue barrier island is increasing 36705.7 cu.m at the end of the 2018 winter monsoon. Area of the beach part is relative the same as the previous period be with some minor changes in the northern and southern part. The sand dune part is increasing in both area and volume. Area of Dingtoue barrier island is increasing 14616.2 sq.m while volume of Dingtoue barrier island is decreasing 23894.1 cu.m at the end of the 2019 summer monsoon. Areas of beach and sand dune are both increasing while volume of the sand dune is decreasing. The mid-part of the beach is occupied by sand dune and the beach is recovering to previous shape.

In general, Dingtoue barrier island is increasing 7% in area and is decreasing 13910.5 cu.m in volume between March 2018 and September 2019. The average surface lowering is 0.05 m in this period. The trend shows that typhoons will increase area of Dingtoue barrier island, but decrease volume. The winter will decrease area of Dingtoue barrier island but increase volume. So the main change of area is at the beach part and the main change of volume is at the sand dune part. From the installation of the eight detached embankments can stabilize Dingtoue barrier island by increasing area, and volume is decreasing during the summer period and increasing in the winter period.

How to cite: Hsu, H.-J., Chyi, S.-J., Jen, C.-H., Ho, L.-D., and Chen, J.-H.: Using unmanned aerial vehicle (UAV) photogrammetry for monitoring seasonal changes of barrier island in the southwestern coast of Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3072, https://doi.org/10.5194/egusphere-egu2020-3072, 2020.

Previous study by Ho et al. (2017) proposed an evolutionary model of the Feng-Chuie-Sha (FCS) clifftop dunes in the Hengchun Peninsula, southeastern Taiwan. In this model, tectonic uplifting, eustatic sea-level falling and the fluctuations of the East Asian winter Monsoon during the late Holocene could be the major forcing factors to the development of the clifftop dune. However, the climbing dune at the bottom of the cliff has not been carefully investigated yet, as the climbing dune is an important link between the beach and the clifftop dune, in terms of aeolian sediment cascades. In this study, we aim to improve our understanding of the unique beach-climbing dune-clifftop dune system in the FCS, and to identify phases of the changing influence of geomorphological forcing factors during the Holocene. For the paleo-environmental reconstruction, a detailed chronological framework will be established by applying numerical dating techniques, such as radiocarbon and optically stimulated luminescence (OSL) dating. Landscape features and sedimentological successions were mapped in the field and samples were taken for high resolution grain size analyses. Preliminary results show that several carbonate-cemented thin layers of aeolian sediment were observed in the outcrop. Based on the sedimentological sequence, the thin layers in two sections can be correlated well. We interpret the correlated thin layers as palaeo-surfaces of the climbing dune, and they may indicate the pause time of sand accumulation. The slopes of the palaeo-surfaces gradually increase from the bottom to the top, demonstrating the morphological development of the climbing dune over time. As the OSL and radiocarbon dates of the outcrop section are still under processing, the accumulation periods and rates of the climbing dune and its relationship with the formation of the clifftop dune will be presented and discussed.

Ho, L., Lüthgens, C., Wong, Y., Yen, J., Chyi, S.(2017): Late Holocene cliff-top dune evolution in the Hengchun Peninsula of Taiwan: Implications for palaeoenvironmental reconstruction. Journal of Asian Earth Sciences 148, 13-30.

How to cite: Ho, L.-D., Lüthgens, C., Chen, C., and Chyi, S.-J.: The Missing link between beach and clifftop dune – Landscape evolution of the climbing dune in the Feng-Chiue-Sha area of Hengchun Peninsula, Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4215, https://doi.org/10.5194/egusphere-egu2020-4215, 2020.

DGPS surveys were undertaken on two beaches of Dundrum Bay (east coast of Northern Ireland, Co.Down, U.K.) and analysed to investigate the short-term morphodynamics of a multiple intertidal bar (‘ridge and runnel’) system, as part of the INTERREG MarPAMM project. Ballykinler (east) and Murlough beach (west) are medium to coarse sand environments subjected to short waves and macrotidal conditions. Since April 2019 (ongoing), monthly surveys consisting of 14 cross-shore profiles and 2 intensive sections (in total) were carried out. Hydrodynamic conditions were extracted from the model WaveWatch3 (WW3) run by Ifremer (France) at a node located offshore of the bay. Intertidal bars were well-developed along Murlough beach (profiles 3 to 11) at the beginning of the experiment, with an increase in the complexity of ridges and runnels morphology toward the inlet (profile 12). In contrast, intertidal bars were only well developed in the western end of Ballykinler beach (profiles 13 and 14) and gradually disappeared toward the eastern end (profiles 15 and 16).

Preliminary results from the summer season show no measurable morphological change to significant accretion and onshore migration of the bar crest with low to moderate hydrodynamic conditions. However, there is a strong alongshore variability in the bay, with response to the summer season and is recorded not only between Ballykinler and Murlough beach, but also along Murlough beach. By contrast, the winter season is characterised by a decrease of bar amplitude due to an expansion of the bars wavelength, and in some cases, an onshore migration of the bars crest (mostly noticeably for Ballykinler). The winter season is, however, highly dependent on storm conditions. During the period of November to mid-December 2019 there were 4 storms including Storm Atiyah, and a signature of those highly energetic conditions was recorded in the beach morphology.

Depending on the location along the bay, ridges and runnels underwent bar crest erosion and sediment deposition into the runnels leading to a flattering of the profile, or cross-shore bar migrations.  Calm summer conditions, therefore, appear favourable for ridge accretion and onshore migrations, while energetic winter conditions seem to actively drive bar erosion and profile flattening. An alongshore variability in Dundrum Bay is a response to both seasonal and event conditions and is demonstrated by the results to date. This variability is probably due to wave orientation, wave energy dissipation and wave reflection linked to both offshore and nearshore bathymetry. The shape, position and number of the ridges and runnels should therefore play a key role in the energy dissipation depending on the tidal phase.

How to cite: Biausque, M., Grottoli, E., Jackson, D., and Cooper, A.: Short-term morphological changes of multiple intertidal bars on macrotidal beaches: from seasonal to storm-scales., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4883, https://doi.org/10.5194/egusphere-egu2020-4883, 2020.

With an area of almost 10,000 km², the project area represents the tidal flats on Germany’s North Sea coast. The tidal flats and their channels as well as morphologically highly active estuarine systems undergo significant erosional and sedimentational processes that prove difficult the assessment of sedimentological composition based on relatively few and temporally far stretched field measurements. The holistic databased simulation of both the internal structure of the soil itself and its sedimentary composition is based on around 21,000 measured surface sediment samples (from 1949 until recent) and yearly consistent digital bathymetric models, starting 1950, spatiotemporally interpolated in a 10 m grid resolution by the Functional Seabed Model. By utilizing the high temporal and spatial resolution of the bathymetric models, it is possible to quantify the seabed depth evolution (sedimentation and erosion) and to solve a differential equation to capture sedimentary evolution, a consistent and continuous three dimensional model of both the surface and the subsurface structures and sedimentary compositions can be generated. To further extend the volumetric extent of the model, around 16,000 sedimentary core samples are used to fill the spatial and consequently the temporal void between the lowest altitudinal range of validity of the aforementioned model segment to the lower boundary of the target model volume. This boundary is set to be the lower limit of the morphologically active or activatable space, which contains the volume of sediment that could be eroded in current climate conditions. The limit, generally speaking, can be expected to somewhat coincide with the base of Holocene sediments, as Pleistocene sediments – especially subglacial tills – generally take higher amounts of bottom shear stress to erode than unindurated Holocene sediments, which usually form tidal flat sediments. The purpose of the generated three dimensional model is to be able to derive sedimentological information in both custom spatial resolution as well as custom sedimentological classification as base and validation data for process based morphodynamic simulation models. With these enhanced models, the quality of the prognosis of morphological developments and stability of coastal areas as a tool for planning processes for coastal protection and maritime economy is expected to be increased.

How to cite: Sievers, J., Milbradt, P., and Rubel, M.: Databased simulation and reconstruction of the near shore geomorphological structure and sediment composition of the German tidal flats, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2566, https://doi.org/10.5194/egusphere-egu2020-2566, 2020.

Coastal monitoring of sandy beach areas requires data gathering at regular time scales to capture daily to weekly geomorphological changes that are modified through tidal and wave action. Regular GPS profiling surveys carried out at medium (weekly) to long-term (month/annual) frequency can lead to misinterpretation of beach changes as they are not able to pick up subtle gross changes and instead usually only capture net changes that have occurred. Recently, a new suite of monitoring devices such as Terrestrial Laser Scanners, airborne LiDAR or the use of Unnamed Aerial Vehicles have become more readily available and have in many cases replaced traditional monitoring methods (e.g. the use of Emery method and GPS) as they can capture contemporary morphological impacts faster and more conveniently. Monitoring coastal systems using video cameras is also an increasingly common monitoring method as it allows a continuous monitoring method through the image capture at different time-scales.

Here, we present the use of high-frequency imagery generated from a low-cost, fixed, time-lapse camera system as an effective method for quantifying intertidal bar migration patterns on a daily to annual time scale. The time-lapse camera system was deployed over-looking a beach-dune complex at Five Finger strand, NW Ireland. It was located on high ground (around 80 m) obliquely overlooking the study site, with a field of view of 59° and set to acquire images every 30 min. Images captured were calibrated using multiple ground truth Ground Control Points (GCPs), positioned at regular geo-located intervals along intertidal profile lengths. Further, average distance of each pixel on the ground was converted into real-world distance using a pre-calculated scaling factor.

The method successfully tracked the leading edge of an onshore migrating intertidal bar using a set of chronological captured images over a shoreward distance of 31.23 m in a 3-month period. The technique can also be used in the monitoring of wave run-up and dune toe encroachment events by waves during high energy events. The use of the camera over long time periods provided a rich dataset for examining both long-term intertidal beach dynamics of sites to help fully compare forcing and response phenomena in between forcing events. We believe that this easy-to-use and low-cost technique will enhance future monitoring of highly dynamic coastal systems enabling a more detailed spatial and temporal analysis of intertidal sandy beach areas.

How to cite: Guisado-Pintado, E. and W.T. Jackson, D.: The use of a low cost, time-lapse camera for high frequency monitoring of intertidal beach morphology , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5184, https://doi.org/10.5194/egusphere-egu2020-5184, 2020.

Coastal dunes are both a vital natural coastal defence and a key ecological habitat; therefore, understanding their evolution is important to inform coastal management. Megatidal environments are the world largest tidal ranges and hence provide a unique endmember of the tidal range continuum. A study site at Crymlyn Burrows, Swansea Bay, UK is monitored here; the area was originally of applied interest due to its identification as a key receptor of the Swansea Bay Tidal Lagoon project. The study site comprises of 2km of dune frontage bounded to the west by hard sea defences and to the east by the River Neath estuary. The intertidal is characterized by a shallow slope and crescentic intertidal bars. Mean spring tidal range at the nearby Mumbles tide gauge is 8.46m; mean wave heights at a wave buoy offshore of the site (depth 9m LAT) are 0.66m and storm wave heights exceed 3m; predominant wind direction is in an alongshore – onshore direction.

A Sensefly Ebee-RTK drone with a Sony RGB camera has been used to map the dune system and the mid to upper intertidal beach on a monthly – bimonthly frequency since October 2018. Initial post-processing was conducted in the Sensefly Emotion3 software; Pix4D was then used to generate a point cloud from the georeferenced images. RTK-GPS surveyed ground control points distributed over the study area were used to improve the accuracy of the solution. Point clouds were cleaned to remove noise using Cloud Compare, an open source point cloud editor, before being interpolated onto a gridded surface. Comparison of the gridded surface against RTK-GPS surveyed points gave a vertical mean absolute error (MAE) of 0.05m over the beach area. Comparison in the dune area is more complex since the raw point cloud includes the vegetation and hence over-estimates height compared to the bare earth. Based on the raw point cloud, MAE over the dune area was 0.22m; however, when vegetation points were removed using artificial neural network based colour discrimination, the MAE was 0.05m.

Longshore variation in dune evolution is clearly evident. At the eastern and western ends of the dune system, dune progradation can be observed whereas in the central portion the frontal dune is cliffed and the dune foot position is static or eroding landward. Pressure transducers have been deployed in a longshore array at the neap high tide level to assess variation in wave energy reaching the upper intertidal over the study area.

This presentation will explore whether this variation in behavior is due to longshore variation in wave energy (erosion potential), variation in sediment availability (accretion potential) or the persistence of antecedent morphology.

How to cite: Fairley, I., Horrillo-Caraballo, J., Mendzil, A., Blow, G., Miller, H., Masters, I., Karunarathna, H., and Reeve, D.: Longshore variation in coastal foredune growth on a megatidal beach from UAV measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8059, https://doi.org/10.5194/egusphere-egu2020-8059, 2020.

Beach morphodynamics are largely controlled by the interaction of wave climate with beach sediments. Local changes in sediment grain size, shape or density can lead to distinct morphological changes of beach systems subjected to similar energetic inputs. Whereas the spatial variation of grain size along beach profiles has been well studied, the temporal variation in beach grain size has received less attention. Moreover, the fate of cliff-eroded sediments along sandy coasts, with limited tidal effect, was rarely studied as most studies focused on shingle beaches (rocky/pebble rich) especially in coastal environments where tide plays an important role.

Here we use grain size data to explore the temporal dynamics of beach sediments in cliff-dominated beaches along Israel’s Mediterranean coast and their relationship to cliff erosion as well as sand abrasion/attrition. Our approach is based on repetitive seasonal-scale sampling of surficial sediments along cross shore transects over 3 years. We found that most samples exhibit unimodal particle size distribution (PSD), with a mode either at the fine sand fraction (180-220 µm) composed of quartz, or at the coarse sand to very coarse sand fraction (900-1,200 µm), composed of eolianite rock chips. The coarse fraction dominants the PSD mostly during winter times, whereas at summer times it is usually absent. In addition, this coarse fraction decreases with time that passed since waves reached the cliff base during sea storms. Our results suggest that: 1) The addition of the coarse fraction during winter is related to high-energy wave storms that mobilize and transport cliff-derived materials (taluses) along the beach, and 2) The disappearance of the coarse fraction towards summer is related to sand abrasion by wave and/or by wind action, i.e. breakage of the ~1 mm eolianite rock chips into ~200 µm quartz grains. Our findings emphasize the importance of cliff erosion and sand abrasion in controlling the temporal variation in PSD along cliff-dominated beaches.

How to cite: Crouvi, O., Shemesh, R., Katz, O., Mushkin, A., Morag, N., and Lensky, N.: Spatial and temporal changes of sediment grain size along Israel’s Mediterranean cliff-dominated beaches, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8252, https://doi.org/10.5194/egusphere-egu2020-8252, 2020.

The Nalón estuary (Asturias, NW Spain) provides a highly varied information about natural and anthropic changes along more of one century since it received the contributions from the major coal basin of Spain and it was the most important coal port of the north of the Iberian Peninsula during 20^th century. As a consequence of these factors, the estuary has undergone important transformations for port uses such as port basin, jetties in the mouth, intense dredging, etc.. These changes triggered the progradation of the dune field of the confining barrier, erosion in neighbouring eastern dune fields, joint to the subsequent changes in the morphology and sedimentary in the most part of the estuary. On the other hand, the historical exploitation in the hydrographic basin of numerous mines, mainly coal mines and some metallic such as Hg, Cu, Fe and Au, with null environmental control, has produce the contribution of carbonaceous mineralogy to the fractions of quartz sands and the modification of the natural geochemistry by the contribution of metals and metalloids along the entire estuary. Furthermore, some borecores and samples throughout the estuary were studied to assess the impact of the historical human footprint on the sedimentary sequence.

Since 70´s of 20^th century, the mining activity decreased and in parallel the intensive dredging decreased until today when they are scare and small definitive interruption of the intensive dredging in the estuary. Nowadays, the role of the sea-level rise and the recurrence of a series of strong wave storms since 2009 has caused the retreat of the dune fields of the confining barrier of the Nalón estuary and eastern beach/dune system of Bayas and the gradual filling of the external sector of the estuary. In addition, the management of the port is complicated because a serious navigation problem in the mouth occurred due to restrictions on dredging activities as a consequence of the high trace element concentration in the sediments and the ongoing inputs of As, Hg, Pb, and Zn into the coastal zone.

How to cite: Flor-Blanco, G., García-Ordiales, E., Flor, G., López Peláez, J., Roqueñí, N., and Navarro-García, V.: Environmental change assesments in response to anthropogenic human footprint in the Nalón estuary (Asturias-NW Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10010, https://doi.org/10.5194/egusphere-egu2020-10010, 2020.

Coastal sedimentary environments are dynamic systems in continuous change responding to different temporal and spatial scales. Their sedimentary record offers invaluable data to unveil the effect of different drivers, such as relative sea-level rise, on their evolution.

The Ría de Arousa is located on the Atlantic coast of Galicia (NW Spain) and represents the largest of the so-called “Galician Rias”, with a total area of 230 km2. It corresponds to a mesotidal tide-dominated incised valley characterized by a complex physiography with numerous smaller bays, islands and peninsulas.

The identification of elements of sedimentary architecture was used to study the sedimentary evolution of this incised valley since the Last Glacial Maximum (ca 20 kyr BP to present). This approach was based on the combined analysis of seismic and sedimentary facies, complemented with radiocarbon, geochemical and pollen data.

During the lowstand of the Last Glacial Maximum, a river basin occupied the deep axial valley whose physiography was controlled by the rocky basement morphology and the presence of preserved older sedimentary units. The postglacial transgression changed the base level of rivers, flooding the valley and leading to the formation of an estuary. Facies distribution during this phase (Late Pleistocene) was characterized by large tidal sandbanks and sandflats in the outer area and a bayhead delta at the river mouths. As the transgression proceeded, during the Early Holocene, the system evolved into a tide-dominated estuary. Tidal sandbanks and sandflats occupied large extensions in the axis of the valley, flanked by mudflats. The presence of a small group of islands in the middle area of the incised valley gave way to the existence of an ancient strait during most of the postglacial transgression (Late Pleistocene and Holocene), modulating the relative influence of hydrodynamic conditions and probably leading to tidal currents amplification due to the local morphological narrowing. These structural highs favored the formation of a rock-bounded tidal inlet in the middle of the valley, characterized by scarce deposition and erosional processes.

During the Middle and the Late Holocene, most of the incised valley became drowned, and wave influence increased. A wave ravinement surface is identified, which was developed around 8 cal kyr BP coeval with the initiation of large storm fans associated with rocky barriers.

Finally, a maximum flooding surface is recognized at ca 5 cal kyr BP while the slow rise of sea level forced river mouths to retreat to its present position and marine processes became dominant in the basin.

How to cite: Cartelle, V., García-Gil, S., García-Moreiras, I., Muñoz-Sobrino, C., and Martínez-Carreño, N.: Sedimentary evolution of a bedrock-conditioned incised valley since the Last Glacial Maximum: the Ría de Arousa (NW Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10251, https://doi.org/10.5194/egusphere-egu2020-10251, 2020.

Understanding coastal responses to relative sea level rise is key to be able to plan for future changes and develop a suitable managing strategy. The sedimentary record of the Late-Pleistocene and Holocene transgression provides a natural laboratory to study the long-term changes induced in coastal landscapes by the rapid sea level rise. As sea level rises, coastal morphology continually adapts towards equilibrium changing the landscape and reshaping the distribution of sedimentary environments.
The Ría de Ferrol is a confined tide-dominated incised valley located in the mesotidal passive Atlantic margin of western Galicia (NW Spain).  A multidisciplinary approach was used to identify the elements of sedimentary architecture within its sedimentary record since the Last Glacial Maximum. The sedimentary evolution was reconstructed combining seismic and sedimentary facies analysis with radiocarbon, geochemical and pollen data.
The Ría de Ferrol is characterised by a particular morphology with a rock-incised narrow channel in the middle of the basin (the Ferrol Strait) connecting an inner shallower sector with an outer deeper sector. The inner sector is characterised by low energetic conditions and is where the main fluvial inputs occur. The outer sector is connected to the shelf.
The main factor influencing the sedimentary evolution of the Ría de Ferrol incised valley was Late Pleistocene and Holocene sea-level rise. However, this evolution was modulated by the antecedent morphology, particularly once the middle strait became flooded during the Holocene transgression. Three main phases of evolution are distinguished: a fluvial valley drained by a braided river system, a tide-dominated estuary and a shallow marine basin (ria).
During the lowstand of the Last Glacial Maximum (ca 20 kyr BP), the ria was a fluvial valley whose sediments are mainly preserved in the inner sector. Sediments cores recovered sediments from ponds and stagnant areas, dated to be older than 10790-11170 cal yr BP.
During the Holocene, the basin turned into a tide-dominated estuary whose facies distribution was conditioned by the strait. The strait acted as a rock-bounded tidal inlet enhancing tidal erosion and deposition at both ends, where an ebb-tidal delta and tidal sandbanks appear. At this time, extensive tidal flats occupied most of the inner sector, dissected by estuarine channels of varied dimensions. Radiocarbon data showed ages from 8610-8910 to 5760-5940 cal yr BP.
An erosive episode is identified after 6 cal kyr BP with the formation of a ravinement surface. Wave and tidal energy were split by the middle strait. A wave ravinement surface is identified in the outer sector, while a coetaneous tidal ravinement surface occurs in the inner sector.
Slow sea-level rise after ca 4 ka BP finally forced rivers to retreat to the present position, causing the dispersion of their energy and leading to the final evolution of the area into a fully marine system.

How to cite: García-Gil, S., Cartelle, V., Muñoz-Sobrino, C., Martínez-Carreño, N., and García-Moreiras, I.: Sedimentary conditioning of a rocky strait during the Holocene transgression: Ría de Ferrol (NW Spain), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10493, https://doi.org/10.5194/egusphere-egu2020-10493, 2020.

The evolution of the cross-shore beach profile is tightly related to the evolution of the coastline in both small and large time scales. Bathymetry changes in extreme maritime events can also have important effects on coastal infrastructures such as geotechnical failures of foundations or the modification of the incident wave conditions towards a more unfavourable situation.

The available strategies to study the evolution of beach profiles can be classified in analytical, physical and numerical modelling. Analytical solutions are fast, but too simplistic for many applications. Physical modelling provides trustworthy results and can be applied to a wide variety of configurations, however, they are costly and time-consuming compared to analytical strategies. Finally,  numerical approaches offer different balances between cost and precision depending on the particular model.

Some numerical models provide greater precision in the beach profile evolution, but incurring in a prohibitive computational cost for many applications. In contrast, the less expensive ones assume simplifications which do not allow to correctly reproduce significant phenomena of the near-shore hydrodynamics such as wave breaking or undertow currents, neither to predict important features of the beach profile like breaker bars.

In this work, a new numerical model is developed to reproduce the main features of the beach profile and hydrodynamics while maintaining an affordable computational cost. In addition, it is intended to reduce to the minimum the number of coefficients that the user has to provide to make the model more predictive.

The model consists of two main modules. Firstly, the already existing 2D RANS numerical model IH2VOF is used to compute the hydrodynamics. Secondly, the sediment transport model modifies the bathymetry according to the obtained hydrodynamics. The new bathymetry is then considered in the hydrodynamic model to account for it in the next time step.

The sediment transport module considers bedload and suspended transports separately. The former is obtained with empirical formulae. In the later,the distribution of sediment concentration in the domain is obtained by solving an advective-diffusive transport equation. Then, the sedimentation and erosion rates are obtained along the seabed.
Once these contributions are calculated, a sediment balance is performed in every seabed segment to determine the variation in its level.

With the previously described strategy, the resulting model is able to predict not only the seabed changes due to different wave conditions, but also the influence of this new bathymetry in the hydrodynamics, capturing features such as the generation of a breaker bar, displacement of the breaking point or variation of the run-up over the beach profile. To validate the model, the numerical results are compared to experimental data.

An important novelty of the present model is the computational effort required to perform the simulations, which is significantly smaller than the one associated to existing models able to reproduce the same phenomena.

How to cite: Garcia-Maribona, J., Lara, J. L., Maza, M., and Losada, I. J.: A RANS numerical model for cross-shore beach profile evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10501, https://doi.org/10.5194/egusphere-egu2020-10501, 2020.

Coarse sediment nourishments are increasingly used as a form of coastal protection at sites where the natural shore is affected by erosion processes. Based on the extent of the erosion effects, they can be just an integration to the backshore or rather an artificial reconstruction of a beach that has been completely eroded. In both cases, the comprehension of the physical processes affecting coarse sediments would be crucial to define the transport patterns, which are not completely understood yet. In this sense, short-term tracing experiments have already proved to be a reliable method to gain a significant amount of data about sediment transport in brief timespans. The aim of this work is quantifying the transport rate of coarse tracers 4, 24 and 48 hours after the injection during a time interval characterized by very low to no wave activity. Pebbles of about 7 cm in mean diameter were sampled on the coarse-clastic beach of the Promenade des Anglais in Nice (France), which needs yearly nourishments because of a reported huge sediment loss to the offshore. Since 1969, around 600 000 m3 have been brought in order to maintain the coastline. Once the pebbles fall downslope, no natural process is able to move them back landward due to the steepness of the shoreface. Passive RFID cylinder glass tags have been inserted into the tracers, which have been measured with a caliper and weighed. A 110 m long portion of the public beach has been selected as the site of the experiment because it is confined within two consecutive boulder groynes, which reduce longshore sediment exchange with the adjacent sectors. The pebbles have been injected along 21 transects, two at the berm crest, two in the swash zone and two at the step crest. The tracers have been inserted in the surface of the beach to avoid immediate displacement due to the uprush and backwash flows. The surrounding size of the sediments was on average slightly finer than the tracers. Visual observations right after the injection allowed us to report a strong downslope movement of the swash zone pebbles. The first detection campaign after 4 hours reached just about 60% of recovery rate, which is surprisingly low compared to previous such experiments at different locations. Topographic surveys made contextually revealed the destruction of the fair-weather berm during the rising tide, which led to the burial of a large number of tracers. During the night, low-energy waves managed to wipe out the thin layer of gravel, unearthing back several marked pebbles that had not been detected before: the recovery rate was beyond 90% after 24 hours. This dataset confirms the high transport rate of coarse sediments in very short timespans and under very low energy state: such condition is responsible of moving downslope the tracers with little chance of getting them back up unless the wave motion increases significantly. Such high mobility might also imply a high wear of coarse sediments, which in turn can contribute to volume loss of the beach.

How to cite: Bertoni, D., Sarti, G., Bruno, G., Pozzebon, A., Doumasdelage, R., and Larraun, J.: Coarse sediment tracing experiment at the Promenade des Anglais (Nice, France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11697, https://doi.org/10.5194/egusphere-egu2020-11697, 2020.

Extensive tidal flats and sandy beaches are a typical coastal landform along the macrotidal west coast of Korea. Macrotidal beaches typically with long stretches and the gentle slope, contain often a series of sand bar, parallel to the coastline. Shinduri beach under macrotidal condition in a semi-closed embayment, western coast of Korea, has three to four lines of sand bars on about 500 m stretches of intertidal zone. Interesting feature is the dynamic behaviour of the multiple bars. These bars appear only in summer and disappear dramatically all in winter, which is opposite pattern in common beaches. In order to understand the seasonal dynamics of multiple bars on a macrotidal beach, three years of topographic survey using a VRS-GPS system have been conducted at three-months interval on six transects placed on the beach. In addition, an ADV/ADCP was deployed to collect wave data on a bar. Shinduri beach is a 4 km in length and about 500 m in width. Topographic survey reveals that three to four bars occur only during summer, and disappear suddenly during winter. In response to bar growth and destruction, beach slopes become steeper in winter and gentler in summer. Mean grain sizes show generally shoreward coarsening trend, ranging from 2.0 phi to 2.75 phi. Sediments get coarser in summer, but finer in winter, which are opposite compared to other beaches in the west coast of Korea. Wave data show strong seasonality, high waves in winter and much gentler waves in summer, suggesting the study area experienced by monsoon climate. The opposite pattern of multiple bar dynamics, growth in summer and destruction in winter, is likely associated with strong winter waves, destroying the bars and hence filling the trough of bars, thereby the beaches becoming flat in topography. From spring the bars start to form under normal wave condition. This signifies that local wave condition is more important for maintaining patterns of multiple bars.

How to cite: Chang, T. S., Youn, H. H., and Chun, S. S.: Multiple sand bar dynamics in the macrotidal Shinduri beach, west coast of Korea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13206, https://doi.org/10.5194/egusphere-egu2020-13206, 2020.

Understanding shoreline evolution, and in particular, the consequences of shoreline erosion is a
major societal concern that threatens to become even more important in the future with the impacts
of climate change. Thus, it is necessary to improve both knowledge of the dominant physical processes
controlling medium to long-term shoreline evolution and the capabilities of morphological evolution
models to simulate beach changes at these spatial and temporal scales.
Empirical models may be an ideal choice for modelling complex and dynamic environments such as
sandy beaches at large spatial (beach) and long temporal (years to decades) scales. They reproduce
the effects of the main morphodynamical processes with low computational cost and relatively high
accuracy, in particular when high quality, long-term data are available for calibration.
Here, to broaden its range of application, a cross-shore equilibrium model, which has demon-
strated its accuracy and efficiency in reproducing shoreline and intertidal beach profile changes at
several micro and macrotidal beaches, is extended to couple it with a longshore beach evolution
modelling approach. The selection of a particular longshore model (based on a one-line approach),
and its implementation and validation with benchmark test cases of shoreline evolution caused by
the effects of diffusion, high angle wave instabilities, and coastal structures are presented.
The new hybrid model is applied at Narrabeen beach to reproduce the long-term evolution of
beach contours near the shoreline. The model is calibrated and tested using the 40-year timeseries of
monthly subaerial beach profile surveys conducted along 5 cross-shore profiles along the 3.6km-long
Narrabeen-Collaroy embayment. The novelty of the current work is to focus on reproducing changes
at different altitudes, with the objective of assessing the cross-shore variability of the longshore
sediment flux, which is assumed constant in most one-line longshore transport models. The coupled
model performance is discussed, and the results are compared to existing studies that have simulated
shoreline evolution at Narrabeen using other morphological change models.

How to cite: Chataigner, T., Yates, M., and Le Dantec, N.: Empirical modelling of beach evolution: implementation of coupled cross-shore and longshore approaches, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17932, https://doi.org/10.5194/egusphere-egu2020-17932, 2020.

Coastal dunes are dynamic landforms whose morphology is governed primarily by climate and vegetation dynamics. Over the last 50 years, coastal sand dunes across the globe have dramatically ‘greened’ and wind speeds fallen (Pye et al., 2014; Delgado-Fernandez et al., 2019; Jackson et al., 2019), reducing aeolian transport of sediment and minimising dune reshaping by near-surface winds.  This rapid vegetation has also been attributed to a dramatic decline of several rare species of plants and invertebrates in several coastal dune systems (Howe et al., 2010; Pye et al., 2014). In an effort to increase habitat diversity, large-scale vegetation removal and dune re-profiling are becoming increasingly common interventions. However sustained aeolian activity following intervention appears to be rare (Arens et al., 2013).

In order to better understand the environmental drivers of long-term dune mobility, this work explores the landscape scale physical factors related to self-sustaining ‘natural’ mobile dunes across the United Kingdom. The analysis presented includes the use of geographically weighted regression, a spatial analysis technique that models the local relationships between predictors (e.g. wind speed, slope, elevation, aspect, surface roughness) and an outcome of interest (mobile dunes). It is hoped that the results of this work will help guide decision-making with regards the location, scale and morphology of future interventions in order to maximise their sustainability, minimising the need for maintenance and further intervention.

References

Arens, S.M., Slings, Q.L., Geelen, L.H. and Van der Hagen, H.G., 2013. Restoration of dune mobility in the Netherlands. In Restoration of coastal dunes (pp. 107-124). Springer, Berlin, Heidelberg.

Delgado-Fernandez, I., O'Keeffe, N., & Davidson-Arnott, R. G. (2019). Natural and human controls on dune vegetation cover and disturbance. Science of The Total Environment, 672, 643-656.

Howe, M. A., Knight, G. T., & Clee, C. (2010). The importance of coastal sand dunes for terrestrial invertebrates in Wales and the UK, with particular reference to aculeate Hymenoptera (bees, wasps & ants). Journal of Coastal Conservation, 14(2), 91-102.

Jackson, D. W., Costas, S., González-Villanueva, R., & Cooper, A. (2019). A global ‘greening’of coastal dunes: An integrated consequence of climate change?. Global and Planetary Change, 182, 103026.

Pye, K., Blott, S. J., & Howe, M. A. (2014). Coastal dune stabilization in Wales and requirements for rejuvenation. Journal of coastal conservation, 18(1), 27-54.

How to cite: Smyth, T., Wilson, R., Rooney, P., and Yates, K.: Landscape drivers of coastal dune mobility , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15230, https://doi.org/10.5194/egusphere-egu2020-15230, 2020.

Coastal dune morphology is largely controlled by the availability of sand to be transferred from the beach and the capacity of the vegetation to trap and retain the moving sand grains. The resultant coastal dune morphology is, in turn, key to achieve maximum efficiency of nature-based solutions that plan the construction of such aeolian features. Therefore, developing approaches that integrate key processes becomes crucial, especially in order to efficiently design and test solutions that meet the timescale requirements of coastal management. The process-based XBeach-Duna model has been developed to integrate nearshore, aeolian and ecological processes across the beach-dune profile, thus allowing long-term simulation of complex coastal features and feedbacks. Here, we explore the potential of this coupled modelling solution to simulate the morphological response of coastal dunes to changes in sediment supply and vegetation cover over decadal timescales. Simulations show the capacity of the approach to reproduce the natural response to changes in sediment supply, shifting the shoreline position and simultaneously modifying the overall shape of the dune, within a range of dimensions that are in agreement with observations. In general, narrow and low dunes are formed under high supply conditions, wide and high dunes develop if sediment supply is low and the shoreline position stable, while narrower and higher dunes are created after a relative drop in sediment supply that induces a negative budget. Denser vegetation coverage, on the other hand, favours taller dune morphologies, however the influence of sediment supply and receding shoreline positions to plant growth are non-linear and, in turn, produce feedbacks that cascade to the morphology of the dune itself. These results demonstrate the capacity of the approach to reproduce different dune states, resulting from alternative evolutionary pathways, and its potential to identify coastal dune (in)stability domains and critical morphological shifts, factors that are key to better understand the efficiency of dunes as nature-based solutions for coastal management.

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

How to cite: Costas, S., Kombiadou, K., and Roelvink, D.: Exploring the role of vegetation and sediment supply to coastal dune states using integrated process-based modelling , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18568, https://doi.org/10.5194/egusphere-egu2020-18568, 2020.