Coasts worldwide face a great variety of environmental impacts as well as increased anthropogenic pressures of coastal zone urbanization and rapid population growth. Over the last decade coastal erosion has emerged as a widespread problem that causes shoreline retreat and irreversible land losses. The attempts of managers and other stakeholders to cope with erosion using different types of hard engineering methods may often aggravate this problem, damaging natural landscape and coastal ecosystems in unexpected and unpredicted ways. Other negative impacts of human activities on littoral environments are chronic and punctual pollution of beach and coastal sediments with associated health risks for human beings. Chronic pollution is often observed in coastal areas close to factories, industries and human settlements - because of waste water discharges, punctual contamination is often linked to beach oiling.
The session gives priority to the subjects of coastal geomorphology: evolution of coastal landforms, coastal morphodynamics, coastline alterations and various associated processes in the coastal zone, e.g. waves and sediment drift, which shape coastal features and cause morphological changes. Contributions to this session will focus on the mechanisms responsible for coastal erosion and shoreline behaviour (advance or retreat) and will address the many natural and human factors involved. The topics may include work on predictions of shoreline change and discussions on the effects of human activities and their continuing contribution to coastal changes. The session will also cover submissions on coastal vulnerability to the combined effects of natural and human-related hazards, any type of coastal and environmental sensitivity classifications, and risk assessments. Globally, coastal dunes are seriously threatened as people tend to modify landforms and habitats through their actions and regulations, and the session invites also studies on natural and human-induced geomorphological changes of sand dunes, and recent projects and examples of dune eco-restoration and re-building.
Last, but not the least, studies related to Marine Spatial Planning (MSP), including Integrated Coastal Management (ICM), are also welcome. For any MSP and ICM, it is essential to consider the dynamics across the land-sea interface, i.e. the Land-Sea Interactions (LSI) that involve both natural processes and the impact of human activities.
We will organize the session in four groups. We stop very shortly on abstracts without displays and spend at least 5 minutes for those with uploaded displays.
GROUP 1: 10:45 - 11:10
1. D1012 | EGU2020-95: “The Future of the World's Sandy Beaches Under a Changing Climate.” Authors: Michalis Vousdoukas, Roshanka Ranasinghe, Lorenzo Mentaschi, Theocharis Plomaritis, and Luc Feyen.
2. D1013 | EGU2020-624: “A Feasibility Investigation for Developing Artificial Beachrocks: A Potential Measure for Coastal Protection in Southeast Yogyakarta Coast, Indonesia.” Authors: Lutfian Rusdi Daryono, Kazunori Nakashima, Satoru Kawasaki, Koichi Suzuki, Anastasia Dewi Titisari, Didit Hadi Barianto, Imam Suyanto, and Arief Rahmadi.
3. D1014D1014 | EGU2020-3174: “Disintegrated coastal zone management (DICZM): an example from Auckland, New Zealand.” Authors: Martin Brook, Alex Palma, Rosemary Garill, Nick Richards, and Jon Tunnicliffe.
4. D1015 | EGU2020-3320: “Enhancing shoreline advance by ploughing the intertidal beach: Physical simulation.” Authors: Erica Pellón, Iñigo Aniel-Quiroga, Mauricio González, and Raúl Medina.
5. D1016 | EGU2020-2124: “Innovative Approach for Addressing Coastal Erosion Protection Using Microbial Induced Carbonate Precipitation.” Authors: Md Al Imran, Kazunori Nakashima, Niki Evelpidou, and Satoru Kawasaki.
GROUP 2: 11:10 - 11:35
6. D1017 | EGU2020-1609: “Controls on coastal overwash morphology in natural and built environments.” Authors: Hannah Williams, Luke Taylor, Evan Goldstein, and Eli Lazarus.
7. D1018 | EGU2020-5716: “Coastal geomorphic response to volcano-tectonic activity in the Campi Flegrei Caldera: new insight from the geoarchaeological study of Portus Julius (Pozzuoli Gulf, Italy).” Authors: Claudia Caporizzo, Pietro Patrizio Ciro Aucelli, Gaia Mattei, Aldo Cinque, Salvatore Troisi, Michele Stefanile, Francesco Peluso, and Gerardo Pappone.
8. D1019 | EGU2020-4628: “Mapping of Coastal Cliff Erosion in Denmark.” Authors: Gregor Luetzenburg, Kristian Svennevig, Anders A. Bjørk, and Aart Kroon.
9. D1020 | EGU2020-11484: “Driving mechanisms of coastal cliff retreat in flysch deposits on the eastern Adriatic coast.” Authors: Goran Vlastelica, Kristina Pikelj, and Branko Kordić.
10. D1021 | EGU2020-20486: “Morphodynamic types of postglacial cliffs of the Southern Baltic.” Authors: Andrzej Kostrzewski, Marcin Winowski, and Zbigniew Zwoliński.
GROUP 3: 11:35 - 12:00
11. D1022 | EGU2020-20386: “Spatial diversity and time variability of erosion and accumulation processes on the unconsolidated cliffs of the Wolin Island (Southern Baltic - Pomeranian Bay).” Authors: Marcin Winowski, Zbigniew Zwoliński, Andrzej Kostrzewski, and Jacek Tylkowski.
12. D1023 | EGU2020-5755: “Geomorphological properties of the island of Hvar beaches (Croatia, Eastern Adriatic Coast).” Authors: Marin Mićunović and Sanja Faivre.
13. D1024 | EGU2020-7140: “Coastal Stability and Micro Morphology; Disturbances due to Human Interventions along West Coast of India.” Authors: Rafeeque Mk, Akhil Thulasidharan, Mintu E George, Suresh Babu Ds, and Prasad Tk.
14. D1025 | EGU2020-1623: “Surface sediments of Richards Bay Harbour, South Africa – potential pollutants (heavy metals, persistent organic pollutants, microplastics) and grainsize distribution.” Authors: Paul Mehlhorn, Marc Humphries, Peter Frenzel, Olga Gildeeva, Annette Hahn, Finn Viehberg, and Torsten Haberzettl.
15. D1026 | EGU2020-7592: “Bank Erosion Processes, Trends and Impacts in a Hypertidal Estuarine System.” Authors: Andrea Gasparotto, Julian Leyland, Stephen Darby, and Paul Carling.
GROUP 4: 12:00 - 12:20
16. D1027 | EGU2020-10272: “The coastal vulnerability of the north-eastern sector of Gozo Island (Malta, Mediterranean Sea).” Authors: Mauro Soldati, George Buhagiar, Anton S. Micallef, Angela Rizzo, and Vittoria Vandelli.
17. D1028 | EGU2020-19785: “A DPSIR analysis of aeolian sand dune mobilization along the coast of Manawatu-Wanganui in New Zealand.” Authors: Dissanayake Mudiyanselage Ruwan Sampath and Joana Gaspar de Freitas.
18. D1029 | EGU2020-11173: “Rapid shifts in the Baltic Sea region climate, detected from the ancient coastal formations and number of other ecosystems – how likely it is to happen again and what are the consequences?” Authors: Sandra Kuusik and Hannes Tõnisson.
19. D1030 | EGU2020-10487: “Aperiodic embayed sandy beach rotation and erosion-risk exposure on a hyper-muddy wave-exposed coast.” Authors: Edward Anthony, Antoine Gardel, Morgane Jolivet, Guillaume Brunier, and Franck Dolique.
WRAP-UP 12:20 - 12:30
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Chat time: Friday, 8 May 2020, 10:45–12:30
The world's coastline consists of more than 30% of sandy beaches, many of which are already eroding. Climate change is expected to put more pressure on sandy shorelines, not only because of rising seas, but also from changing weather patterns, affecting the characteristics of marine storms. Here we discuss projections of coastline dynamics along the world's sandy beaches in view of climate change. Using Bruun's rule combined with new global wave projections1 and a dataset on beach slopes2, we find that sea level rise will result in median retreat around -28 m and -35 m under RCP4.5 and RCP8.5, respectively, by the year 2050. the shoreline retreat is projected to climb to around -63 m and -105 m, respectively, by the end of the century. The impact of episodic erosion during storm events will most likely become more severe as sandy beaches will shrink, however, changes in the intensity and characteristics of storms seem to leave an noticeable footprint only in few locations worldwide. Ambient change, extrapolated from historical behaviour3, is expected to contribute signicantly to future sandy beach erosion. However, ambient change can also drive accretion, as is the case along a big part of East Asia. The present findings imply that many sandy beaches worldwide will experience retreat of more than 100 m, i.e. they are very likely to vanish, especially in the absence of accommodating space. The socio-economic implications to tourism, quality of life and the economy can be devastating, especially in small, tourism dependent communities.
- Vousdoukas, M. I. et al. Global probabilistic projections of extreme sea levels show intensication of coastal flood hazard. Nature Communications 9, 2360, doi:10.1038/s41467- 018-04692-w (2018).
- Athanasiou, P. et al. A global dataset of coastal slopes for coastal recession assessments. Earth System Science Data Discussions, 29, doi:https://doi.org/10.5194/essd-2019-71 (2019).
- Mentaschi, L., Vousdoukas, M. I., Pekel, J.-F., Voukouvalas, E. & Feyen, L. Global long-term observations of coastal erosion and accretion. Scientic Reports 8, 12876, doi:10.1038/s41598-018-30904-w (2018).
How to cite: Vousdoukas, M., Ranasinghe, R., Mentaschi, L., Plomaritis, T., and Feyen, L.: The Future of the World's Sandy Beaches Under a Changing Climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-95, https://doi.org/10.5194/egusphere-egu2020-95, 2020.
Erosion prone sandy beaches are frequently covered by cement and mortar to preserve the coastal zone, but the conventional approach has an adverse impact on the environment, altering the coastal landscape and processes unfavorably. The term “beachrock” refers to cemented coastal sediments through a long-term formation of CaCO3 cement, and which is an important feature in many tropical coastlines as it appears to have a substantial anchoring effect against wave effects and erodibility. Therefore, the objective of this study is to evaluate the feasibility in progressing the formation of artificial beachrocks using natural materials (e.g., microbes, sand, shell, pieces of coral, and seaweed etc.) within a short-term, and to introduce the method as a novel candidate for coastal protection. In this study, both resistivity survey and multi analysis seismic wave (MASW) survey along the same lines were performed at first to elucidate the subsurface structure of existing beachrocks in the Southeast Yogyakarta coastal area (Indonesia), followed by the laboratory analysis, which is aimed understand the basics in the formation mechanism. Peloidal micrite cement, the cement comprised of aragonite needles, micritized granules and the cover of micritic were observed in natural beachrocks. Mimicking the mechanism, an attempt has been undertaken to develop artificial beachrocks in the laboratory via microbial induced carbonate precipitation (MICP). Finally, the physical and mechanical properties were well compared between the artificially formed beachrocks and natural beachrocks collected from the survey lines. The results suggest that the artificial deposits treated for 14 days under optimum conditions, achieved a peak unconfined compressive strength of around 6 MPa similar to that of weak-consolidated natural beachrock. The comparison further reveals that the variables such as porosity, Vp, Vs, and strength are primarily rely on the precipitated morphology of the crystals.
How to cite: Daryono, L. R., Nakashima, K., Kawasaki, S., Suzuki, K., Titisari, A. D., Barianto, D. H., Suyanto, I., and Rahmadi, A.: A Feasibility Investigation for Developing Artificial Beachrocks: A Potential Measure for Coastal Protection in Southeast Yogyakarta Coast, Indonesia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-624, https://doi.org/10.5194/egusphere-egu2020-624, 2020.
Typically, integrated coastal zone management (ICZM) uses the informed participation and cooperation of all stakeholders to assess the societal goals in a given coastal area. ICZM seeks, over the long term, to balance environmental, economic, social, cultural and recreational objectives, all within the limits set by natural dynamics. We outline coastal instability in the Auckland region of New Zealand, where the effects of natural coastal dynamics appear to have been underplayed, or even overlooked, during the residential land development process. Auckland is New Zealand’s largest city, with the Auckland region encompassing c. 3,300 km of coastline, with a highly variable wave climate and coastal geomorphology. The sparsely inhabited high energy west coast records significant wave heights of 2-3 m for much of the year. In contrast, the eastern bay coastlines are lee coasts, protected by offshore islands in the Hauraki Gulf and the Coromandel Peninsula. Nevertheless, significant coastal cliff instability does occur along these eastern coasts, which are heavily populated, with houses often constructed within 10 m of the cliff edge. Coastal instability in the Beachlands area in particular, is part-conditioned by engineering properties of the cliff materials, which include soft, Pleistocene sediments. In particular, shear surfaces develop along clay-rich tephra layers, which are of low-permeability, leading to increased porewater pressure, and cliff failure. Despite the clear failure mechanisms, coastal protection works and routing of domestic stormwater over the cliffs has led to further coastal instability.
How to cite: Brook, M., Palma, A., Garill, R., Richards, N., and Tunnicliffe, J.: Disintegrated coastal zone management (DICZM): an example from Auckland, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3174, https://doi.org/10.5194/egusphere-egu2020-3174, 2020.
Understanding shoreline behaviour and developing tools to deal with erosion has increasing interest nowadays. Coastal erosion and accretion produce changes on the beach width. These changes condition the uses given to dry beach and coastal areas. As the beach becomes narrower, the hazard of coastal areas increases. Additionally, due to the tourism, the demand and interest for wider beaches in early spring have risen.
Natural and human factors determine shoreline evolution. Storms erode beaches during winter, and calm weather conditions produce accretion. Assisted recovery techniques aim to propose new soft engineering methods that enhance accretion during calm periods. These human interventions need to be thoroughly analysed to ensure their effectiveness. In this study, we propose the ploughing of the intertidal beach area to accelerate the natural recovery process of the beach.
The effect of ploughing the intertidal area of a beach has been analysed through real scale physical simulations in the wave-current-tsunami flume (COCoTsu) of IHCantabria. The effect of the ploughing was monitored by measuring the sand transported shoreward with cell pressures beneath sediment trap boxes. The channel was longitudinally split into two equal channels (1 m wide each), one of them with plane sloping sand and the other including five crests and holes emulating a real plough made by a tractor. The comparison of both sides derives the effect of the ploughing.
Simulated geometry includes wave generator, 11 m of flat bottom, 17 m of concrete variable sloping fixed bed, 10 m of sand with D50 = 0.318 mm movable bed, 2 m of trap box for continuous capturing and weighting shoreward transported sand and 10 m of wave dissipators. Concrete and sand slopes were designed to mimic the real geometry of a sandy beach intertidal accreting bar.
Sixteen experiments were conducted with fixed wave dynamics and bottom geometry and varying water level. Wave conditions were irregular waves with Hs = 0.3 m and Tp = 7 s, which produce dimensionless fall velocity Ω ≤ 1.5 ensuring accretion over the sandy bottom. Water level ranged from the level of the top of the sand to 50 cm above it. Additionally, one test was conducted with rising water level from -20 cm to 50 cm (from the top level of the sandy area), emulating a rising tidal cycle.
Hydrodynamics and morphodynamics were measured continuously during each experiment by means of 16 free surface elevation sensors, 4 ADV, 2 OBS, 8 pressure cells and 6 video cameras. Bottom load sediment transport was calculated as the difference of the measured total load (pressure cells beneath the aforementioned sand trap boxes) and suspended load sediment concentration measured by the OBS. Additionally, the laser scanner accurately determined the initial and final 3D geometry of the movable bed area.
All this data allows the analysis of the suitability of ploughing technique for accelerating natural accretion processes. Preliminary results show that ploughing affects the roughness of sandy bottom, increasing the wave dissipation and with a variable effect on sediment transport depending on the water level.
How to cite: Pellón, E., Aniel-Quiroga, I., González, M., and Medina, R.: Enhancing shoreline advance by ploughing the intertidal beach: Physical simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3320, https://doi.org/10.5194/egusphere-egu2020-3320, 2020.
Considering the global climate change and the ensuing sea level rise, the subsequent acceleration of coastal erosion is evident. Phenomena of coastal erosion, coastal flooding and shoreline retreat are expected to show a significant increase in frequency and intensity, in global level. The effects of coastal erosion are worsened by storms, and the reduction of sediment supply associated with global warming and anthropogenic modification of rivers and coastlines. As a countermeasure to coastal erosion, this work focuses on the development of coastal artificial in-situ rocks. We developed a new method that encompasses microbes and the related mechanism is called “Microbial Induced Carbonate Precipitation” (MICP). We successfully isolated three microorganisms, Micrococcus sp., Pseudoalteromonas sp., and Virgibacillus sp., from the selected area, and investigated their effectiveness in order to make a solidified sand sample. The precipitated bounding material has also been confirmed as calcite by XRD and XRF analysis. We successfully demonstrated that all of these bacterial species are very sensitive with certain environmental parameters, such as temperature, pH, culture type, culture duration, etc. In laboratory scale, we successfully obtained solidified sand by syringe (d = 2.3 cm, h = 7.1 cm) solidification method bearing UCS (Unconfined Compressive Strength) up to 1.8 MPa using 0.5 M CaCl2 and urea as cementation solution at 30°C. In addition, we propose a new sustainable approach for field implementation of this method through a combination of geotube and MICP mechanism, which will contribute to coastal erosion protection. The proposed approach is more economic, energy-saving, eco-friendly, and sustainable for bio-mediated soil improvement.
How to cite: Imran, M. A., Nakashima, K., Evelpidou, N., and Kawasaki, S.: Innovative Approach for Addressing Coastal Erosion Protection Using Microbial Induced Carbonate Precipitation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2124, https://doi.org/10.5194/egusphere-egu2020-2124, 2020.
Overwash is a key mechanism controlling the flux of sediment from the front of a barrier island to the top and back of an island during a storm event. The process is essential for barrier environments to maintain their height and width relative to sea level. Barrier topography and vegetation – and also road networks and buildings – can direct overwash flow, and thus the shape and size of sedimentary deposits that overwash leaves behind. Controls on overwash deposition have been examined more closely in natural settings than in developed zones. But overwash poses a major hazard to coastal infrastructure, and accurate prediction of storm impacts requires quantitative insight into the dynamics of overwash morphology in built settings. Here, we compare barrier floodplain controls across a range of spatial "fabrics", both natural and built (e.g., sparse to dense vegetation coverage; sparse to dense configurations of roads and buildings), to explore how these fabrics affect scaling relationships for overwash morphology. Integrating empirical measurements from post-storm imagery, trials of an analogue model in a small experimental basin, and results from a numerical toy model, we identify thresholds at which floodplain fabrics cause scaling relationships to change, or "break". Our findings illustrate a continuum in overwash pattern formation between endogenous self-organisation and exogenous forcing templates, and set up further inquiry into the dynamics of flood deposition in built environments.
How to cite: Williams, H., Taylor, L., Goldstein, E., and Lazarus, E.: Controls on coastal overwash morphology in natural and built environments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1609, https://doi.org/10.5194/egusphere-egu2020-1609, 2020.
The nowadays submerged Roman harbour of Portus Julius, located inside the Campi Flegrei caldera (Pozzuoli Gulf, Naples, Italy) and within the Underwater Archaeological Park of Baia, is one of most important coastal archaeological site in Italy and, in the past, it has been the subject of several geoarchaeological studies, since anthropic structures, reflecting the former coastal morphology, are still clearly visible.
High precision data from direct underwater surveys carried out on many reliable archaeological sea level markers are allowing us to evaluate ancient relative sea level (RSL) positions and the amount of the vertical ground movements (VGM) since Roman Time. In this study, we present data regarding the coastal area of the famous archaeological site of Portus Julius.
In the 37 BC, the study area was chosen by Agrippa for the construction of the new military harbour of Portus Julius, equipped with an entry channel sheltered by several pilae structures. In the 12 BC, the site has been transformed into a commercial hub and renewed through the construction of a fish tank and systems of warehouses.
By using a metric roll and a depth gauge, three direct surveys were carried out on as many sea level markers, each of them related to a precise constructive phase of the port. The submersions of some living floors belonged to a maritime villa of the Republican Age (before 37 BC) have been measured at -3.2 m asl. Instead, nearby the main entry channel, we measured the submersion of the concrete change (i.e. limit between the areas in hydraulic concrete set underwater and the areas in concrete totally laid in subaerial environment) of five roman pilae (37 BC) located at the entrance at -2.6 m asl and the submersion of the top of the sluice gate, belonging to the fish tank built directly on the channel bank (12 BC), at a depth of -2.7 m asl.
From these submersion measurements, corrected with respect to the indicative meaning, the tidal height and the barometric pressure, we have determined a RSL of -4.7/-5.0 m related to the period before the 37 BC from the living floors, a RSL of -3.1 related to the 37 BC from the pilae and a RSL of 3.1 m related to the 12 BC from the fish tank.
Comparing the oldest RSL value with the one obtained by the pilae, it is evident that a subsidence has already occurred before the 37 BC leading to an increase of the water depth and favouring the construction of the port facility. On the other hand, the RSL stationing at -3.1 m asl between the 37 BC and the years after the 12 BC proofs that the area lived a period of VGM stability. As geomorphic response, a local sea level rise of about 1.5 m occurred all over the I century BC, not balanced by neither other coastal processes and anthropic forces, resulting in a coastal retreat up to 0.6 km.
How to cite: Caporizzo, C., Aucelli, P. P. C., Mattei, G., Cinque, A., Troisi, S., Stefanile, M., Peluso, F., and Pappone, G.: Coastal geomorphic response to volcano-tectonic activity in the Campi Flegrei Caldera: new insight from the geoarchaeological study of Portus Julius (Pozzuoli Gulf, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5716, https://doi.org/10.5194/egusphere-egu2020-5716, 2020.
Coastal cliff erosion is often an underestimated process to understand shoreline evolution in Denmark. Cliff failure occurs episodically and not always coincides with the highest waves, making prediction difficult. Therefore, comprehensive knowledge about the spatial distribution of cliff erosion in Denmark and the historical rates of change are required to predict future shoreline change and sediment distribution. The erosion of coastal cliffs delivers a substantial amount of sediment in to the coastal zone and future changes in storm intensities and frequencies might influence the rates of cliff erosion.
Historical aerial images, dating back to the mid of the 20th century are combined with a current high resolution digital elevation model (DEM), to map coastal cliff erosion across Denmark’s coastline and to calculate rates of change over the last decades at selected sites. Countrywide oblique aerial images further assist in mapping coastal cliff erosion processes. Landslides are characterized by polygons in the DEM. Morphometric indices are calculated out of the length, width and height of each site to distinguish between different processes. Furthermore, the rate of change is derived from the spatial displacement of the crown.
In this presentation, we present a map of around 1500 (and counting) coastal cliff erosion sites mapped all across Denmark. Steep cliffs mainly occur at the Danish inner coast and along the fjords, determining the presence of coastal cliff erosion. The multi-temporal analysis of shoreline changes revealed erosion rates up to 30 m in the last 20 years leading to considerable loss of land and sediment redistribution. The spatial distribution of the mapped coastal cliff erosion processes (e.g. topples, slides etc.) shows a connection to the maximum extend of the ice sheet covering most parts of Denmark during the Last Glacial Period (Weichsel). This indicates an ongoing emergence for the postglacial landscape and highlights the importance of coastal cliff erosion in landscape evolution.
How to cite: Luetzenburg, G., Svennevig, K., Bjørk, A. A., and Kroon, A.: Mapping of Coastal Cliff Erosion in Denmark, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4628, https://doi.org/10.5194/egusphere-egu2020-4628, 2020.
Cliffs formed in soft rocks are rare coastal forms along the Croatian eastern Adriatic and most exposed to natural erosional processes. The rare example of such cliff affected by anthropogenic activities was developed in marl-dominated flysch in the Split urban zone. Due to the panoramic view cliff-top area is being massively occupied since the 1980ies mostly for the tourism industry and urban development. In spite of increased pressure, little attention has been given to the cliff stability. Ongoing cliff erosion seriously endangers both, coastal infrastructure on the cliff-top, as well as at the narrow shore platform used as a recreational beach area, demanding the urgent development of erosion management plan. In order to do so, fundamental knowledge is needed to understand the cliff erosion driving mechanisms.
The non-vegetated cliff face was scanned 11 times by terrestrial laser scanner during the 6-year period (2012-2018). Four representative profiles along the study area were compared on precisely georeferenced point clouds. Additionally, a close examination of the cliff-top, cliff face and shore platform was carried out over 15 times during various seasons and weather conditions in order to recognize erosional processes involved. Cliff retreat rates obtained from our monitoring ranged between 3 and 18 cm/y. Extreme erosion rates of 25-34 cm/y occurred during the 2014/2015 and 2017/2018 monitoring period. Both extremes occurred after autumn and spring high precipitation periods. A causal link between intensive rain periods and erosion was further observed after two landslides during spring 2018. Furthermore, many gullies caused by surface runoff were carved after heavy rains. At the same time, increased amount of groundwater caused seepage along structural discontinuities, inducing surface erosion below the seeping line. All observed erosional processes occasionally lead to the occasional formation of marly talus cones at the cliff toe. Their duration depends on wave climate, and are being gradually removed by waves.
Obtained results showed that monitored coastal cliff is predominantly subjected to various processes of surface erosion related with high precipitation, while wave abrasion is of subordinate role. Predominant marl lithology is likely to cause further surface mechanical erosion, highlighting the need for erosion management to be developed.
How to cite: Vlastelica, G., Pikelj, K., and Kordić, B.: Driving mechanisms of coastal cliff retreat in flysch deposits on the eastern Adriatic coast, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11484, https://doi.org/10.5194/egusphere-egu2020-11484, 2020.
The contemporary morphogenetic system of the South Baltic Sea is clearly changing, both in the annual and long-term weather cycle. Morphogenetic seasons are subject to change, both in terms of duration and types of morphogenetic processes and related forms of relief. The duration of the late-autumn and early-spring season is clearly increasing, which is associated with the occurring climate change and related hydrometeorological conditions. All this means that the morphodynamic types of the South Baltic coast are subject to change, the nature of which is conditioned by geological structure, relief, land cover and, hydrometeorological conditions. Undoubted individuality of the geo-diversity of the South Baltic coast in Poland are postglacial cliff coasts (50 km long).
Systematic geomorphological mapping of cliff coasts carried out since 1975 which have recently been supported by GIS methods, allow the recognition of cliff coast development mechanisms, emerging landforms and associated morphodynamic types of the South Baltic coast.
Based on repetitive geomorphological mappings, the following morphodynamic types of the South Baltic cliffs can be distinguished: landslide-type, rock fall-type, talus-type, slump-type and flow-type.
The basis for the typology of morphodynamic types of cliff coasts was the dominant types of relief forms, including lithology, exposure, land cover and hydrometeorological conditions. It can be unequivocally assumed that the morphodynamic types of the cliff coast is a good indicator feature of monitored morphogenetic systems and their space-time variability.
The effect of the observed climate change is the increasing frequency of storm surges that initiate denudation processes of an extreme nature. Another consequence of the observed climate changes is the increasing variability of morphodynamic types of the South Baltic cliff coast in the analyzed morphogenetic seasons with a greater share of landslide and rock fall-types.
How to cite: Kostrzewski, A., Winowski, M., and Zwoliński, Z.: Morphodynamic types of postglacial cliffs of the Southern Baltic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20486, https://doi.org/10.5194/egusphere-egu2020-20486, 2020.
During the period of climate change affecting the increase in the frequency of extreme events, the problems of the functioning of the sea coasts become very important. The process of raising the World Ocean level directly threatens coastal areas which are inhabited by more than half of population.This situation is also observed on the Polish coast. The very intense and often uncontrolled tourist and economic development of this region requires the introduction of protection systems aimed at limiting the adverse changes caused by extreme processes. Anthropogenic coastal transformations commonly contribute to the modification of natural morphogenetic processes. As a result, the development of the coastal zone goes in an unknown direction.
The current problems related to the functioning of cliff coasts prompted the authors to conduct research on the impact of storm surges on the transformation of the morphology of the unconsolidated cliffs of Wolin Island. The research consisted of annual measurements of cliff morphology using terrestrial laser scanning (TLS) in the period 2013-2019. The obtained measurement series allowed to demonstrate spatial diversity and time variability of erosion and accumulation processes in various hydrometeorological conditions. Differential analyzes allowed to quantify of the sediment budget on different sections of the cliffs. Based on the proposed denudation indicators, spatial and temporal differentiation of cliff dynamics and efficiency was presented.
How to cite: Winowski, M., Zwoliński, Z., Kostrzewski, A., and Tylkowski, J.: Spatial diversity and time variability of erosion and accumulation processes on the unconsolidated cliffs of the Wolin Island (Southern Baltic - Pomeranian Bay), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20386, https://doi.org/10.5194/egusphere-egu2020-20386, 2020.
Beaches are sedimentary forms, situated at the land-sea contact formed of unconsolidated material which can range in size from sand up to cobbles and boulders. They are one of the most dynamic coastal forms and particularly sensitive to changes (natural and/or anthropogenic).
Geomorphological properties of the island of Hvar beaches were analyzed by means of field mapping, ortho-photo and GIS analysis. All the beaches have been mapped and measured at four different points in time by means of Ortho-photos from State Geodetic Administration (2011, 2014 and 2017) and HERE maps (2019). Larger beaches have been also measured in the field with a GPS receiver (from July 2018 until July 2019). GIS and statistical calculations and visualization were done in ArcGIS 10.4 software.
Hvar is the longest Croatian island with a length of 67,8 km, and the fourth in size with a surface of 297,4 km2. Along its 254 km long coastline 247 beaches have been mapped which make up 3,8 % of total coastal length. The beaches are rather small relating generally to pocket beaches. Only 14,7 % of beaches are larger than 500 m2 and 59,95 % are smaller than 200 m2. According to the sediment size gravely beaches predominate with 95,5 %, while only 4,5 % relates to sand beaches.
This study revealed that on the island of Hvar four major morphological types of beaches can be distinguished: beaches formed in fan material at the gully mouth (82,6 %), beaches under the cliff (9,3 %), beaches formed in Aeolian deposits (4,45 %) and artificial or anthropogenic beaches (2,4 %). 1,2 % are undefined. The majority of beaches, 75%, are today under anthropogenic impact while only 25% is completely natural.
Along the eastern Adriatic coast most of the beaches are formed in torrential material derived from the land accumulated at the gully mouth. Here we revealed that this is also the case on the Island of Hvar (82,6 %). Those beaches are parts of a larger geomorphological system which links the backward drainage basin with the beach. Consequently, here we test if the surface of the beaches correlates with the surface of the drainage basins. Taking into account all the beaches of that morphological type (204 beaches) the correlation revealed to be rather low (r2=0,37). However, taking into account only the beaches without any anthropogenic impacts the correlation becomes more significant (r2=0,64). This probably points to the disturbing effects of the anthropogenic activity on beaches sediment budget of the island of Hvar.
A part of this research was made with the support of the Croatian Science foundation (HRZZ-IP-2019-04-9445).
How to cite: Mićunović, M. and Faivre, S.: Geomorphological properties of the island of Hvar beaches (Croatia, Eastern Adriatic Coast), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5755, https://doi.org/10.5194/egusphere-egu2020-5755, 2020.
Coastal areas are known as cradles of civilization from the beginning of human settlements and the coastal belts in tropics experience high density of population all over the world. Indian coastal region is one of the most populated coastal belts of the world. Kerala coastal region of South West Peninsular India hosts 2931 person per sq. km. Stability of coastal zone helps to prevent the intensity of coastal hazards like extreme waves, coastal flooding and coastal erosion, which is quite noticeable in the northern part of Kerala state, when compared to the southern coastal region. The paleo-shoreline of Kozhikode coast in northern Kerala is identified as 2.5 to 5 km landward from the modern shoreline in the Beypur – Kallayi sector, 1 to 2 km in the Kallayi – Korapuzha Sector and 1 to 2.5 km in the Korapuzha – Quilandi Sector. This proves that the area is an accreting one over the recent geological history. The sediment discharge of Chaliyar, Korapuzha, Kadalundi and Kallayi rivers along with micro morphology leads to the evolution and development of this coastal plain for last few centuries. Paleo channels of this area changed its direction in many places during Holocene – Pleistocene period under the tidal influence. Nearshore bottom features of the area got diversified with parallel and transverse bars, reefs, exposed and buried rocks. The major nearshore features are demarcated as Kadalur Cape, Thoovappara, Elathur Cape, Thikkodi reef, Kadalur reef, Anchorage reef, Coote reef, Calicut reef, Rocky It, Gilham rocks, Rocky points, Black rock and Puthiyangadi bay. As a fast growing urbanised coastal city of the state, the Kozhikkode coast line is subjected to intense human interventions and thereby adversely affect sustainability of the coastline. Construction of two major fishing harbours, vis. Puthiyappa and Quilandi and Beypur port in 1990s re-defined the coastal morphology and nearshore bottom features of the sector. Shoreline towards the south of Puthiyappa harbour and Beypur breakwater is accreted and vast beach was developed while the Quilandi harbour doesn’t have much influence on sediment drift. Rocky coast, sand bed, seasonal sand bar and exposed and buried rocks have been properly documented in the paper. Along with those natural features, the artificial landforms and coastal protection measures have been analysed for understanding the disturbances in the coastal stability of the area. One-meter contour of the bathymetry line runs parallel to the coast except in the near shore of the Elathur and Kadalur headlands. Current investigations show that 48 percent of the total coastline can be considered as stable (Quilandi - Korapuzha and Korapuzha – Kallayi sectors), while 36 percent is erosion prone (Kallayi – Beypur Sector) and the rest is accreting.
How to cite: Mk, R., Thulasidharan, A., George, M. E., Ds, S. B., and Tk, P.: Coastal Stability and Micro Morphology; Disturbances due to Human Interventions along West Coast of India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7140, https://doi.org/10.5194/egusphere-egu2020-7140, 2020.
Richards Bay harbour, on the Indian Ocean coast of South Africa, is one of the largest coal export facilities in the world. Since its founding in 1976, the bay and its associated estuary (the Mhlatuze) have undergone fundamental changes linked to the development and expansion of the port. Today, the industrial impact in Richards Bay is centred on coal export, aluminium smelters and fertilizer plantations and its associated ship operations add to a heavily impacted environmental system, especially in the managed harbour basin.
Based on surface sediment samples, this study analyses the harbour-system as final sink for pollutants. Sedimentological aspects are combined with environmental influence indicators such as potential pollutants (e.g., heavy metals, persistent organic pollutants, microplastics), eutrophication indicators (total organic carbon, biogenic silica) and erosion indicators (e.g., grain size).
The harbour-interior is periodically dredged to maintain a constant water depth for the ocean going vessels, which is reflected in its bathymetry and grainsize distribution patterns. Multiple parameters infer a hydrodynamically controlled environment, due to their correlation with the harbours bathymetry, such as grainsize or elemental distribution. Short sediment cores from the entire harbour interior are of very young age and therefore indicate very high sedimentation rates.
Analyses reveal a link between the distribution of Polyethylene terephthalate (PET) measured in bulk sediment samples and remains of microplastics. Additionally the distribution of microplastics shows strong similarities to the hydrodynamic regime in the harbour system as seen in bathymetry and grainsize distributions. In contrast, the distribution of certain environmental pollutants, e.g., cadmium and chromium, appear to be influenced by point sources, such as the main bulk port or the small craft harbour. Results of elemental concentrations complement previous studies, but reveal increased maximum concentrations values (e.g., max. Cu concentrations in Wepener & Vermeulen (2005): 53.5 mg*kg-1; current study: 353 mg*kg-1). Additionally, the measured maximum concentration values exceed findings of other comparable studies on South African ports (e.g., max. Cu concentration in Fatoki & Mathabatha (2001) for Port Elizabeth: 68.5 mg*kg-1 and East London: 106 mg*kg-1). The otherwise even distribution pattern of Organochlorine pesticides (OCPs) indicates a sink for pesticide pollution within the harbour centre. OCP values suddenly decrease towards the harbour mouth and imply their discharge by (tidal) currents towards the Indian Ocean.
Wepener, V., & Vermeulen, L. A. (2005). A note on the concentrations and bioavailability of selected metals in sediments of Richards Bay Harbour, South Africa. Water SA, 31(4), 589-596.
Fatoki, O. S., & Mathabatha, S. (2001). An assessment of heavy metal pollution in the East London and Port Elizabeth harbours. Water SA, 27(2), 233-240.
How to cite: Mehlhorn, P., Humphries, M., Frenzel, P., Gildeeva, O., Hahn, A., Viehberg, F., and Haberzettl, T.: Surface sediments of Richards Bay Harbour, South Africa – potential pollutants (heavy metals, persistent organic pollutants, microplastics) and grainsize distribution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1623, https://doi.org/10.5194/egusphere-egu2020-1623, 2020.
Estuarine systems represent the dynamic transition zone between fluvial and marine systems and as such they are sensitive to changes in both domains resulting from impacts of climate change and human activities related to coastal and water-flow management especially in densely inhabited areas. Further, these tidally influenced systems are subject to a unique set of driving conditions linked to bidirectional flow processes. The potential growing risks of shoreline erosion in coastal, estuarine and inter-tidal environments have been identified by a number of studies in recent years. However, bank erosion processes in tidal settings remain poorly understood, especially when compared to the large volume of research concerning fluvial bank erosion. In general, the well-established fluvial bank erosion literature suggests that bankline erosion involves two main sets of processes: hydraulic erosion and gravitational collapse. Given the additional complexity of the process mechanics involved in tidal settings, arising mainly from the presence of bi-directional flows, process insights gained from studies of fluvial bank erosion might not be appropriately applied in a tidal context.
The present study aims to improve our understanding of estuarine bank mobility dynamics through investigation of the evolution and rates of bank retreat/accretion acting in the Severn Estuary (UK). The Severn Estuary has one of the highest semidiurnal tidal ranges in the world (about 15 m in the outer estuary, up to 8-9 m in the middle parts of the system, and 2 to 3 m in the inner river-dominated sector). Here we estimate bank mobility throughout the estuary from the river-dominated to the tidal-dominated zones during the last 119 years, via analysis of historical maps and recent satellite images. We use the findings from this analysis coupled with recent data collection to propose an empirical model of bank mobility throughout the entire estuary, highlighting the characteristics and the differences between riverine and coastal erosive processes. The model indicates that (i) the highest bank mobility (both in term of erosion and deposition) is located in the mid part of the estuary, close to the bedload convergence zone (BLZ), with other ‘hot spots’ of change linked to major anthropogenic disturbances either in the outer and inner estuary, and (ii) that the erosive mechanics associated to severe lateral land losses in the estuary are mainly driven by impulses in energy delivery to the bank surface in occasion of very high tidal oscillations (particularly in spring overbank tides) and severe storms triggering mass wasting in form of toppling and rotational failures.
How to cite: Gasparotto, A., Leyland, J., Darby, S., and Carling, P.: Bank Erosion Processes, Trends and Impacts in a Hypertidal Estuarine System, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7592, https://doi.org/10.5194/egusphere-egu2020-7592, 2020.
Coastal hazards, including marine-related and gravity-induced processes such as landslides, coastal erosion, storm water runoff and coastal flooding, may have different impacts mainly due to local geomorphological characteristics and natural and anthropogenic settings. The sustainable conservation of coastal areas represents a worldwide issue and therefore, coastal vulnerability and risk assessments are of paramount importance for ensuring appropriate coastal management.
This study is focused on the assessment of coastal vulnerability along the NE sector of the Island of Gozo (Malta, Mediterranean Sea), which is characterized by diverse landforms, including plunging cliffs, sloping coasts, pocket beaches, shore platforms and a large sandy beach partly backed by dunes. Results of detailed geomorphological investigation, integrated with the analysis of marine geophysical data, show that the study area is particularly susceptible to mass movements, coastal flooding and erosion processes.
From the economic point of view, Gozo Island is considered an attractive geotourist destination due to its high environmental, cultural and geological heritage. In particular, the study area hosts Roman remains and two important natural protected areas included in the Natura 2000 network. Moreover, the presence of quarrying areas contributes to increase the economic value of the study area.
The evaluation of coastal vulnerability refers to the methodological approach proposed in the framework of the EU-funded RISC-KIT project, partially modified to adapt the method to the context of the study area and to the available information. Specifically, the method is based on the evaluation of the exposed elements in the investigated area by applying a set of indicators related to the local land use, anthropogenic and natural assets, and economic activities. Furthermore, a social vulnerability indicator is applied to evaluate the socio-economic characteristics of the population potentially exposed to coastal hazards. Available data is overlaid and reclassified by means of specific GIS tools in order to obtain the overall vulnerability level of the investigated area, represented on a coastal vulnerability map.
Results highlight that 18.3% of the study area is characterized by high to very high vulnerability: including Marsalforn Bay, which hosts an extensive urban centre, and the area nearby Dahlet Qorrot Bay, where a natural protected site is located. Ramla Bay, a very important tourist attraction hosting the largest sandy beach in Gozo, is characterized by very high vulnerability. Most of the investigated area (61.3%), is however characterized by a medium level of vulnerability, while areas characterized by low vulnerability (20.4%) mainly correspond to abandoned agricultural fields and bare rocks outcrops.
This research represents a first attempt at the assessment of coastal vulnerability in the Maltese archipelago, and shows that the method used can be easily applied to other Mediterranean coastal areas providing policy makers with comprehensive coastal vulnerability information. The latter is crucial to approaching sustainability, through integrated coastal management.
How to cite: Soldati, M., Buhagiar, G., Micallef, A. S., Rizzo, A., and Vandelli, V.: The coastal vulnerability of the north-eastern sector of Gozo Island (Malta, Mediterranean Sea), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10272, https://doi.org/10.5194/egusphere-egu2020-10272, 2020.
Coastal sand dunes are multifunctional landscapes with rich biodiversity and provide ecological goods and services. They play a dual role as a sediment sink or a source to maintain the long-term stability of a coastal system. These landscapes have been affected by human settlements, economic activities and recreational purposes. Sand dunes in the Manawatu-Wanganui region, in New Zealand, have been subjected to such forcings during Maori settlements and, in particular, since the establishment of Europeans since 1840. Consequently, dunes have evolved from a transgressive system to a parabolic one, while the rate of dune drifting is still observed to be one of the highest in the world.
Because this was a problem for populations living in the area, there were several attempts to arrest dune drifting. Using the Driver-Pressure-State-Impact-Response (DPSIR) cyclic framework, we analyzed these interventions during two-time frames: 1) from the 19th to 20th century and 2) during the early 21st century. We checked for data in historical records and literature including the Parliamentary debates of New Zealand. Historical evolutionary trends were inferred by analyzing a series of maps since 1773. The present-day impacts were derived from a series of georeferenced google images from 1983 using the ESRI ArcGIS tools. The coastal management responses were obtained through scientific literature and reports of the Horizon Regional Council.
According to the analysis, drivers of dune drift before the 21st century were 1) settlements resulting in burning shrubs, deforestation, grazing, agriculture, mining, and building, 2) introduction of non-native animals. The pressures were: 1) mobile dunes and 2) blowouts. The assessment of the state of the environment included: 1) soil fertility, 2) habitat quality, 3) river navigability and 4) air quality. The assessed impacts were 1) increase of wasteland and loss of fertility, 2) foredune erosion, 3) impact on transportation and 4) creation of swamps as river mouths were closed. The management responses included 1) introduction of 1903 and 1908 Sand Drift acts for reclamation of affected areas, 2) introduction of exotic vegetation (e.g. Marram grass) and 3) foredune building using sand trapping fences.
The main drivers of the 21st century are 1) intensive urbanization, 2) introduction of exotic vegetation and 3) global fossil fuel burning. The invasive character of marram resulted in the loss of biodiversity. The coastline erosion due to sea-level rise during the 21st century will be moderated due to its progradational nature. The study revealed a significant spatial variability of the rate of dune drift. The responses include 1) a consolidated “One Plan” as mandated by 1991 Resource Management Act; 2) removal of exotic vegetation to support native biodiversity by enhancing natural processes of dunes (a paradigm shift in dune management); 3) enhancing awareness while encouraging the public participation in mitigating measures.
In conclusion, historical data combined with DPSIR framework tools showed that management interventions should be implemented considering long-term and interdisciplinary analysis to better understand the systems’ evolution and the full consequences of human actions.
How to cite: Sampath, D. M. R. and Freitas, J. G. D.: A DPSIR analysis of aeolian sand dune mobilization along the coast of Manawatu-Wanganui in New Zealand , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19785, https://doi.org/10.5194/egusphere-egu2020-19785, 2020.
Estonia is located in the norther part of the Baltic Sea region characterized by land uplift and prograding coasts. On uplifting sedimentary coasts, a variety of coastal landforms can be found. Sometimes the partly buried or elevated coastal formations appear as extensive stripe-like patterns populating coastal plains up to 5–10 km inland. These ridge systems are mostly called beach ridge plains, strandplains and foredune plains. The ridge systems are offering a unique opportunity to examine the events over at least the last 7,000 years when the Baltic Sea mean water table has been consistently dropping and a steady shoreline advancement has been punctuated by rare extreme events. We have found that the signs of past storms are clearly reflected in the internal structure and size of the ancient ridges. It can be assumed that high ridge systems containing extensive seaward-dipping layers formed 3,500–3,000 years ago are reflecting period of extreme storms and high influence of maritime climate, while the following small, nearly unnoticeable ridges, formed 3,000-2,200 years ago are reflecting calm period, probably with more continental climate. The current study is focusing on this shift in climatic conditions and is trying to find shifts in different ecosystems during the same period.
In this study, GIS analyses based on LiDAR topography were carried out in the coastal ridge systems. Number of study areas with different exposure to the storms and different rates of land uplift were selected. Ridge system patterns from the age of 3500-2200 BP were analysed. The ages for this study were acquired from published luminescence and radiocarbon dating results. Additionally, land uplift rates were used to determine approximate age of the formations. These results were compared with other studies based on the literature analyses. These analyses included: ground penetrating radar studies; records of aeolian sand influx into the coastal peat bogs in Estonia and in Northern Europe; past climatic records of northern Europe; and number of studies related to other ecosystems.
We have found that during the period of increased storminess and more maritime climate, 3500-3000 years ago, an increased sand influx was reported into the coastal peatbogs. Moreover, number of ground penetrated radar studies along Estonian coast have detected several extensive erosional layers in the internal structure of coastal landforms. In contrast, during the following period, such markers are completely missing. Additionally, notable change has been found in wetland ecosystems where we can find rapid shift from fen phase to raised bog phase around 3000 years ago. All these results are indicating that, for some reason, the climate in our region changed rapidly from western cyclones dominated maritime climate to much calmer and dryer continental climate.
What where the reasons behind this climatic shift, how it might have influenced different ecosystems, how likely it might happen again as a result of global warming and how we need to take it into account in coastal management plans will be also discussed in this poster.
How to cite: Kuusik, S. and Tõnisson, H.: Rapid shifts in the Baltic Sea region climate, detected from the ancient coastal formations and number of other ecosystems – how likely it is to happen again and what are the consequences?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11173, https://doi.org/10.5194/egusphere-egu2020-11173, 2020.
The 1500 km-long wave-exposed coast of the Guianas, South America, is characterized at any time by up to 20+ large distinct mud banks with suspended mud concentrations of up to 1000 g/l migrating from the Amazon delta to the Orinoco delta under the influence of wave-driven longshore transport. Banks can be up to 60 km-long, strongly dissipate waves, and are separated alongshore by ‘inter-bank’ sectors of similar length. The latter are affected by shoreward propagation of much less dissipated waves that can generate rapid muddy shoreline erosion and reworking of beaches and cheniers formed from sand supplied by rivers draining the crystalline rocks of the Guiana Shield.
About 500 km northwest of the mouths of the Amazon, the pervasive mud and its effects on the nearshore wave regime determine, for the embayed, headland-bound beaches in French Guiana, outcomes that are important from a long-term management perspective. These beaches have come under urban pressures and assure recreational and ecological functions such as provision of nesting sites for marine turtles. The sand-mud interactions, processes of sand segregation from mud, sediment transport modes, and morphodynamics associated with these beaches over timescales ranging from weeks to several decades, were analyzed from aerial photographs, satellite images, aerial photogrammetry, and field experiments. The longer bay beaches are exposed to longshore transport when mud is temporarily scarce (inter-bank phases), and subject in parts to overwash. During inter-bank phases, ‘normal’ westward sand transport along these beaches is generated by waves from E to NE, but is counter-balanced during bank phases by eastward drift at the leading edge of a bank as waves are refracted over the bank. This counter-drift prevails at a ‘mobile’ rotation front that moves with the bank’s leading edge migrating at rates of 1 to 2.5 km a year. As the bank passes, it further shelters beaches from wave reworking, with eventual re-exposure to waves and ‘normal’ drift following complete mud-bank passage. In the context of the ‘closed’ sand budget of these beaches, headlands spatially constrain sand mobility, and the unique mode of rotation induced by mud-bank refraction of waves plays an important role by counter-balancing unidirectional longshore transport that could otherwise result in permanent deprivation of updrift beach sectors of sand. Due to variability in bank-migration rates and spacing, normal drift and counter-drift may prevail, respectively, over periods exceeding two years but of unknown duration. The variability of this time frame of rotation poses a challenge to the implementation of set-back lines necessary to avoid the impingement of urbanization and sea-front activities on the long-term (>decadal) bandwidth of beach affected by rotation, which involves aperiodic and variable erosion and accretion in different parts of the beach. In this context of aperiodic beach rotation, prediction of mud-bank migration rates downdrift of the Amazon and of the imminent arrival of a mud bank, coupled with the firm implementation of shoreline development setback lines, are necessary to mitigate risks from erosion and overwash events.
How to cite: Anthony, E., Gardel, A., Jolivet, M., Brunier, G., and Dolique, F.: Aperiodic embayed sandy beach rotation and erosion-risk exposure on a hyper-muddy wave-exposed coast, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10487, https://doi.org/10.5194/egusphere-egu2020-10487, 2020.