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Coastal Zone Geomorphological Interactions: Natural versus Human-Induced Driving Factors

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.

Co-organized by NH5
Convener: Hannes Tõnisson | Co-conveners: Giorgio Anfuso, Andreas Baas, Guillaume BrunierECSECS, Margarita Stancheva
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Wed, 28 Apr, 13:30–14:15

Chairperson: Hannes Tõnisson

Patrick Barrineau

Some beaches regularly experience a rapid decrease in volume due to ‘coastal flow slides’. These events visually resemble subaerial landslides, but are subaqueous and located along river or tidal channels. Along a steeper shoreface, material eroded from the upper beach can be stored in deep water. In some cases, these events can remove thousands of cubic meters (m3) of beach sand in a few hours.

On several occasions in recent years, a flow slide has formed at Seabrook Island, South Carolina (USA). As of January 2021, there have been five events observed since July 2016. Surveys of a January 2017 event show the slide displaced ~25,000 m3 into deep water (15–20 m) along North Edisto River Inlet. This volume is comparable to hillside-scale slides observed in mountainous regions like the Blue Ridge, and similar-scale failures have been observed in the Netherlands, France, and Australia (Mastbergen, 2019).

The Seabrook flow slide is consistently located along a marginal flood channel of a relatively large ebb-dominant inlet, just below a quarrystone revetment protecting an upland development. In this particular location, erosion of the dry beach could cause undermining of the revetment. Historical charts suggest a small inlet was located along this portion of the beach as recently as ~1920. Reviews of available rainfall and water level data suggest exceptional (ie – near-record daily total) rainfall events and spring tide levels may coincide with observed flow slide events.

This study analyzes available meteorological, water level, geotechnical, and historical shoreline data to identify mechanisms affecting repeat coastal flow slide events at Seabrook Island (SC). A combination of excessive rainfall, spring tidal currents, and sediment characteristics all appear to affect these events. Because of the unpredictability of these events, and the dynamic nature of the inlet channel adjacent to this portion of the island, it is difficult to observe events in situ and identify specific mechanisms triggering flow slides. While a hard structural solution is unlikely to effectively mitigate the hazard in this location, providing an excess of beach sand may help maintain a shallower shoreface slope and mitigate future flow slides.

How to cite: Barrineau, P.: What mechanisms trigger coastal flow slides?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8397, https://doi.org/10.5194/egusphere-egu21-8397, 2021.

Björn Mehrtens et al.

Natural coastal dunes covered by vegetation are an essential component on many sandy coastlines worldwide and often provide the only physical protection against flooding by dissipating wave energy and enhancing erosion resilience. However, sea level rise, changing and widely intensifying coastal wave climates and storm surges constitute severe exacerbated stresses, calling into question the perseverance of such unique coastal ecosystems as dunes and their protective functions taken for granted.

Here we investigate the extensive coastal dune system of St. Peter-Ording, a major tourist draw of the German North Sea within a marine high energy zone. Lining the coast along 15 km, extending up to 1.5 km in cross-shore direction it covers an area of 18 sqkm characterized by overgrown dunes separating the tidal foreshore from the topographically flat hinterland. Featuring a dedicated, Germany wide unique, coastal protection function sets it apart from other national coastal dune systems - potentially creating a role model for mitigating coastal squeeze related driving factors, further adding to its awe-inspiring landscape character.

Consequently, the joint-research project ''Sandküste St. Peter Ording'' examines whether the local flood protection dune “Maleens Knoll”, a 16.6 m high natural coastal dune stretching a roughly 1.2 km long gap in the sea-dike defense, will continue to offer adequate protection in the future. Current hypothesis is, that due to the overgrowth with non-endemic and invasive vegetation species, the natural dynamic and self-adaptation of the system is impaired and will not withstand projected changes in coastal drivers. Therefore, the long-term goal is to develop a variety of nature-friendly flood protection measures to reinforce the dune and reduce its probability of failure during an extreme storm surge.

Possible options comprise the installation of hybrid systems, combining the existing dune core with one of the following structures: 1) a vertical wall to gain more stability during erosion of the sand cover, 2) rock filling to increase wave dissipation and reduce wave reflection and erosion and 3) geotextiles to provide a temporary and more environmentally protection against runup. The built-in materials will be covered with sand, to mimic the original landform and yield its previous degree of freedom regarding topographic adaptation. Another approach is to strengthen the resistance of the sand surface against aeolian and fluvial erosion. Through a microbiological process based on calcium carbonate precipitation (MICP), the strength can be increased in a particularly environmentally friendly way that saves raw materials. Furthermore, adapted or additional planting with a site-typical vegetation can promote sand accumulation at the surface and thereby stabilize the dune.

Large-scale physical model experiments will be performed in a wave flume to investigate the protection potential of the dune. First, the natural dune condition will be recreated and tested under a combination of water levels and wave conditions to investigate current and future load cases. Based on the findings, a second series of experiments will be conducted to determine which engineering methods are most appropriate to reinforce the dune and ensure its coastal protection character and retain its naturalness at the same time.

How to cite: Mehrtens, B., Kosmalla, V., Lojek, O., Schürenkamp, D., and Goseberg, N.: Coastal dunes - A nature based coastal protection element exposed to exacerbated anthropogenic stress, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10779, https://doi.org/10.5194/egusphere-egu21-10779, 2021.

Teresa Drago et al.

Beach nourishment is an increasingly recommended solution for reversing the erosion process that affects nowadays the coastal zone. Usually, it is used in emergency situations as a local and short-term solution or as a regional and long-term management strategy.

From April 2017 to November 2019, sediment samples and beach profile data were collected seasonally, before and after a sand nourishment (100.000m3) that increased 30m of width in Belharucas beach (south Portugal, Algarve).

The main objective of the work was to evaluate the nourishment impact in the beach ecosystem, aiming at contributing to seafloor integrity assessment, in the scope of Descriptor 6 of the Marine Strategy Framework Directive. 

Methodology included grain size and macrobenthic fauna analyses in two profiles of the nourished area and another one further away, selected as a control area. Each profile was sampled at three intertidal zones: supralittoral (beach berm), mediolittoral (beach face) and infralittoral (low tide terrace).  Beach profile data were collected with the main objective of measuring the beach width and evaluate nourishment longevity.

Results show that grain size variability, higher at beach face, is dominated by local energy beach conditions rather than to changes related to the nourishment.

Morphological data shows that beach nourishment had a relatively low longevity as two years after the nourished beach present roughly the same width as priori to nourishment.

While supralittoral samples were defaunated, medio and infralittoral ones exhibited extremely low diversity. Assemblages were dominated by small-size polychaetes, bivalves and isopods. No statistically significant differences were found in assemblage composition regarding pre- and post-sand nourishment, year seasons, tidal zones and control stations.

In conclusion, Belharucas beach exhibited high resilience to the sand nourishment, preserving its morphodynamics and ecosystem conditions.


The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL and through the strategic project UIDB/MAR/04292/2019 - MARE and ECOEXA project (MAR-01.04.02-FEAMP-0016)

How to cite: Drago, T., Teixeira, S., Rosa, M., Tuaty-Guerra, M., Gaudêncio, M. J., Lobo-Arteaga, J., Veronez, A., Taborda, R., and Cascalho, J.: Morphosedimentary and ecosystem evolution at Belharucas beach after a sand nourishment (Algarve, south Portugal), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15838, https://doi.org/10.5194/egusphere-egu21-15838, 2021.

Nienke Vermeer et al.

Sand nourishments are carried out along numerous sandy coasts worldwide to counteract coastal erosion, with the sand added to the inter- and supratidal beach or to the subtidal nearshore profile. Since the early 1990s beach and shoreface nourishments have been carried out along the Dutch coast, with a total nourished volume of 10 to 15 Mm3/year. Although we have a reasonable understanding of how an individual nourishment temporarily affects the evolution of nearshore morphology, it is not clear how repeated nourishments influence the long-term dynamics of the nearshore zone. This understanding is crucial, not only for the safety of beachgoers or marine life, but especially in view of the expected increase in the number of nourishments and total nourishment volume given expected accelerating sea-level rise in the decades to come.

This contribution aims to analyse how repeated nourishments affect the long-term evolution of the shoreline and the two subtidal sandbars at the Dutch beach town Noordwijk aan Zee using Argus video imagery available since 1995. Between 1998 and 2014 four shoreface and three beach nourishments were carried out at the study site. The low-tide time-exposure images of the Argus station were used to determine  sandbar and shoreline position along a 6-km stretch of coast.
The results show that prior to the first nourishment the sandbars migrated seaward slowly but persistently. The repeated nourishments permanently decreased this seaward directed migration rate of the sandbars to only a few m/year. The sandbars showed alternating periods of seasonal to multi-year onshore and offshore migration superimposed on this very weak decadal offshore trend. Furthermore, the various sand nourishments gave rise to forked shoreline-sandbar morphology. This large-scale alongshore variability was undone within 1 – 2 years by switches, in which the landward part of a sandbar or the shoreline on one side of the fork realigned with the seaward part of a bar on the other side. These switches appear to be a direct consequence of the repeated nourishments. For example, the 2013-2014 sequence of a beach and a shoreface nourishment resulted in 4 bar switches within the subsequent 2 years, compared to a total of 12 switches in the total dataset of 24.8 years. Further analysis will focus on the effect of repeated nourishments on the temporal and spatial persistence of rip-channel morphology and on the wave conditions that caused the forked morphology to switch.

How to cite: Vermeer, N., Ruessink, G., and Price, T.: The effect of repeated sand nourishments on long-term nearshore evolution: a case study for Noordwijk aan Zee, the Netherlands, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3021, https://doi.org/10.5194/egusphere-egu21-3021, 2021.

Marin Mićunović and Sanja Faivre

Beaches are dynamic coastal forms. However, nowadays, natural processes are intertwined with anthropogenic influences. The island of Hvar has 247 beaches from which we selected those which evolution could be studied by means of repeat photography method using archive maps and old photographs. More than 150 old photographs dating between the 1900s and 1980s have been collected and analyzed. The recent period is studied using unmanned aerial vehicles (UAV).

In total 12 beaches have been selected for precise study. The benchmarks from old photographs were marked and geolocated during the fieldwork using GNSS Trimble receiver. In November 2020, all locations were recorded by quadcopter DJI Phantom 4 Pro v2.0 with approximately 80% overlapping. On each beach, 6 - 12 ground control points (GCP), mostly benchmarks from the old photographs, were marked and measured. Data collected from UAV has been generated by photogrammetric techniques in ESRI Drone2Map software. Orthophoto and digital surface model (DSM) has been processed with a spatial resolution of 0,02 m and 0,1 m for the digital elevation model (DEM). All analyses were made using the ArcGIS Pro software. In this work, the analysis will be presented on two sites: Mina sand beach, formed in Aeolian deposits, on the northern side of the island and Mola Milna gravel beach, found on the southern side. Beaches have been studied in three points in time, in the 19th, 20th and 21st century.

On the Franciscan Cadastre (1834), Mina beach was mapped as an individual cadastral parcel with an area of 222 Klafter Quadrimeter (written in the Cadastral supplement), that is 799 m2. Recalculating in GIS we obtained a similar value, that is, 782 m2. The beach area from the beginning of the 20th century was reconstructed from old photographs and was approximated to 450 m2. Consequently, since 1834 the beach area reduced by ~43%. In 2020, the area further drops to 226 m2, so its surface diminishes by 55% since the beginning of the 20th century or even 72% from 1834.

In 1834 the Mola Milna beach was ~1073 m2, ~900 m2 in the 1950s (16% smaller) and finally 802 m2 in 2020 (11% less than in the 1950s, or 27% smaller compared to 1834).

Thus, we observed that during the last two centuries the sand beach Mina reduced for more than 2/3 of its size since 1834, while the gravel beach Mola Milna reduced for around 1/3. Similar results have been observed previously on the Zogon gravel beach which lost ½ of its size since the 1960s. Even if the reconstructions of the beach area from the Cadaster maps and old photographs are less accurate than the model generated from UAV photos, obtained values clearly reveal the trend of beach erosion during the studied period.

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.: Analysis of morphological changes of the island of Hvar beaches using archive maps, old photographs and UAV (Eastern Adriatic Coast, Croatia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10021, https://doi.org/10.5194/egusphere-egu21-10021, 2021.

Florin Tatui et al.

Shoreline, as the interface between the upper shoreface and the beach-dune system, is sensitive to all changes from both the underwater and sub-aerial parts of the beach at a wide range of temporal scales (seconds to decades), making it a good indicator for coastal health. While more traditional techniques of shoreline monitoring present some shortcomings (low temporal resolution for photointerpretation, reduced spatial extension for video-based techniques, high costs for DGPS in-situ data acquisition), freely available satellite images can provide information for large areas (tens/hundreds of km) at very good temporal scales (days).

We employed a shoreline detection workflow for the dynamic environment of the Danube Delta coast (Black Sea). We focused on an index-based approach using the Automated Water Extraction Index (AWEI). A fully automated procedure was deployed for data processing and the waterline was estimated at sub-pixel level with an adapted image thresholding technique. For validation purposes, 5 Sentinel-2 and 5 Landsat based results were compared with both in-situ (D)GPS measurements and manually digitized shoreline positions from very high-resolution satellite images (Pleiades – 0.5 m and Spot 7 – 1.5 m). The overall accuracy of the methodology, expressed as mean absolute error, was found to be of approximately 7.5 m for Sentinel-2 and 4.7 m for Landsat data, respectively.

More than 200 Landsat (5 and 8) and Sentinel-2 images were processed and the corresponding satellite-derived shorelines between 1990 and 2020 were analysed for the whole Romanian Danube Delta coast (130 km). This high number of shorelines allowed us the discrimination of different patterns of coastline dynamic and behaviour which could not have been possible using usual surveying techniques: the extent of accumulation areas induced by the 2005-2006 historical river floods, the impact of different high-energy storms and the subsequent beach recovery after these events, the alongshore movement of erosional processes in accordance with the dominant direction of longshore sediment transport, multi-annual differences in both erosional and accumulation trends. Moreover, a very important result of our analysis is the zonation of Danube Delta coast based on multi-annual trends of shoreline dynamics at finer alongshore spatial resolution than before. This has significant implications for future studies dealing with different scenarios of Danube Delta response to projected sea level rise and increased storminess.

The presented approach and resulting products offer optimal combination of data availability, accuracy and frequency necessary to meet the monitoring and management needs of the increasing number of stakeholders involved in the coastal zone protection activities.

How to cite: Tatui, F., Anghelin, G., and Constantin, S.: Satellite-derived shorelines reveal fascinating dynamics for the last three decades on Danube Delta coast, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13031, https://doi.org/10.5194/egusphere-egu21-13031, 2021.

Freya Fenwick et al.

Wave-dominated sandy coastlines worldwide are susceptible to change under the expected climate-change induced developments in sea level rise, mean wave conditions and storm events. For coastal management it remains important to observe and quantify these coastal changes, especially in low-lying developed coastal areas susceptible to flooding. The beaches surrounding an ocean basin have a variety of orientations, tidal ranges and management strategies, to name a few, which will lead to a range of morphological responses to future changes in hydrodynamic conditions within the basin. In addition, the conditions under which these varied responses mainly occur (e.g., under regular conditions or only during storm conditions) is not clear. Here, we used satellite imagery to compare the morphological response of a selection of beaches surrounding the North Sea.

The position of the shoreline is generally considered as a key variable to monitor the morphological evolution of sandy coasts. This research used the open-source software toolkit CoastSat (Vos et al., 2019) to automatically map shorelines from publicly available satellite imagery from 1984 to present, which are retrieved via Google Earth Engine (Gorelick et al., 2017). We selected five long, sandy beaches around the North Sea with varying tidal ranges, orientations and wave exposure for our analysis: (1) Skallingen in Denmark, (2) Egmond aan Zee and (3) the barrier island of Schiermonnikoog, both in the Netherlands, (4) Groenendijk in Belgium, and (5), Theddlethorpe in the UK. Approximately 2000 images per site were used for the shoreline extraction. Offshore wave buoy measurements and numerical model output provided the tidal water levels and wave conditions for the different sites. To account for tidal correction of the shoreline to a reference elevation, we used the dataset of Athanasiou et al. (2019) to estimate characteristic beach face slopes. At the conference we will present our analysis of the shoreline responses around the North Sea over the last few decades.

Athanasiou, P., Van Dongeren, A., Giardino, A., Vousdoukas, M., Gaytan-Aguilar, S., & Ranasinghe, R. (2019). Global distribution of nearshore slopes with implications for coastal retreat. Earth system science data, 11(4).

Gorelick, N., Hancher, M., Dixon, M., Ilyushchenko, S., Thau, D., & Moore, R. (2017). Google Earth Engine: Planetary-scale geospatial analysis for everyone. Remote sensing of Environment, 202, 18-27.

Vos, K., Splinter, K. D., Harley, M. D., Simmons, J. A., & Turner, I. L. (2019). CoastSat: A Google Earth Engine-enabled Python toolkit to extract shorelines from publicly available satellite imagery. Environmental Modelling & Software, 122, 104528.

How to cite: Fenwick, F., Price, T., and Ruessink, G.: Detecting coastal change around the North Sea from open-source satellite images, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8266, https://doi.org/10.5194/egusphere-egu21-8266, 2021.

Errol Wiles et al.

It is well-known that the coastal zone attracts human populations like no other. There is ever increasing demand on the coastal zone by tourists, residents and developers alike; it is named as the fastest growing sector of the blue economy. Hence, coastal zone processes require better understanding to be effectively managed in the context of sustainable development of the asset. While sea level may be rising over the long-term, and many global-resolution studies lament the loss of sandy beaches as a result, coastal managers work over far shorter time-scales thus require site specific information to manage the coastal zone on a daily and monthly basis. In this contribution we discuss nine years of morphological change along a sandy beach in Durban, South Africa. The beach is managed by the eThekwini Municipality who are responsible for maintaining a coastal zone of ca. 100 km. The morphological data of this study were collected on a near monthly basis over 9.6 km of sandy beach between October 2011 and March 2020. From these data, beach volume and area are calculated and the variation is documented over time in conjunction with wave data recorded from a proximal directional wave buoy. Over the study period, the beach has experienced a net loss of 177 885m3 and 29 375 m2 in volume and area respectively. However, the beach response has not been uniform throughout the study area. The southern three-quarters of the beach were affected by significant losses while the northern one-quarter gained in volume and area over the same period. The summer wave climate is characterised by increased variability in swell origin with greater easterly contributions than other seasons, and typically lacks the frequency of large swell (>3.5 m) events (7% of events) common to autumn (20%), winter (35%) and spring (38%). Winter, followed by spring then autumn seasons have more focused swell origins and southerly contribution, particularly in terms of large swell events. During periods of reduced event frequency allow for partial beach recovery, while erosion is associated with periods of increase event frequency. The sediment budget has been significantly reduced though impoundments on the proximal river catchment, compounded by un-managed sand mining. Rather than sea level, these near-field controls on sediment availability likely play a major role in beach volume and area in response to wave climate. There is less sediment available to nourish the beach naturally following erosional events; artificial nourishment will likely be required to maintain the sandy beach in the future.

How to cite: Wiles, E., Leuci, R., Thakeray, Z., and Vella, G.: Trends in shoreline variability driven by anthropogenic change, EThekwini Municipality, South Africa, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11099, https://doi.org/10.5194/egusphere-egu21-11099, 2021.

Mar Roca Mora et al.

Coastal areas are being rapidly transformed in the last 50 years due to anthropogenic causes. New infrastructures and intensive activities have changed the natural behaviour of coastal ecosystems, promoting problems related to water quality, eutrophication and coastal erosion. This situation increases the vulnerability to climate change, requiring important efforts in monitoring and defining protocols for optimizing operational decision-making and strategic management. Remote sensing techniques are becoming a key tool for coastal mapping in terms of resolution, effectiveness and cost reduction. In the last decade, the European Commission launched the Copernicus programme for Earth Observation as a way of improving coastal monitoring with higher resolution. Sentinel-2A/B twin satellites are part of this free and open policy programme available since 2015, but atmospheric corrections or cloud cover are still challenges to face. In order to process this data, cloud computing platforms such as Google Earth Engine (GEE) have revolutionized the way satellite images are processed, without the need to download and store local data. The present study aimed at developing a GEE-based technique for selecting cloud-free Sentinel-2 Level-2A images in the Guadiaro estuary in the Western Mediterranean (Spain) during the last four years (2017-2020).  It has been used to analyse the evolution of the sand bar and to identify hotspots in its sedimentary variation along the coast, at 10 m and 5 days spatial and temporal resolution respectively. NDWI index was evaluated using 0.05 to 0.15 threshold, revealing 0.1 as the best threshold to be used for land/water mapping, easily incorporated in the GEE platform. In addition to Sentinel-2 potential, this study also demonstrates the power of GEE, computing more than 400 images for statistical analysis in terms of seconds, which enabled the automatic filtering method developed for cloud-free images selection with a 95% of effectiveness. Moreover, ACOLITE processor has been used on Sentinel-2 L1A images for atmospheric and sunglint correction to generate Level-2 data and for analysing turbidity and water quality patterns during extreme rainfall events, providing key information as early-warning indicators development. This improvement will be useful for near future implementation of remote sensing applications for coastal managers, ensuring a continuous and detailed monitoring and helping to support an ecosystem-based approach for coastal areas.

How to cite: Roca Mora, M., Navarro Almendros, G., García Sanabria, J., and Caballero de Frutos, I.: Use of Sentinel-2A/B satellites and Google Earth Engine for monitoring estuarine systems: a study case in the Western Mediterranean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15400, https://doi.org/10.5194/egusphere-egu21-15400, 2021.

Peter Gitau et al.

Coastal deltas worldwide are under risk of degradation due to the increasing impacts of sea-level rise, and continuous human alterations of river basin hydrology. This research highlights the geomorphological changes that have occurred within the Tana River delta in Kenya, an important deltaic ecosystem of high biodiversity value in East Africa.

The geomorphological features (river channels, floodplain, coastal dune system) and their evolution over the past two centuries were described. Aerial and satellite imagery was used to assess the magnitude and distribution of coastal changes from the 1960s to present.  Additionally, sediment cores recovered within the mangrove environment were analysed to establish the succession of sedimentation periods and patterns. Finally, we explored the response of the coastal processes of deposition and erosion under anthropogenic alterations of the hydrological system.

It was established that over the past two centuries Tana River has changed its main channel and outlet to the Indian Ocean on three occasions. A first river avulsion occurred in the 1860s, followed by a second avulsion in the late 1890s that was promoted by human interference through channel expansion and dyke construction. The third change in river course has occurred gradually over the past 20 years, amid human efforts to engineer the river channels.

From the sediment analysis and radiocarbon dating, it is ascertained that the lower deltaic region developed rapidly over the past ~180 years, facilitated by increased sedimentation from the main Tana River. On the other hand, analysis of the coastline changes indicate that there has been increased erosion of the coastal dune system and mangrove vegetation along the former river outlet, leading to rapid marine intrusion into local subsistence farming areas. By analysing the combined impacts of both natural river dynamics and human alteration we highlight how the integrity of the Tana River delta has increasingly become vulnerable under present sea level rise and continued upstream river alteration.

How to cite: Gitau, P., Duvail, S., Verschuren, D., and Guillaud, D.: Geomophological and vulnerability analysis of a coastal delta under increased hydrological alteration over the past two centuries: Tana River delta, Kenya, East Africa., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15404, https://doi.org/10.5194/egusphere-egu21-15404, 2021.

Sílvia Nave and Luís Rebêlo

The Portuguese coastal zone, where ¾ of the population lives and where the contribution to GNP is estimated at 85%, assumes an important role in the national economic context, which is not only presently reflected on the budget dedicated to the management and mitigation of current risks associated to climate change, but also for its strategic importance in environmental, social and leisure industry perspectives.

The geological and hazardous cartography of the coastal zone, of mainland Portugal, on a 1:3000 scale, has been developed, at LNEG, as an instrument to support the sustainability, protection and enhancement of the coastline [2018, JCC, 22:1031-1043].

The increase in knowledge concerning coastal hazard, based on the historical evolution of the shoreline, expressed on a systematic and digital cartographic basis at a scale of detail, in addition to the important contribution to the development of regional geology, is a vital contribution for the correct use and sustainable development of the coast. So far, shoreline evolution evaluation has been determined for two coastal sectors covering approximately 140 km of coastline: the western sector of Figueira da Foz to Nazaré and the southern sector of Algarve between Faro and Vila Real de Santo António [2021, JCC, https://doi.org/10.1007/s11852-020-00791-3].

The western sector evolutionary trend, from 1947 to 2015, shows an overall erosional behaviour, even though a prograding tendency is observed in some areas. Coastline evolution assessment reveals an average retreat of -13.6 m and a 702,558 m2 land loss area. However, when looking only for the sectors where erosion occurred, a total of 1,164,888 m2 of land loss was observed. Erosion, that is more severe in the northern part, reaching a maximum coastal retreat of -145 m and an erosion rate of 2.46 my-1, seems to be induced by a reduction of the littoral drift, but also by human interference in coastal dynamics, namely by the introduction and enlargement of the original rigid constructions and groins installation.

Regarding the south eastern Algarve coastline displacement, from 1950 to 2015, a seaward shift prevails, with a prograding coastline occupying approximately 54% of the studied sector. However, this progradation is mostly associated to human intervention on the coast, being related to up-drift accumulation against inlet jetties / groins. The erosional trend prevails predominantly in the central barrier island system of Ria Formosa, namely in the Culatra (with a maximum displacement of -163 m), Armona (maximum displacement of -83 m), the Tavira, (maximum displacement of -116 m), and the Cabanas islands, where maximum displacement observed is up to -235 m. Regarding land area changes, some sectors lost and others gained area due to coastline displacement. However, the overall analysis showed an area increase of 1.05 × 106 m2 for this south-eastern coastal fringe.

The achievement of high-resolution, continuous and updated data, at a regional scale, likely favour successful application of the needed mitigation measures (as beach/dune nourishment, sand-shots and others) at the exact key target locations.

How to cite: Nave, S. and Rebêlo, L.: National geological and hazardous systematic cartography of the coastal zone as a contribution to the promotion of sustainability, defence and valorisation of the Portuguese coastal area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8965, https://doi.org/10.5194/egusphere-egu21-8965, 2021.

Jakub Mares and Jiri Bruthans

Cavernous weathering is a typical example of the degradation pattern of both natural outcrops and cultural heritage. It is described from all environments on Earth and also on Mars. The most common examples are honeycombs and tafoni. Honeycombs are known from arid, humid, and cold deserts, but best developed honeycombs are often described from coastal areas. There are many ideas on the origin of cavernous weathering (case hardening, chemical alteration), but currently most authors believe that the origin is caused by salt weathering. Huinink et al. (2004) described a theory that inside the pits, the capillary zone is closer to the surface and therefore the intensity of the evaporation is higher than in walls separation the pits. As more evaporation accumulates more salts the pits enlarges faster than surface outsides the pits is retreating. To verify this theory, in the environment of coastal honeycombs in Tuscan (Italy), the depth of the vaporization plane (interface between dry surface zone and deeper capillary zone) was measured by the "uranine-probe" method (Weiss et al., 2020) inside and outside the ten honeycombs. From the depth of the vaporization plane and climatic conditions on the study site, the intensity of evaporation was calculated and from the mineralization of water the amount of precipitated salts was estimated. To determine the effect of case hardening, the tensile strength of honeycomb pits and walls was measured. The vaporization plane measurements show that for all honeycombs, the vaporization plane was closer to the surface in pits than outside. The evaporation intensity was calculated for the mean depth of vaporization plane inside the honeycombs (2 mm) and the mean depth outside the honeycombs (7 mm). In marine environment a solution on a vaporization plane should be saturated with halite which has an equilibrium relative humidity of 75 %. The evaporation intensity inside the honeycombs is 9.4 mm/year for 75 % RH and 2.7 mm/year outside the honeycombs. Considering that the evaporated water is of the same composition as seawater, 0.1-0.4 g salts precipitate from 1 m2, most of which is NaCl. Inside the honeycombs precipitate 3 times more salts than outside. The tensile strength inside the honeycombs is approximately the same as outside considering standard deviation (354±339 and 284±157 kPa, respectively), so case hardening does not have any effect. The results correspond to the theory of origin according to Huinink et al. (2004). For a detailed description of the moisture behavior in future studies, it is necessary to better understand the moisture conditions (especially relative humidity on the vaporization plane) and it is vital to perform repeated measurements during various seasons.



Huinink HP, Pel L, Kopinga K., 2004. Simulating the growth of tafoni. Earth Surface Processes and Landforms 29: 1225–1233.

Weiss T, Mareš J, Slavík M, Bruthans J. 2020. A microdestructive method using dye-coated-probe to visualize capillary, diffusion and evaporation zones in porous materials. Science of The Total Environment 704, 135339.

How to cite: Mares, J. and Bruthans, J.: Moisture distribution, tensile strength, evaporation rate and origin of coastal honeycombs (Tuscan, Italy) , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8770, https://doi.org/10.5194/egusphere-egu21-8770, 2021.

Thomas Oliver et al.

Sediment budgets on wave-dominated coastlines are important in understanding shoreline behaviour. Coastal sediment compartments provide a means to investigate sediment budgets over a range of time and space scales. This study reconstructs the sediment budget over the mid- to late- Holocene for a secondary coastal compartment on the New South Wales (NSW) south coast ~26 km in length and containing five adjacent but discrete barriers: Barlings Beach, Broulee Beach, Bengello Beach, Moruya Heads Beach and Pedro Beach. Building upon existing morphostratigraphic studies in this region, a new set of Optically Stimulated Luminescence (OSL) ages are reported for foredune ridge successions at previously un-dated sites. Additional Ground Penetrating Radar (GPR) transects complement earlier stratigraphic data, and topographic and bathymetric LiDAR datasets capture the morphology of subaerial coastal deposits and the inner shelf. The results demonstrate two different sediment sources promoting shoreline progradation and coastal barrier construction. A quartz-rich sand, transported onshore from the shoreface as it evolved towards equilibrium, dominates the barrier sequences. Skeletal carbonate sand augmented the quartz sand supply for the northern Barlings and Broulee beaches after ~3000 years ago. Shoreline progradation at Bengello Beach was steady throughout the mid-to late- Holocene. Bengello Beach contains the largest volume of Holocene sand and accreted at an average rate of 3.1 m3/m/yr (for the current shoreline length). Changes in sediment accumulation rate has occurred for the other barrier systems as their shorelines prograded resulting in changes to their alongshore interconnectivity. Rapid infilling of the Pedro Beach embayment by ~4000 years ago initiated headland bypassing northwards to Moruya Heads Beach which only then commenced progradation. In contrast, as Broulee and Bengello Beaches prograded, a tombolo formed in the lee of Broulee Headland which restricted northward sand drift into the Broulee embayment. As these once continuous shorelines became two, a marked increase in skeletal carbonate content at Broulee occurred attesting to shoreline separation and independence of sediment budget. This study emphasises the importance of quantifying the long-term temporal variability in sediment budget and embayment interconnectivity in order to better understand shoreline response to contemporary anthropogenic influences and changing boundary conditions such as sea level and wave climate.

How to cite: Oliver, T., Tamura, T., Brooke, B., Short, A., Kinsela, M., Woodroffe, C., and Thom, B.: Holocene sediment budget for wave-dominated Moruya coastline, southeastern Australia: sediment sources, transport and embayment interconnectivity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8104, https://doi.org/10.5194/egusphere-egu21-8104, 2021.

Roland Freisleben et al.

Abstract. Tectonically active coasts are dynamic environments characterized by the presence of multiple marine terraces formed by the combined effects of wave-erosion, tectonic uplift, and sea-level oscillations at glacial-cycle timescales. Well-preserved erosional terraces from the last interglacial sea-level highstand are ideal marker horizons for reconstructing past sea-level positions and calculating vertical displacement rates, which can be subsequently compared to short-term coastal deformation patterns associated with the earthquake cycle. We carried out an almost continuous mapping of the last interglacial marine terrace along ~5,000 km of the western coast of South America between 1°N and 40°S. We used quantitatively replicable approaches constrained by published terrace-age estimates to ultimately compare elevations and patterns of uplifted terraces with tectonic and climatic parameters in order to evaluate the controlling mechanisms for the formation and preservation of marine terraces, and crustal deformation. Uncertainties were estimated on the basis of measurement errors and the distance from referencing points. Overall, our results indicate a median elevation of 30.1 m, which would imply a median uplift rate of 0.22 m/ka averaged over the past ~125 ka. The patterns of terrace elevation and uplift rate display high-amplitude (~100–200 m) and long-wavelength (~102 km) structures at the Manta Peninsula (Ecuador), the San Juan de Marcona area (central Peru), and the Arauco Peninsula (south-central Chile). Medium-wavelength structures occur at the Mejillones Peninsula and Topocalma in Chile, while short-wavelength (< 10 km) features are for instance located near Los Vilos, Valparaíso, and Carranza, Chile. We interpret the long-wavelength deformation to be controlled by deep-seated processes at the plate interface such as the subduction of major bathymetric anomalies like the Nazca and Carnegie ridges. In contrast, short-wavelength deformation may be primarily controlled by sources in the upper plate such as crustal faulting, which, however, may also be associated with the subduction of topographically less pronounced bathymetric anomalies and varying distances to the trench. Latitudinal differences in climate additionally control the formation and preservation of marine terraces. Based on our synopsis we propose that increasing wave height and tidal range result in enhanced erosion and morphologically well-defined marine terraces in south-central Chile. Conversely, river incision and lateral scouring in areas with high precipitation may degrade marine terraces. Our study emphasizes the importance of using systematic measurements and uniform, quantitative methodologies to characterize and correctly interpret marine terraces at regional scales, especially if they are used to unravel tectonic and climatic forcing mechanisms of their formation.

How to cite: Freisleben, R., Jara-Muñoz, J., Melnick, D., Martínez, J. M., and Strecker, M.: Eemian marine terraces along the Pacific coast of South America (1°N-40°S) allow regional assessments of tectonic forcing from earthquake cycle to glacial-cycle timescales, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7638, https://doi.org/10.5194/egusphere-egu21-7638, 2021.

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