GM6.6

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.

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.

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.

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 19^th, 20^th and 21^st 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 20^th 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.

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.

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.

References:

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.

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.