NH5.3

In order to estimate the size and the origin of giant tsunamis, it is useful to investigate “tsunami far-field” as the coastal area far from the source. However, it is challenging to distinguish a tsunami deposit from an extreme storm deposit in these areas. In this study, we report sand layers induced by extreme waves on the coast of Hokkaido, Japan, facing the southern Kuril Trench. In the study area (central part of the Hidaka coast), it is said that the tsunamis caused by observed earthquakes have never exceeded the dune or beach. However, geological evidence indicates that giant earthquakes and tsunamis occurred at intervals of several hundred years in the Kuril Trench, and the traces of these tsunamis are still unclear in the Hidaka region.

The study area can be classified into the inland zone consisting of peatland and the seaward zone consisting of floodplain muds by the paleo beach ridge. We identified three volcanic ash layers and three to four sand layers with clear boundaries to the ordinary mud layer in each zone. However, there are large gaps in the ages of the sand layers discovered in both inland and seaward zones, and their distributions are limited (ranging from a few tens to 100 m from the ridge at that time) and do not overlap. To understand the peculiarities of the depositional age and distribution of the sand layers, we clarified the sedimentary environmental changes and sea-level index in the late Holocene by analyzing the diatom assemblage and CNS of the mud layer. The inland zone showed the paleoenvironments from the sandy tidal flat formed by the transgression in the mid-Holocene to the beach ridge formed by the regression, and the sheet sand layers were formed only during the period of the beach ridge development. On the other hand, the seaward zone showed various changes due to the formation of meandering rivers and beach ridges associated with the regression, and the formation of recognizable event layers is accompanied by changes in the depositional environment, such as the opening of lagoons and rapid changes to upland. Thus, especially in the tsunami far-field, the preservation potential of the event layers is strongly influenced by the coastal development and relative sea level, and such geological information will provide clues to identify the origin of the sand layer. In the presentation, the numerical simulation of the paleo-tsunami considering the reconstructed relative sea-level change and topographic development will be reported.

How to cite: Nakanishi, R., Ashi, J., Yokoyama, Y., and Miyairi, Y.: A study on the interaction between extreme waves and coastal development processes for identification of tsunami and storm deposits in the tsunami far-field, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1875, https://doi.org/10.5194/egusphere-egu21-1875, 2021.

Investigating palaeotsunami deposits is a primary way to extend the tsunami database beyond relatively short instrumental and historical records. Such information is essential to reconstruct the frequency and magnitude of past coastal flooding events, which are a key to assess the impact and risk of tsunami to the coastal community. However, palaeotsunami studies are limited as most of the proxies, such as microfossil and geochemical signals, can be modified or degraded with time. Here, we present the application of DNA analysis to investigate a series of palaeotsunami deposits up to ~2800-years-old from a coastal beach ridge sequence on Phra Thong Island (Thailand). Our result shows that it is possible to accurately discriminate palaeotsunami deposits from intercalating organic mud layers using the microbial communities recovered from DNA preserved in the sediment of the geological record. Our work demonstrates that environmental DNA represents a new and promising tool for investigating historical and pre-historical tsunami records.

How to cite: Yap, W., Switzer, A., Gouramanis, C., Marzinelli, E., Wijaya, W., Dominey-Howes, D., Labbate, M., Jankaew, K., and Lauro, F.: Investigating the sedimentary DNA of palaeotsunami deposits in Thailand., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1935, https://doi.org/10.5194/egusphere-egu21-1935, 2021.

On 28 September 2018, a magnitude 7.5 earthquake struck north of Palu, Central Sulawesi, Indonesia. The earthquake generated a tsunami with inundation depths of up to 7.5 m and run-up of up to 10 m above sea level. Inundation distances were only partly topography dependent and reached less than 400 m inland even where terrain did not rise steeply beyond that point. A subsequent tsunami was generated by a combination of minor fault displacement and multiple submarine landslides. In places, co-seismic coastal subsidence of >1 m exacerbated the tsunami inundation. During a post-event field survey in November 2018, we sampled three transects for sediment analysis; two in Palu City and one on the eastern coast of Palu Bay. The tsunami deposits in Palu City are predominantly massive, fine- to medium-grained sand in thin layers (<5 cm) with patchy distribution of sediments. In contrast, sediments present near Pantoloan on the east coast of Palu Bay were coarser (medium- to coarse-grained sand), thicker (up to 12 cm) and more continuous. These tsunami deposits exhibited fining and thinning landwards, and are characterized by a continuous sand sheet that extends up to 250 m inland with few post depositional changes. The grain size ranges from coarse-grained sand to silty-fine-grained sand at the landward extent. The Pantoloan site also contained wave-transported blocks of sea wall weighing up to 4.7 t in addition to sandy deposits. The blocks together with grain size data suggest that water velocities reached 3 m.s^-1 at more than 130 m from the coast. The tsunami deposits of Palu Bay generally exhibit sedimentological and stratigraphic characteristics shared by storm and tsunami deposits, which maybe be ascribed to the short wave length, relatively low power and short-term inundation of the tsunami and the limited availability of sediments in the nearshore environment.

How to cite: Switzer, A. D., Majewski, J. M., Guan, R. YS., Benazir, B., Meilianda, E., Parham, P., Weiss, R., Martin, S., Jordan, C., Pilarczyk, J. E., and Horton, B. P.: Sediment analysis and modelling reveal short inundation distances and low onshore flow speed of the 2018 Palu-Donggala tsunami in Indonesia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6822, https://doi.org/10.5194/egusphere-egu21-6822, 2021.

Foraminifera are commonly used to examine patterns of tsunami inundation occurring over centennial to millennial timescales. However, the impacts of post-depositional change on geologic reconstructions is unknown. In tropical environments, the taphonomic character (i.e. test surface condition) of a foraminifer can deteriorate, rendering them unidentifiable, and in the worst case, dissolve them entirely. Here, we investigate the rates and extent of post-depositional change associated with the foraminiferal assemblages found within the 2004 Indian Ocean Tsunami (IOT) deposit over a 15-year time interval in Aceh, Indonesia from 2007 to 2019. 

The IOT deposit consisted of a 13-18cm thick, medium-fine sand unit that sharply overlays a muddy sand contact. During the 15-year time series analysis, the IOT deposit remained a consistent thickness and maintained easily recognizable stratigraphical contacts between the overlying soil layer and the underlying mud layer. The overlying soil layer increased in thickness from 2cm in 2007 to 6cm in 2019 and resulted in roots bioturbating the IOT deposit. Calcareous taxa dominated the IOT deposit assemblage, where hyaline taxa accounted for 62% of the assemblage, porcelaneous taxa for 34% of the assemblage and agglutinated taxa for 4% of the assemblage. The concentration of calcareous foraminifera within the tsunami deposit decreased by 5% from 2007 to 2019. This trend is attributable to the high abundance of delicate porcelaneous tests, which are more susceptible to post-depositional processes than the more robust hyaline tests. The taphonomic character of the foraminiferal assemblage became more corraded (dissolved, abraded and/or pitted) over the 15-year period. The relative abundance of corraded individuals within the foraminiferal assemblage increased by 4% in the IOT deposit, to reach a relative abundance of 50% by 2019 compared to 46% in 2007. Our results indicate that there is minimal change occurring within the deposit and presents good evidence that microfossils can be used as reliable indictors of tsunami origin and to identify characteristics of a tsunami deposit. While it is minimal, we recommend that post-depositional change should still be considered, especially with regards to the more delicate porcelaneous tests and over longer taphonomic timescales.

How to cite: Pearson, L., Pilarczyk, J., Hawkes, A., Gouramanis, C., Majewski, J., Ismail, N., Afrizal, T., and Horton, B.: Assessing the rate of post-depositional change within the 2004 Indian Ocean tsunami deposit: implications for long-term records of paleotsunamis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10419, https://doi.org/10.5194/egusphere-egu21-10419, 2021.

The 2004 Indian Ocean Tsunami tragically underscored the practical implication of understanding the Sumatran subduction zone and its tsunami potential. Further paleo-tsunami research is needed to fully inform the assessment of future tsunami hazards for coastal regions of the Indian Ocean. However, the Covid-19 pandemic has severely limited the ability of many international teams to conduct field investigations of paleo-tsunami sites in Southeast Asia. In collaboration between Syiah Kuala University in Indonesia and Nanyang Technological University in Singapore our team has been investigating the paleo-tsunami history of Sumatra for more than 10 years. This year, in order to facilitate training of junior staff at Syiah Kuala University we recorded a number of coring, sampling and sediment description videos combined with virtual workshops. Written material, as well as regular meetings via zoom, have made co-ordination of fieldwork possible. We also uploaded all data to cloud services immediately following fieldwork to allow everyone in the project to have access to it quickly. This data now consists of more than 500 photographs, field description files, and field reports.

Previously our efforts have concentrated along the northern half of the Aceh province, which was devastated by the 2004 Indian Ocean Tsunami. Over the past 12 months, we have extended our field research ~250 km southwards along the western coastline of Sumatra. Despite the pandemic, we have been able to investigate 4 new coastal wetland sites and identify between 1 and 5 potential paleo-tsunami layers at each site at depths of 1.7 to 4.5 m. More than 50 samples of the sediments have been sampled and are currently being analyzed to confirm their marine origin and the chronology of the events they represent.

How to cite: Majewski, J., Daly, P., Switzer, A., Ismail, N., Afrizal, T., and Horton, B.: Extending the paleo-tsunami record along the west coast of Sumatra, Indonesia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9341, https://doi.org/10.5194/egusphere-egu21-9341, 2021.

The 2004 tsunami killed more than 200,000 people in Asia, but fewer than 300 in all East Africa. As a result, the search for ancient precursors has focused primarily along the coastlines of the Northern and Eastern Indian Ocean. No efforts to study past events were made in East Africa, leading to an underestimation of the tsunami risk in the region. Here we document a 1,000-yr old event that devastated a coastal Swahili settlement in Tanzania. Our study suggests a tsunami wave as the most likely explanation, in agreement with coeval tsunami deposits elsewhere across the Indian Ocean.  Numerical simulations of tsunami flooding suggest a megathrust earthquake from the Andaman-Sumatra subduction zone as a potential source, with a larger magnitude than the 2004 event. Our findings indicate that tele-tsunamis represent a serious threat to coastal societies along the Western Indian Ocean, with implications for future tsunami hazard and risk assessments.

How to cite: Maselli, V., Oppo, D., Moore, A., Gusman, A., Mtelela, C., Iacopini, D., Mshiu, E., Mjema, E., Taviani, M., and Ortiz, J.: Historical evidence warns about disastrous tele-tsunami risk on the coast of East Africa., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3354, https://doi.org/10.5194/egusphere-egu21-3354, 2021.

Tsunamis and other extreme wave events draw a severe threat to coastal populations today and in historic times. The ancient settlement of Cerro del Villar located in present-day Málaga, southern Spain, was built by Phoenicians around the second quarter of the 8^th century BCE on a small sand bar (island) in the wide estuary of the Guadalhorce River. Later, the sand bar connected to the southern river bank and alluvial plane. Due to the low height above mean sea level, the site has been prone to river floodings, as well as extreme wave events of the Mediterranean Sea. In order to understand the palaeoenvironmental evolution and settlement history, as well as its vulnerability, it is important to analyse the nature of the events by dating and interpretation of the sedimentary record.

Here, we present first results of a short field campaign carried out in October 2019 at the western end of the Guadalhorce River palaeo-estuary, outside the boundaries of the archaeological zone. Two sediment cores (MAL-CV-1; ca. 3.70 m length and MAL-CV-2, ca. 4.69 m) were drilled southwest of the Phoenician site. A total of eight non-invasive ground-penetrating radar (GPR) profiles were carried out in the surroundings of the cores, and additional GPR profiles close to the beach were taken to understand the changes in the depositional environment along the coast. The cores cover a stratigraphy of three different sediment units: a basal sand unit representing a palaeo-beach, followed by a large silt and clay unit developed in a lagoon environment, and topped by another silt and clay unit representing floodplain conditions. At MAL-CV-1 two possible high-energy event units (Ey and Ez) interrupt the low-energy silt and clay units. At MAL-CV-2 event unit Ey is preserved as well, the other event unit Ez is concealed by an anthropogenic unit rich in ceramic, brick and glass fragments. GPR profiles show the same stratigraphy and allow a lateral continuation of the different units and event deposits. With the help of these GPR profiles, event unit Ez can be traced in-between the anthropogenic unit of MAL-CV-2. In terms of chronology, two radiocarbon dates establish the transition between the basal palaeo-beach and the lagoon at 4352-4325 cal. BC (6274-6301 cal. BP) and the anthropogenic layer to be younger than 2201-2126 cal. BC (4075-4150 cal. BP). The establishment of coastal freshwater lagoons with plentiful Hydrobia gastropods and ostracods resembles the last stage of post-glacial sea level rise in the Mediterranean. In the future, these promising first results will be extended by additional radiocarbon dates and a palynological study to better understand the climate and palaeoenvironmental evolution.

How to cite: Feist, L., Val-Peón, C., Mathes-Schmidt, M., Broer, L., Álvarez-Martí-Aguilar, M., Machuca-Prieto, F., Suárez-Padilla, J., Martín-Casado, J. M., and Reicherter, K.: The Phoenician settlement of Cerro del Villar (Málaga, southern Spain) and its natural vulnerability, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12185, https://doi.org/10.5194/egusphere-egu21-12185, 2021.

New field investigations along the East Tunisian near Sfax coastline reveal sedimentary deposits that may account for a catastrophic event. The sedimentary unit is made of sand coarse gravels, limestone beach-rock, mixed with broken shells of marine gastropods and lamellibranch mollusks, bones and organic matter. Near Thyna, at El Amra site located north of Sfax city, 3.2 m to 3.6 m high late Quaternary coastal terraces are spread over the coastline; they contain a catastrophic deposit that often cover archeological sites of the Roman period. The stratigraphic units show a succession of sandy-silty paleosol truncated by 40 to 70-cm-thick catastrophic unit which is covered in some sites by fire remains overlain by a relatively thin (~10 cm) sandy-silty aeolian unit. The sedimentary succession ends with about 1-m-thick of alluvial deposits and paleosol units. Charcoal samples collected at 10 cm below and 4 cm above the catastrophic units provide radiocarbon dating 236 - 385 cal AD and 249 – 541 cal AD (2s), respectively. Radiocarbon ages bracket the catastrophic unit that may refer to the major tsunamigenic earthquake of 21 July 365 (Mw ~ 8) in west Crete (Greece) reported to have inundated coastlines of Sabratha in Libya and Alexandria in Egypt. The nonlinear shallow water Tsunami-HySEA code is used to perform numerical modelling using 2 different seismic sources comparable to that of the AD 365 Crete earthquake. They feature 2 principal mechanisms that accommodate the Nubia-Aegean convergence along the Hellenic Arc, namely a shallowly dipping thrust-faulting on the subduction interface, as well as a steeper splay faulting in the overriding material. The maximum tsunami wave heights distribution calculated along the Tunisia coast peak in both cases at about 3 meters. The run-up caused by these sources, also considering that we have used uniform slip on the causative fault, can be significantly higher. This proves that the tsunami waves may have reached Tunisia where several coastal cities where severely damaged and reported to have stopped their economic activity. With the identification of the 365 tsunami deposits in eastern coast of Tunisia, the tsunami hazard and risk associated with a major earthquake from the western Hellenic subduction zone cannot be ruled out.

How to cite: Bahrouni, N., Meghraoui, M., Bayraktar, H. B., Lorito, S., Zagrarni, M. F., and Bel Mabrouk, N.: Evidence of tsunami deposits in East Tunisia coastline contemporaneous of the AD 365 Crete earthquake: Field data and modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9104, https://doi.org/10.5194/egusphere-egu21-9104, 2021.

Within Oceania, the vast Central and Western Pacific (CEWEP) is an intriguing anomaly because of the scarcity of historical tsunami observations and the complete absence of dated palaeotsunami events.  This study establishes the first dated high-magnitude palaeotsunami event within the CEWEP region.  Both geological data and oral legend are presented for a palaeotsunami that struck remote Makin atoll in northernmost Kiribati towards the end of the 16^th century.  Narration of the euhemeristic myth by the Wiin te Maneaba, traditional storyteller on Makin, offered important details supporting a tsunami hypothesis.  The legend preserves credible information surrounding the giant-wave origin of Rebua and Tokia, two prominent named subaerial reefblocks of megaclast size that were produced and transported shorewards away from the reef edge by the event.  The youngest U-Th age-dates for fossil coral samples in the reefblocks give a maximum age for the palaeotsunami of circa AD 1576.  Several far-field Pacific Rim and regional possibilities exist for tsunamigenesis.  These include subduction-zone seismicity and catastrophic volcanic eruption, both of which have been linked to late 15^th century palaeotsunamis recorded elsewhere in the Pacific Islands.  Available evidence, however, suggests that the ~AD 1576 Makin event was more likely locally generated by tsunamigenic submarine slope failure associated with the giant arcuate bight structure that characterises the northern atoll rim.

How to cite: Terry, J., Karoro, R., Gienko, G., Wieczorek, M., and Lau, A.: Oral legend and geological evidence for a 16th century giant tsunami in Kiribati, central Pacific, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-586, https://doi.org/10.5194/egusphere-egu21-586, 2021.

Physical experiments and numerical models support investigations on the transport of boulders by tsunamis, and based on this, on hydrodynamic characteristics of the tsunami itself. We conduct physical experiments in a flume applying idealized (cuboid) as well as a naturally shaped boulder, the latter representing a downscaled model from a field site on the island of Bonaire (Lesser Antilles, Caribbean Sea). Besides the boulder shape, we study the influence of shoreline morphology and pre-transport setting on boulder transport by a tsunami (Oetjen et al., 2020).

The physical experiments show that the interaction between bore and boulder differ significantly for the idealized and natural boulders. In our experiments, the natural boulder model consists of a lower drag coefficient, leading to lower flow disturbances and transport distances, and an increased entrainment threshold compared to the cuboid boulder. Subsequently, the natural boulder is thus transported approximately 30 % shorter than the cuboid one of same volume and weight. Since idealized shapes like cuboids are non-existent in nature, the results indicate that existing equations predicting entrainment thresholds or transport distances, overestimate the actual values. However, it is not clear to which amount the influence of the boulder shape is superimposed by other boulder and wave properties (e.g., ratio between wave velocity and boulder volume or weight) or local conditions (e.g., initial boulder submergence).

Furthermore, especially for experimental setups leading to high transport distances, significant fluctuations, of the transport distances are observed (up to 650 % in a single setup). This shows the sensitivity and complexity of coastal boulder transport and clarifies, that evaluating such processes in nature need to be conducted as accurate as possible while attributing to the large uncertainties associated with the transport process which might not be solvable for particular events (e.g., due to remobilization processes or unknown transport mode).

For most transport properties our findings are in line with previous studies. However, in contrast to some of them, we only observe sliding transport and higher variations in the transport distances. A large percentage of the deviations between our results and other studies, may also be related to divergent experimental setups, especially in terms of wave - boulder property ratios (e.g., increased ratios between boulder density and wave height/velocity).

Subsequently, comparisons between the findings of different studies are not straightforward. For simplifying this, we suppose a more coordinated research approach based on a standardized experimental setup. Such a setup would allow research to focus on single parameters and an easier comparison of results from other research groups, flumes and experimental campaigns.

Oetjen, J., Engel, M., Pudasaini, S.P., and Schuettrumpf, H.: Significance of boulder shape, shoreline configuration and pre‐transport setting for the transport of boulders by tsunamis. Earth Surf. Proc. Landforms, 45, 2118–2133, 2020, https://doi.org/10.1002/esp.4870.

How to cite: Oetjen, J., Engel, M., and Schüttrumpf, H.: Significance of boulder shape for the transport of boulders by tsunamis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8583, https://doi.org/10.5194/egusphere-egu21-8583, 2021.

EGU Abstract

Reconstructing Boulder Deposition Histories: Extreme Wave Signatures on Malta

The Maltese archipelago, a group of 5 small island sits in the Central Mediterranean Sea, some 90-100 km directly south of Sicily.  It is ideally located to capture evidence of major events through the Mediterranean Sea. Its eastern seaboard, in particular, is able to record tsunamis arising from the Hellenic Arc, some 600 km to the east at elevations up to ~ 20 m asl.  We here study extreme wave signatures at Zonqor in SE Malta (the main island), on a strip of coastal terrain unsullied by urbanisation on which tsunami signatures are abundant and well preserved. 

The Zonqor coastline displays an exceptional range of geomorphic signatures of extreme sea wave events. This study brings together evidence acquired from field survey, analysis of time-sequential aerial and satellite imagery, and hydrodynamic modelling to investigate the histories of boulder groups identified by their intrinsic and contextual characteristics.

Clear differences are revealed between the distribution of boulders recently moved (Recent Movers) and those evidently of considerable age (Ancient Movers). Tracking the movement of boulders by aerial photography since 1957, and satellite imagery and field observations more recently, confirms that storms of surprisingly frequent interval are capable of driving complex boulder movements. This includes the lifting of boulders of up to 7 m in length. Scrutiny of the ancient boulders, including extreme weathering features is indicative of longterm in-situ post transportation residence. It also reveals fascinating landward-facing (reverse) imbrication indicative of a very powerful return flow, cautiously suggesting  tsunami(s) as the agent of their emplacement.   

A novel method, including due attention to the Froude number, is developed for depicting velocity decay profiles of hypothetical design waves, thereby overcoming some of the limitations of the Nott approach. Applied here, the wave run-up context further sets the ancient movers apart from their recent mover companions. 

The combined evidence implies a palimpsestic landscape where storm waves are regular geomorphic agents that add to and rework the distribution of boulders close to the shoreline, whilst over long time periods the boulder landscape becomes reset by tsunami, a concept that is of value to agencies in Malta responsible for coastal safety, planning and  management.                                                   

Derek Mottershead

01/2021

How to cite: Mottershead, D.: Reconstructing Boulder Deposition Histories: Extreme Wave Signatures on Malta, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7686, https://doi.org/10.5194/egusphere-egu21-7686, 2021.

Tsunamis and land-falling hurricanes pose an economic and environmental hazard to coastlines of the Caribbean and Gulf of Mexico.  Patterns of their frequency and intensity remain unclear in part because detailed long-term records are limited to only a few locations, but also because of uncertainties associated with interpreting the geologic record (e.g., preservation/erosion of older deposits, distinguishing between storm and tsunami deposition).  The seemingly unprecedented generation of four intense storms during the 2017 hurricane season highlights the uncertainty surrounding the geographic and temporal controls on hurricanes in the Atlantic region.  Similarly, the historical record and recent modeling studies indicate that the region is susceptible to both far-field (e.g., 1755 Lisbon tsunami) and near-field (e.g., originating from the Puerto Rico trench) tsunamis.  We improve upon this uncertainty by comparing the sedimentological characteristics of two modern analogues from Anegada, BVI: sediments deposited by the 1755 Lisbon tsunami and those deposited in 2017 by Hurricane Irma.  The 1755 Lisbon tsunami sediments were collected from hypersaline ponds via trenching and shovel cores.  The Hurricane Irma sediments were collected during a post-event survey of Anegada four months after the storm tracked 35 km south of the BVI as a Category 5 system. During this survey, we investigated the coastal areas affected by Hurricane Irma in an effort to: (1) document the storm surge parameters and associated sedimentary deposits of a known Category 5 hurricane; (2) assess the depth of scour and distance of sediment transport by storm surge; and (3) use the Hurricane Irma deposit as a basis for comparison with older overwash records, including a series of inferred tsunami deposits (e.g., 1755 Lisbon tsunami) preserved within coastal ponds.

The Lisbon tsunami deposited a laterally-extensive graded shell-rich layer composed of medium to coarse sand and abundant Homotrema, an easily recognisable foraminifer with a defined provenance in the reef.  Hurricane Irma’s storm surge reached a maximum flow depth of up to 3 m and deposition was limited to thin (<40 cm) lobes of sand consisting of well-sorted fine to medium Homotrema-bearing carbonate sand.  Homotrema is a red organism that bleaches and rounds predictably following detachment from the reef.  Intertidal mollusks were observed in lobate sediment fans deposited by Hurricane Irma on the southern side of the island, whereas sand sheets with faint laminations were found in trenches along the northern and western coastlines.  While similar in terms of composition, the tsunami and hurricane deposit were slightly different in terms of Homotrema taphonomy (preservation state of individual Homotrema fragments). The 1755 Lisbon deposit contains high abundances of Homotrema that are generally large (250-500 μm) and vibrantly coloured, suggesting scouring and transport by tsunami, followed by rapid burial on the coast.  In contrast, the Hurricane Irma deposit contains bleached and non-bleached Homotrema in near equal proportions, suggesting that the sediment was sourced from the fringing reef to the north of the island as well as the reef flat. Constraining the origin of overwash deposits at this location is essential to the establishment of effective coastal hazard mitigation policies.

How to cite: Pilarczyk, J., Spiske, M., and Mitchell, S.: Distinguishing between hurricane and tsunami deposition using modern analogues from Anegada, British Virgin Islands (BVI), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10233, https://doi.org/10.5194/egusphere-egu21-10233, 2021.

Severe storms, their extreme waves and surges pose the greatest natural hazard to the coasts of northwestern Europe, commonly resulting in infrastructural damages and high financial losses. Proxy records of past storminess are important for assessing future risks that may arise from storm surges and assessing whether storm activity has increased in recent decades. High-resolution records of North Atlantic storminess are generally limited to instrumental weather data or historical documentation of the past 50 to 200 years. Since the most destructive and severe storms passing over Europe originate in the North Atlantic, the Shetland Islands serve as a window to cyclogenesis in this region. In our research, we extracted lacustrine sediments of the coastal freshwater lake Loch of Flugarth on Mainland, Shetland Islands, that is separated from the ocean by a low sand and gravel barrier. A series of distinct sand layers intercalated in the otherwise fine-grained, organic-rich lake deposits and examined using particle-size analysis, microfossils, TOC and XRF, may represent storm overwash or aeolian transport mechanisms, both either pointing towards individual storm events or shorter phases of high storm activity. Based on radiocarbon data of some selected layers, the investigated sediment sequence covers ca. 1500 years and a Bayesian age-depth model is being established. In combination with a hydrodynamic wave model based on Delft3D-Flow, we simulate a critical threshold value at which waves may reach the lake to determine the sensitivity of the sedimentary archive. With the inclusion of historical documentation and observations, our multi-methodological approach aims at reaching a better understanding of the recurrence pattern of extreme storm events on the Shetland Islands over the last 1500 years. This implies further insights into the parameters driving extra-tropical storms in the wider region as well as the role and variability of the North Atlantic Oscillation across the targeted time frame.

How to cite: Hess, K., Engel, M., Oetjen, J., Patel, T., Schön, I., Dawson, S., and Heyvaert, V. M. A.: Historical records of storm frequency on the Shetland Islands (UK) – Preliminary insights from lake sediment cores and coastal wave modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8773, https://doi.org/10.5194/egusphere-egu21-8773, 2021.

The chronostratigraphy of coastal sedimentary records such as washover fans or beach-ridge sequences may be used to reconstruct storm chronologies on centennial to millennial time scales. However, modern analogues are crucial for interpretations of depositional processes and for reducing uncertainty in evaluating the typically complex chronostratigraphic architecture of these landforms. Such a modern analogue was provided by category 3 tropical cyclone (TC) Olwyn in 2015, which caused a significant storm surge in the Gulf of Exmouth, Western Australia, and which activated large washover fans located in the southwestern part of the Gulf. Pre- and post-TC Olwyn geomorphological surveys and high-resolution drone-derived topographical data of one of these washover fans document a detailed history of erosion and deposition during the event. The modern analogue deposits provided an excellent opportunity to evaluate the use of luminescence-based proxies (luminescence inventories) including quartz single-grain age distributions and associated remnant ages, as well as quartz and feldspar luminescence signal comparisons for tracing event-related sediment source environments and understanding transport processes (May et al., 2020). Sediments deposited during Olwyn show a systematic relationship between luminescence characteristics and washover fan position. Seaward and central washover sections are indicated by well-bleached deposits due to the beach as the dominant source and/or long transport distances across the fan. Lateral washover deposits, in contrast, are characterised by rather local source areas and short transport distances, resulting in higher remnant ages of 70-140 a. This data shows that the combination of sediment source environments and sediment transport length across the fan represents the main control in resetting the luminescence signal and enabling reliable depositional ages to be calculated. It documents the benefit of investigating luminescence inventories when establishing chronologies from complex sedimentary records, thereby demanding a careful consideration of local processes and source areas when interpreting sedimentary TC records.

May, S. M., Callow, J. N., Brill, D., Hoffmeister, D., & May, J.-H. (2020). Revealing sediment transport pathways and geomorphic change in washover fans by combining drone-derived digital elevation models and single grain luminescence data. Journal of Geophysical Research: Earth Surface, 125, e2020JF005792. https://doi.org/10.1029/2020JF005792

How to cite: May, S. M., Brill, D., Callow, J. N., Hoffmeister, D., and May, J.-H.: Combining drone-derived digital elevation models and single grain luminescence data of modern analogue deposits to reveal sediment transport pathways and geomorphic change in washover fans, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1740, https://doi.org/10.5194/egusphere-egu21-1740, 2021.

Within the Baltic Sea basin, the frequency and intensity of coastal flooding caused by storms is influenced by wind direction and exposure of the coast. Strong (north)westerly winds associated with the North Atlantic Oscillation hit large parts of the Polish coast, while (north)easterly winds affect Puck Bay. Following from that, the research on the frequency and magnitude of past storminess within the Baltic Sea sheds light onto regional climatic conditions indicating changes in wind-field directions and storminess in north-western and northern Europe. Despite the fact that the Baltic Sea basin area bears potentially vast amount of information on the past storminess and climatic conditions on the regional scale, no systematic, basin-wide research on storm deposits and analysis of frequency and intensity of storminess is available.

The spatially variable occurrence of sedimentary evidence for coastal flooding caused by storms indicates necessity for multisite investigations in order to develop reliable records of past storm frequency and intensity. As the historical written sources and instrumental records are insufficient to draw informative conclusion on the past storminess, the survey in search for the depositional evidence of catastrophic coastal flooding has been undertaken along the Polish coast, from the Puck Bay to Wolin Island. Following from that, detailed research on the storm deposits has been undertaken in 4 key locations (one is presented here, Mechelinki).

The survey allowed to create the list of features common for the locations where sedimentary evidence for coastal flooding is preserved. These include flat, inundational character of the coast, prevailing organic deposition in lowlands close to the shore and non-tidal character of adjacent marine basin.

Mechelinki peatland (Puck Bay) is separated by a N-S extending sand barrier from the open sea and exposed to (north)easterly winds. Investigated sedimentary succession comprises ca. 450 cm of peat interdigitated with few centimetres thick layers of sand. The origin of sand has been established based on multiproxy investigation including: particle size, shape (automated MorphologyG3, thin sections), diatom, XRF and heavy mineral analyses. Geochronology has been established based on ¹⁴C and ²¹⁰Pb/¹³⁷Cs measurements. In the Mechelinki research site, the evidence for about 20 coastal flooding events which took place during the last 5000 years has been discovered. The results prove, there is no universal method to differentiate the storm deposits from the sediments of other origin and only the combination of multiproxy analyses bears unambiguous results.

The research project CatFlood is funded by National Science Centre, Poland, OPUS grant nr: 2018/29/B/ST10/00042

How to cite: Leszczyńska, K., Statteger, K., Szczuciński, W., Moskalewicz, D., Kokociński, M., and Niebieszczański, J.: Tracing the late Holocene storminess at the Polish Baltic Sea coast – regional survey and local in depth research., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13967, https://doi.org/10.5194/egusphere-egu21-13967, 2021.