Hurrikan Irma auf den Britischen Jungferninseln – Dokumentation und Analyse von Überflutungsparametern und Hurrikan-Sedimenten
Zusammenfassung der Projektergebnisse
The record-breaking 2017 hurricane season drew renewed attention to the risk of hurricane-related coastal flooding. The Caribbean has a relatively short written record of hurricanes. Long-term patterns of hurricane frequency and intensity are essential to hazard vulnerability assessments. Thus, sediment archives that store evidence of past storm events provide valuable data, but one needs to be able to interpreting the geologic record. Studies on recent analogues, like hurricane Irma are a beneficial tool to finally be able to decipher the geological record. So far, hurricane Irma was the strongest recorded hurricane to form in the Atlantic Ocean. It reached the Caribbean Islands as a category 5 hurricane on Sept 5th 2017. A post-hurricane survey was conducted 3 months after the event on Anegada (British Virgin Islands). During this survey, we documented I) the extent of the storm surge, II) distribution of associated deposits, III) typical sedimentary structures, IV) the type and extent of coastal erosion. The findings serve as a basis of comparison with older overwash deposits preserved in the geologic record which provide the opportunity to examine long-term patterns in frequency and intensity. Irma passed ~35 km south of Anegada with max. wind speeds of ~256 km/h and max. offshore wave heights of ~17 m. Irma had a surprisingly weak storm surge of up to 3 m along the northern and western sides of the island, as indicated by trim lines. The measured max. onshore flow depth was 1.6 m. The inland inundation was between 30-40 m, evidenced by storm deposits and wrack lines. Microtopographic features and low-lying shrubs were effective elements that prevented farther inundation. The small surge may also be explained by the fact that Irma’s eyewall passed south of the island and winds were mainly shore parallel. Thus, Irma does not represent the worst-case scenario for a storm event on Anegada. This is also underlined by a rubble ridge along the island´s central north shore that was seemingly emplaced by one or several stronger hurricanes in the past. Substantial coastal erosion occurred along the north shore. Erosional scarps of 1-1.5 m height were created and the coastline locally retreated by 8 m. Beaches vertically lost up to 0.3 m of sand. A resurvey, 18 months after the event, assessed the degree of the natural coastal recovery. Sand that was eroded during the Irma and stored in the shallow water, acted as a nearshore source for beach reconstruction which set in only days after the event. Beach recovery peaked in February 2018, when beaches accreted within hours during a nor’easter-like storm that transported large volumes of nearshore sand back onto the beaches. Depositional evidence of Irma is present along the northeastern and southern shore. Deposits in the north are up to 35 cm thick sand sheets that depict a faint lamination and occasionally erosively overly soil horizons. Along the south, thin (max. 5 cm) washover fans contain intertidal bivalves and gastropods. The sandy deposits contain Homotrema rubra, a red foraminifera that bleaches following detachment from the reef. The high concentration of vibrantly coloured Homotrema within the Irma deposit indicates that these taxa were freshly scoured during the storm. The hurricane entirely reworked a pre-existing coast-parallel coral rubble ridge in the central north. The crest of the ridge shifted up to 10 m inland due to the landward transport of cobbles and boulders (max. 0.5 m3). Hardly any fresh material was added to the ridge. This underlines the limited availability of bioclastic material due to the demise of the coral reefs, but also proves that much stronger storms are required to initially emplace such a ridge. The presence of the ridge documents that more intense storms happened in the past and may do so in the future. Thus, this study underlines the importance of geoscientific hazard research to learn from past events to be better prepared for any future events. Both the depositional and erosional evidence differ significantly from features left by two historical tsunami events on Anegada. The tsunami breached the coastal dunes and transported large living corals to up to 2.1 km on land. In contrast, hardly any coral rubble was washed onshore by Irma and the max. inundation distance was only a few tens of meters. Sandy tsunami deposits are found in all depositional environments, including salt ponds, coastal plains and on top the Pleistocene platform. In comparison to the Irma deposits, tsunami deposits depict normal grading, are coarser, and contain numerous marine shells and gastropods. The tsunami scoured material from the reef flat to crest, including offshore planktics. Whereas the hurricane sourced sediment only from the fringing reef and the reef flat. The documentation of differences between storm and tsunami deposits at this location assists in developing long-term records of their frequency and intensity.
Projektbezogene Publikationen (Auswahl)
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2018: Overwash deposits and coastline erosion caused by hurricane Irma on Anegada (British Virgin Islands). AGU Fall Meeting (10-14 December 2018, Washington D.C., USA)
Spiske, M., Pilarczyk, J., Mitchell, S., Halley, R.E.
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2018: Sedimentary and microfossil differences between sediments deposited by Hurricane Irma and the 1755 Lisbon Tsunami. AGU Fall Meeting (10-14 December 2018, Washington D.C., USA)
Pilarczyk, J., Spiske, M., Mitchell, S., Halley, R.E., Jaffe, B.E.
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2019: Sedimentary and erosional features caused by hurricane Irma. INQUA (15-31 July 2019, Dublin, Ireland)
Spiske, M., Pilarczyk, J., Mitchell, S., Halley, R.E.
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2019: Sedimentary evidence of tsunamis and hurricanes impacting Anegada, BVI. AGU Fall Meeting (09-13 December 2019, San Francisco, USA)
Jaffe, B.E., Buckley, M., Pilarczyk, J., Spiske, M., Watt, S., Mitchell, S.
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2021: Distinguishing between hurricane and tsunami deposition using modern analogues from Anegada, British Virgin Islands (BVIs). European Geosciences Union General Assembly, Wien (Austria), 19-30 April 2021
Pilarczyk, Jessica; Spiske, Michaela & Mitchell, Stephen
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Nearshore microfossil assemblages in a Caribbean reef environment show variable rates of recovery following Hurricane Irma. Sedimentology
Mitchell, Stephen; Pilarczyk, Jessica E.; Spiske, Michaela & Jaffe, Bruce
