Modern coastal tempestite deposition by a non‐local storm: Swell‐generated transport of sand and boulders on Eleuthera, The Bahamas

Abstract

This paper presents an analysis of the transport conditions of a storm deposit (i.e. tempestite) produced by a non-local cyclone. Observations and analysis of 'sand to boulder' transport and washover deposition in March 2018 at Gaulding Cay Quarry, Eleuthera, The Bahamas, confirm that swell waves can cause coastal change and affect the depositional record >1000 km from the storm centre. Drone video, news reporting, deposit stratigraphy, grain-size measurements and wave data were all used to define three phases of washover fan construction: an aggradational phase associated with baselevel rise as the quarry filled with water; a progradational phase associated with quasi-constant base-level; and fan incision tied to base level fall as discharge through an outlet channel exceeded input discharge by overtopping waves. Washover fan location was controlled by antecedent topography and represented only a fraction of the swell-impacted coastline. Sand and boulders were transported simultaneously, forming a complex poorly sorted deposit. Drone video, bedrock erosion and sediment-transport estimates all indicated that overwash exceeded sediment availability. As a result, the measured washover fan was estimated to be an order of magnitude smaller than its potential volume if conditions had been transport-limited. This study highlights the importance of pre-event topography and independent measures of event duration on accurately reconstructing storm properties from the sedimentary record, as well as the challenges in reconstructing storm location, and therefore storm intensity and frequency, from sedimentary deposits alone.

More Frequent Hurricane Passage Across the Bahamian Archipelago During the Little Ice Age

“…It should be acknowledged that extreme hydrodynamic events other than those associated with tropical cyclones may also deposit coarse sediment beds in some settings in The Bahamas (e.g., Wilson & Mohrig, 2021), conceivably resulting in an over‐counting bias in sediment‐based hurricane reconstructions. Wilson and Mohrig (2021) documented that a far‐field long‐period swell event from an unnamed extratropical cyclone that stalled off the coast of Massachusetts, USA in March 2018 deposited a “sand to boulder” washover fan over a 3,700 m 2 area at Gaulding Cay Quarry of northern Eleuthera (240 km to southeast of LDBH). However, Gaulding Quarry is directly exposed to the North Atlantic with a geometry that maximizes runup from long‐period swell, and the extreme flooding that drove sediment transport and produced the washover fan was limited to a relatively narrow area of the northern Eleuthera coastline.…”

“…It should be acknowledged that extreme hydrodynamic events other than those associated with tropical cyclones may also deposit coarse sediment beds in some settings in The Bahamas (e.g., Wilson & Mohrig, 2021), conceivably resulting in an over-counting bias in sediment-based hurricane reconstructions. Wilson and Mohrig (2021) documented that a far-field long-period swell event from an unnamed extratropical cyclone that stalled off the coast of Massachusetts, USA in March 2018 deposited a "sand to boulder" washover fan over a 3,700 m 2 area at Gaulding Cay Quarry of northern Eleuthera (240 km to southeast of LDBH).…”

Signatures of Pleistocene Marine Transgression Preserved in Lithified Coastal Dune Morphology of The Bahamas

“…Wave energy on the Atlantic coasts is also related to the nearshore bathymetry. Narrow shelves (1-2 km) along a deep margin (e.g., 4000 m) [28] allow large (8-10 m), long-period (>10 s) open-ocean winddriven and swell waves to attack the modern coast [29], whereas the shallow, protected The open Atlantic shorelines of the islands experience consistent east-west blowing easterly winds and frequent Atlantic Basin cyclonic storms. Wave energy on the Atlantic coasts is also related to the nearshore bathymetry.…”

“…Wave energy on the Atlantic coasts is also related to the nearshore bathymetry. Narrow shelves (1-2 km) along a deep margin (e.g., 4000 m) [28] allow large (8-10 m), long-period (>10 s) open-ocean winddriven and swell waves to attack the modern coast [29], whereas the shallow, protected banks, are characterized by tidal currents but low wave energy. Therefore, bankward sediment transport dominates due to high-energy conditions along the Atlantic coastlines and quiescent lagoon and tidal flats on the bank side.…”