Sedimentological characteristics of the 2015 Tropical Cyclone Pam overwash sediments from Vanuatu, South Pacific

Abstract

The interpretation of sediments deposited by prehistoric tropical cyclones (TC's) is limited by a lack of modern analogues, particularly in the South Pacific. On 13 March 2015, TC Pam made landfall on Vanuatu, reaching Category 5 intensity with 10-minute sustained wind speeds as high as 270km/h. Three months after landfall, we measured flow height (terrain elevation plus storm flow depth) and inland extent of TC Pam's maximum coastal inundation (composed of astronomical tides, storm surge, and superimposed storm waves), and described the sedimentological characteristics of the TC Pam overwash sediments from trenches and transects at two sites (Manuro and Port Resolution Bay).At Manuro (a mixed-carbonate embayment), the maximum flow height was 5.29m mean sea level (MSL), with an inland extent of 106m. The TC Pam sediments transition from a coarse to medium grained (mean: 1.07 Φ) carbonate sand (≤10cm thick) to pumice (≤18cm thick) that extends 400m inland into Lake Otas. The TC Pam overwash sediments are characterized by a coarsening upward sequence (1.45 to 0.23 Φ) followed by a finer grained eolian cap. At Port Resolution Bay (a volcaniclastic beach, PRB), the maximum flow height was 3.30m MSL (1.51m flow depth), with an inland extent of 117m. The TC Pam overwash sediments transition from a medium grained (mean: 1.76 Φ) volcanic sand (≤44cm thick) to pumice (≤5cm thick) that extends 320m from the shoreline. A subtle fining upward sequence was present in trench PRB2, whereas PRB1, PRB3, and PRB4 contained TC Pam sediments that were laminated and showed little to no vertical gradation in grain size. At PRB, we applied an inverse sediment transport model to reconstruct maximum flow depths using laboratory derived settling velocities and the distance from the berm. The reconstructed flow depths at PRB2 (1.43m), PRB3 (1.36m), and PRB4 (1.34m) compare favorably with the observed estimate (1.51m), illustrating the applicability of the inverse sediment transport model to reconstruct flow depths of prehistoric landfalling TC's.