Distant-Source Swells Cause Coastal Inundation on Fiji’s Coral Coast

Abstract

Distant-source swells are known to regularly inundate low-lying Pacific Island communities. Here we examine extreme total water level (TWL) and inundation driven by a distant-source swell on Fiji's Coral Coast using observations and a phase-resolving wave model (XBeach). The objective of this study is to increase understanding of swell-driven hazards in fringing reef environments to identify the contribution of wave setup and infragravity waves to extreme TWL and to investigate coastal flooding during present and future sea levels. The maximum TWL near the shore was caused by compounding mechanisms, where tides, wave setup, infragravity waves, and waves in the sea swell frequencies contributed to the TWL. Waves and wave setup on the reef were modulated by offshore wave heights and tides with increased setup during low tide and increased wave heights during high tide. Numerical simulations were able to reproduce the mechanisms contributing to the extreme TWL and allowed an estimation of the inundation extent. Simulations of the same swell under the RCP8.5 sea-level rise scenario suggest the area of inundation would increase by 97% by 2100. A comparison between the numerical model, a multiple linear regression model, and two commonly used parametric models reveals that both XBeach and the linear regression model are better suited to reproduce the nearshore wave setup and TWL than the empirical equations. The results highlight the need for customized, site-specific coastal hazard assessments and inundation forecast systems in the South Pacific.

Severe Flooding in the Atoll Nations of Tuvalu and Kiribati Triggered by a Distant Tropical Cyclone Pam

“…In this study, we link a widespread series flooding, island overwash and erosion events in 2015 in the atoll nations of Tuvalu and Kiribati to locally extreme values of total water level (TWL), composed of regional sea-level, astronomic tide and wind-wave components. Similar to other studies (e.g., Hoeke et al, 2013;Wadey et al, 2017;Ford et al, 2018;Wandres et al, 2020), extreme wind-waves associated with a distant storm (swell) was the proximate cause, or trigger, of the flooding and erosion reported here. In contrast to (or perhaps complimenting) these other studies, the swell triggering this event was generated not by distant mid-latitude storms, but by a distant tropical cyclone (TC Pam).…”

“…It thus neglects any approximations of local wave transformations required to predict sub-island scale extreme water levels and subsequent flooding. A number of researchers are developing meta-modeling methods (e.g., Pearson et al, 2017;Beetham and Kench, 2018;Rueda et al, 2019) capable of including local details of reef morphology; a number of statistical/analytic methods based purely on a limited number of nearshore observations (Merrifield et al, 2014;Wandres et al, 2020) are also under development. Both of these approaches show promise for much more accurate estimation of local wave transformations over reefs (as relevant to island flooding) compared to the extremely simple approach taken in this study.…”

“…In this study, we investigate extensive flooding and erosion impacts which occurred in Tuvalu and the Gilbert Islands of Kiribati during March 2015. Unlike the case studies of Hoeke et al (2013), Wadey et al (2017), Ford et al (2018, and Wandres et al (2020), all of which focused at least primarily on events triggered by distance-source swell waves generated by midlatitude (temperate) cyclones in the western Pacific (primarily the Federated States of Micronesia, Papua New Guinea, and Solomon Islands), the Maldives, Majuro Atoll (Marshall Islands) and Fiji, respectively, the events in Tuvalu and Kiribati described here are associated with the passage of tropical cyclone (TC) Pam. TC Pam formed in the central south Pacific in early March 2015.…”

The Influence of Reef Topography on Storm-Driven Sand Flux

“…There is, however, no practical method to evaluate and map the values of c f and f w apart from model calibration field data. Swart [56] proposed a formulation to calculate f w based on the size of roughness elements but mapping the roughness of reef environment is still a developing research topic [38,57,58].…”

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

“…High‐energy cyclones can affect coastlines located hundreds to thousands of kilometres from a storm’s eye through the creation and propagation of long‐period swell waves (e.g. Munk et␣al ., 1963; Snodgrass et␣al ., 1966; Wandres et␣al ., 2020). The work presented here characterizes both the overwash and washover fan that formed 1800 km from the eye of an extratropical cyclone with hurricane‐force (Category 1) winds.…”