Observations of wave transformation over a fringing coral reef and the importance of low‐frequency waves and offshore water levels to runup, overwash, and coastal flooding

Abstract

Many low‐lying tropical islands are susceptible to sea level rise and often subjected to overwash and flooding during large wave events. To quantify wave dynamics and wave‐driven water levels on fringing coral reefs, a 5 month deployment of wave gauges and a current meter was conducted across two shore‐normal transects on Roi‐Namur Island in the Republic of the Marshall Islands. These observations captured two large wave events that had waves with maximum heights greater than 6 m with peak periods of 16 s over the fore reef. The larger event coincided with a peak spring tide, leading to energetic, highly skewed infragravity (0.04–0.004 Hz) and very low frequency (0.004–0.001 Hz) waves at the shoreline, which reached heights of 1.0 and 0.7 m, respectively. Water surface elevations, combined with wave runup, reached 3.7 m above the reef bed at the innermost reef flat adjacent to the toe of the beach, resulting in flooding of inland areas. This overwash occurred during a 3 h time window that coincided with high tide and maximum low‐frequency reef flat wave heights. The relatively low‐relief characteristics of this narrow reef flat may further drive shoreline amplification of low‐frequency waves due to resonance modes. These results (1) demonstrate how the coupling of high offshore water levels with low‐frequency reef flat wave energetics can lead to large impacts along fringing reef‐lined shorelines, such as island overwash, and (2) lend support to the hypothesis that predicted higher sea levels will lead to more frequent occurrences of these extreme events, negatively impacting coastal resources and infrastructure.

Mechanisms of Wave‐Driven Water Level Variability on Reef‐Fringed Coastlines

“…The generation of waves at the natural frequencies of the reef flat are of particular interest for assessing coastal hazards along reef‐fringed coastlines, as their resonant amplification can potentially enhance runup and thus coastal flooding (Becker et al, ; Cheriton et al, ; Gawehn et al, ; Nakaza & Hino, ; Nwogu & Demirbilek, ; Roeber & Bricker, ). Natural modes of the reef flat occur at specific frequencies where standing wave nodes are located at the reef crest; this phase relationship allows for waves trapped on the reef flat to be in phase and thus amplified by incoming waves of the same frequency propagating on to the reef flat from offshore.…”

“…Coral reefs are often viewed as natural buffers to coastal storms by causing dissipation of SS waves through depth‐limited breaking and bottom friction (Cheriton et al, ; Ferrario et al, ; Quataert et al, ; Storlazzi et al, ). However, there have been few observations to quantify runup on reef‐fringed coastlines, and although reef morphologies clearly reduce the height of SS waves reaching a shoreline, there is evidence to suggest low‐frequency wave motions (Becker et al, ; Beetham et al, ; Merrifield et al, ; Nakaza & Hino, ; Nwogu & Demirbilek, ; Péquignet et al, ; Roeber & Bricker, ) and wave setup (Becker et al, ; Buckley et al, ; Quataert et al, ) can be enhanced in steeper reef environments relative to milder slope open coastlines.…”

Assessing storm surge hazard and impact of sea level rise in the Lesser Antilles case study of Martinique

“…This can be the case for fringing coral reefs, for example, where the water level can be dominated by large lowfrequency (e.g., infragravity) waves. Indeed, during extreme events, the spectral wave energy at reef crests shifts into lower frequencies, which can be amplified due to resonance modes (e.g., Roberts et al, 1992;Péquinet et al, 2009;Cheriton et al, 2016). Even if (to our knowledge) large infragravity waves were not reported in Martinique, we see no reason to rule them out.…”

Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

“…Consequently, concurrent high wave events and spring tides leading to island overwash and seawater flooding occur with little warning [48]. Although the overwash event recorded in this study caused conductivity (salinity) increases that recovered after approximately one month (Figure 8) in affected groundwater wells, more severe overwash events have the potential to be catastrophic, leading to FWL recovery times of 22-26 months [11][12][13].…”

“…The oceanographic forcing that lead to this event has been documented in detail by Quataert et al [27] and Cheriton et al [48]. The large wave event, which had almost 7-m high waves with 15-s periods, coincided with a spring high tide and caused ocean water surface elevations, combined with wave run-up, to be 3.7 m above the reef flat, resulting in minor seawater flooding of select low-lying inland areas (Figure 7).…”