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

Abstract

Wave‐driven water level variability (and runup at the shoreline) is a significant cause of coastal flooding induced by storms. Wave runup is challenging to predict, particularly along tropical coral reef‐fringed coastlines due to the steep bathymetric profiles and large bottom roughness generated by reef organisms, which can violate assumptions in conventional models applied to open sandy coastlines. To investigate the mechanisms of wave‐driven water level variability on a reef‐fringed coastline, we performed a set of laboratory flume experiments on an alongshore uniform bathymetric profile with and without bottom roughness. Wave setup and waves at frequencies lower than the incident sea‐swell forcing (infragravity waves) were found to be the dominant components of runup. These infragravity waves were positively correlated with offshore wave groups, signifying they were generated in the surf zone by the oscillation of the breakpoint. On the reef flat and at the shoreline, the low‐frequency waves formed a standing wave pattern with energy concentrated at the natural frequencies of the reef flat, indicating resonant amplification. Roughness elements used in the flume to mimic large reef bottom roughness reduced low‐frequency motions on the reef flat and reduced wave runup by 30% on average, compared to the runs over a smooth bed. These results provide insight into sea‐swell and infragravity wave transformation and wave setup dynamics on steep‐sloped coastlines, and the effect that future losses of reef bottom roughness may have on coastal flooding along reef‐fringed coasts.

Relative Magnitude of Infragravity Waves at Coastal Dikes with Shallow Foreshores: A Prediction Tool

“…7(a). These locations of minimum and maximum C ηη ( f ) correspond to nodes and antinodes in a standing wave (Buckley et al 2018;Klopman and van der Meer 1999;Symonds et al 1982). This is further corroborated using Eq.…”

Forcing Factors Affecting Sea Level Changes at the Coast

“…Wave setup is a particular concern for island shorelines that are otherwise protected from wave energy by coral reefs (Vetter et al 2010;Becker et al 2014;Hoeke et al 2015;Buckley et al 2018). At these locations, wave setup can be the largest single non-tidal contributor to extreme water levels (Merrifield et al 2014).…”

Extreme Water Levels for Australian Beaches Using Empirical Equations for Shoreline Wave Setup

“…An approach which could be considered to have separated the contribution of bathymetric depth‐induced breaking and beach swash processes, though not explicitly indicated, is the spectral partitioning analysis of the swash height (Buckley et al, ; Stockdon et al, , ). The bathymetric depth‐induced breaking effect could be largely (but not exactly) attributed to spectral significant height of frequencies lower than the chosen threshold, for example, 0.05 Hz (infragravity waves) and the beach face swash effect to significant height of frequencies higher than that threshold (incident waves) considering the studies such as Guza and Thornton () and Symonds and Bowen ().…”

“…Australia has a wide diversity of coastal beaches, including fringing reef coastlines and rocky platforms, each of which produces a different shoreline wave effect (Buckley et al, ; Merrifield et al, ; Power et al, ). As first step to identifying the contribution of waves to extreme water levels, this paper will focus on estimates for natural sandy beaches directly exposed to open ocean wind waves only.…”