Water level effects on breaking wave setup for Pacific Island fringing reefs

Abstract

The effects of water level variations on breaking wave setup over fringing reefs are assessed using field measurements obtained at three study sites in the Republic of the Marshall Islands and the Mariana Islands in the western tropical Pacific Ocean. At each site, reef flat setup varies over the tidal range with weaker setup at high tide and stronger setup at low tide for a given incident wave height. The observed water level dependence is interpreted in the context of radiation stress gradients specified by an idealized point break model generalized for nonnormally incident waves. The tidally varying setup is due in part to depth-limited wave heights on the reef flat, as anticipated from previous reef studies, but also to tidally dependent breaking on the reef face. The tidal dependence of the breaking is interpreted in the context of the point break model in terms of a tidally varying wave height to water depth ratio at breaking. Implications for predictions of wave-driven setup at reef-fringed island shorelines are discussed.

Wind and Wave Setup Contributions to Extreme Sea Levels at a Tropical High Island: A Stochastic Cyclone Simulation Study for Apia, Samoa

“…This is primarily due to decreased wave dissipation on outer reefs and is consistent with a number of other studies [32,44]. Seasonal and interannual sea-level variability, which is on the order of 30 cm at many Pacific islands, most likely similarly affects momentum balances in many locations.…”

“…At such locations, the results presented here closely approximate a number of analytic/empirical solutions of wave setup which assume straight and parallel alongshore topography and wave conditions, e.g., [8,43,44]. In particular, water levels within several hundred meters shoreward of the reef crest fronting the Mulinu'u Peninsula (e.g., the "reef" location) simulated by the wave forcing ensemble are within approximately 10 centimetres of the solution described by Becker et al [44], which is plotted for comparison with model output in Figure 9. Closer to shore, or at other locations without a neighbouring shore parallel reef crest of some length, these analytic methods greatly overestimate wave setup.…”

“…Ensemble (wave forcing, baseline, and SLR scenarios) significant wave heights (Hs, red) and water levels (WL, blue), along a transect through the "reef" location (shown in Figure 5); offshore is on the left side, landward direction towards the right; topography (Z) is indicated with a black line. The analytic estimate of wave setup described by Vetter et al [8] and Becker et al [44], based on Hs from the Apia model, is plotted with black circles, with the caveat that breaking parameter λb is set to 1.0 (consistent with the Apia model), rather than varying with water level.…”

“…These local changes in momentum balances will result in changed patterns of inundation and sediment transport, particularly under progressively changing sea level. A problem with the assumption of static (constant) topography used in this and past studies is that it is probably unrealistic; however assuming alternate trajectories is far from straightforward as future changes in reef calcification rates remain unclear [44,45] and synergistic effects between water quality and sea level are likely, e.g., [46]. Changes in these processes may lead to large changes in reef morphology, complexity, and roughness, which would then also lead to large local changes in momentum balances.…”

Investigating the evolution and formation of coastlines and the response to sea-level rise

“…Transformation of waves over the reef flat is characterized by increased wave energy dissipation Monismith et al, 2013) due to increased bottom friction and wave breaking at the edge of the reef flat (Péquignet et al, 2011;Lugo-Fernández et al, 1998;Becker et al, 2014). Bottom friction factors are found to be at least an order of magnitude greater than for sandy bottoms, but with significant variability (Quataert et al, 2015;LugoFernández et al, 1998).…”