A Bayesian‐Based System to Assess Wave‐Driven Flooding Hazards on Coral Reef‐Lined Coasts

Abstract

Many low‐elevation, coral reef‐lined, tropical coasts are vulnerable to the effects of climate change, sea level rise, and wave‐induced flooding. The considerable morphological diversity of these coasts and the variability of the hydrodynamic forcing that they are exposed to make predicting wave‐induced flooding a challenge. A process‐based wave‐resolving hydrodynamic model (XBeach Non‐Hydrostatic, “XBNH”) was used to create a large synthetic database for use in a “Bayesian Estimator for Wave Attack in Reef Environments” (BEWARE), relating incident hydrodynamics and coral reef geomorphology to coastal flooding hazards on reef‐lined coasts. Building on previous work, BEWARE improves system understanding of reef hydrodynamics by examining the intrinsic reef and extrinsic forcing factors controlling runup and flooding on reef‐lined coasts. The Bayesian estimator has high predictive skill for the XBNH model outputs that are flooding indicators, and was validated for a number of available field cases. It was found that, in order to accurately predict flooding hazards, water depth over the reef flat, incident wave conditions, and reef flat width are the most essential factors, whereas other factors such as beach slope and bed friction due to the presence or absence of corals are less important. BEWARE is a potentially powerful tool for use in early warning systems or risk assessment studies, and can be used to make projections about how wave‐induced flooding on coral reef‐lined coasts may change due to climate change.

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

“…The exception was the rough run with the lowest wave height ( H rms , SS ,0 = 0.03 m), which demonstrated a higher contribution from setup due to the very weak swash motions (see swash spectra in Figure b). These observations are consistent with the numerical simulations of Pearson et al ().…”

“…The relative strength of the amplification, inferred from the normalized swash spectral density, of mode 0 for smooth runs appears to have been constant with changes to offshore SS wave height, but increased with both SS peak period and still water depth over the reef flat (Figure ). Previous studies of resonance on fringing reefs have also found that resonant responses can increase with increasing water depth and with lower frequency incident SS waves (Gawehn et al, ; Pearson et al, ; Péquignet et al, ), although in general the natural frequencies and amplification are specific to the reef geometry. The increased resonant amplification was likely due to reduced frictional wave dissipation (Péquignet et al, ) and the forcing (the incident wave groups) becoming closer in frequency to the natural frequencies, which increased with increased water depth.…”

“…The separation frequency between IG H and IG L was specifically chosen so that the IG L frequency band incorporated the dominant (first two) natural frequencies of the low‐frequency motions associated with the reef morphology used in this study (see section 2.3). We note that some previous studies of fringing reefs, have chosen lower frequencies (typically 0.005 Hz) to divide the lower frequency portions of the wave spectra and have termed the frequency band either “far infragravity” (Péquignet et al, ) or “very low frequency” (Gawehn et al, ; Pearson et al, ); we have not adopted either of these terms due to our choice of a higher separation frequency, which is more appropriate in the present study as it separates the specific physical processes responsible for these low‐frequency motions.…”

Impact of Coral Reef Mining Pits on Nearshore Hydrodynamics and Wave Runup During Extreme Wave Events

“…The offshore bottom elevation is kept at a constant level throughout this study ( z 0 = − 30 m; k peak h < 1.4) and extended in offshore direction in order to let the model adjust to the boundary conditions. The beach extends from the toe upward to a fictitious elevation of z b = 30 m for all simulations, similar to Pearson et al (), in order to focus on runup as a proxy for overtopping (Matias et al, ). Based on Quataert et al (), a distinction is made between the hydrodynamic roughness of the fore reef ( c f = 0.4) and reef flat ( c f = 0.01) due to characteristic differences in coral reef structure (live/branching coral at the fore reef and deposited/cemented coral sands and fragments on the reef flat and beach).…”

“…A number of parameters were varied to create a large set of artificial reefs with different geometries (Table ), following the approach of Pearson et al (), Pomeroy, van Dongeren, et al (), and Quataert et al (). The presented parameters were selected based on a sensitivity analysis that assessed the impact of each parameter on reef hydrodynamics and on the effects of excavation pits (Klaver, ), as well as on the results of previous research on fringing reef hydrodynamics and wave runup (Pearson et al, ; Quataert et al, ). Wave runup and subsequent flooding of reef‐lined coasts is influenced most by hydrodynamic forcing, fore reef slope, reef width, reef submergence, and beach slope (Pearson et al, ; Quataert et al, ; Shimozono et al, ).…”