Presenter: Mark Buckley (USGS Coastal and Marine Science Center)

Description:
Predictions of wave runup are necessary for early warning systems and long-term coastal planning. Both the swash and wave setup contributions of the total wave runup are dependent on how offshore wave and water level conditions interact with coastal morphologies. Yet, the complexities of surf zone and swash dynamics make accurate predictions difficult. This is particularly true for coral reef-lined coasts that may have steep, irregular bathymetry and large bottom roughness. Here we compare nearshore observations from two recent field experiments conducted at sites with contrasting coral reefs. The Molokaʻi, Hawai’i, study site is generally alongshore uniform with the reef crest separated from the shoreline by a 700 m-wide reef flat. In contrast, the Ningaloo, Western Australia, study site has a 2 km-wide reef flat-lagoon system and is bound laterally by deep (~8 m) channels. These contrasting bathymetries drive differences in circulation that ultimately result in differences in wave runup. The deep lateral channels at Ningaloo allow for wave-driven on-shore depth-integrated mass flux over the reef crest to be returned offshore through the channels. Whereas for Molokaʻi, its more alongshore uniform bathymetry limits net mass flux across the reef. Field observations and a phase-resolving wave-flow model (SWASH) are used to quantify the mean momentum balances governing wave setup at the two contrasting sites. Time-averaged bottom stresses generated by the combined wave and current velocities over the reef flat at Ningaloo result in a shoreward taper of the wave setup profile. In contrast, at Molokaʻi the cross-shore wave setup profile across the reef flat is more constant. As a result, the wave setup contribution to the total wave runup in the lee of Ningaloo reef is significantly less than if it had a more along-shore uniform bathymetry like Molokaʻi. We further explore the effect of reef channels using SWASH to model variations in net mass flux across the reefs.