Physical and numerical modeling of random wave transformation and overtopping on reef topography

Widespread flooding of reef-fringed islands demonstrates the insufficient protection provided by reef topographies and suggests the necessity of artificial structures for hazard prevention and mitigation. As an indicator of the structural protection efficiency, the wave overtopping rate is difficult to estimate in reef environments because hydrodynamics over reefs violate the application conditions of standard overtopping formulas. A combined physical and numerical modeling was performed to investigate wave transformations over reefs of different topographies and overtopping at vertical seawalls on the reefs. Results showed that, in addition to the off-reef wave height and still water level, the off-reef wave period also played significant roles. The intensity of short-wave motions and the level of wave setup near the seawall were positively correlated with the off-reef wave period, and the intensity of infragravity-wave motions depended largely on it. Steeper fore-reef slopes would lead to more intensive wave motions and higher setup on reef flats and, consequently, higher overtopping rates. Furthermore, the overtopping rate exponentially decreased with the distance between the seawall and reef edge, primarily due to the decay of short waves in the surf zone.