Two-dimensional modelling of wave dynamics and wave forces on fringing coral reefs

Abstract

Two-dimensional numerical modelling of swell wave dynamics on idealized fringing reefs is performed using SWAN, covering a wide range of bathymetries, climate forcing conditions and water depths over the reefs. The results illustrate the impact of reef geometry and bathymetry, coral species and sea level rise on key hydrodynamic parameters on the reef and on forces on corals. The modelling demonstrates that one-dimensional models underestimate the wave action on the reef flat. Wide short reefs and narrow long reefs have similar wave heights at the centre of the reef flat. For a given reef length, the wave height first decreases with increasing reef width, then increases to a local maximum when reef width is approximately equal to the reef length, and then decreases for further increases in width. This pattern is a result of combined dissipation and refraction processes, which combine to lead to different zones of cross-reef wave transformation. Provided that a reef retains its hydrodynamic functions in breaking and refracting the waves, sea level rise enhances the wave heights and wave orbital velocities on the reef flat. If vertical coral growth does not keep pace with sea level rise, loss of the hydrodynamic functions of the reef may occur on deeper reefs, and result in a reduction of near bed velocities with sea level rise. Hydrodynamic forces on corals vary by coral species and SLR changes the magnitude of the forces on different species in different ways, which may lead to less favourable conditions for certain coral species. For long period swell, the intermediate size corals are drag-dominated and behave similarly to branching corals, whereas for short period swell their behavior is similar to that of the inertia-dominated massive corals. For intermediate corals different responses to SLR may therefore be expected for different overall regional wave climates. Over time, this process may contribute to changes in the structural complexity of reefs. The influence of sea level rise on the forces on corals on the reef flat is different under swell and cyclonic wind conditions since wind influences wave period in the latter case.