Connecting roughness and hydrodynamics of tropical reefs

Coral reefs are characterized by highly rugose, complex bed geometries, with fluctuations in bed elevation occurring over a wide range of horizontal scales. The interaction of waves and currents with this roughness results in high energy dissipation and in strong vertical mixing with important implications for reef ecology. Here we present analysis of observations of current boundary layers along with AUV-based roughness measurements from several tropical forereef environments. Roughness, quantified using spectral analysis, is highly variable, but is generally characterized by a broad-banded spectral distribution over the range of length scales that influence wave and current boundary layers. At all sites, steady bed stress depends strongly on wave forcing. Wave-current boundary layer model predictions correlate well with observations using measured RMS roughness values, but significantly underestimate bed stress. The physical roughness required to match the observations increases with wave orbital amplitude indicating that relevant hydrodynamic scales vary with wave conditions, consistent with the observed red spectral roughness distribution.