Wave dissipation due to spatially varying bottom roughness: Observations from Oahu, Hawaii

The influence of bottom friction on waves shoaling over reef topography prior to breaking is examined using across-shore array of wave sensors on the Leeward side of Oahu during the winter of 2014. Offshore wave heights ranged from 2 to 5 m during a succession of northwest swell events. Wave energy flux is approximately conserved between an offshore wave buoy and the deepest wave sensor at 15 m depth. Between 15 m and 6 m depths, where rough reef bathymetry replaces sand, significant dissipation is inferred with a ~60% reduction in the wave energy flux. Equating the energy flux divergence to bottom friction dissipation results in a wave friction factor of 0.38 ± 0.03, higher than typical wave factors reported in previous reef field studies. The estimated friction factor between 6 m and 5 m reduces to 0.14 ± 0.01. AUV surveys of the study site provide high resolution depth measurements that are used to relate bottom roughness to the variable friction factor estimated across the transect. A numerical model is used to assess the influence of the high friction factors on simulations over two-dimensional bathymetry. The results highlight the need for spatially dependent bottom roughness estimates for accurate wave predictions in reef environments.