Flow and Sediment Transport on a Shallow Reef Flat: South-central Molokai, Hawaii

Two instrumented tripods were deployed off south-central Molokai to examine the physical processes and suspended sediment transport on the shallow (h = 0.5-1.5 m) reef flat. This study was conducted as part of the U.S. Geological Survey's Coral Reef Project that focuses on the health and sustainability of coral reef systems. The tripods carried a suite of sensors to measure waves, currents, suspended sediment, temperature, and salinity. The instruments were deployed in a shore-parallel line half way between the shoreline and the reef crest from January - March, 2001; both collected 8.5 minute data bursts at 2 Hz every hour. The period of deployment encompassed four complete spring-neap tidal cycles and times of both large North Pacific winter storm swell and trade wind-driven waves. Mean tidal fluctuations were approximately 0.4 m while the maximum tidal range observed was almost 0.7 m. Across-shore and alongshore flows related to the semi-diurnal tides were typically on the order of 0.02-0.15 m/sec. During the deployment, strong trade winds blew towards the west at 10-12 m/sec most every afternoon, generating up to 0.30 m waves on the reef flat. Near-bed wave-orbital wave velocities (rms) ranged between 0.02 and 0.13 m/sec and are strongly modulated by tidal fluctuations at the tripods' locations. During peak trade winds, near-bed currents and suspended sediment transport were oriented predominantly offshore. This data suggests increased suspended sediment concentrations occur during periods of high tidal elevation when the strong trade winds can generate larger depth-limited waves and stronger onshore wind-driven surface currents. The greater water depth during high tides also allows longer-period deep-water wave energy to propagate over the reef crest and onto the reef flat. These mechanisms force water onto the inner portion of the reef and cause strong offshore-directed near-bed currents that transport fine sediment suspended by the waves. These mechanisms likely contribute to the offshore flux of fine-grained terrestrial sediment resuspended from the inner reef that is hypothesized to be causing harm to corals offshore.