Wave- and tidally-driven flow and sediment flux across a fringing coral reef: Southern Molokai, Hawaii

Abstract

The fringing coral reef off the south coast of Molokai, Hawaii is currently being studied as part of a US Geological Survey (USGS) multi-disciplinary project that focuses on geologic and oceanographic processes that affect coral reef systems. For this investigation, four instrument packages were deployed across the fringing coral reef during the summer of 2001 to understand the processes governing fine-grained terrestrial sediment suspension on the shallow reef flat (h ¼ 1 m) and its advection across the reef crest and onto the deeper fore reef. The time-series measurements suggest the following conceptual model of water and fine-grained sediment transport across the reef: Relatively cool, clear water flows up onto the reef flat during flooding tides. At high tide, more deep-water wave energy is able to propagate onto the reef flat and larger Trade wind-driven waves can develop on the reef flat, thereby increasing sediment suspension. Trade wind-driven surface currents and wave breaking at the reef crest cause setup of water on the reef flat, further increasing the water depth and enhancing the development of depth-limited waves and sediment suspension. As the tide ebbs, the water and associated suspended sediment on the reef flat drains off the reef flat and is advected offshore and to the west by Trade wind-and tidally-driven currents. Observations on the fore reef show relatively high turbidity throughout the water column during the ebb tide. It therefore appears that high suspended sediment concentrations on the deeper fore reef, where active coral growth is at a maximum, are dynamically linked to processes on the muddy, shallow reef flat. Published by Elsevier Ltd.

Effects of two sediment types on the fluorescence yield of two Hawaiian scleractinian corals

“…Beach and nearshore seafloor sediment in Hawaii is predominantly marine carbonates, while the small size fractions (silts and clays) are often terrigenous in origin (Harney et al, 2000;Calhoun and Field, 2002;Storlazzi et al, 2004;Bothner et al, 2006). Terrestrial sediment is likely to be the most dangerous to reef corals, since sediment runoff from urban and agricultural areas often contains harmful organic compounds, heavy metals, or nutrients (Glynn et al, 1984;Glynn et al, 1989;Bastidas et al, 1999;Jokiel et al, 2004; see also review by Fabricius, 2005).…”

The large-scale influence of the Great Barrier Reef matrix on wave attenuation

“…On the GBR, waves can propagate through the reef matrix without completely dissipating, leading to transmission of significant amounts of wave energy (Hardy and Young 1996). This is unlike mainland beaches and fringing reef-lagoon systems (Hardy and Young 1996), where waves generally terminate and energy is dissipated or transformed into changes in water level and currents (e.g., Lugo-Fernández et al 1998;Storlazzi et al 2004;Lowe et al 2009;Gallop et al 2012). Little is known about how the porosity of reef matrices influences wave attenuation.…”

Numerical modeling of the impact of sea-level rise on fringing coral reef hydrodynamics and sediment transport

“…An underlying assumption in this approach is the dominance of wind-and wave-driven processes. This assumption is justified based on the small tidal velocities observed in the study area Storlazzi et al 2004;Presto et al 2006). Model validation therefore was focused on accurate representation of the wave-breaking processes.…”

“…The in situ observations used for model calibration and validation were presented by Storlazzi et al (2004) and Presto et al (2006) and are summarized here. Current velocity data were collected via acoustic Doppler current profilers, acoustic Doppler velocimeters, or single-point electromagnetic current meters.…”

“…Due to shadowing by the surrounding islands and the island of Molokai itself, however, most of south Molokai's fringing reef is sheltered from large North Pacific swell but is exposed to the other wave climates. Hawaii has a mixed, semidiurnal microtidal regime, with the mean daily tidal range of approximately 0.6 m and the minimum and maximum daily tidal ranges are 0.4 and 0.9 m, respectively Storlazzi et al 2004).…”