Hanauma Bay Nature Preserve is a shallow bay (<30 m depth) on the island of O'ahu, Hawai'i, offshore of which tidal flow over deep ridge topography (500-1000 m depth) is known to generate semidiurnal frequency internal tides. A field experiment was conducted during March to June 2009 to determine whether the deep internal tides propagate shoreward to influence variability in temperature and currents in the bay environment. Temperature observations in the bay exhibit a diurnal cycle that is strongest near the surface (upper 10 m) and is associated with solar heating. In early summer (May-June), as the upper mixed layer warms and a shallow seasonal thermocline develops, temperature fluctuations in deeper bay waters (>15 m depth) become dominated by large semidiurnal variations (up to 2.7 °C) that are attributed to the internal tide. These temperature drops caused by the internal tide occur consistently twice a day under summer stratification at
This paper describes the modeling work by Makai Ocean Engineering, Inc. to simulate the biochemical effects of of the nutrient-enhanced seawater plumes that are discharged by one or several 100 megawatt OTEC plants. The modeling is needed to properly design OTEC plants that can operate sustainably with acceptably low biological impact.Ocean Thermal Energy Conversion (OTEC) uses large flows of warm tropical seawater and cold deep seawater to generate non-polluting electric power. The magnitude of the global OTEC resource dwarfs that of other other marine renewable energy technologies, and OTEC power is non-intermittent, making it suitable for utilities and manufacturing. Small demonstration OTEC plants using commercially-available equipment have generated 50 -270 kilowatts of electricity. Recent advances in offshore design and cold water pipe technologies have renewed interest in developing large 100 megawatt plants that would be cost-competive with local island utilities. Such plants would have several seawater pumps, each equivalent to tugboat engines, that would guide 750 tonnes per second of seawater thorugh the OTEC system.At most potential OTEC sites, the tropical ocean is thermally stratified into a well-mixed warm upper layer overlying cooler and denser seawater. This stratification hinders the supply of nutrients upwelled into the photic zone, which results in a nutrient-limited "oligotrophic" phytoplankton community having low biological productivity. Discharging deep seawater nutrients (primarily nitrates) into the upper waters from the OTEC plant could potentially enhance phytoplankton growth, shift community species composition, or cause algal blooms. It is desirable to discharge the seawater flows deep enough so that the discharged nutrients are diluted and remain below the photic zone. Thus, the size and depth of the large seawater ducts affect both the overall architecture of an OTEC plant as well as the extent of the perturbation to the ambient phytoplankton populations.In order to quantify the effect of discharge configuration and phytoplankton response, Makai Ocean Engineering implemented a biological and physical model for the waters surrounding O`ahu, Hawai`i, using the EPA-approved Environmental Fluid Dynamics Code (EFDC). Each EFDC grid cell was approximately 1 square kilometer by 20 meters deep, and used a time step of three hours. The biological model was set up to simulate the biochemical response for three classes of organisms: Picoplankton (< 2 um) such as prochlorococccus, nanoplankton (2-20 um), and microplankton (> 20 um) e.g., diatoms. The dynamic biological phytoplankton model was calibrated using chemical and biological data collected for the Hawaii Ocean Time Series (HOTS) project. Peer review of the biological modeling was performed by Dr. John Hamrick, the author of EFDC, and by Dr. Matt Church of the University of Hawai'i, a leading marine microbiologist.The physical oceanography model uses boundary conditions from a surrounding Hawai'i Regional Ocean Model, (ROM) operated by...
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