Evaluation of current and wave measurements from a coastal Buoy

The Chesapeake Bay Interpretive Buoy System (CBIBS) collects real-time measurements of currents and directional waves from a series of small buoys designed for coastal regions. The buoys use an AXYS Technologies TRIAXYS directional wave sensor and a downward looking 1 MHz Nortek Aquadopp Profiler for current profiles. In this experiment, a bottom mounted Nortek acoustic wave and current (AWAC) profiler was deployed next to one buoy in 7 m water for 1 month (April 2008) to collect reference current and wave data to evaluate the performance of the buoy wave gauge in the small, short waves common to the region, and to understand how buoy motion affects the fidelity of the current measurements. Over 75% of the total current velocity energy was well described by a regional tidal model. The buoy-mounted current profiler compared well with the bottom-mounted current profiler in both speed and direction at all depth cells. For all currents greater than 0.1 m/s, the mean of the absolute value of the difference in current magnitude was less than 0.01 m/s. The mean of the absolute value of difference in current direction was 5deg. An analysis of average intervals demonstrated that an average interval of 2 minutes achieved 96% of the best possible measurement compared to a 10 minute average. Significant wave height ranged from 0.1-0.7 m and peak periods varied from 2-4 s. The buoy mounted wave gauge reported wave period accurately, but may under-report significant wave height. Wave direction between the buoy and AWAC agreed well for all waves greater than about 0.25 m.