Current and wave measurements in support of the Chesapeake Bay Interpretive Buoy System

Abstract

The Chesapeake Bay Interpretive Buoys System (CBIBS, www.buoybay.org) is - at present - a 9-buoy system of observational buoy platforms located around the Chesapeake Bay, operated bythe NOAA Chesapeake Bay Office. The buoys themselves are AXYS Watchkeeper buoys based on the Tideland Signal SB 138 P hull, moored with all chain rode with a 2.5∶1 scope. All of the buoys have downward looking NORTEK AquaDopp 1 mHz profilers mounted in the hulls; six of the buoys are equipped with AXYS TriAXYS OEM wave measurement modules. To evaluate the performance of these buoy mounted sensors, concurrent wave and current profile data were collected at one site (34 days of data at the SN ‘Six Foot Knoll’ buoy, 21 foot depth) using an adjacent bottom-mounted 1 mHz NORTEK Acoustic Wave and Current (AWAC) instrument. Using one Hz overlapping single ping current profile data, the following conclusions were reached: • Ten minute averages calculated and transmitted by the AXYS Watchman controller accurately represent internally recorded current profiler data (however the comparison revealed a correctable Watchman firmware error); • Over four 1-meter bins, absolute differences in magnitude among 10 minute means of AquaDopp and AWAC currents are less than 0.005 meters/second, with standard deviations of 0.02 – 0.03 meters/second; • Absolute differences in direction over the same bins were 7–10 degrees. By only including velocities in excess of 0.1 meters/second, direction errors were reduced by nearly one half; • AWAC and buoy currents were subjected to harmonic tidal analysis at each level. Differences in the orientation of the major axis of the tidal ellipses were 0.3 to 1.6 degrees; • Current measurement accuracy was not affected by sea state / buoy motion, at least up to the 1.5 meter waves heights experienced during this comparison; • An analysis of the ‘errors’ (assuming AWAC data as the standard) shows that accuracy does not improve after 120 pings (2 minutes) of averaging. In a power-limited environment such as a buoy, this is a significant result. Typical Chesapeake Bay wind waves - one to three feet height, two to three second period - are difficult to measure with a 1300-pound 1.75 meter diameter buoy. But accurately distinguishing between 1 and 2 foot waves is important to many of the small craft boaters using the CBIBS system. In comparing the TriAXYS (20 minute sample) and AWAC (2 Hz, 2048 samples) wave measurements, we found that for simple wave parameters of interest to us - maximum wave height and mean direction - the instruments were in good agreement. A linear fit to maximum wave heights had a slope / intercept of 1.05 / .02 (meters) . Similarly, comparison of mean wave direction from both instruments agreed well, with slope/intercept of 1.01 / –5.1 (degrees).