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USGS Steers Science at Massachusetts Maritime AcademyMeasuring Tidal Flows in the Cape Cod Canal
A new pilot study in Buzzards Bay, Massachusetts, is the first step toward better understanding of the bay's water quality, ecosystems, and potential for tidal power. As part of the study, the U.S. Geological Survey (USGS) Woods Hole Coastal and Marine Science Center deployed a sideward-looking acoustic Doppler current profiler (ADCP) to measure water currents and tidal flows in the Cape Cod Canal. The Massachusetts Maritime Academy (MMA), located at the southwest end of the canal, is ideally situated for measuring the canal's influence on upper Buzzards Bay. While deploying such an instrument isn't anything new, the location of the instrument definitely raised some eyebrows.
During the first reconnaissance trip to the MMA in October 2009, MMA's director of Marine Operations Billy Klimm noted that the hulking steel rudder of the 550-ft Training Ship (TS) Kennedy was aimed directly perpendicular to the Cape Cod Canal, an optimal orientation for mounting the ADCP. Tied securely to a dock on the MMA campus, the ship would remain in place until a training cruise in January 2010—plenty of time for a 4-week deployment of the ADCP. Once the Massachusetts Department of Transportation and the ship's captain approved a mount design, USGS divers Chuck Worley and Mike Casso installed the ADCP 3 m below the water surface using a low-impact wood frame designed by Jon Borden, Worley, and Bob Barton. Borden also deployed a conductivity, temperature, and depth (CTD) sensor on the seabed near the vessel.
The measurements will address two goals for the USGS and the MMA. Firstly, they will support the USGS development, recently begun, of a 3D numerical model of Buzzards Bay, using the USGS-sponsored Community Sediment-Transport Model. The modeling will support water-quality and ecological studies in Buzzards Bay and West Falmouth Harbor, in collaboration with the USGS Geochemistry group, Cornell University, and the Marine Biological Laboratory. Modeling Buzzards Bay requires water-velocity data at both the seaward end and the canal end of the bay. The seaward data can be extracted from larger, regional-scale models, but there is little information on the water velocity through the Cape Cod Canal. Secondly, the MMA is currently planning a pilot tidal-energy study adjacent to the TS Kennedy. Velocity measurements will aid the MMA in estimating the power available from tidal currents at different locations in the canal.
To measure current velocities, the ADCP relies on the Doppler effect: sound emitted by a source moving toward you reaches your ear at a higher frequency than sound emitted by a source moving away from you. For example, a train horn has a higher pitch (higher frequency) as the train approaches and a lower pitch as it recedes. Transducers on the ADCP emit pulses of sound energy at a known frequency and then receive the sound energy that bounces back to the instrument after hitting particles suspended in the water. Sound bounced back by particles moving toward the ADCP will arrive with a higher frequency than the original sound pulses, whereas sound bounced back by particles moving away from the ADCP will have a lower frequency. The instrument uses these frequency shifts to calculate how fast the particles and the water around them are moving. To learn more about ADCPs, visit the Woods Hole Oceanographic Institution's Acoustic Doppler Current Profiler (ADCP) Web page.
An ADCP mounted on the seafloor, sometimes called an upward-looking ADCP, can measure current velocities over a range of depths, from the bottom all the way up to the surface, thus creating a vertical "velocity profile" of currents at a particular site. Sideward-looking ADCPs measure water velocities along a horizontal line at a fixed depth; they can create a horizontal velocity profile across an entire channel, for example. Sideward-looking ADCPs are typically used to aid in port management and navigation, as well as to develop flow-rating curves—graphs of the mathematical relationship between water level and flow—in open channels. The sideward-looking ADCP used in this study was deployed at the entrance to West Falmouth Harbor earlier this year as part of the pilot study, and a continuous record of tidal water flow over a 2-month period was generated using the data.
The Cape Cod Canal presents a very different challenge from West Falmouth Harbor, owing to the width of the canal (approximately 200 m) and large turbulent eddies that form during flood and ebb tides. Ebb tide carries water from Cape Cod Bay through the canal to Buzzards Bay. Cape Cod Bay is typically cooler than Buzzards Bay. Therefore the rate of tidal flow affects the temperature field in northern Buzzards Bay. Variations in habitat quality (for example, the amount of dissolved oxygen) might be linked to variations in water temperature, and so accounting for canal water in the numerical model will be important.
Perhaps the biggest challenge in deploying the ADCP was mounting the unit with minimal tilt. Because the instrument has only two beams (that is, it emits sound pulses and receives their echoes in just two directions), it can only resolve currents that are aligned with the transducer. Open channels typically have the largest velocities in the horizontal, along-channel direction, so the transducer must be aligned in the same direction. Any rotation of the unit will result in current measurements in a direction not consistent with horizontal flow. After securing the ADCP to the rudder, Worley and Casso came to the surface concerned that there might have been a slight rotation of the unit. The deployment software was not as concerned, however, and reported a nearly negligible tilt of just 0.09 degrees!
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