Current studies were performed using a 0.4 MHz Nortek Aquadopp acoustic Doppler current profiler (ADCP). The ADCP was mounted in a downwards-looking orientation on a retractable ADCP boom attached to the side of the research vessel. The retractable ADCP boom-mount used by Ocean Ecology was a highly modified adaptation of the concept presented by Hench et al. (2000) (“A portable retractable ADCP boom-mount for small boats“).
A photo of the ADCP mounted on its deployment boom in the raised position is shown below.
To reduce drag and wave generation from the boom, a custom tear-drop shaped fairing was mounted just above the transducer heads. Studies by Colbourne et al. (1993) (“Improved ADCP Performance Using a Hydrodynamically Designed Boom Mount“) have shown that the use of a fairing can reduce the longitudinal (along-ship) force resulting from drag and wave generation by the boom by as much 50% as compared to that on unfaired ADCP mounts. The effectiveness of the fairing in reducing turbulence around the transducer heads can be seen in the photo below.
The ADCP parameters were set as follows:
- Profile interval: 1 s
- Number of cells calculated using the following formula:
- Number of cells = [(Maximum depth + Tidal range)/Cell size] – (Blanking distance + Deployment depth). This value is multiplied by 1.10 (e.g., 110%) and rounded up to the nearest whole value to get a 10% margin of error.
- Cell size: the minimum cell size for unit for shallow water surveys (2.00 m)
- Blanking distance: the minimum blanking distance for unit for shallow water surveys (1.00 m)
- Compass update rate: 1 s
- Coordinate system: magnetic ENU
A salinity refractometer was used to measure the approximate salinity of the water at a depth of 1 m, and this value was used by the ADCP for automatic speed of sound compensation.
The shipboard survey methodology used by Ocean Ecology was a modification of the methodology described by Epler et al. (2010) (“Shipboard Acoustic Doppler Current Profiler Surveys to Assess Tidal Current Resources“). During an ebb or flood survey, the vessel completed continuous laps around a designated track through the peak of the tidal cycle. Each lap measured the currents along the survey track at a different stage of the tidal cycle. Laps were conducted both before and after the time of peak ebb or peak flood currents. In order to achieve the best results, the duration of the survey was designed to capture both the peak currents and more than two hours of the tidal cycle before and after the peak. Vessel speed was maintained in the range between 3 and 6 knots. This speed was slow enough to allow sufficient time for data collection by the ADCP, while keeping sufficient velocity to provide desirable spatial coverage and allow effective steerage in strong currents. Unlike the methodology used by Epler et al. (2010), Ocean Ecology’s methodology did not involve bottom tracking, and as a result, the upper vessel speed limit was not so critical.
Tidal current predictions were used to provide an estimate of the peak current timing on which to base survey start time and duration. If a survey pattern was too long in duration, the timing between each lap resulted in large gaps in the time series for each bin and inaccurate identification of peak currents. Thus, the size of the laps was chosen such that the lap duration was approximately 20 minutes. This struck a balance between adequately resolving the spatial variability in the currents and maximum vessel speed for good ADCP returns.
During the survey, data were logged to a notebook computer. Three data logs were generated: (1) a raw log of the NMEA sentences as output by the DGPS unit using Franson GpsGate as the logging software; (2) a log of the ADCP data using Nortek’s AquaPro software; and (3) a track log of the lap positional data using ArcGIS. Track position was viewed real time using ArcGIS as a navigational program. Incoming ADCP data was also viewed in real time, thus allowing the status of the instrument to be monitored.