Abstract:
The design and operation of a unique flow measuring instrument for
bottom boundary layer studies in the marine environment is documented.
The effectiveness of the instrument in acquiring data with which models
of near bottom flows in the ocean can be tested is demonstrated by the
results of a field experiment in Vineyard Sound.
The instrument uses four sensors which measure the mean and fluctuating
parts of the three components of the velocity vector at four
heights above the sea bed. The sensors employ the acoustic travel time
difference technique, and are designed to minimize sensor-induced flow
disturbances. BASS, an acronym for Benthic Acoustic Stress Sensor, has
a resolution of .033 cm/sec per least bit, a range of ±62 cm/sec, noise
of .07 cm/sec in 10 sec, and an estimated accuracy of ±.5 cm/sec, referred
to an in situ zero point. A complete set of velocity measurements
is made every .750 seconds, each measurement being the vector
component averaged over 15 cm. The data is internally recorded on
digital cassette tape. Eight hours of continuous data can be recorded.
BASS was deployed in a tidal flow in Vineyard Sound at a depth of
10 m where a time series of u, v, and w velocities at 26 cm, 46 cm,
96 cm, and 210 cm above the bottom was recorded. The mean velocity was
determined by fitting each 6 hour series with a sixth order polynomial
and the deviations from the polynomial, the fluctuating velocity components,
were correlated to produce Reynolds stress profiles. The
stress series shows very few negative stress events while the dominant
positive events have an average duration of 5 seconds and exceed
30 dynes/cm2.
Zero offset was removed from the mean by assuming a log profile at
maximum ebb. Deviations from a log profile developed when the current
dropped below 40% of maximum, i.e., when the flow could no longer be considered
steady. A break in the Reynolds stress profile at 1 m suggested
a larger length scale than the 1 cm bottom roughness was present in the
flow. A value of u* was determined by using the quadratic drag law
(u* = 1.56 cm/sec), the log profile method (u* = 1.60 cm/sec), and the
eddy correlation method (u* = 1.91 cm/sec). Integral length scales
of 5 m cross-stream, and 2.5 m vertically were identified by correlation
calculations. Two length scales were present in the downstream
direction, 5 m within 1 meter of the wall and 8 m further from the
wall.
