Coherent sonar measurements of water motion and turbulence in a tidal inlet

Vertical profiles of water motion and turbulence obtained using a 200 kHz coherent Doppler sonar installed in a tidal inlet between Biscayne Bay, Florida and the Atlantic Ocean are presented. The Doppler sonar operates at 200 kHz in a monostatic mode and is equipped with four separate sequentially switched beams. The frame supporting the transducers is installed at 1.5 meters above the bottom with the beams locking toward the bottom. All of the beams are tilted at 12 degrees from vertical in two orthogonal planes. The positioning and tilt of the downward looking transducers maximizes the overlap between the beams at the seafloor. Ancillary measurements of tide level, wave height wind velocity and air and sea temperature are also collected at the experiment site. The water velocity information is recorded at 64 range gates spaced by 3 centimeters. Several estimates of mean Doppler velocity, Doppler spectrum variance and mean backscattering signal intensity are calculated. Vertical profiles or the two orthogonal horizontal velocity components, vertical velocity, radial velocity variance and u´w´ covariance are computed and displayed after processing the recorded data. The results of the independent observations at each range gate show highly reproducible tidal velocities with small random variability (standard deviation or a few cm/sec with 10 minute time average) and small scale velocity variance. The vertical profiles at the peak of the ebb and the flood regimes show a consistent decrease of mean velocity moving down from the water surface. Observations of backscattering signal intensity also indicate a strong correlation between “sonic turbidity” end the tide stage, i.e. the sonar targets are much more numerous in an ebb regime compared to a flood regime. The directly measured Reynolds stress at the bottom is larger than the stress derived from the mean velocity profile during accelerating flow and is smaller during decelerating flow