ADCP measurement of tidal dynamics

Moving vessel Acoustic Doppler Current Profiler data can be used to give measurements of the spatial pattern of dynamical terms . The measurements were made over 13 hours in the entrance to Tauranga Harbour on the Port of Tauranga's survey vessel. The vessel track is shown by the dashed line on the right. The measurements show up to 1.8m/s or 3.5 knot currents through the narrow entrance,

Below are the ADCP measured depth, velocity and dynamical terms. Horizontal advection is the largest term by an order of magnitude. It was also found possible to use the ADCP's current measurments to give observations of the surface displacement by inverting the horizontal momentum equation.

See:

ADCP Measurements of Momentum Balance and Dynamic Topography in a Constricted Tidal Channel

Ross Vennell - Journal of Physical Oceanography, (2006) Vol. 36, No. 2, pages 177–188

Abstract

The dynamics of tidal flow through inlets are not fully understood; observations are scarce due to the small spatial scales over which the flow varies. This paper gives the first detailed measurements of the 2D structure of tidal currents and the dynamical terms of the momentum equation within a tidal inlet, leading to an improved understanding of the physics of tidal inlets. In the 180 cm/s peak flow the near steady state momentum balance is dominated by horizontal advection and pressure gradient, with bottom friction playing a secondary role. At slack water there is a balance between local acceleration and pressure gradient. Numerical integration of the ADCP measured terms in the momentum equation yields 60 m resolution dynamic topography which shows a 7 cm variation at peak flood consistent with Bernoulli's equation. The surface topography due to friction forms a linear ramp with a peak irreversible head loss of 2 cm over 600 m. Tidal velocities were extracted from the ADCP measurements by extending an existing spline analysis technique. This technique is known to be sensitive to the number and location of the nodes where weights are applied to the spline. Simulations with artificial data representative of the tidally varying ADCP measurements show that, provided there are sufficient nodes to resolve the smallest spatial scale of interest, then velocities predicted by the spline technique are insensitive to the number or locations of the nodes. .