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

The dynamics of tidal flow through inlets are not fully understood; observations are scarce because of 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−1 peak flow the near-steady-state momentum balance is dominated by horizontal advection and the pressure gradient, with bottom friction playing a secondary role. At slack water, there is a balance between local acceleration and the pressure gradient. Numerical integration of the ADCP-measured terms in the momentum equation yields 60-m- resolution dynamic topography that shows a 7-cm variation at peak flood consistent with Bernoulli’s equation. The surface topography because of 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, velocities predicted by the spline technique are insensitive to the number or locations of the nodes.