Tidally induced pressure gradients in sea level drive mean estuarine tidal currents that can have horizontal spatial variability across a bounded channel or inlet. Strong cross-channel gradients in along-channel mean velocity set up extremums in the background potential vorticity that can support instabilities of tidal currents flowing through narrow, bounded estuarine channels and tidal inlets.  The instabilities may lead to horizontal mixing of momentum across the channel, which can have implications for estuarine dynamics such as the fate and transport of organic and inorganic matter, navigational safety, and tidal energy resource assessment.

The first and second parts of the dissertation examine barotropic instabilities of inlet tidal currents.  The dispersion relation is found analytically and the cross-channel velocity structure, bathymetry, and geometry can be altered to approximate typical natural inlet geometries allowing for a range of scenarios to be examined. The presence of instabilities of tidal currents is observed in the Hampton-Seabrook Inlet, NH using a spatially-lagged array of current meters. The wavenumber-frequency spectra are estimated on both the flood and ebb tides.  Dominant wavenumbers (± 0.002 - 0.02 m-1) of the low frequency motions (0.0006 - 0.01 s-1) with corresponding wavelengths (± 314.2 – 3141.6 m) and periods (628.3 – 10472 s) are resolved and consistent with those predicted by the dispersion equation. The lack of breaking wave group modulations within the inlet and the presence of the seaward (shoreward) propagating instabilities on the ebb (flood) flow indicate that the presence of the instabilities can be attributed to the shear of the tidal current.

In the third part of the dissertation, a numerical model is used to better understand the forcing mechanisms driving intensification of velocity over the shallow lateral shelf in the Piscataqua River observed during both the flood and ebb of the spring tide. Results show that the along-channel flow is intensified over the lateral shelf under high Reynolds number conditions, where the inertial forces dominate over the frictional and viscous forces, during both quasi-steady flooding and ebbing currents.  The velocity intensification is driven by both the potential vorticity balance and the conservation of volume and leads to areas of strong shear that can support instabilities and mixing of momentum.