Tidal variability of lateral advection in a coastal plain estuary

• Contribution of v′u′y to the along-channel momentum balance was of the same order of magnitude as the gHρ′x/ρo.
• The impact of v′u′y on u′ was strongest during or just after the peak flood and weakest at the end of ebb.
• Observed v′u′y exhibited a 4-cell circulation as predicted by an analytical model: (−) at the southern side near surface or the northern side near bed; (+) in the rest of the channel.

Tidal variability of lateral advection of momentum (vuy, where u and v are along-estuary and lateral flows, respectively, and the subindex indicates differentiation with respect to the cross-estuary direction) was investigated in a coastal plain estuary with observations at Hampton Roads, which is the transition between the James River and Chesapeake Bay. Towed current velocity profiles and hydrographic profiles were captured during 9 expeditions in 2004 and 2005, to determine the intratidal and spatial changes in lateral advection of momentum and its contribution to along-channel flow. Curvature effects and lateral density gradients were important in driving lateral circulation and in modifying intratidal lateral advection of momentum. Lateral advection had the same order of magnitude as the baroclinic pressure gradient. Its contribution to the along-channel momentum balance was greatest during or just after peak flood and weakest at the end of ebb. During peak flood and peak ebb, the spatial distribution of vuy was seaward at the southern (left) side near surface and at the northern side (right) near bed (looking up-estuary), and landward in the rest of the channel. During slack periods the vuy structures were mostly landward. Observations were in good agreement with analytical model results during peak ebb and flood, but inconsistent during slack periods. The discrepancies between model results and field measurements can be attributed to bathymetry–density gradient interactions, which enhanced ebb-to-flood asymmetries in the along-channel and lateral flow.