Lateral structure of tidal asymmetry in vertical mixing and its impact on exchange flow in a coastal plain estuary

• ATM induced residual flows were of the same order of magnitude as density-driven flows.
• Residual flow showed either a two or a three-layer structure in the channel, two-layer structure over shoals.
• Regular tidal straining favored gravitational circulation, but reversed straining opposed it.
• The contribution of residuals induced by asymmetric mixing increased with stratification.
• The models are verified with observations showing ATM induced flow opposing density-driven flow.

Residual currents induced by asymmetries in tidal mixing (ATM) were determined using a series of underway-current velocity profiles and salinity measurements, estimates of vertical mixing, and an analytical expression. Six different 12-hour transect surveys at the mouth of the James River were carried out under typical spring and neap tides and a range of river flows. The ATM-induced flows were found to be of the same order of magnitude as the density-driven flows. These flow profiles showed either a two or a three-layer structure in the channel and a two-layer structure over shoals. More stratification during ebb than flood periods (tidal straining) favored gravitational circulation, whereas reversed straining (more stratification at flood) opposed gravitational flow. The contribution of ATM-induced residuals increased as water column stratification increased. Lateral variations in ATM favored lateral shear in along-channel flow, enhancing the laterally sheared exchange. Observations were in good agreement with previous numerical and analytical model results. In particular, observational validation is presented on reverse tidal straining inducing residuals that act against gravitational circulation. Inconsistencies arise from the assumptions made in the models, namely no Coriolis forcing, no topographic features such as headlands, and simplified nature of stratification.