Estuarine residual flow induced by eddy viscosity-shear covariance: Dependence on axial bottom slope, tidal intensity and constituents

Abstract

Residual flow generated by Eddy viscosity-Shear Covariance (ESCO) in a narrow estuary is investigated with a numerical model. New aspects concern the dependence of the spatial structure of ESCO flow on (1) longitudinal depth variation for fixed semi-diurnal tidal forcing, (2) varying amplitude of the semi-diurnal tidal forcing, and (3) mixed tidal forcing for fixed stratification at the mouth. Regarding (1) and (2), it appears that ESCO flow merely involves the components due to the semi-diurnal tide (uτ,M2 ) and quarter-diurnal tide (uτ,M4 ). For a periodically stratified estuary, uτ,M2 is stronger than uτ,M4 in the middle reach, and weaker in the other reaches, while both show a two-layer structure with seaward flow near the surface. For weak stratification, uτ,M2 has a three-layer structure with seaward flow in the middle layer, while uτ,M4 has a two-layer structure and contributes significantly to the total residual flow in the upper and lower reach. For a highly stratified estuary, uτ,M2 dominates the ESCO flow (being weak compared to the total residual flow), and it has a reversed two-layer structure (seaward flow near the bottom). Regarding (3), if diurnal and semi-diurnal tides are of similar order, the ESCO flow due to diurnal tides dominates and it has a two-layer structure. If diurnal tides prevail, the ESCO flow induced by the long-periodic tide (due to joint action of two diurnal tides) is the main contributor to the total residual flow in the upper and lower reach of the estuary.