Large Estuaries (Effects of Rotation)

Abstract

This chapter explores the effects of Earth’s rotation and friction on the main drivers of subtidal motion in semienclosed basins, in general, and in estuaries and lakes in particular. Subtidal motion is considered as that having variations with periods longer than one tidal cycle. In semienclosed and enclosed basins, the main drivers of overall subtidal motion are tides, winds, and density gradients. In order to balance the drivers of subtidal motion, frictional effects can compete with the Earth’s rotation effects (through Coriolis accelerations). This friction vs Coriolis competition can be characterized via nondimensional numbers. In subtidal motions driven by tides, the friction/Coriolis competition may be assessed by comparing bottom stresses and depth-averaged Coriolis accelerations, analogous to an Ekman number. For wind- and density-driven motions the competition is assessed through an Ekman number. In any case of the nondimensional number, the depth of frictional influence is compared to the total water depth and can be regarded as an indicator of the basin’s nondimensional dynamic depth. When friction overwhelms Coriolis, the Ekman or frictional number is >>1 and the basin is dynamically shallow. When friction is negligible, Coriolis influences the dynamics under frictional number <<1, rendering a dynamically deep basin. Such effects are explored with analytical, numerical, and observational results. Interactions among density-driven with tidally driven and wind-driven flows are also depicted. Illustrations of such interactions center on whether the lateral structure of exchange flows is vertically sheared or laterally sheared. The main message of the discussion is that the dynamical depth of a basin determines the lateral structure. Another relevant point is that the dynamical width matters, too.