Influence of river discharge on tidal and subtidal flows in a microtidal estuary: Implication on velocity asymmetries

Abstract

Freshwater discharge into an estuary can affect its horizontal density gradients, water-column stratification, and subtidal circulation. An incomplete understanding of the effects of river discharge on tidal and subtidal flows motivates this study. The focus is on how river discharge effects translate into temporal variations in velocity asymmetries within a microtidal estuary with mixed tides. In particular, this study quantifies the relative contribution of overtides, compound tides, and subtidal flows to velocity asymmetries by modifying the method proposed by Nidzieko (2010). Quantification is carried out with a three-dimensional hydrodynamic model, and the modified method is applied in a microtidal estuary with mixed tides. Numerical simulations show that the upper estuary (toward the river) responds differently from the lower estuary (toward the ocean) to changes in freshwater discharge. Throughout the estuary, the horizontal salinity (density) gradient tends to migrate to the lower estuary as river discharge increases. In the upper estuary, subtidal flows become vertically unidirectional, mainly because fresh water displaces salty water toward the lower estuary. In the lower estuary, the horizontal salinity gradient increases with a corresponding enhancement of the baroclinic circulation (exchange flow). Estimates of velocity asymmetries show that the interaction between subtidal flows and semidiurnal tidal flows is the main contributor to the asymmetries. As freshwater discharge increases, changes in the velocity asymmetry manifest as attenuated interactions between semidiurnal and diurnal flows and enhanced interactions between semidiurnal flows and subtidal flows.