Wind effects on the circulation of a geometrically-complex small estuary

Abstract

Local geometry and bathymetry set bounds on how estuarine circulation and salinity respond to river and tidal forcing. Although often considered secondary, wind can drive variations in the salinity field, as well as inducing locally strong along and across-estuary salinity and water level gradients. Here, we use observations and numerical simulations to look at the effect of winds on estuarine dynamics in the Coos Estuary in the Pacific Northwest. The small, strongly tidally-forced estuary, does not conform to the traditional funnel-shaped estuary, instead it is shaped like an inverted U. The numerical simulations use idealized forcing to separate the contribution of tides, river discharge, and winds, on subtidal salinity and velocity fields. We find that wind can lead to reversals in the out-estuary surface flow despite the tidal dominance on subtidal circulation, in accordance with the limited available observations. Northward winds pile fresher waters in the north side of the estuary, and decrease exchange flow due to the winds opposing the main channel surface outflow, which may ultimately enhance the transport of particles along estuary. Southward winds pile fresher waters on the southern sides of the estuary, where most of the flats are found, and act to enhance the loss of salt. These transient winds drive non-transient changes to salt content in the estuary: high discharge cases show a general increase of salt, while low and moderate discharge show a reduced loss of salt in the estuary after the winds are turned off. The wind-driven spatial and temporal variability quantified here in the salinity and velocity distribution underscores the importance of local geometry constraints on estuarine dynamics, especially as many estuaries continue to evolve either due to natural environmental changes or to anthropogenic impacts.