Tidal and subtidal transport in short, tidally-driven estuaries with low rates of freshwater input

Abstract

Tidal and subtidal current velocity structure and forcing mechanisms are important to forecasting estuary conditions linked to water quality problems in coastal areas. This study combined current velocity, salinity, temperature, and water level data with analytical and numerical models to understand dynamics in a small estuary in Downeast Maine, USA. Both the tidal and subtidal flow, as well as the drivers of the subtidal flow, were found to have substantial spatial variation along the estuary. An analysis of the along-channel momentum balance indicates that advection and friction alternate in balancing Stokes drift and the barotropic pressure gradient along the estuary, switching at locations of estuary width constriction or expansion. These dynamics influenced along-channel subtidal flows producing highest velocity at the shoals and lowest velocity in the channel in the mid-reaches of the estuary when advection was more influential than friction. This pattern reversed when friction dominated over advection, producing the highest velocity in the center of the channel, which is corroborated by field measurements. The idealized model produced a spatial structure of along-channel residual flows that do not align with field measurements and numerical simulation results due to the simplicity of the estuary geometry used in the model. The outcome of the analyses suggests that slight variations in along-channel geometry and bathymetry can alter tidal and subtidal flows and transport, even in small (<10 km in length) tidally dominated estuaries with relatively little spatial variation in hydrodynamic conditions within a tidal cycle.