Structure, variability, and salt flux in a strongly forced salt wedge estuary

TLDR

The study investigates how strong tidal and river flows shape circulation, stratification, and salt flux in a shallow salt‑wedge estuary with weak baroclinic currents. Field observations and an unstructured‑grid hydrodynamic model are combined to simulate and validate the Merrimack River estuary’s dynamics. The model demonstrates that discharge controls the estuary’s length and stratification, with a transition when the salinity intrusion equals the tidal excursion, and that salt flux is dominated by tidal processes whose asymmetries depend on tidal amplitude and river discharge, while the model reproduces observed water‑level, salinity, and velocity patterns, with accuracy hinging on detailed bathymetry and low diffusivities.

Abstract

The tidally varying circulation, stratification, and salt flux mechanisms are investigated in a shallow salt wedge estuary where fluvial and tidal velocities are large and the steady baroclinic circulation is comparatively weak. The study integrates field observations and numerical simulations of the Merrimack River estuary. At moderate to high discharge the estuary is short and highly stratified, while at lower discharges it shifts to a longer, more weakly stratified estuary; the transition occurs when the length of the salinity intrusion is similar to the tidal excursion. The Merrimack is highly variable at tidal time scales owing to the advection and mixing of a bottom salinity front. Salt flux is predominantly due to tidal processes rather than steady baroclinic or bathymetric shear. Tidal pumping is important near the mouth, but inside the estuary salt flux is due to tidal asymmetries in the elevation and thickness of the halocline that depend on the tidal amplitude and river discharge. Conditions in the Merrimack, including the salinity intrusion length and stratification, vary more with event to seasonal shifts in river discharge than with spring‐neap changes in tidal amplitude. An unstructured grid hydrodynamic model is used to simulate conditions in the Merrimack and model results are compared quantitatively against field observations. The model achieves a high skill against time series of water level, salinity, and velocity and captures the spatial structures of salinity, velocity, and salt flux observed in along‐ and across‐estuary transects. High model skills depend on accurate and well‐resolved grid bathymetry and low background vertical and horizontal diffusivities.