THREE-DIMENSIONAL MODELLING OF HYDRODYNAMICS AND THERMOSALINE CIRCULATION IN THE SAINT JOHN RIVER ESTUARY, CANADA

From sources in Canada and the United States, the Saint John River traverses more than 670 km before entering the Bay of Fundy at Saint John, New Brunswick. Mixing and exchange of fresh and salt water between the Saint John River, the adjoining Kennebecasis Fjord and the sea is driven by strong tides in the Bay of Fundy and fluvial flows, which are seasonal and highly variable. Upstream of the Port of Saint John, the bathymetry features a series of shallow sills and deep gorges. Estuarine mixing processes are controlled by a natural sill near the mouth of the river, known as the Reversing Falls. The restriction causes a turbulent rapid to form flowing upstream with the flood tide, which then reverses and flows downstream with the ebb tide. Although tides are damped by the Reversing Falls, intermittent discharges of saline water over the sill during the flood tide can accumulate in deeper parts of the estuary and penetrate more than 30 km upstream depending on seasonal river flows. A three-dimensional, baroclinic numerical model of the Saint John Estuary was developed based on the TELEMAC-3D finite volume solver to simulate hydrodynamics and circulation. The model setup, calibration and validation processes relied on field data derived from a multi-year campaign of bathymetric surveys and oceanographic measurements, conducted by the University of New Brunswick’s Ocean Mapping Group. The data included high resolution, multibeam bathymetric survey data, acoustic Doppler current profiles, and conductivity-temperature-depth measurements. The model has become a useful tool for assessing water exchange, water quality issues (e.g., fate and transport of effluent discharges), and the potential impacts of development within this dynamic estuary.