3D Flow and Sediment Transport Modelling at the Reversing Falls - Saint John Harbour, New Brunswick

Abstract

Hydrodynamic and sediment transport modelling of the Reversing Falls channel was conducted as part of a feasibility study for a small craft fishing harbour in Saint John Harbour, New Brunswick, on the Bay of Fundy. The Reversing Falls represent a unique site of interest. A shallow rock ridge and narrow gorge act as a hydraulic control point for the reversing flow regime. The Falls regulate the Bay of Fundy's tidal inflows into the Saint John River estuary at high tide, and the large river outflows into the Harbour at low tide when the water level gradient is greater than 4 m over only 3 km. The flows do not reverse during the spring freshet when river levels are above high tide. The channel downstream of the Falls and upstream of the Harbour constitutes a dynamic interface between the tidal and river regimes. Existing field data in the most dynamic reach of the channel are sparse, due to the difficulties and costs in surveying the area. Hydrographic and spot current measurements were made at the site to complement existing water level, salinity and current datasets. The Danish Hydraulic Institute's MIKE3 hydrodynamic model was calibrated to field data and compared with results from past studies. Based on the limited current and discharge measurements available for the Reversing Falls channel, the accuracy of the modelled flows in this extremely dynamic area is estimated at about 20%. This error range could be improved in the future if long- term field measurements are collected as part of further design studies. At the studied wharf site, during ebb and low tide, the strong down-channel flow sheds a large-scale 400 m-long back eddy along the cove. The model helped identify source mechanisms of the eddy as a combination of strong flows hitting a rock outcrop and locally causing water level gradients large enough to force a back flow. The ample sediment supply from the tides and river, combined with the constant renewal of water in the harbour, cause sedimentation over dredged areas. Local sediment transport processes were investigated using the numerical model based on measured suspended sediment concentrations and dredging records for the Port of Saint John. Despite the uncertainties associated with sediment modelling in such a dynamic area, the model evidenced the significant role of the bottom density current in sedimentation processes in dredged areas of the City Harbour. The model also helped understand why observed sedimentation rates in the Harbour are un- correlated with the duration or intensity of the spring freshet.