Rapid reduction of tidal amplitude due to form drag in a narrow channel

Abstract

Reversing Falls is a large sill in a narrow section of the Saint John River on the northern coast of the Bay of Fundy, Canada. Reversing Falls limits tides from propagating farther up the river and impedes river water flowing seaward. This work aims to describe the physical processes that control flow over Reversing Falls by using output from a regional numerical model. We find form drag is more important than frictional drag for limiting the M2 tide propagation over Reversing Falls. Despite large tides and strong stratification suggestive of estuarine processes, the dynamics at Reversing Falls are dominated by a steady balance between drag and the surface pressure gradient, with the distinction that all terms in this river-like dynamical balance change sign on a semi-diurnal tidal cycle.