Undular bores

A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising, forming during the spring tides when the tidal range exceeding 4 to 6 m is confined into a narrow funnelled estuary. The existence is based upon a fragile hydrodynamic balance between the tidal amplitude, the freshwater river flow conditions and the river channel bathymetry, and it is shown that this balance may be easily disturbed by changes in boundary conditions and freshwater inflow. The very large majority of tidal bore have an undular shape, with the leading wave followed by a train of well-developed undulations or whelps. The undular bore propagates upstream relatively slowly and the free-surface undulations have a smooth appearance; little wave breaking is observed but close to the banks or above sand banks. The meteorological equivalent is the undular bore cloud pattern observed in Northern Australia and over the Arabian Sea for example. Herein the basic hydrodynamics of undular bores is reviewed and presented using both field and laboratory data, and the analogy with the atmospheric bore is discussed