Bores and Swash on Natural Beaches: Fid-Bau Portal

Bores and Swash on Natural Beaches

M. Bradshaw

A field study has been carried out to investigate the behaviour of bores in the inner surf zone of natural sandy beaches and to examine a theory which links bores to swash through a process of bore collapse at the shoreline. The theory models subsequent swash as a lens of water moving up the bech slope under the decerlerating influence of gracity only. Cine-photography was used to collect all data. Bores were filmed on a flat profile in shallow water (<0.5m) which was either at rest or flowing seaward with velocities up to approximately 1 m/sec. In all cases it was found that the theoretical velocity of the bore front correlated closely with observed velocity. Film records of bore collapse on a steep and a flat beach and of subsequent swash flows indicate that the bore disappears at the shoreline in both cases. This occurs as a gradual flattening of the steep bore face over distances ranging from 1.5m (small bore-flat beach) to 5m (large bore-steep beach). Swash velocities at the base of the steep beach were found to be high compared to those hitherto reported. The theory of bore propagation over a sloping bottom is used to simulate the behaviour of multiple bores in a surf zone.