Experimental study of bore run-up

Abstract

Bore propagation near the shoreline, the transition from bore to wave run-up, and the ensuing run-up motion on a uniformly sloping beach are investigated experimentally. As a bore approaches the shoreline, the propagation speed first decelerates by compressing its wave form and then suddenly accelerates at the shoreline. Although this behaviour is qualitatively in agreement with the inviscid shallow-water wave prediction (often called the ‘bore collapse’ phenomenon), unlike the genuine bore-collapse phenomenon, the acceleration is caused by the ‘momentum exchange’ process, i.e. collision of the bore against the initially quiescent water along the shoreline. Owing to this momentum exchange, a single bore motion degenerates into two successive run-up water masses; one involves a turbulent run-up water motion followed by the original incident wave motion. The transition process from undular bore to wave run-up appears to be different from that of a fully developed bore. The bore front overturns directly onto the dry beach surface, and the run-up is characterized by a thin splashed-up flow layer.