Upstream Propagation of Surges and Bores: Free-Surface Observations

Abstract

In a free-surface flow, a sudden increase in water depth induces a positive surge, also called compression wave or bore. Herein a physical study was conducted in relatively large-size rectangular channel with a smooth bed. The upstream propagation of breaking and undular bores were investigated with a broad range of Froude numbers Fri ranging from 1.1 to 2.3. Both instantaneous and ensemble-averaged free-surface measurements were performed nonintrusively. The observations showed the occurrence of undular bores for 1 < Fr1 1.2 to 1.3, breaking bores for Fr1 > 1.4 to 1.5, and breaking bores with secondary waves for 1.2-1.3 > Fr1 1.4-1.5. The propagation of a breaking bore was associated with an upward free-surface curvature immediately before the roller toe for Fri < 2, and an abrupt increase in free-surface elevation with the passage of the breaking roller. The propagation of undular bores was characterized by a smooth upward free-surface curvature, followed by a smooth first wave crest and a train of secondary quasi-periodic undulations. For all tidal bores, the passage of the bore front was always associated with large free-surface fluctuations, occurring slightly after the arrival of the front. During the generation process, the positive surge formed very rapidly and the surge celerity increased very rapidly, reaching maximum values excess of the fully-developed bore celerity. With increasing time, the surge decelerated and the bore propagated at an early constant celerity for (xgate —x)/xgate > 10.