Turbulent Advances of Breaking Bores: Physical Modelling in a Large Facility

Xinqian Leng, Hubert Chanson

Thursday 2 july 2015

17:15 - 17:30h at South America (level 0)

Themes: (T) Water engineering, (ST) River and coastal engineering

Parallel session: 13C. Engineering - Industrial

In an estuary, a tidal bore is a positive surge generated at the leading edge of the flood tidal wave during the early flood tide under spring tide conditions into a narrow funnelled channel. After its formation, the bore front may be analysed as a hydraulic jump in translation. For Froude numbers greater than 1.4 to 1.8, the leading edge of the bore is characterised by a breaking roller. The roller is characterised by a sudden increase in water depth, a highly turbulent flow with large-scale vortical structures, some kinetic energy dissipation, a two-phase air-water flow region and strong turbulence interactions with the free surface associated with splashes and droplet ejection. New experiments were conducted in a 19 m long 0.7 m wide canal to investigate the breaking bore roller propagation. The upstream propagation of the roller toe was a highly turbulent process. The toe perimeter shape fluctuated rapidly with transverse distance and time and its transverse fluctuations were quantified in terms of the standard deviations (X-Xmedian)'/d1 = 0.145. The characteristic wave length of the toe perimeter was approximately 1.2 times the initial flow depth d1. Both the standard deviation of toe perimeter location and characteristic wave length were comparable to field observations in the Qiantang River bore (China). The celerity of the roller toe fluctuated rapidly with time and space, although in a quasi-two-dimensional manner on average. The sidewalls had little effect on the upstream propagation of the roller within the experimental flow conditions. The instantaneous longitudinal free-surface profile of the roller showed significant temporal and spatial fluctuations. The standard deviation of the free-surface elevation was maximum in the first half of the roller and the data were comparable to previous studies in stationary hydraulic jumps for a similar Froude number. New air-water flow measurements highlighted some distinctive air bubble entrainment at the toe of the roller. Bubbles with larger chord times were detected at higher vertical elevations in a more intermittent manner. Overall the study demonstrated that the propagation of breaking bore is a very turbulent, three-dimensional process.