Turbulence and turbulent events in tidal bores: field observations

A tidal bore is a compressive wave of tidal origin, propagating upstream as the tidal flow turns to rising when a macro-tidal flood flow enters a funnel shaped river mouth with shallow waters. New field measurements were conducted in the Garonne River tidal bore at Arcins in 2015. Instantaneous velocity measurements were performed continuously at high-frequency prior to, during and after each bore. The bore occurrence had a marked effect on the velocity and turbulent Reynolds stress field, with large and rapid fluctuations, during the bore passage and the early flood tide. A turbulent event analysis was conducted in the highly unsteady rapidly-varied tidal bore flow. The method detects bursting events by comparing the absolute value of an instantaneous turbulent flux with its standard deviation. This analysis, based upon basic concepts, was extended to the rapidly varied, highly-unsteady tidal bore flood flow motion. The turbulent event data analysis showed relatively close results for all studies and all fluxes. A very large majority of turbulent events had a duration less than 0.01 s, with on average 20 turbulent events per second. During all studies, the event duration showed some tidal trend, with longer turbulent events immediately after the tidal bore passage, occurring simultaneously with sediment erosion processes