The rumble sound generated by a tidal bore event in the Baie du Mont Saint Michel

A tidal bore is a sharp rise in free-surface elevation propagating upstream in an estuarine system at the leading edge of the flood tide. It generates a powerful noise that was sometimes compared to the sounds of a horse cavalcade. Herein the sounds generated by a tidal bore event in the Baie du Mont Saint Michel were carefully recorded. The data showed three distinct periods. These were the incoming tidal bore when the sound amplitude increased with the approaching bore front, the passage of the tidal bore in front of the microphone where the impacts of the bore on the bank, rocks, or jetty generated powerful noises, and the upstream propagation of the bore when the flood flow motion caused additional loud noises. During the arrival of the tidal bore, the sound levels were less energetic and a lower-pitch sound was noted than during the subsequent record. For the breaking bore process, the analysis of the sound record indicated a dominant frequency around 76-77 Hz. The low-pitch rumble had a frequency comparable to the collective bubble oscillations, suggesting that air entrapment in the bore roller might play a major role in the acoustic signature.