Physical Modelling of Tidal Bore Dyke Overtopping: Implication on Individuals' Safety

Hubert Chanson, Dong-zi Pan

Wednesday 1 july 2015

11:30 - 11:45h at Africa (level 0)

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

Parallel session: 9F. Engineering - River

A tidal bore is a surge of waters propagating upstream as the tidal flow turns to rising and the flood tide rushes into a funnel-shaped river mouth. The bore forms during the spring tides when the tidal range exceeds 4–6m and the rising tidewaters are confined to the narrow funnelled estuary. The tidal bore can be a major tourism attraction. In China, the Qiantang River bore attracts more than 300,000 people each year for the Moon festival while the bore propagation is seen live on television by over 15 millions of television spectators. All the year around, tens of thousands of tourists come to see the tidal bore during spring tide conditions. In Europe, the Dordogne and Severn Rivers are the sites of well-known tidal bore surfing competitions that many individuals come to watch. In the early 1960s, the mascaret of the Seine River attracted more than 20,000 people during the weekends. When the river banks are protected by dykes, these become attractive view points, despite the hazards caused by the risks of bore overtopping. For the last 20 years in China, over 80 people are drowned in the Qiantang River bore flood tide motion. Herein a physical study was conducted in a relatively large size facility. The upstream propagation of a tidal bore in a compound channel was investigated based upon a Froude similitude. The data highlighted the occurrence of transient secondary currents in the wake of the bore front. The results demonstrated that these currents constituted major hazards for individuals standing on the dyke.