A steep front, from a ripple to a breaking wall more than a metre high, that forms when a large tidal range meets a shallow, narrowing river. The incoming flood overtakes the outgoing river water and steepens into a travelling step, followed by a rapid rise in level. Famous bores include the Severn, the Qiantang and Turnagain Arm, and their timing and size are predicted from range and river flow.
Bore's due at the bridge 40 minutes after low water at the mouth, keep the boats off the bank.
Bores are largest on big spring tides with low river flow, and surfers and kayakers follow the published bore timetables closely.
Also heard as
- tidal bore
- eagre
- mascaret
- pororoca
Related terms
Flood
Tidal current running in from the sea and up rivers and estuaries, mostly while the water is rising.
Tidal range
The vertical difference between a high water and the neighbouring low water.
Spring tide
A tide of larger than average range, occurring around new and full moon when Sun and Moon pull in line.
Overtide
A higher-frequency constituent, such as M4, generated when a tide distorts in shallow water.