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Weird Science: Tidal Bores: The Longest Waves Ever Ridden

NGSS Crosscutting Concepts:

Tidal currents can be very strong at the mouths of rivers and in the narrow inlets of bays and harbors. For example, all the water moving into and out of San Francisco Bay must pass through the inlet to the bay under the Golden Gate Bridge.


In places where an incoming high tide enters a shallow and sloping estuary, river, or harbor, the higher water level can form a wave called a tidal bore (SF Fig. 6.18). Tidal bores occur during flood tides when the tide is flowing towards land, often upstream. To an observer on the bank, an approaching tidal bore looks like a turbulent wave or “wall” of water that suddenly raises the water level. Tidal bores can advance rapidly and travel for great distances up a river against the direction of the current. Although tidal bores have been recorded worldwide, they are most common in areas with tidal ranges larger than 6 m between high and low tide.

<p><strong>SF Fig. 6.18.</strong>&nbsp; (<strong>A</strong>) A tidal bore in the Qiantang River, Hangzhou, China</p><br />
<p><strong>SF Fig. 6.18.</strong>&nbsp;(<strong>B</strong>) A tidal bore in the River Ribble, Lancashire, United Kingdom</p><br />
<p><strong>SF Fig 6.18.</strong>&nbsp;(<strong>C</strong>) Surfers riding a tidal bore in the Severn River, Gloucestershire, United Kingdom</p><br />


Tidal bores break in a manner similar to plunging breakers. When a tidal wave slows down to the point where the motion in the crest area—the high tide of a tidal bore—exceeds the speed of the wave, the tidal bore wave breaks. In contrast to a plunging breaker, however, there is a tremendous amount of water behind the wave that keeps the wave breaking as the tidal bore continues to move.


Tidal bores can be very large and potentially dangerous to unsuspecting or unknowledgeable boaters and swimmers, but they can also provide a great deal of enjoyment if you understand the phenomenon. One of the most famous tidal bores occurs on the Severn River in Gloucestershire, United Kingdom, which has one of the largest tidal ranges in the world (approximately 15.4 m, SF Fig. 6.18 C). Surfers track tidal bore events on the Severn River by using tide forecasts. Hundreds of surfers and thousands of spectators show up to the Severn when a large tidal bore is predicted (see The Severn Bore for forecasts).

Special Feature Type:

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.