Tuesday, September 19, 2006

Cowboy Question #1

I had rancher friends visiting from western Canada (Alberta) this past week. They'd never been to the Bay of Fundy before and sure had some interesting questions about the tides. I like to think I'm pretty well versed in matters tidal but a couple of questions caught me by surprise. I'll research and answer these questions in my next few postings...

Q1. Why are the tides so much higher in this funnel shaped bay when there are lots of other funnel shaped bays on various coasts around the world?

The high tides of the Bay of Fundy have less to do with the fact that the bay is funnel shaped and more to do with two other factors:
a) the bottom topography and depth of the bay, and
b) the unlikely fact that the bay's natural resonance coincides almost perfectly in "wave length" with that of the Atlantic ocean.

So, let me explain...

The mouth of the Bay of Fundy is 100 km (62 miles) wide and between 120 and 215 meters (400-700 feet) deep. The bay gradually narrows and becomes more shallow until it splits to form Chignecto Bay and the Minas Basin. The distance across the bay in either of these two smaller bays is only a few km (miles) and the depth at low tide about 14 meters (45 feet).

The gradual tapering and shallowing constricts the tidal flow, causes the water to rise from an average of one meter (3 feet) found elsewhere in other tides to the 16-meter (52 feet) tidal range found at the head of the Bay of Fundy.

The second factor contributing to the highest tides....

Every basin of water has its own natural rhythm and at 290 km (180 miles) long, the time it takes for the tide to flood the length of Bay of Fundy is nearly identical to the time it takes for the tide to come in from the adjoining Gulf of Maine.

This resonance - the meshing of these two rhythms - means that the tidal range is amplified. Called the "Seiche Effect", this amplification is frequently compared to the wave action produced by a child sloshing water back and forth in a bathtub, each wave higher than the one previous. It is this comparison which led to the Bay of Fundy being called 'the world's largest bathtub'.

When the tide is fully out here in our harbour, it really does look like somebody pulled the plug on the tub!

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