Ungava Bay: Giving the Bay of Fundy a Run for its Money

Growing up in Canada, we were always taught that the Bay of Fundy, is home to the world’s highest tides. The difference between the highest high tides and lowest low tides is 17.0 m, plus or minus 0.2 m [Arbic et al., 2007].  When I was younger (long before I ever knew that coastal engineering was a career option), I had the good fortune of visiting the Bay several times, usually while en route to visiting my Mom’s family in eastern Canada.

When the tide retreats, tidal flats stretch for tens of kilometers, making you question which direction the ocean is actually in.  The Hopewell Rocks (pictured above) are revealed, and tourists wander the base of beautiful red cliffs.  Then right on schedule, the tide returns, and everything is bathed deep in chocolate milk for 6 hours.

However, 1600 km due north of Fundy lies Ungava Bay, which is also fighting for first place in the World’s Highest Tides Competition.  Flanking Hudson Strait (which connects Hudson Bay to the Atlantic Ocean), Ungava Bay has a maximum tidal range of 16.8 m, plus or minus 0.2 m [Arbic et al., 2007].  This means that the difference between the Bay of Fundy and Ungava Bay is so small that it is within the limits of what we can measure- the jury is still out!

How do the tides grow so large in these bays?  The answer is resonance.  Tides generated by the gravitational pull of the moon rise and fall every 12.4 hours (one tidal period).  By coincidence, the time it takes a tidal wave (i.e. one full cycle of low water to high water and back to low water) to travel the length of Ungava Bay is 12.7 hours [Arbic et al., 2007].  This means that a new tidal wave is coming into the bay just as the old one is coming out.   If you push a child on a swing, they will travel much higher if you properly time your pushes with the rhythm of their swinging.  The same holds true with tides moving in and out of a bay, and we call this phenomenon resonance.

The speed at which a tidal wave travels depends on the depth of the water, the shape and size of the bay or estuary it moves through, and friction from the seabed.  The consequence of this is that changes like sea level rise and the construction of tidal power stations or storm surge barriers can actually modify the behaviour of the tides.  For instance, an increase in sea level rise could bring Ungava Bay even closer to that 12.4 hour resonant period, and further increase the tidal range there [Arbic et al., 2007].  However, this would require 7 m of mean sea level rise to increase the tidal range by just 2%, which I think would be a much bigger problem…

Usually friction from the seabed is one of the main factors that affects tidal waves in estuaries, dissipating energy and damping out their range.  However, a recent study by my TU Delft colleagues Kleptsova & Pietrzak [2018] found that friction at the surface of the water from sea ice can also have a moderating effect on tidal amplitude.  As a consequence, the tidal range in the far-north Ungava Bay actually decreases during the winter. This suggests to me that Fundy remains the World Champion at Christmas, but that during the summer, it’s still anybody’s guess.

An interesting question (and the one that sparked my curiosity to write this post in the first place) then becomes: how might our tides change in a world where the Arctic no longer fully freezes over?  To me, this is just one more example of the scary Pandora’s Box that humanity has opened with rapid climate change.  If something as seemingly dependable as the tides can change, what other surprises will nature have in store for us?

Sources:

Arbic, B. K., St‐Laurent, P., Sutherland, G., & Garrett, C. (2007). On the resonance and influence of the tides in Ungava Bay and Hudson Strait. Geophysical Research Letters, 34(17).

Guo, L., Wu, Y., Hannah, C. G., Petrie, B., Greenberg, D., & Niu, H. (2020). A modelling study of the ice-free tidal dynamics in the Canadian Arctic Archipelago. Estuarine, Coastal and Shelf Science, 106617.

Kleptsova, O., & Pietrzak, J. D. (2018). High resolution tidal model of Canadian Arctic Archipelago, Baffin and Hudson Bay. Ocean Modelling, 128, 15-47.