4.8 The Bay of Fundy and the tides of climate change

1.1 The highest tides in the world

Probably the most known characteristics of the Bay of Fundy are the exceptional tides, according to NOAA (2014) the highest in the world with an amplitude of 16.3 meters in the Minas Basin, and even 17 meters at the Burncoat head tidal gauge. The reason for these record tides is the configuration of the Bay of Fundy as a semi-enclosed basin that resonates at the same frequency as the lunar semidiurnal tide (figure 2). The period of a resonant standing wave (seiche) in a rectangular body of water bounded on one side (where the effective wavelength is twice the length of the basin) is given by Merian’s formula: T = 4L/ √(gh) where T is the period of the wave, L the length of the basin, g the gravitational acceleration on earth (9.8 m/s2) and h the height of the water column. With a length of 290 km and an average depth of 66 m (decreasing from 200 m at the boundary to the Gulf of Maine), the resonance period of the Bay of Fundy of about 12 h 40 min (13.3 h in other sources), which nearly coincides with the semi-diurnal tidal period (M2) in the North Atlantic of 12 h 25 min (Pinet, 2006; McCully, 2006), thus leading to exceptionally high amplitude tides1. These great amplitude tides also create strong tidal currents, which at the far end of the Bay of Fundy exceed the theoretical limit for a shallow water wave and are also at the origin of the famous tidal bore observed notably on the Petitcodiac, Macaan, Hebert, Chigonois, Salmon, Shubenacadie and Meander rivers and are also responsible for other unusual phenomena like the “Old Sow” whirlpool in the Passamaquoddy Bay, thunder holes or reversing falls.

1For a more detailed discussion of the tidal resonant, see i. a. Duff, G. F. D., 1970. Tidal resonance and tidal barriers in the Bay of Fundy system, Journal of the Fisheries Research Board of Canada 27, 1701-1728,DOI 10.1139/f70-192; Ku, L.-F., D. A. Greenberg, C. J. R. Garrett, F. W. Dobson, 1985. Nodal Modulation of the Lunar Semidiurnal Tide in the Bay of Fundy and Gulf of Maine. Science 230, 69-71, DOI: 10.1126/science.230.4721.69; Garrett, C., 1972. Tidal Resonance in the Bay of Fundy and Gulf of Maine, Nature 238, 441 – 443, doi:10.1038/238441a0; Garrett, C., 1974. Normal Modes of the Bay of Fundy and Gulf of Maine. Canadian Journal of Earth Sciences 11, 549-556, DOI 10.1139/e74-049; Greenberg, D. A., 1979. A numerical model investigation of tidal phenomena in the Bay of Fundy and Gulf of Maine. Marine Geodesy 2, 161-187.
Amplitude and periods of the tides in the Bay of Fundy.

Figure 2. Amplitude and periods of the tides in the Bay of Fundy.

Source: Garrison (2006)

The high tides have also contributed to shape the landscape of the Bay of Fundy, such as the iconic Hopewell rocks (figure 3). The strong vertical mixing of the water column also means that water temperatures at the surface are generally low, rarely exceeding 13°C and that the water column is rarely stratified. It also contributes to a rapid vertical recycling of nutrients. The consequences of the particular hydrology also affect the ecology of the region, through the presence of large intertidal zones, and of course the human occupation, which has to adapt its infrastructures to the large differences in water levels.

The Hopewell Rocks at low tide.

Figure 3. The Hopewell Rocks at low tide.

Source: S. Weissenberger, 2009

The exceptional amplitude of the tides allows for the generation of tidal energy. The Annapolis generation station, a 20 MW tidal plant, was completed in 1984 and is the only of its kind in North America (2 other exist in France and Russia). An in-stream turbine technology is to be tested in the Minas passage by the Fundy Ocean Research Center for Energy, which over time could grow to 17.5 MW of electricity from five sites, benefiting from a feed-in tariff into the Nova Scotia Power distribution system (NRCan, 2016).

The tidal resonance and hence the amplitude of the tides are dependent on the exact configuration and water levels in the region and are thus affected by changes in sea level. Tidal resonance began in the early Holocene and 80% of present day amplitude was reached 5000 years ago (Shaw et al., 2010). Over the 20th century, the Gulf of Maine and Bay of Fundy tides experienced an acceleration until the 1980s followed by a discontinuity (Ray, n.d.)