Tobermory Seiches

A Review of a study by John Greenhouse

A seiche (SAYSH) is a wave in an enclosed or partially enclosed body of water. Like water in a jostled tub, energy sloshes back and forth between the boundaries of the water body creating what are known as “standing waves”, not unlike those that can be seen in a skipping rope. Seiches and seiche-related phenomena can be observed on lakes, reservoirs, bays, harbours and oceans. Some can be very dramatic, but others are all but imperceptible.

On the Great Lakes a seiche derives its energy (the “jostling”) from the atmosphere. A sharp change in pressure or wind speed will produce a wave in the body of water (Figure 1). The wave is reflected repeatedly from the shores, generating standing waves with one or more peaks. The repetition time, or “period” of the oscillation is determined by the size and shape of the water body, and by its depth.  In large water bodies the period of oscillation will be several hours. As shown in Figure 1, the thermocline can also oscillate as a standing wave.

Figure 1. Wind generating standing waves on the surface and the thermocline of a lake.
Frankemann [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)%5D

In bays leading off Lake Huron the water level will rise and fall with the lake, but they also have oscillations with periods determined by the size, shape and depth of the bay itself. These harbour oscillations or seiches, have periods typically from a few minutes to an hour.

Large seiches occur every few years and, are exceptional events. They are generated by extreme weather out in the big lake and, when they arrive at a harbour, they can be hugely amplified. They are extreme events but a seiche is continual. The lake and its harbours are in almost constant but very low level oscillation because the atmosphere itself is never completely static. This continual seiche activity is most often imperceptible to observers on land or in boats due to the extremely long period and low amplitude of the wave.

John Greenhouse is fascinated by these harbour oscillations. They have already been studied quite extensively but there are nothing more than anecdotal accounts of the seiche activity in individual bays and harbours around the peninsula. Do they have characteristic and repeatable periods independent of the those in Lake Huron itself? Are these characteristics explainable in terms of the size, shape and depth of bays? Do these characteristics determine what happens when a major seiche comes from the big lake outside? Despite an extensive body of literature on the subject, there seemed to be a local knowledge gap to fill.

Figure 2. Numbered red dots indicate the location of measuring stations in this study.

Accordingly, over a four year period, John Greenhouse set up water-level monitoring stations (Figure 2) at 19 locations in Tobermory and along the western shore of the peninsula.

The frequency of a wave is the number of times per second that the wave cycles. Frequency is measured in cycles per hour (cph). The period of the wave is the time between wave crests. The period is also measured in time units. The period and frequency are inverse of each other. Frequency, the inverse of period, is measured in cycles per hour (for example, 4 cycles per hour implies a period of 15 minutes).

John has characterized each harbour in terms of the frequency of its oscillations. He found that each harbour had its own characteristic frequency that was consistent over time and it was unique to that harbour. In other words, each harbour or bay had its own “signature”. 

The dominant frequency of the harbours he studied ranged from 0.8 cph at Stokes Bay (period of 75 minutes) to 13 cph (period of 4.5 minutes) at Little Tub Harbour.  These harbour frequencies are broadly consistent with predictions based on the size, shape and depth. They are quite distinct from oscillations in the big lake, which also can be measured in the harbours but have much longer periods.

As noted above, a large seiche coming in from the lake can hugely amplify a seiche in a harbour like Little Tub with possibly damaging effects. The next step in this study may be to obtain simultaneous measurements of a large seiche on the lake before it reaches the harbour and measure the harbour’s response. From that one might be able to predict a large seiche and mitigate the effects on property.

The full paper will soon be published on the SOK website (www.sourcesof knowledge.ca). In the meantime you might enjoy reading Chapter 7 of Sherwood Fox’s book, The Bruce Beckons, who gives a wonderful description of  a very large seiche in Stokes Bay in the late 1940’s.

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