Baptist Harbour ‘Observatory’ Seasonal Report

The SOK Baptist Harbour Temperature Profiler has been shut down for the season. This array of 4 thermistors (temperature measuring devices) was installed on the end of the pier you will see in the accompanying video link. Each thermistor was read every 3 minutes and the data transmitted to the web via a $29 Arduino computer installed in Eric Goodyear’s workshop.

thermistor-at-baptist-harbour-sok-500As shown in the diagram, the upper two thermistors T1 and T2 were installed to span the average water level (T1 10 cm above, T2 10 cm below) so that rises and fall in water level would be indicated by changes in temperature as the thermistors were alternately covered and uncovered. T3 at a depth of 55cm and T4 on the bottom at 110 cm were to record temperature fluctuations within the water column. (Not shown in the diagram are the connecting cables to the thermistors, and the fact that they are installed in a tube to damp local wave action.)

Time Lapse Video of Seiche Baptist Harbour by Eric Goodyear

Time Lapse Video of Seiche Event
Baptist Harbour by Eric Goodyear

On September 4th a moderately strong seiche occurred in Baptist Harbour and at other locations on the Huron shore.The record of the upper three thermistors over this event is shown in the next figure. The temperatures on both T1 and T2 oscillate by about 1 degree Celsius between the higher water and lower air temperatures as they are alternately covered and uncovered by the seiche. T3 meanwhile is unaffected. The vertical grid lines are spaced by 16 minutes to show that the periodic covering and uncovering occurs with this quite regular frequency. You can see Eric Goodyear’s spectacular time-lapse photography of this event at https://www.youtube.com/watch?v=cx1Ww-0-HoI.

Throughout the 2 month period that the profiler operated there occurred many oscillations in temperature with strong 15-18 minute periodicities, suggesting that the seiche oscillations are more or less continuously ongoing though at much lower energies than on Sept 4th.

Thermistor String Baptist Harbour

Thermistor String Baptist Harbour

Why this particular periodicity? Think of a basin of water; disturb it briefly and the water will wash from side to side with a repeat rate or “periodicity” dependent on the dimensions of the basin and the depth of water. A harbour, with its openings to the larger lake, is a more complicated basin but it turns out that 16 minutes is consistent with theoretical models for the slosh periodicity (more formally known as the “Helmholtz” period) of a water body with Baptists Harbour’s general shape and water depth.

In a paper recently submitted for publication entitled “Amplification of long period waves in shallow coastal embayments of the Great Lakes”, Bogdan Hlevca and Mathew Wells from U of T and our Parks Canada’s Scott Parker examine the periodicity of seiches in Cove Island Harbour (“La Ronde” as it is often called) and Boat Passage just north of La Ronde. Of the two Boat Passage, a long carrot-shaped inlet, most resembles Baptist Harbour and Hlevca et al. found it also had a Helmholtz period of about 16 minutes while La Ronde (more rounded as the name suggests) had a period of 12 minutes.

What initiates the sloshing? The initial “kick” setting a harbour in motion could be a sharp change in barometric pressure, strong waves piling up on its entrance, or (unlikely here) an earthquake. Once the disturbance abates the harbour water will slosh back and forth, slowly losing energy to the friction of water moving over the bottom.

Large seiche events can cause considerable damage. Local residents may remember the June, 2011 event which grounded tour boats in Little Tub Harbour. So can they be predicted in advance? As far as I know – and I need to do more reading – no attempt has been made to do this around the Northern Bruce Peninsula. We can expect the prediction to be highly harbour-specific, depending on orientation and shape amongst other things. It is an interesting challenge! By combining these seiche measurements with Bill Caulfeild-Browne’s detailed climate records perhaps we can make some inroads on this problem in 2015.

© 2017 Sources of Knowledge