The difficulty of recording tides at Port Arthur in 1840

Let’s say the date is 1840, the place is Port Arthur in the penal colony of Van Dieman’s Land and you are Thomas Lempriere. You’re trying to work out when the high and low tides occur.

So you watch a tide register at the dock. You calculate approximately when the tide is supposed to reach its maximum and minimum. But its complicated because the harbour has a seiche:

A seiche (pronounced /ˈseɪʃ/saysh) is a standing wave in an enclosed or partially enclosed body of water. Seiches and seiche-related phenomena have been observed on lakes, reservoirs, swimming pools, bays, harbors and seas. The key requirement for formation of a seiche is that the body of water be at least partially bounded, allowing the formation of the standing wave.

The term was promoted by the Swiss hydrologist François-Alphonse Forel in 1890, who was the first to make scientific observations of the effect in Lake Geneva, Switzerland.[1] The word originates in a Swiss French dialect word that means “to sway back and forth”, which had apparently long been used in the region to describe oscillations in alpine lakes.

Of course this is 1840 and you know nothing about seiches which won’t be described for another 50 years.

But two guys from the future suddenly appear and give you this:

Tide Register from data collected by Hunter and Coleman

Now where do you begin? The seiche makes it complicated to say exactly when high and low tide occur and the times between high and low tide appear to vary considerably because of that and the near resonance of the estuary with the solar tide, making those people from the future think that you’re an incompetent recorder.

Here’s the satellite view of the estuary
and here’s a closer view of Port Arthur harbour:

 

Port Arthur harbour from Google Earth

Clearly you’re going to have a problem if you’re going to work out where the mean level of the sea is.

Advertisements
This entry was posted in Uncategorized. Bookmark the permalink.

4 Responses to The difficulty of recording tides at Port Arthur in 1840

  1. Gary says:

    John,

    What’s the period of the seiche?
    Translation of the graph axes to common form with labels rather than scientific notation would be appreciated if you can manage it easily. Not an engineer here and I’m lazy.

  2. John A says:

    What’s the period of the seiche?

    50 minutes

    Translation of the graph axes to common form with labels rather than scientific notation would be appreciated if you can manage it easily. Not an engineer here and I’m lazy.

    I’ll try in future to make the axes look more sensible. The y-axis is in mm

  3. John A says:

    Actually its not as clear as all that. The seiche has a peak amplitude at 50 mins but can be as low as 49 minutes or as high as 51 minutes.

    Yes I’m being fussy

    • Gary says:

      OK to be fussy, sometimes. Looks like the seiche amplitude varies from roughly 10 to 100 mm and with some periodicity too.

Comments are closed.