Day n - A mathless look at tides and currents; Rod

If you ever find yourself asking how something works or why something behaves as it does, there's a good chance you are, or could have been, a scientist of some kind. I have an academic background in physics, so I have a fondness for observing oscillations in natural phenomena. When I can't understand something physical it becomes food for thought.

Such was the case when observing that the predicted time for slack current though the Chatham Narrows was two hours after low tide. That's the kind of thing that sits in my brain and bugs me. Oscillations generally come to rest at the highest and lowest points, like springs or pendulums do. Yet here is a current that continues to flow for two hours after low tide. How is that possible?

The introductory material to most reading on the subject of tides in the area will tell you that there is a lot of river outflow that reduces the predictability of tides, but that explanation doesn't satisfy me. Two hours feels like too much to be explained by river outflow.

Lying in bed on the morning we were to leave for the narrows, I had an epiphany: Imagine we have a current station at each end of the narrows and we can generate tide charts for each station (Figure 1). If the water flow supplying each end is different, then high and low tide will arrive at different times. When the tide is higher at one end than it is at the other, current will flow. When the two tides are equal, we have slack.

In Figure 2 we see the green areas where the tide is higher at station B than it is at station A - current flows from B to A; and the orange areas where where the tide is higher at station A than it is at B - current flows from A to B. Where the lines cross, the tides are equal and the current is slack.

If you look at the timing of these events though, the time from high tide to low tide is roughly six hours, so high tide to slack is less than an hour. Something else is at work that explains how the time from high tide to slack can be two hours.

Then it occurred to me that better flow to one station and diminished flow to the other can mean more than a difference in timing of the tides; it can also mean higher or lower tidal swings. Figure 3 shows what happens when one station has smaller tidal swings than the other. The effect is to increase the currents and delay the onset of slack. With that we finally see slack current that has been delayed by about two hours.

As the Greeks would say: Quad Erat Demonstrandum (QED); and all without math, which should leave Tom feeling a little unsatisfied.