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post #1 of 25 Old 07-11-2008 Thread Starter
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Tidal Current Speed

When looking at graphs of tidal current speed, they describe the current changing in a way that seems counter-intuitive to me. They typically show a sine wave form with a horizontal line across line across the chart that denotes no current. Above the line is the current speed coming in (flooding), below going out (ebbing).

Interpreting this chart, I expect that at slack tide, when the curve crosses the center line, the water should be basically still, but a moment after that precise moment, it starts to accelerate very quickly to near maximum speed.

When I observe the tides, however, I see something different. It rises quickly, but as it approaches its azimuth, the rate of rise slows until it reaches the peak, at which time there is no movement for a while. Then the speed gradually starts to increase. For that to be charted, the rise would be less steep than what I see on the above chart.

Am I missing something? How does it really behave?

TIA
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Last edited by jbondy; 07-11-2008 at 12:06 PM. Reason: Inserting graph
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post #2 of 25 Old 07-13-2008
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It depends on where you are. Sometimes it's like you noted, others like Deception Pass, it's like that chart.
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post #3 of 25 Old 07-14-2008
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When you are watching the tide, it behaves as you describe. The currents associated with this behaviour are based on the difference in water level on either side of the narrows. The current reversal can be quite sudden as the levels either side momentarily equalize, but the driving force of the tide has still gotten ahead so the current will build relatively quickly.

In some narrows the slack time is very very short, others you get about 10-20 minutes grace.. it all depends on the geography and the tidal range.

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post #4 of 25 Old 07-14-2008
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Current predictions are much more complicated than tide height predictions. The current is a vector (direction+speed). Consider that in many places in the open water, the current flows are rotary. They do not change much in speed, but rotate around the full 360 in direction.
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post #5 of 25 Old 07-14-2008
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someone once gave me the percent of water movement as the tide changes - to use as a rule of thumb - it went something like 20% in the first 2 hrs ect - and that is not correct -
does anyone know what that rule of thumb is?
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post #6 of 25 Old 07-14-2008
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The rule is called the Rule of Twelveths.

Assuming a six-hour high-tide to low tide cycle

In the 1st hour 1/12 of the water will move.
In the 2nd hour 2/12 of the water will move
In the 3rd hour 3/12 of the water will move
In the 4th hour 3/12 of the water will move
In the 5th hour 2/12 of the water will move
In the 6th hour 1/12 of the water will move

So in an area with 6' tides, the height of the water will drop:

6" in the first hour
12" in the second hour
18" in the third hour
18" in the fourth hour
12" in the fifth hour
and 6" in the sixth hour.

In a twelve hour cycle, it will be every two hours.

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post #7 of 25 Old 07-14-2008
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For tidal height it is the "rule of 12" 1-2-3-3-2-1. 1/12 the total change in the first hour, 2/12 the second hour, etc. I know of no similar general rule for currents. Current flows are just too variable. Consider the Cape Cod canal, 3 ft tides on one end, 9 ft tides on the other.
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post #8 of 25 Old 07-14-2008
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thanks - great planning tool
i would also assume that the greater the volume the faster the flow - or am i an assume
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post #9 of 25 Old 07-14-2008
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In tidal passes and narrows the timing of maximum current and slack will NOT coincide with high and low tide times so be sure to use the current tables appropriately.

Largish bays in areas of large tide ranges often do turn into "toilet bowls" with rotary and reversing currents depending where in the bay you are. These are typically "known" by locals, esp racers and tow boat operators, but not usually documented. Nevertheless if you know the pattern using these currents can be helpful (or less hurtful).

We have some extreme tidal narrows in BC and the power of these passes is not to be ignored. Some can run up to 15 knots on large tides, and many regularly used ones run up to 6 knots, effectively closing them to most sailboats during those times.

An example below:


Ron

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post #10 of 25 Old 07-14-2008
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It does look a bit strange that tide table. More water going out than in, it seems.
I would have thought that at high water and low water the tide would not be moving as the flow is about to reverse, but no-doubt there is a complication somewhere.
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