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bluestar
09-11-2006, 10:17 AM
Should slack currents correspond exactly to high/low tides?

I was checking the current and tide condition for Golden Gate on Dec 26th because I'm intending to entertain a couple of guests by taking them under the bridge then back. I found that, according to the sites, the slack current and high/low tides are off by about 2 hours. That sounds incredible!

Here is current info grabbed from webpage:

San Francisco Bay Entrance (Golden Gate), California Current
2006-12-26 02:42 PST 3.45 knots Max Flood
2006-12-26 06:07 PST -0.01 knots Slack, Ebb Begins
2006-12-26 08:41 PST -2.88 knots Max Ebb
2006-12-26 12:20 PST 0.00 knots Slack, Flood Begins
2006-12-26 14:57 PST 2.17 knots Max Flood
2006-12-26 17:46 PST -0.00 knots Slack, Ebb Begins
2006-12-26 20:49 PST -3.48 knots Max Ebb

Here are tide info:

Tides for San Francisco (Golden Gate) starting with December 26, 2006.
26 High 4:26 AM 5.4
26 Low 10:04 AM 2.4
26 High 3:42 PM 4.7
26 Low 9:55 PM 0.5

Is this normal???

Fish_R_Us
09-11-2006, 10:44 AM
I've pondered over those observations a long time ago. Perhaps it's the tilt of the earth in respect to the moon's orbit, or the 15 degree difference between magnetic and true north? Where are the astro-physics experts?

cabbagehead
09-11-2006, 11:33 AM
Should slack currents correspond exactly to high/low tides? *
No. Not at the Gate.

There are 2 types of tidal waveforms: Progressive waveform and standing waveform. In a progressive waveform, maximum currents are at peak high and low tide. This is the common tidal pattern offshore, beyond the influence of land and shallow water. For a standing waveform, slack current coincides with peak high and low tide. Standing waveforms are common in restricted areas, such as bays, inlets, and estuaries. What you are seeing at the Gate is a mixture of the two: peak flow would be off from peak tides ~3 hours if progressive and 0 hours if standing. In your example, it was off 2 hours, so it is a mix, but more influenced by a progressive wavefront.

Hope that helps...it is all confusing. I'll make up a graph if enough people want to see difference. I recently read up on this stuff in The Fisherman's Ocean by David A. Ross. The subtitle of the book is "How marine science can help you find and catch more fish." Enough said. *;)

cabbagehead
09-11-2006, 12:32 PM
...peak flow would be off from peak tides ~3 hours if progressive and 0 hours if standing.
Oooops, I read this again and managed to confuse myself. Reverse that above...peak current off from peak tide ~3 hours if standing wavefront and 0 hours if progressive...doh!