Colvos Passage is very strange. Here in our part of the world, we are used to large tidal swings, large tidal currents. And we expect to see those currents reverse twice a day, as the tide goes out, comes in, goes out, etc.In Colvos Passage, the current always flows North.
How can this be? Even when there is a prodigious amount of water pouring South thru the Narrows, there will be a weak flow North in Colvos. I don't know - perhaps Vashon and Maury Islands sit in the middle of a permanent, clockwise-rotating gyre.
(Please recall that all these pictures are larger than the thumbnails shown on this web page. To see the full-sized version, click on the picture.)
When heading South to the Tacoma Narrows (what looks like a river entering the lower left corner of this segment of chart I captured) and the South Sound, you have to decide if you will go East around Vashon and Maury Islands (by far the longer route, because of the shape of the islands), or take the direct route down the West side of Vashon Island, thru Colvos Passage.
For the Southbound sailor, this is a real dilemma. If there is a good North wind, you will take your lumps against the current in Colvos.
Ah, but if the wind is strongly out of the South, you would be tempted to take East Passage where you will have room to tack, but have to travel nearly twice as far, even before accounting for the distance lost tacking. And you will need to dodge commercial shipping.
If the wind is light, you will need to fire up the engine, and then you will again take your lumps against the current in Colvos.
The current is not constant. As you would expect, it is far stronger during an ebb tide. The Southbound sailor will time his passage thru Colvos to correspond with maximum flood tide, when the Northbound current is at its weakest. He will also take advantage of eddies which form behind points along the shorelines on either side, riding the counter currents closer to shore.
Other than reaching water that is finally moving in your direction, the prize for clearing the Southern end of Vashon Island is when you look to port and Mount Rainier, rising from sea level to more than 14,000 feet smacks you between the eyes.
5 comments:
Why does it have a constant northerly current?
I don’t know...
i would guess the current generally flows north here due to the constriction of water flow capable of flooding over the shallow sill and narrow passage of the tacoma narrows into the southern basin.
It does flow South at times. I have first hand experience of that, having lost a knot in my already slow sailboat on the way North.
I'm an oceanographer (and a sailor) and this is a phenomenon called 'tidal rectification' - it occurs because the tides don't just go up and down (and in and out) symmetrically. The currents generated by tides carry momentum, and because of that, can generate flows (like recirculating eddies) that are reinforced to flow in the same direction on both the rise of fall of the tide. Three Tree Point (on the other side of Vashon) is another example of this - currents right along the shore flow preferentially towards the point regardless of whether the tide is rising or falling. This is from an eddy that forms downstream of the point. River kayakers (and fish!) use these features all the time to get upstream.
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