Monday, December 7, 2015

No, It's Not That Simple

The tide comes in, the tide goes out. The current flows in, the current flows out. That's it, right?

Well, no, actually.




First a little orientation for those of you not familiar with the area - this image shows the upper left-hand part of Washington State.  The important features to note are:
  • The Strait of Juan de Fuca - the large strait connecting the Pacific Ocean to the entire Salish Sea
  • Puget Sound - the body of water reaching from a little north of Everett southward - basically everything south of the large word "Sea"
  • The San Juan Islands - the group of islands just north of the large word "Sea" and south of the border with Canada
  • The Strait of Georgia - the body of water running north of the San Juan Islands, between Vancouver Island and the Canadian mainland

So the water coming in via the Strait of Juan de Fuca (and it is a LOT of water - imagine how much has to flow thru here to raise the water level in all of Puget Sound and the Strait of Georgia 12-16 feet in just 6 hours...) meets a decision point when it reaches the eastern end of the strait...  it is forced to split, part going north into Canada, and part going south into Puget Sound.  That's a lot of momentum, forced to make a sharp turn in one direction or the other.  You might imagine that things could be a little, umm, confused at the eastern end of the Strait.  And you'd be right.

But like Ron Popeil says...  "Wait!  There's more!"  

Tho the water heading south to Puget Sound has a pretty straight shot past Point Wilson and into Admiralty Inlet, the northbound flow faces the obstruction of the San Juan Islands:


The image above shows a realistic picture of what the flows look like (the entrance to Admiralty Inlet and Puget Sound is at the bottom right-hand corner).  First, notice that two major gyres have developed, just south of Victoria and at the entrance to Admiralty Inlet, and some minor ones off of Port Angeles and at Cattle Pass just south of San Juan Island.  And the flow paths thru the San Juan Islands themselves are by no means simple.

But wait!  There's more!  

The picture gives the impression that things are static.  Reality isn't.  The flows morph and change throughout the entire tidal cycle, changing and reversing directions in the most amazing ways.  And the flows are different for each day in the tidal cycle, and for each month of the year.  In our area we have two tidal cycles that, like two wave trains on the ocean, some times nearly cancel each other out, and at others reinforce each other.  How could a mariner ever keep track of all of that?

Well there is an answer.  In 1999 the Canadian government compiled a series of nearly a hundred charts like the one above and published them in book form (the chart above is a portion of one of them):




But wait! There's more!

It's not easy to select the chart that applies to a particular date and hour.  I'm not going to go into how the published instructions in the book select the particular page you need.  Just know that it uses a couple of nomographs and is not simple.

Because of this, there are a couple of published tables (Washburne's, Murray's) that you just enter with a date and time and they tell you which page in the Current Atlas applies.  But tho the Current Atlas is timeless, these published tables need to be re-purchased each and every year.

But wait! There's more!


There is a solution to that too!  A sharp individual named Emanuel Borsboom puts equivalent tables online for you to view or download and print (free for non-commercial use).  If you are computer savvy, he also makes the code that produces the tables available.  With that code, you can produce your own tables forever, without depending on Internet access.  Or Borsboom's continued interest in publishing them online.

So, armed with this information the astute mariner should not get caught unintentionally stemming an adverse current. With flows approaching 5 kt in some places this is not unimportant, especially for sailors whose boat speeds mean that they could actually be moving backwards.

Our copy of the Current Atlas is on its way, and we are looking forward to speedier and more relaxed passages next season!  No surprises!




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4 comments:

Rick said...

Wow. I had no idea. And I thought the salmon fishing rules were complicated. . .

Anonymous said...

This is a tremendous find, Robert. Kudos to Mr. Borsboom for the effort and to you for the publicity!

Alaskaman said...

What's your source for the current atlas?

Robert Salnick said...

Amazon has the Current Atlas

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