I’ve got a question about what currents at the edge of very large channel (20+ miles wide) are doing compared to what the closest current station in the middle of the channel is indicating. Please see below data for this Saturday for the middle of the entrance to the Strait of Juna de Fuce in Washington State. At 14:43 this data is showing a slack before the flood at the middle of the entrance to the Strait. My understanding is that with such a large channel the same slack before the flood will likely start about 2 hours or so earlier at the edge of the entrance to the channel, for instance at Cape Flattery, the northwesternmost point of the Olympic Peninsula in Washington State. Is this understanding correct?
Thank you for your help.
Strait of Juan de Fuca Entrance, Washington Current
25 June 2005
48.4500° N, 124.5833° W
2005-06-25 00:07 PDT Moonrise
2005-06-25 04:26 PDT 0.00 knots Slack, Flood Begins
2005-06-25 04:50 PDT 0.02 knots Max Flood
2005-06-25 05:13 PDT -0.00 knots Slack, Ebb Begins
2005-06-25 05:17 PDT Sunrise
2005-06-25 09:26 PDT Moonset
2005-06-25 10:47 PDT -2.03 knots Max Ebb
2005-06-25 14:43 PDT 0.00 knots Slack, Flood Begins
2005-06-25 17:44 PDT 1.37 knots Max Flood
2005-06-25 20:52 PDT -0.00 knots Slack, Ebb Begins
2005-06-25 21:24 PDT Sunset
Tough question
Tidal currents behave much like rivers with respect to the fastest cuurent being found mid-channel (like mid-river). Typically the current runs more slowly at the edges, often due to a shallower depth and or proximity to the stationary bank.
But more importantly there is the potential for eddies and/or upwellings that could increase or decrease the current and wreck havoc with timing versus what’s happening in the main channel at the tide station.
Eddies and upwellings will also change dependent on variations in the tidal range, just as river features change dependent on flow levels.
Heavy rains that increase river flows retard estuary flood currents and delay tides.
Sounds like you would be best off talking to people with local knowledge and maybe find a tidal atlas for that area.
Good luck!
Jed
wow
what a great technical question!
I would try to email some people at bodyboatblade.com
about the straits of san juan de fuca, if I were you, I think they paddle there?
What data are you using that indicates the flood would start 2 whole hours earlier in once place than another?
kwikle
Thanks for the suggestion. I hadn’t thought about that. I’ll call over there and see what Shawna and Leon’s thoughts are.
I’ve read several places that current at the edge and mouth of a channel will turn earlier that where the main body of current is in the middle of the channel. I’ve also talked to one of the Shellbacks (a fairly exclusive kayaking club in this area–ask flatpick–see www.shellback.net ) who’s been in this area and that’s where I got the about 2 hours timeframe.
Thanks again for your suggestion.
Juan de Fuca
Howdy,
I paddle here quite alot as I live in the sound region. There are two current stations that I know about, though there may be more. The first is the American one in the West Entrance to the strait, just east of Neah Bay, located in the middle of the strait. The second is the Can’eh’dian one in the middle of the strait, south of Victoria, effectively Central Waters of the strait. Just Google the Canadian Hydrographic Service for current readings. Also, there is a tide and current prediction service that will also have the information.
http://tbone.biol.sc.edu/tide/sitesel.html
Some things that you should have in your arsenal for understanding the tides and currents are a copy of the Current Atlas-Juan de Fuca Strait printed by the CHS and a copy of Captain Jacks Tide and Current Almanac-Puget Sound, including the strait, etc. On the latter source, your question can be answered directly by referring to Appendix B, Tidal Current Corrections.
Appendix B lists the lats and longs of the current station and then it gives time corrections, true compass bearings of current flow, speed ratios and average speed and directions for 20 locations along the south side of the strait. By consulting it, you will be able to identify exactly what is going on where you intend to paddle. Very easy to use
The former source, the CHS Atlas to the strait, gives you 3 color pictures of most of the strait and the waters of Georgia Strait to boot. You use a yearly published pamphlet of tables that give you the page numbers of the atlas to refer to as to what’s going on with the current. A given page of the atlas depicting the strait will detail a collection of arrows of varying sizes. Each arrow has a detailed speed associated with its size. In otherwords the smaller the arrow the lighter the current. The arrow is directed where the current is flowing towards. Very easy to use.
There are other tables to use, such as Washburne’s Tables, similar to the CHS version and other tide and current guides. Most nautical chart dealers, bookstores have something decent, but I prefer Captain Jack’s the most.
There are some other things you will want to know. Look at the length of ebb that you posted in your query, it is MUCH longer than the flood. The 6 hours of current going one way or the other does not apply here. Usually, the ebb is about 2 hours longer than the flood on the south shore of JDF. This is because of the huge volume of water that empties from Puget Sound and Georgia Strait. The ebbs on springs are usually much longer. In the Capt Jack’s almanac you will often see max ebb followed by slack, then another max ebb, followed by a weak flood. I once returned to an empty bay and suffered a long portage because I failed to account for these.
Finally, obtaining the current NOAA and Canadian charts for the strait are important. The canadian version gives current directions and tidal diamonds not found on the NOAA version. They will tell you a lot about the area. Have fun!
Rob G
basically correct
But this is where you would need either a lot of recorded data, ie, lots of current stations to compare, or rely on local knowledge.
But, yeah, you seem aware of something many are not. For instance, “slack” water is not water that is not moving, just that the set and drift are not consistent. In areas with reversing currents (as opposed to rotary), the new current will seek the path of least resistance. That would be in such places as eddies caused by the previous event (an eddy caused by the ebb, can increase in velocity around the beginning of flood, after all,it is already going that way), and/or shallow water and shoreline.
I know of no prediction calculation for this, it is based upon local topography, and highly variable.
In Deception Pass, the middle can still be ebbing, but the sides will start to flood, and the sheer line it forms starts to move towards the middle. Wild!
For Cape Flattery, I have only been there less than a dozen times, but I did not notice anything at the edges in the time of two hours. Probably much less, and generally the currents are not that strong ('tho there is a nice tide race just west of the Cape)
At the Columbia River Bar, when the ebb is above 3kn, the eddy current outside Jetty A increases to 5+kn 20min ahead of slack before flood. Can get some into real trouble. I have seen something similar in Anglesey.
Just being aware of it gives you an advantage!
karl
Thanks Rob G
I didn’t realize that Cap’n Jacks would have data for currents in the Cape Flattery area. I’ve seen the publication many times, but haven’t really looked at it. I’ll have to study closely the info you mention for the south side of the strait. I’ve got the CHF Current Atlas and Washburne’s tables for it so I’ll consult those further. I’ve also got the NOAA charts for that area, but not the CHF ones.
Thanks for the tips!
Thanks otterslide
Ok, so it sounds like that the 2 hours timeframe is off. Maybe I didn’t read the email right from my initial source.
I talked to the folks at Body, Boat, Blade and they thought the current changes, etc. there at Cape Flattery would be much closer than that to the the current station in the middle of the staight as well.
We were thinking of heading up there this last weekend, but I was just unsure enough of my local knowlege there that we decided to wait until we had a better handle on the current/wind/bottom topography interaction there at the entrance to JDF (thanks for the abbreviation Rob G) and with Tatoosh Island, the big underwater rocks about mid channel and that fractured and high cliff Cape Flattery coastline. We settled for a paddle to Shi Shi and getting worked and maytagged in the surf in the middle of Shi Shi for hours…man that is fun!
Thank you for your insights.
Keep me in mind…
when you head out that way. We go out there all the time. If you can handle Shi Shi, Portage Head/Anderson Point, to my way of thinking there is not too much of a jump to the Cape itself. There are some races to deal with, but really not that big of a deal. Pay attention to the NOAA swell report and shoot for a low swell weekend with low wind for the first time. Going holistic on your prediction, try to see to it that you aren’t jumping out on a new/full moon weekend or a couple days afterwards.
Rob G
Thanks Rob G
Thanks for the offer! I will keep it in mind.