Canadian Current Atlas Update - Salish Sea

[mick_allen]

Updated from the 1984 atlas by Richard Dewey, formerly Associate Director of Science at Ocean Networks Canada - based on a more fine-grained model. General info here:

www.dewey.ca

A new set of Salish Sea Tidal Current Atlases are available.

www.dewey.ca

Specifics:

www.dewey.ca - SSTCA

An updated tidal current atlas for the Salish Sea!

www.dewey.ca

large files, supplemental details at 10 min intervals around high/low slack for an hour and references to also be published ea year.

 

[chodups]

Updated from the 1984 atlas by Richard Dewey, formerly Associate Director of Science at Ocean Networks Canada - based on a more fine-grained model. General info here:

www.dewey.ca

A new set of Salish Sea Tidal Current Atlases are available.

www.dewey.ca

View attachment 19667
Specifics:

www.dewey.ca - SSTCA

An updated tidal current atlas for the Salish Sea!

www.dewey.ca

large files, supplemental details at 10 min intervals around high/low slack for an hour and references to also be published ea year.

Excellent! Thanks for posting

 

[JohnAbercrombie]

The differences between the Dewey charts and the CHS Current Atlas are dramatic enough that they can't both be correct. I'm encouraging my paddling friends to make careful notes when paddling in the Victoria Oak Bay area to see which charts are more trustworthy.

 

[dimit]

Those will be very useful. Thanks!!

 

[JohnAbercrombie]

The differences between the Dewey charts and the CHS Current Atlas are dramatic enough that they can't both be correct.

Here's an example from noon on March 4, 2026:


There are differences between the CHS Tide Atlas info and the Dewey chart

CHS:

Notice the southbound current near Gonzales Point (golf course). The CHS chart also shows the current direction at the entrance to the channel between Discovery and the Chathams.
Also notice the southbound current on the E side of Discovery, and the strong current flowing from Trial, NE toward Discovery.


Dewey:

The Dewey chart shows only northbound current through Oak Bay, and also northbound current on the outside of Discovery.
No indication of the current direction through the Discovery/Chathams channel.
On the Dewey chart there is a weak current flowing from Discovery toward Trial Is.

 

[cougarmeat]

Which one is correct?

 

[JohnAbercrombie]

Which one is correct?

I wasn't paddling that day; I'm still waiting for a report from some friends who were on the water then.

But similar differences are visible on most of the charts. As the saying goes: "Inquiring minds want to know!"

 

[dimit]

I've sent an email to Mr Dewey, pointing him to this thread and the discrepancies JA has highlighted. I'll keep y'all posted on what he says.

 

[alexsidles]

I wasn't paddling that day; I'm still waiting for a report from some friends who were on the water then.

Ground-truthing this stuff isn't the easiest task in the world. As Dewey himself reminds us in the introduction to the new atlas, there's a lot more to currents than just the tidal harmonics. Other important ocean dynamics also affect currents, including wind, buoyancy, and estuarine influences. The Fraser River, especially, is a notorious confounding factor in our waters—and we have had a lot of rain lately! Dewey also warns that the models may not capture fine details, such as back-eddies and flow separations at abrupt headlands. Could this be the source of the discrepancy between the two atlases at Gonzales Point, for example?

In any event, it seems to me that a fair ground-truthing effort would have to either somehow exclude the non-tidal variables on the day of testing, or else run enough test days to average out the non-tidal variables. A single day's visit on a randomly selected day doesn't strike me as a fair test of Dewey's model or any other.

I agree with John that there are significant discrepancies between the old atlas and the new atlas. Like John, I picked a few random locations and times and found several examples of localized currents flowing opposite one another in one atlas compared to the other (although in general, the two atlases agree more than they disagree). I assume the new, more precise modeling in the new atlas is more accurate than the old, less precise modeling in the old atlas, but who knows? I'll be interested to see if Dewey weighs in.

Alex

 

[JohnAbercrombie]

I assume the new, more precise modeling in the new atlas is more accurate than the old, less precise modeling in the old atlas, but who knows?

The old atlas data was based on modelling, which was checked against actual measurements. I don't know how the Dewey modelling was checked.

 

[JohnAbercrombie]

Dewey also warns that the models may not capture fine details, such as back-eddies and flow separations at abrupt headlands. Could this be the source of the discrepancy between the two atlases at Gonzales Point, for example?

Perhaps.
The statement that the new charts are 'better' because they are based on more powerful computer modelling caught my attention. I've paddled quite a bit in Oak Bay, and the currents have hardly ever seemed straightforward. So my bias is to give more credence to the charts for the Oak Bay area which show more complex current patterns.

 

[alexsidles]

The old atlas data was based on modelling, which was checked against actual measurements. I don't know how the Dewey modelling was checked.

Dewey based his new atlas on Foreman's 2004 tidal model in "M2 Tidal Dissipation Around Vancouver Island: An Inverse Approach." The old atlas was based on Crean's 1976 and 1988 tidal models efforts in "Numerical Model Studies of the Tides Between Vancouver Island and the Mainland Coast" and "Mathematical Modelling of Tides and Estuarine Circulation."

If I'm understanding Foreman correctly, his checking process was to compare his model's predictions against the measured water levels at the tide gauges. Of course, Crean had done the same thing back in the 1970s! The difference, if I'm understanding correctly, is that when Crean's model didn't match the measured tide levels, Crean arbitrarily juiced whatever variables he needed in order to make the model fit reality. In particular, Crean tweaked his bottom friction coefficients at different locations until his model matched the measurements, even if there was no data to support the notion that the actual, physical bottom friction at these locations was different in reality. Supposedly, Foreman relied on a different approach, leaving the bottom friction coefficient alone while tweaking the rates of tidal dissipation to make his model match the measurements.

Perhaps Foreman's is a more reliable approach, because the model would need to add more eddies and swirlies and whatnot to supply the needed tidal dissipation? And perhaps those additional eddies and swirlies and whatnot would more closely match the reality of the water's movement?

Frankly, I don't see a principled difference between Crean arbitrarily cranking the bottom friction coefficient and Foreman cranking the tidal dissipation mechanisms. We know Crean was probably wrong to tweak the bottom friction coefficient because there was no data to support the notion that the actual bottom was different in the way he was tweaking it. But why is Foreman's tweaking of the dissipation mechanisms any more trustworthy? There's an intuitive appeal to tweaking the water movement instead of tweaking the bottom composition—after all, we know the bottom composition tweaks must be wrong, whereas the water-movement tweaks might at least be correct. But it strikes me that it is still guesswork, and I don't know enough to say whether these newer guesses are any better-founded than the previous guesses.

Alex

 

[dimit]

I got an interesting and informative reply from Richard:

Dick,

Thank you and the other paddlers for the posts and interesting discussions.

I registered (westcoastpaddler), but it seems I can follow and watch the thread, but not reply. Premium membership?

Anyway, good discussion. In general I agree with all the posts (if that’s possible). One experience on the water is likely not enough. Fine scale features do not get modelled exactly. The new model was verified / adjusted against tidal heights to minimize the differences between modelled and measured heights. It does remarkably well (95-99% agreement). When compared to observed current stations, good but not as great (80-120%). Mike Foreman, one of the best and most knowledgable tidal modellers in the world couldn’t do better at the time, and acknowledges that current structures can be small and complex, giving rise to many small differences. To get the heights so good, the model must be getting the net current flows right, if not the fine scale eddies etc. So take any “interesting ” feature at the smallest scale with a grain of salt. The character of turbulence is a certain amount of randomness, and in many locations, the scale of “turbulent” flow is 100m.

I did not develop the model. I received from Mike a compact (8 constituent) output from the model that allows a fairly robust reconstruction throughout the region. My contribution is really just computing and plotting output in what I hope is a helpful product. I did check my 43 maps against the original 1984 Atlas, and am confident I got the correct tidal phases and general patterns right.

I was clad to see your users also bothered to read the preamble in the new Atlas, which tries to qualify these complex ideas. I will continue to monitor this discussion and would be happy to rejoin the group at a future SISKA meeting to explore all these ideas. I could even invite Mike, as he is a super presenter.

Feel free to cut and post.

Richard Dewey

I'm spending time this spring and summer paddling in the central Salish Sea, the Discovery Islands, and the north end of the Island, and will be using Richard's charts for route planning and timing. I'll report back here, and to Richard, with my comments and observations.

 

[RKDewey]

All,

Thank you all for your interest and feedback on the updated Atlases. There are always limitations to model output, especially complex flow models, so remember: “all models are wrong, some are useful.”

It should be noted that I could run the model for any time and location. So if there is some exact site and exact time when you observe some tidal flow / feature, let me know and I’ll run the model for that exact time and we can see how it does.

The Atlases have several compromises. First, the time step is one hour, which is large. I’d rather use something smaller, or even variable, like 15 or 30 minutes, but then the Atlases would be over 1000 pages and truly huge (GB).

As an example of this, I ran the model at finer time steps for a high slack period and low slack period, at 10 minute intervals. These are already posted at https://www.dewey.ca/

I also generated 1 mjnute = 1 frame animations for these slack transitions, and I’ll post those there as well.

The approach adapted by Pat Crean, and I followed to leverage existing use, was to model only six tidal phases, three floods and three ebbs, for small, medium, and large tides (each). THEN, I do my very best to match up any future hourly time/tide to one of the hours within those six cycles (43 maps). This can be a fairly good match, or not so much. For Volume 2 (Puget Sound) I use 8 tidal cycles, and more than 60 maps. So the matching is a best approximation, not a perfect independent model run for that time. The lookup table shows the time match is sometime close, but may be as much as 30 minutes off from even the best fit. I could look into a means of assessing ‘goodness of match’ and colour code the boxes in the lookup tables to provide some estimate of confidence.

Please continue to assess and comment of the utility of these Atlases, as I can adjust and regenerate them if the updates are warranted.

Richard

 

[Oceaneer]

On theme

If you can find a copy of this book its worth having .An out of print Environment Canada publication , it is a labour of love including real science and observations by experienced and working mariners . It took me a few years to acquire 2 copies hunting Vancouver Island bookstores, which is a whole story in itself . At one point amazon was asking $350 dollars for it . Bonus art and anecdotes by Emily Carr.

 

[SWriverstone]

No disrespect to anyone intended...but does all this matter much? What I mean is, I know I can easily paddle 4 knots in relatively calm (or slightly rough) conditions for a few hours (and assuming I'm not paddling straight into strong winds). So when looking at tides, the only currents I really care about are those 3 knots or higher. Below 3 knots, I simply view it as "noise" that might slow me down a bit but won't be life-threatening. Am I a fool? LOL (My assumption is that most of these current charts will agree when it comes to the BIG currents...but perhaps not?)

I admit perhaps I'm overlooking some common situation where a 1.5kt random current could (maybe in combination with other currents) kill me? Admittedly I have limited coastal paddling experience compared to many here...but the only times I've ever been slightly worried in coastal waters is when the current is more powerful than I am.

Just thinking out loud.

 

[JohnAbercrombie]

does all this matter much?

You'll find out.
Besides 'draining the energy tank', currents can have a big effect on wave heights.
You may not be paddling solo, so you don't want to put your companions into unanticipated 'problems'.
Some years ago a group paddle (I wasn't paddling that day, but friends were) had a lot more excitement than planned going around Trial Island (Victoria). Everybody was OK (eventually) but it wasn't a happy day for some novice paddlers.
It can also affect your navigation - results vs prediction - on a crossing.
Taking hours to cover the last few miles to your planned campsite isn't fun.

 

[CPS]

Paddling against a current for the entire duration doesn't seem like a big deal until the wind piles on and you still have miles until the next viable (even marginal) campsite.

A 12 nautical mile day seems like a cakewalk until it takes 12 hours...

 

[alexsidles]

does all this matter much?

Yes, it matters a great deal. If you paddle at four knots, but you lose three knots to an adverse current, then you have just cut your speed by 75 percent. That eleven-mile crossing you thought was going to take 2.5 hours is now going to take you 10 hours. That's the difference between arriving by lunchtime versus arriving in the dark.

An adverse current also robs you of the opportunity to rest. Every minute you spend resting means two minutes' additional paddling to make it up. This at a time when the current is already causing you to travel slower and work harder than you had planned.

Currents also make waves worse, as John points out above. On a day when there are wind-waves, especially wind-waves traveling opposite the current, the current will interact with the waves to produce steep chop. A sufficiently fast current can generate breakers even in deep water far offshore, where you would not ordinarily expect to encounter surf. Even if you don't encounter full-on breakers, you will still have to bash through the chop, which will slow you down even more than the wind and current already are. Your chances of capsizing will also be far higher than if you were only facing current alone or wind-waves alone.

Alex

 

[SWriverstone]

Thanks, LOL. Though I might add that I **do** understand how paddling against the current can ruin your day. (I have *some* experience in coastal waters.) Your point @alexsidles was exactly the one I made: a 3kt current is a big deal, and that's the kind of current I'd be concerned about! (And I also understand how opposing winds and currents can generate big waves.)

My question (which I didn't clarify) wasn't about currents in general, but being concerned about the small ones—the 0.5kt current, the 1kt currents, or even 2kt currents. I've never had a problem making at least 2-3kts against all those smaller currents. (And you'd never typically need to paddle against those for more than a few hours, if that.)