Northwest Dive Club

I'm a beginner (nine dives) and I'm trying to understand the effect of tidal exchange in planning. (Sorry if this post is much like the thread on Hood Canal)

Here are the tides for Sunday: And here is the prediction of current for Edmonds:

Edmonds, 2.7 wsW of, Washington Current
18 May 2014
47.9500° N, 122.5833° W
2014-05-18 00:23 PDT -0.47 knots Max Ebb
2014-05-18 04:26 PDT 0.00 knots Slack, Flood Begins
2014-05-18 05:27 PDT Sunrise
2014-05-18 05:35 PDT 0.07 knots Max Flood
2014-05-18 07:25 PDT -0.00 knots Slack, Ebb Begins
2014-05-18 09:26 PDT Moonset
2014-05-18 11:50 PDT -0.69 knots Max Ebb
2014-05-18 15:53 PDT 0.01 knots Slack, Flood Begins
2014-05-18 18:29 PDT 0.29 knots Max Flood
2014-05-18 20:46 PDT Sunset
2014-05-18 22:02 PDT -0.00 knots Slack, Ebb Begins

And here it is for Alki:

Alki Point, 0.3 miles W of, Washington Current
18 May 2014
47.6667° N, 122.4333° W
2014-05-18 03:06 PDT 0.00 knots Slack, Flood Begins
2014-05-18 04:45 PDT 0.22 knots Max Flood
2014-05-18 05:28 PDT Sunrise
2014-05-18 06:59 PDT -0.00 knots Slack, Ebb Begins
2014-05-18 09:26 PDT Moonset
2014-05-18 10:52 PDT -0.69 knots Max Ebb
2014-05-18 14:33 PDT 0.01 knots Slack, Flood Begins
2014-05-18 17:39 PDT 0.88 knots Max Flood
2014-05-18 20:44 PDT Sunset
2014-05-18 21:36 PDT -0.00 knots Slack, Ebb Begin

(source http://tbone.biol.sc.edu/tide/tideshow.cgi)

Things I don't really understand:

• I think of Edmonds being near Alki and yet Max Ebb is an hour later at Edmonds than at Alki. Why?

• Is -.69 knots ebb current significant for a beach dive? Do ebb currents matter much more than flood currents?

• Is Sunday the 25th (a week from this Sunday, see below) likely to be an easier day because the tidal exchange in the mid-morning is 10 ft instead of 12 feet and it flooding instead of ebbing?

Thanks for any understanding you can share.
- Carl
p.s. We're also very interested in viz reports this weekend, especially from Edmonds.
p.p.s. Tides for Sunday the 25th:

Hi Carl,

carlk3 wrote: Things I don't really understand:
I think of Edmonds being near Alki and yet Max Ebb is an hour later at Edmonds than at Alki. Why?

They are roughly 10.5 miles apart as the crow flies, that's a pretty significant distance for water.

carlk3 wrote: -.69 knots ebb current significant for a beach dive?

Not particularly, but it may seem brisk, depending on your level of comfort, strength, etc. Ideally you would be diving it at slack, not at the max at ebb or flood though.

carlk3 wrote:Do ebb currents matter much more than flood currents?

It varies and is absolutely dependent on the site. There is a preference for Edmonds but I don't know if offhand (I believe it's in Fischnaller's book?). Hopefully someone else will pipe up.

carlk3 wrote: Is Sunday the 25th (a week from this Sunday, see below) likely to be an easier day because the tidal exchange in the mid-morning is 10 ft instead of 12 feet and it flooding instead of ebbing?

The exchange is smaller on the 25th, so technically yes, currents should be less.

Edmonds, 2.7 wsW of, Washington Current
25 May 2014 - 27 May 2014

47.9500° N, 122.5833° W

2014-05-25 00:13 PDT 0.15 knots Max Flood
2014-05-25 02:45 PDT -0.00 knots Slack, Ebb Begins
2014-05-25 03:44 PDT Moonrise
2014-05-25 05:20 PDT Sunrise
2014-05-25 06:54 PDT -0.72 knots Max Ebb
2014-05-25 11:08 PDT 0.00 knots Slack, Flood Begins
2014-05-25 13:24 PDT 0.23 knots Max Flood
2014-05-25 16:38 PDT -0.00 knots Slack, Ebb Begins
2014-05-25 17:48 PDT Moonset
2014-05-25 19:43 PDT -0.40 knots Max Ebb
2014-05-25 20:54 PDT Sunset
2014-05-25 23:18 PDT 0.00 knots Slack, Flood Begins

But...they are both large exchanges. I have been diving Edmonds for 9 years, and have ALWAYS experienced current, sometimes quite significant, when there is a large exchange, regardless of what the current predictions may be. Plus it is a looooonngg walk in and out when there's a super low tide. IMHO it's not an ideal place to dive on days with really large exchanges. YMMV.

Happy diving!

And the vis is pretty brutal (worse than normal) there during large exchanges. My opinion, I would wait until you can dive a slack with smaller exchange.

edmonds is current sensitive.. cove 2 is not.. on a day like sunday, you would seek out sites to dive that are not current effected

H20doctor wrote:edmonds is current sensitive.. cove 2 is not.. on a day like sunday, you would seek out sites to dive that are not current effected

+1 on that. With your total dive count, this is extremely good advice. Heck with my dive count its still good advice!

Thanks for all the info! It prompted me to try to learn more about title exchange. Here is some of that:
http://www.scubadiving.com/drive-and-di ... uget-sound

• When to Go: Winter has the best viz, but summer months allow long diving days. Avoid spring’s algae blooms and glacier runoff.
...
• Best Tip: Plan your trip around the tides, looking for minimal tidal exchange to maximize flexibility. On full and half moons, some of the sites are not accessible because tidal flow can be 16 to 20 feet.

http://www.scubaboard.com/forums/socal/ ... -site.html

[from SoCal]
Tidal Exchanges:
Sites that are often swept by large currents (Pt. Dume) are best dived when the tidal exchange is small. The difference between the high tide and the low tide is "the exchange." 2 foot exchange is small. 8 foot exchange is NOT. The bigger the tidal exchange, the more water is moving which could potentially turn into massive currents.
WOTADs (Weird One Tide A Day) mean the water is barely moving during the exchange. The slope on the tide graph is very shallow. Check out November 3, 2008, on http://www.tidelines.com. THAT is a WOTAD.
If your dive day has a big exchange, try to dive just before the highest tide (pushing tide), so there is less water moving.
Try NOT to dive on the steepest slopes of the tide graph.
The Lowest tide often creates murkier water due to debris being dragged out into the water as it recedes.. but not always.

http://www.shorediving.com/Earth/USA_We ... unds_Park/

04/30/2011 Ken 2 (Avg: 3.40 Review) - [...] It can be a bit of a surface swim out to the cool stuff during high tide. There's rarely any real current here, but visibility is usually best during smaller tidal exchanges. Visibility often goes to zero during larger exchanges, so it's always good to see what the tides are doing. If you've never been here before, you may hear the nearby ferry coming/going. It can sound like it's right on top of you, but as long as you're in the roped area, you're safe. The Cabezons get a little irritable from about Dec-March when they're sitting on eggs. They'll charge and give you a head-butt if you're too close, but they can't really hurt you. If one comes after you, just give them some room, and make sure they don't hit your mask/regulator.

- Carl