Tidal Question

[RenaB]

Ok, so I have a few questions. I have studied dive tables a little because I have always loved tidal pools and need to know when low tide is to enjoy these. The ebb part is what is confusing. I have done some reading online, and I don't think it's very easy to grasp how to figure when this is. So, first let me try to write what I think I know.....

Ebb is when you get no increase tidal activity or decrease tidal activity. IF you were basing this prediction solely off the ocean (meaning not considering you have a river/lake connected) it would appear that the ebb would be the moment inbetween the lowest point and the highest point. I know this is a simplification of the process, but bare with me.

FIRST Question.....IF the above statement is correct, then there would be two "ebbs". The time between the high and low and then another between the low and high. It would seem to avoid being swept out to sea, that you would choose the ebb just before the one between the low and high. That way the tide is just beginning to come in. (First presumption correct?)

SECOND QUESTION...How long before the one in between? If I have chosen the one correctly, should I start 10 minutes before the ebb? ---enter water at that time, not start doning.

I know that for the most part everything I have read states, ask the dive guys in the area for the correct time to dive, and when to come out of the water. Well, that's nice for everyone who knows everyone, and as you can see I am not shy about asking, but it makes it difficult to plan if I have to goto the shop, and say hey, this is what I was planning, he says, great idea, but you have 15 minutes to get in the water.

I looked up the tidal table thing we have on the site to see the ebbs for certain areas, and this was what my researched produced.

Thanks in advance.

[Don-B]

The Quick answer is "ebb" is The period between high tide and the next low tide in which the sea is receding.
That is the outgoing tide.

Then you have "Flood" that is the incoming tide.

The period between flood and ebb is "slack" .

I could spend the next hour explaining all the nuances' about tides and the best time to dive.
There are just a lot to factor in. It will take you a little while to figure it all out.
Ask questions , get advise on the dive site and the best time to dive it.

[Linedog]

Kinda like Top Dead Center and Bottom Dead Center of a piston driven engine.

[RenaB]

Ok, yes, sorry I had the ebb and slack words mixed. So, the period between any ebb and flood is slack time, correct? So slack is not on both sides, but only between flood and ebb, not the time between ebb and flood as well?

I guess this is my main question: Only by logic (albeit maybe flawed) it would seem first that there is a "slack" on each side of ebb and flood. AND, again logic, it would be better to dive in the "slack" (for lack of a better term--if there is a different term for this time, please fill me in) between the slack of ebb and flood. And not during flood to ebb (although, maybe exiting would be easier during ebb).

And yes, I am aware there many factors, I am just trying to get the jest down for now.

[RenaB]

I understood you analogy as well Linedog, and that would mean slack would be at top dead center AND bottom dead center of the piston. Which would support the two "slacks" one on the up side, one on the down side.

[mpenders]

Ok, yes, sorry I had the ebb and slack words mixed. So, the period between any ebb and flood is slack time, correct? So slack is not on both sides, but only between flood and ebb, not the time between ebb and flood as well?

"slack" (low tide) ---> "flood" ---> "slack" (high tide) ---> "ebb" ---> "slack" (low tide) ---> "flood" ---> etc.

I guess this is my main question: Only by logic (albeit maybe flawed) it would seem first that there is a "slack" on each side of ebb and flood.

Yes.

AND, again logic, it would be better to dive in the "slack" (for lack of a better term--if there is a different term for this time, please fill me in) between the slack of ebb and flood. And not during flood to ebb (although, maybe exiting would be easier during ebb).

Timing your dives here in Puget Sound is HUGELY dependent upon the dive site. At some sites, diving is possible just about any time, regardless of the status of the tides. Then again, some sites are severely current sensitive (timing, size, whether it is a flood or ebb...or a combination thereof). Failure to understand and plan around the nuances of the site you intend to dive can result from anything from a good butt-kicking to risking your life.

Do research and ask questions before exploring a new site on your own. There is a ton of experience here at your fingertips if you're willing to look and/or ask for it.

[Penopolypants]

Slack happens somewhere between the maximum flood and maximum ebb each time. It's not exactly between.

If you are trying to determine slack you need to look at current tables, NOT tide tables. Current and slack predictions for major points around Washington can be found under the "Other Current Stations" pulldown near the bottom right on this page: http://www.dairiki.org/tides/ - two common ones used in the Puget Sound area are Admiralty Inlet (north Puget Sound) and the Narrows (near Tacoma). To find predictions for specific sites, you can start with Fishnaller's book, which give corrections to apply to data from one of the major points.

I am attaching an image from today's current predictions for Admiralty Inlet. The four slacks for today are marked.

More refined current information can be found here: http://tbone.biol.sc.edu/tide/sites_uswest.html , if you know more about where your site is located.

Determining which time to dive would depend on each dive site and how the currents interact with local conditions - some are better at flood, some at ebb.

[RenaB]

Alright, so I did have the words mixed in my first post, but my understanding is correct, penopolypants, Thanks. Yes, I understand that it happens somewhere in between, but not exactly. I have pulled the first site up before that has you picture. So, just so I have this correctly understood. On your picture the peak of the brown sine wave farthest to the left is flood, the point that is crosses, what would be zero on the graph is the slack (and now that I look closer is actually marked) followed then by the maximum point of the green peak is the ebb followed by the slack again at zero.

Yes, I am not at all trying to dive anywhere with current tables, so don't misunderstand that from my questions. I just hear the terms and wanted to grasp what they are saying for certain, and not guess that I understand. And, yes, you can see from the graph that slack does not occur even at this site (even by theoretical graphs) at the exact center of the ebb and flood. If it did the graph would be more gaussian in shape (although it is close for this particular example).

Thank you, this definitely helps. Along with the idea that the best time may not be the slack, flood, or ebb time. (ie the point is taken that you do have to determine from locals the best time to dive for that particular site). Also, connecting water ways could and does shift the parameters.

Thank you guys. That helped.

[RenaB]

"slack" (low tide) ---> "flood" ---> "slack" (high tide) ---> "ebb" ---> "slack" (low tide) ---> "flood" ---> etc.


I don't mean to poke at this, but this looks like it might be a little off to me.

more like:

ebb (low tide)--->Slack--->flood (high tide)----> slack---->ebb (low tide)

but maybe I am misunderstanding your arrows.

[mpenders]

Yes, I am not at all trying to dive anywhere with current tables...

I think that's the opposite of what P-pants (and many more of us) actually are recommending when planning dives in the Sound. Current tables are much more valuable than the tide tables when figuring out if/when a site is dive-able. It's generally more applicable to know if and how fast you're going to get blown off a site (i.e. a butt-kicking), and which direction you'll be heading.

Tide tables, however, are great for figuring out when you should explore tide pools, launch your boat, or head out to go clamming.

Be safe, have fun.

[YellowEye]

Hi.
Welcome to the board!

Check out _Northwest Shore Dives_ by Fischnaller, there's some good stuff on tides and planning dives there.

If you really want to understand how tides work (less so dive planning) search Wikipedia. You'll learn tides are mostly affected by the moon thus go on a 14 day cycle (28days but symmetric), and shift by one hour a day. The weakest tides/currents are on half moons, and in the spring and fall. See the neap tides section.

Look at planyourdive.com, it provides some good visualizations of predictions with corrections at various sites.

You may have seen me recommend 3tree over Redondo on an ebb on another thread. That is because Redondo can have some sporadic strong currents during a large ebb (search this site for 'Redondo river'). Three tree is relatively benign during an ebb - looking at its location on a map might suggest why - it is north facing and protected from the rush of the water leaving the sound. Ali junkyard is the same way.

Hope that helps. If in doubt ask here.

Happy diving!

-Eric

[mpenders]

"slack" (low tide) ---> "flood" ---> "slack" (high tide) ---> "ebb" ---> "slack" (low tide) ---> "flood" ---> etc.


I don't mean to poke at this, but this looks like it might be a little off to me.

more like:

ebb (low tide)--->Slack--->flood (high tide)----> slack---->ebb (low tide)

but maybe I am misunderstanding your arrows.

"ebb" and "flood" indicate moving water. "ebb" meaning water leaving the Sound after the high tide, and "flood" meaning water returning to the Sound after the low tide.

"slack" indicates still water (even when it only indicates a change of direction and happens for only a fraction of a second, such as in Deception Pass or the Tacoma Narrows).

So, even though "slack" and "high tide" or "low tide" rarely (if ever) happen at the same time - referencing "slack" with "high/low tide" is probably more accurate than referencing it with "ebb" or "flood". That's all I was trying to indicate.

[Dusty2]

This can be a real brain teaser for a particular site considering all the factors. As Ppants said always go by the current maps for the site you are diving because tides may be quite different timing wise. The sample shown above is a day I would go elsewhere if it was a current site. The steeper the angle where the graph crosses the center line the shorter the safe diving window will be.

This is what you want to see for a current site the area between the brakets is the proper time at this site. The lower closeup view shows how to determine these points. The distance between the center line and the black bar above is one knot of current. I feel anything over an 1/8 of a knot is to much to swim against for any distance so the black lines that I put in above and below center is approximately and 1/8 knot. The points where the lines cross the graph are your splash and exit times. As you see you have about an hour on that slack. If you did the same on P's graph you can see that you would have a very short window or maybe none depending on the site. Some sites on a big exchange like that will have no slack at all.

[RenaB]

Yes, although the wording isn't coming across, this is what I understood. Except for slack being with the high or low tide. That isn't what I was understanding. So, maybe I don't have it. With a current map and the sine wave I was reading high tide as the top of the peak, and low tide as the bottom of the peak, which would be the fastest current on graph. I know they are different graphs, one is the current and one is the tides (I understand the difference). ie, the highest point on the current graph would be the middle somewhere on the tidal graph? This makes sense to me now. If that's correct anyway, lol. So, at the top of the peak of the sine wave on the current graph would NOT be the flood (highest tide?) Just the fastest current.

[RenaB]

Hi.
Welcome to the board!

Check out _Northwest Shore Dives_ by Fischnaller, there's some good stuff on tides and planning dives there.

If you really want to understand how tides work (less so dive planning) search Wikipedia. You'll learn tides are mostly affected by the moon thus go on a 14 day cycle (28days but symmetric), and shift by one hour a day. The weakest tides/currents are on half moons, and in the spring and fall. See the neap tides section.

Actually I did read that
Look at planyourdive.com, it provides some good visualizations of predictions with corrections at various sites.

You may have seen me recommend 3tree over Redondo on an ebb on another thread. That is because Redondo can have some sporadic strong currents during a large ebb (search this site for 'Redondo river'). Three tree is relatively benign during an ebb - looking at its location on a map might suggest why - it is north facing and protected from the rush of the water leaving the sound. Ali junkyard is the same way.
Ok, yes, this makes sense
Hope that helps. If in doubt ask here.

Happy diving!

-Eric

[LCF]

It would seem quite intuitive that, as the water reaches its highest point and starts to fall, the current would drop to nothing and then reverse. I've never quite grasped WHY that isn't so, but it's not. That is why you need to use current charts and not tide charts for dive planning.

"Slack" is the period where there is no water movement. It can be further described as "slack before flood", or "slack before ebb". The terms mean exactly what they appear to mean -- slack before ebb is the slack before the water begins to flow back out to the ocean. Some sites are better dived on one or the other, because the behavior of water coming INTO the Sound can be quite different from water going OUT of the Sound.

As an example, let's look at Keystone Jetty. Keystone, which is the eastern terminus of the dotted line you see on the map, lies just south and east of a big "bulge" in Whidbey Island:



This means that, as water pours into the Sound, it is deflected off that bulge and heads south. Keystone is somewhat protected from water movement during the flood. On the other hand, you can see that, as the water moves back north and west, it runs along the edge of Whidbey Island into the bay, and pours along the shore, creating very strong currents at Keystone that will run OUTWARD -- precisely what you don't want to dive against. (What is not intuitive is that the geography of the bay can create an eddy current that can move outward, even during floods; it's generally much weaker, however.)

So it's important to understand timing, but it's also very important to understand the effect of the local geography on water movement. The Fischnaller book referred to above is excellent, because the author actually spent time studying the specific movement of water at the dive sites, rather than at some arbitrary current station which might be removed enough to have somewhat different characteristics.

[RoxnDox]

Adding to what others have said about the influence of local geography on the currents, you can see a really nice data set and visualization tool at http://maps.ngdc.noaa.gov/viewers/bathymetry/

Zoom in on the Puget Sound region, click the check marks "OFF" for the "____ Surveys", and check the box for "BAG Hillshades". At a certain zoom level you'll see very detailed (and brightly color-coded) maps of the areas where they have the high-resolution data collected. Very fun to explore - and even in the areas with less detail you still get a really good sense of the underwater topography. The ridges and bumps and current-scoured areas can really show up.

You can think of the Sound and the channels as massive rivers (that just happen to change direction regularly). They run generally "downstream" on a large scale, but the the local, small-scale flows that affect us as divers are mostly dependent upon (a) the magnitude of the flow (size of tidal exchange), (b) the local topography, and (c) where in the tidal cycle you're looking. Item (a) is also affected by where you're at - the more water "backed up" and trying to get past you out of the Sound the bigger your flow would be, given equal sized channels and all those "all other things being equal" qualifiers... :-)

All in all, fluid mechanics integrated with astrophysics and geomorphology gets complicated in a hurry - enough to make ya


Jim

[Dusty2]

Lynn's point is excellent and very important. Eddy currents can be quite confusing at many sites. Sites like Titlow for instance are quite contrary to what you might get from a current chart based on flow out in the mainstream of the narrows. Not only is Keystone effected by this so is Pt Hudson on the other side. Because of it's position at the point of the peninsula the currents are mostly outgoing even on an incoming current. The water coming into the bay on incoming flows tends to head out into the bay then swirl around and fill the area near the point making it seem like the current is always going out. Also while the current in the bay may be small near the point it is magnified because the water must force it's self around the point and out into Admiralty inlet.

[RenaB]

Thank you everyone for your responses, and detailed information and resources. This was quite helpful.

The intent was to try to understand when the best time for a particular location was to dive, then contact a close shop (or post here) to get a confirmation. Ultimately, I am a researcher, and like to try to figure these things out, and then see how well I can get to the correct answer. I meant that I wouldn't be diving based solely on current or tide tables alone.

You are great!