Help me understand tides and currents

[3ricj]

Ok. So many questions.

In short, I'm trying to figure out when a good time to dive sites might be. I have tide books, and various online resources, and yet I still seem to get dragged away sometimes.

Example: Saturday night (10/26/13) splashed at just after 7pm at Alki Junkyard. Everything I've looked at tells me that should have been just after slack, during a low exchange. In fact, I got dragged westerly most of the dive.

Here are my questions:

- What specific geographic layout causes the ebb and flow to flow in the same direction? I'm told this is what happens at Three Tree Point. I've looked at maps and depth profiles and it doesn't make any sense to me.

- I understand that slack current may or may not relate to high or low tide. These terms seem to be standing wave or progressive wave. What geography causes this relationship between current and tides to change?

- How can I figure out which "site" on NOAA is the best reference point for any given dive site? EG: http://tidesandcurrents.noaa.gov/curren ... c2.html#17

- Most of the time when I ask people about this stuff, I get shrugged off with a mumble about "well, that's why it's a prediction, just like the weather". The scientist in me thinks that's a garbage answer. While I'm sure that wind has some dynamic impact on the tides, the greatest impact must just be from the geography of the land and the flowing of the water. To that end, it shouldn't be impossible to get pretty good predictions, provided we know how the local and global water flow is influenced by this. It's not like the sun or moon is doing anything unexpected. I just feel like I'm missing some key knowledge.

Thanks in advance,
-3ric Johanson

[LCF]

The overall tidal movement of water is largely determined by the moon, and is predictable. But the local behavior can be heavily influenced by geography. For example, Keystone Jetty is a very current-sensitive site, but can be dived on most floods, because Admiralty Head deflects the incoming water -- but it can also eddy around and result in an inshore current running north and then OUTWARD along the jetty, so one needs to be prepared for that. Ebb tides result in much stronger currents there.

Skyline Wall is a very current-sensitive site, but needs to be dived on a moderate exchange, because on a small one, the current may never reverse. The water coming around both sides of Burrows Island can result in very muddled currents on small exchanges.

Redondo Beach is generally NOT a very current-sensitive site, but on large ebbs, can have a quirky and very strong current that can blow up out of nowhere (the "Redondo River").

Overall, if your dive site is in the Fischnaller book, that's a superb reference for planning. Many other sites, like Plan Your Dive and Emerald Diving, will have information about planning for specific sites. But no matter what the advice and the predictions are, one should always sit and observe the currents for a little while before getting in at very current-exposed sites.

[Mortuus]

In short: aim to start your dive right before slack, but also look up additional info on a site. This may include information such as whether or not a site can be dove on slack, ebb or both, adjusted slack times (because a lot of the times the current station is not positioned exactly at the site), and other divers' prior experiences. Past trip reports are a very good place to gather data imo. This board can be a great resource for all of the above. One website I would check out is planyourdive.com. They have an interactive map of dive sites, shops, and current stations that is very easy to use, and intuitive. I have used it for planning many a dive. Welcome to the board!

- Dylan

[3ricj]

"The overall tidal movement of water is largely determined by the moon, and is predictable. But the local behavior can be heavily influenced by geography. For example, Keystone Jetty is a very current-sensitive site, but can be dived on most floods, because Admiralty Head deflects the incoming water -- but it can also eddy around and result in an inshore current running north and then OUTWARD along the jetty, so one needs to be prepared for that. Ebb tides result in much stronger currents there."

Ok, thanks, I learned something. Does anyone have a real computer model of the pacnw tides, which can account for the impacts of the jetty, etc? NOAA? NASA? GUE?

I still don't understand what causes the deltas between the time of slack tide and slack current. It looks like xtide (http://www.flaterco.com/xtide/) uses a harmonic file which independently adjusts the phase of the tide and current for each subordinate station. At a glance, this implies that the relationship between tide and current isn't relational. I find this confusing.

[CaptnJack]

Ok, thanks, I learned something. Does anyone have a real computer model of the pacnw tides, which can account for the impacts of the jetty, etc? NOAA? NASA? GUE?

There are some out of print current maps but nothing is available in print right now. And these are all for surface currents only anyway, subsurface currents esp at places like the Narrows and Admiralty inlet sometimes go different directions. If you sit in a boat at plaes like these you can see the water swirling and going in circles which is really eye opening too!

I still don't understand what causes the deltas between the time of slack tide and slack current. It looks like xtide (http://www.flaterco.com/xtide/) uses a harmonic file which independently adjusts the phase of the tide and current for each subordinate station. At a glance, this implies that the relationship between tide and current isn't relational. I find this confusing.

Its rational but not linear. Since the sun and the moon are circling in elipses while the earth is rotating. With all these elipses, and local geography, and seasonal wind patterns, you can imagine currents are predictable but non-linear.

[H20doctor]

there are many tide apps, and you would want to check more than one to get a prediction ..depending on the dive site you would have to make a judgement or calculation on when is the best time to get in ... what is that best time ? that is the mystery for some of the sites here in Puget sound... other sites dont need tide planning they are diveable all day long... As for the junk yard, i havent dove it and i dont know the characteristics of that site.. ? here is the tide for that day

from this picture the red line reps your splash ... i looked at other tides sites and slack was at 8pm.. But this maybe a site where you needed to be in later, or earlier... this is part of the problem diving here on some sites.. its a guessing game , with trial and error , the more you dive a site the more you get a feel for it and what the water is doing...

[fmerkel]

As you've seen, NWSD book has a bunch of predictions. For the most part they are OK. BUT, the man couldn't dive all the sites for all the variations of volume exchange. For some sites, Sunrise and Skyline come immediately to mind, he doesn't have enough data. His description for Sunrise makes me shake my head. Data for Skyline is only partially accurate.

Other sites simply don't have readily available data or are predicted by word of mouth/internet. The best methodology I've found is to get a basic feel for a site of interest, try to garner as much info as possible from divers who successfully dive that site, and keep your own logs of data, predictions, and real life results. Then you can start modifying a bit at a time to find where it 'breaks', but hopefully not catastrophically, merely uncomfortably.

[BillZ]

As you've seen, NWSD book has a bunch of predictions. For the most part they are OK. BUT, the man couldn't dive all the sites for all the variations of volume exchange. For some sites, Sunrise and Skyline come immediately to mind, he doesn't have enough data. His description for Sunrise makes me shake my head. Data for Skyline is only partially accurate.

I was speaking to Rick and Jackie from Bandito Charters a few months ago about the currents at Sunrise and how over the last few years that, using the NWSD corrections, I've never been able to hit slack exactly right at the site. They theorized that since the new narrows bridge went in the cement stations caused a change in the timing of slack and that it was now a bit unpredictable.

[WaGigKpn]

I used to think that slack meant no current and was always perplexed when fishing out of Sekiu that even two hours after slack the current would still be ripping at 2-3 knots. Made me think about it. What i realized that tide predictions are only water level, not water flow (current). If the point you are diving has a large volume of water beyond it then you can expect the current to keep flowing long after you hit low/high tide. The water level wont change but water will be flowing past to "fill" the areas beyond. Look at a river running past you. The water is flowing very fast but the water level is not changing.
Hope this helps.

[fmerkel]

They theorized that since the new narrows bridge went in the cement stations caused a change in the timing of slack and that it was now a bit unpredictable.

A couple of concrete pillars several miles away....sounds like BS to me.
My wife hits it 90% of the time and been doing so for years. The new bridge hasn't made any difference. She's damn good for a difficult site. :luv: