Understanding tides

[PfcAJ]

 

[TSandM]

Thank you, Hickdive! I learned something today.

 

[TMHeimer]

Another place to find info. is the Encyclo. of Rec. Diving (PADI), but I think it's all been covered here. I learned nothing in OW course about tides--I guess it's assumed that you are aware that here it is regular standard semidiurnal tides (of a difference of about 6' from low to high). The only thing that ever concerns me is if I'm shore diving and the tide is ebbing, the very slight current here does give you a slight push heading out. A little harder swimming back against it, so you don't want to got out too far. But as mentioned, tides are a local thing that you must figure out. Just 50 miles from here is the Bay of Fundy with the record 50' tides. If I ever dive there I'll seek local advice. Some places can be very dangerous when not diving at slack tides.

 

[swimmer_spe]

That's how I learned mostly. We covered some about tides and current in OW but we had 192 hours of training so we had a lot more time than they get today. Depending on where you dive / yak you'll find that sometimes the current isn't the same in the same place under the same conditions as it was the last time you where at that site. Tides and currents are tricky. It takes a while to learn the currents in the areas where you dive regularly. If you dive in BC regularly you'll learn.

I have dove half in the Halifax area and half in the Victoria area.

---------- Post added August 12th, 2013 at 10:22 PM ----------

Low tide has a height, because tidal heights are measured from a specific reference. (I don't know how that reference is chosen, though.) If the low tide is higher than the reference, it will have a positive number; if it is below the reference, it will have a negative number. Tides, since they largely an effect of the moon, cycle through the month, with periods of small exchanges (small difference between high and low tide) and periods of larger exchanges.

In many sites in our area, the currents are tidal and fairly predictable. Books listing dive sites will often say something like, "Dive this on slack before ebb," which means you should center your dive time around slack current when the tide is changing from flood (getting higher) to ebb (getting lower). It's important to know that, although tides and currents are related, they are not the same, and slack current may not be the time when the tidal height crests or reaches trough. Check current tables before diving current-sensitive sites, as well as tide tables. In some places, the tidal height affects the safety of the entry more than the exchange magnitude affects the currents at depth!

It's also important to know that some sites are more affected by wind, for example, than by tides. One of our local dive sites can be a bear with any significant south wind, even on a small exchange. And some sites simply aren't entirely predictable, particularly those out in large areas like Rosario Strait. It's important to plan dives in those places with plenty of wiggle room, and watch indicators like tag lines or kelp for when the current starts to die off.

Here, Ten Mile Point is very much like that. Only dive on slack, otherwise make sure you have your passport with you.

---------- Post added August 12th, 2013 at 10:23 PM ----------

For the meaning of more terms than you ever thought existed about tides and currents, take a look at http://co-ops.nos.noaa.gov/publications/glossary2.pdf .

Basic terms:
Datum: The height tide levels are measured from, in the US the datum is MLLW.
MLLW: Mean Lower Low Water - The average of the lower low water level over a long period of time (the time period is irrelevant for this discussion.
High Tide: The two periods of the day when the water level is high.
Low Tide: The two periods of the day when the water level is low.
Ebb Current: Current flowing toward the sea.
Flood current: Current flowing away from the sea.
Slack: No, or little, tide related current.

Thank you. That helps a lot.

 

[electrician]

There is a book that explains all of this:

"The Sea Around Us" by Rachel Carson

 

[Rich Keller]

I for the life of me, cannot understand tides, the currents they create, and the various lingo:ebb, flood, slack...etc.
So, being the smart person I am, I decided to look in my OW book. The PADI book has a few paragraphs on it, but nothing useful.
At what point should this have been taught?
Is there something online that explains it in simple terms? Like, why does low tide have a height?
I also kayak, so, have been getting better understand when high and low tides are, and when to avoid certain spots because of the current they create.

Everything seems to have been covered very well, I would just add a few minor points. You can google a locations tide for any given day, in the US that will usually come from NOAA, I do not know what the Canadian equivalent agency is called but it should be easy to find out. Most people dive in the summer and during those months the highest tide occurs during a full moon. If you are diving in the winter months the highest tides occur at the new moon not the full moon. As for kayaking, currents will run faster in shallower water so if you are paddling against the tide you want to be further off shore, paddling with the tide you will want to be closer to shore to gain the maximum benefit from the tide change.

 

[NAM001]

low tides have height associated with them for navigation and ability of passage. say for instance we are looking at a cargo ship. it when loaded and pulls a 75 foot draft. in the channel at high tide you have 90 feet of water and at low tide you have 70 ft. You then have to clear the channel at time of high tide or become grounded. Or load the cargo to pull less than 70 ft of draft and pass at any time.

 

[captain]

From a divers standpoint the two most useful are the time of high tide which usually brings with it clearer water near shore and slack tide which is good to know when diving passes or channels subject to tidal currents. The NOAA tide and current tables has calculated times for both but times must be adjusted for the distance your location is from the datum location that is used in the prediction.

NOAA Tide Predictions

 

[NAM001]

I guess the obvious should be added. low to high tide will drift you to shore. After high tide the currents are sea bound. The greater the tide swing the greater the currents. Knowing the time of high tide and diving before it is the best and safest of all worlds