Understanding tides

[swimmer_spe]

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.

 

[supergaijin]

Try this very cool internet application for 'Understanding Tides'.

 

[AfterDark]

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.

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.

 

[triggerman365]

Here is link with pretty straight forward explanation:
The Ocean's Tides Explained

 

[CT-Rich]

The basic concept of tides is pretty straight forward. The gravity of the Sun and moon pull on the liquid portion of the Earth. The relative position each affects the time and height of any given tide. They are greatest when the Moon and Sun line up (full and new), and weakest when the They are perpendicular. But the devil is in the details. The local geography is what really matters for divers, boaters or yaks. Some land masses act like funnels and force pile ups of water flowing into a given area. The bay of Fundy off Nova Scotia and Long Island sound have very large tidal ranges because they are wedged shaped bodies of water and focus the movement of water. Wind direction and intensity can add to or subtract from given tide.

The currents created by moving all that water can have a huge effect on the quality of a dive site and its safety. When trying out a new site it is best to research it. Most of the time the a site is most dive able in the hour or so before high tide. After High tide, silt and run off from the surrounding area will lower the visibility. The best, time tested way to figure out how strong the currents are is to look at a tide table for the area you want to dive, big numbers in height mean big currents in the open channel. Ask people that have been there themselves and already know the dive conditions. Go and LOOK at the place a few times and see what the conditions are like (pay attention to how water and buoys (or pilings or boats) interact. Each spot will have its own characteristics, and you need to respect that. Poor reading of the current in a spot will get you into more trouble faster than just about anything.
The flip side is that high water flow can also mean high concentrations of marine life, so you need to weigh your options.

 

[phoenix31tt]

i had used this link to help understand it... but from watching seamen and going out with boats etc... even to the most seasoned person sometimes the currents aren't 100% predictable by using tides... too many other factors affect it (at least where i live)

 

[TSandM]

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.

 

[Hickdive]

Chart Datum, the point at which depths and tidal heights are calculated from, is normally the Lowest Astronomical Tide i.e. the lowest sea level that will be reached purely from the gravitational effects of the Moon and Sun. Atmospheric pressure and wind speed also affect tidal height but they are ignored for the purpose of Chart Datum.

Tidal heights are stated as heights above Chart Datum. Thus a tidal height of 3 metres means the height of water is 3 metres above Chart Datum. If a wreck is charted as being at a depth of 10 metres and the tidal height is given as 5 metres at High Water (HW) then you know that if you dived that wreck at HW then the depth would be 15 metres, not allowing for atmospheric pressure or wind.

You can, very roughly, calculate the height of water over a site at any state of the tide by using the rule of twelfths.
Rule of twelfths - Wikipedia, the free encyclopedia

However, for more accurate prediction, you should use a tidal curve.

The above is ok for Standard Ports but you're unlikely to find your site is near a Standard Port so for more accuracy, if using Admiralty Charts, you can use the nearest tidal diamond on the chart to your intended location. For even greater accuracy and to allow for the fact that geography can have an enormous effect on tides, you can get a Tidal Atlas for coastal areas.

 

[corvettejoe]

Try this very cool internet application for 'Understanding Tides'.

I saw the beginning of the link and couldn't help but laugh.

 

[SSpiffy]

lingo:ebb, flood, slack...etc.

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.