High or Low Tide?

[ClayJar]

Googling [google]"Rule of 12ths"[/google] returns several pages of explanations and even diagrams.

(Since I tagged in mere moments after Charlie, I added the link. )

 

[mike_s]

Question here. The average tide in PCB is up/down maybe a foot or two.

I think the average tide in England (where String is located) goes up/down many feet.


how would the "rule of 12th's" affect differently in these locations based on how far north (larger tide swing) or how far south (smaller tide swing) you were?

 

[String]

These "rules" work best for open coastline and you can forget them where it comes to narrow passages and islands etc. For those the only "prediction" is to ask for local knowledge a lot of the time as slack period may not coincide with high or low water and so on.

Rule of 12s for tide height firstly take the tidal range for that day.

Assume low water is 2.0m and high water is 6.0m for the day. That's a range of 4m. Assume you have a dive site of 10m depth

Lets say low water is at midday. Depth at the site then will be chart datum plus 2.0m so 12.0m

12ths work as the folllow (time is the hour after low water).

1/12th of 4.0 is 0.33m so

1300 = 1/12. Depth = 12.0m + 0.33m
1400 = 2/12. Depth = 12.33m + (2x 0.33m) = 13m
1500 = 3/12. Depth = 13m + (3 x 0.33m) so 14m
1600 = 3/12. Depth = 14m + (3x 0.33m) so 15m
1700 = 2/12. Depth = 15m + (2x 0.33) so 15.66m
1800 = 1/12. Depth = 15.66m + (1x 0.33m) so 16m

There you can see the range of 4m taking you from low to high water.

It should be clear from that the periods where there is greatest increase in depth, therefore the greatest water movement therefore the highest current. Going from that you can apply some basic rules to get a rule of 1/3s to work out speed.

First thing you need is from local knowledge or a chart the maximum current flow for the area (use tidal diamonds or similar).

In the example above lets say its extreme and maximum current on a spring tide is 9kts (it makes the maths easier!)

As above we can do a chart for hours after low water (or high water) so

1300 = 1/3 max speed so in this case its 1/3 of 9 which is 3kts
1400 = 2/3 max speed so in this case its 2/3 of 9 which is 6kts
1500 = 3/3 max speed so in this case its 3/3 of 9 which is 9kts
1600 = 3/3 max speed so in this case its 3/3 of 9 which is 9kts
1700 = 2/3 max speed so in this case its 2/3 of 9 which is 6kts
1800 = 1/3 max speed so in this case its 1/3 of 9 which is 3kts

As you can see current flow is highest during mid tide and lowest around the turn point. Note in this example there is very little if any slack water time so aim to dive maybe 15 mins before high water and end it 15 mins later and so on.

There are other rules people such as kayakers use such as the 50/90 rule which some claim is more accurate for some areas of coastline but not others.

Thats basically:

Flow rate at end of first hour = 50% of that tides max flow rate (4.5kts)
90% at end of 2nd hour so 8.1kts
100% at end of 3rd hour so 9kts
90% 4th hour so 8.1kts
50% 5th hour so 4.5kts
0 6th hour so no movement

None of these "rules" hold true for narrow passages, islands and places where flow is confused. They are based on standard sinusoidal tide movement with a clear 6hr cycle between high and low water as is generally found on open coastline and open ocean. Local knowledge is still by far the best way to get information about depth, tides, currents and safe times to be on a site.

Im sure google can turn up a far better description than the above too.

 

[String]

Question here. The average tide in PCB is up/down maybe a foot or two.

I think the average tide in England (where String is located) goes up/down many feet.


how would the "rule of 12th's" affect differently in these locations based on how far north (larger tide swing) or how far south (smaller tide swing) you were?

Its not the range that affects those rules - they're designed for tides that follow the standard sinisoidal pattern with an interval of roughly 6hrs between high and low water. Open ocean, straight coastline and so on.
Some areas have maybe 2 high water bumps before a definite low, others have even more confused patterns. In these areas you can forget any "rules" and its trial and error along with local knowledge to sort out.

You can use a prediction site to see if your choice of dive area follows a nice smooth curve.
http://easytide.ukho.gov.uk/ for example. Look at the graph and see if its nice and smooth. Southampton for example on there isnt, Milford Haven is.
Currents you can read off a nautical chart of the area or tidal flow atlas to work out speeds.

If the range is about 3ft as in some places in the US or if its 30ft (as in some places here) the "rules" will hold provided its a proper smooth curve. Obviously for a place with low tidal range its hardly worth bothering. Bigger ranges can lead to stronger currents. Obviously the difference between 0.5kt and 1.0kt isnt huge but the difference between 3kts and 9kts is as far as diving is concerned!

In places with large tidal ranges its the tides responsible for the majority of currents and are fairly easy to predict. In places with lower ranges these effects can be masked by other factors such as the weather, upwellings and so on which cant be guesstimated with the rules.

 

[mike_s]

None of these "rules" hold true for narrow passages, islands and places where flow is confused. .

So none of this applies to St Andrews Jetties where the original poster asked about? right?

It's a narrow passage (I'd guess max 100 yards wide at its narrowest point) that is maybe a 1/2 mile long. The water that flows in/out of it flows into a very large bay inland to the jetty.

See this map reference for an overview of the inlet channel and bay in question
http://maps.google.com/?ie=UTF8&z=11&ll=30.135626,-85.691986&spn=0.409153,0.572662&om=1

 

[String]

If thats the case then its down to local knowledge more than anything else.

Edit:- For Panama city you can see the clear double peak on a few days where local variations cut in:

http://easytide.ukho.gov.uk/Easytide/EasyTide/ShowPrediction.aspx?PortID=2612&PredictionLength=7

 

[Charlie99]

When trying to figure out tidal currents in an inlet, the key bit of local knowledge is when slack water will be. Typically it will be "xxx hours or xxxx minutes" after (or before) high tide and xxx hours after low tide.

You can get a reasonable feel for what sort of relationship there is if you can get info on the tidal range on the two side of the channel. If the tidal range is about the same, then the slacks will be very close to the high and low tides.

If the tidal range on the bay or other protected water is much smaller than on the ocean side, then the slack waters will be a couple of hours after high tide and low tides.

If the channel is between two separate tidal driven areas (like at Cape Cod canal, Wood's Hole, etc.), the all bets are off and hopefully you can find a set if tide and CURRENT predictions for the point in question.

http://tbone.biol.sc.edu/tide/tideshow.cgi?site=Channel+entrance,+St.+Andrew+Bay,+Florida&units=f this site has tide height predictions for both the inside and outside of the channel entrance, and also for places like Panama City. It looks like the tidal range in the bay is not that much less than the ocean, so the slack waters will probably be pretty close to the high and low tides.

Another good rule of thumb is that the tidal range (difference between high water level and low water level) will be the greatest near the full moon and the new moon, and the tidal range will be smaller around 1st and 3rd quarter moons. Tidal currents are usually proportional to the tidal range, so expect the higher currents and shortest slack times around full and new moons.

Remember that all you are going to get off of these websites are predictions, and things like winds will make it so that the actual tides and currents aren't exactly as predicted. Don't treat any prediction as a hard and fast time at which you can jump into the water. In real life, you may find it best to simply use the predictions as the guide to when you should be onsite and ready to dive. Then you observe the current and enter shortly before slack water.

 

[av8er23]

Mike, you are pretty familiar with St. Andrews jetties right?
If so what would be the best time to dive in that sat.

 

[Nemrod]

It may be a small tide in terms of height but the volume of water in the back bay that exchanges there is large and therefore significant currents can be set up. These currents can be hazardous to divers and swimmers and to small craft were at inlets an outgoing tide stacks ups incoming waves. The Jupitor Inlet in south Florida is one known for being especially hazardous to small craft.

You want to dive most jettys on a waning or slack high tide. The water will be clearest and the current will be small or none. Despite the small height of these tidal changes, significant currents exist and they are stronger than any diver and capable of sweeping a diver to an undesired direction if not given respect.

N

 

[rawls]

Just google on "rule of 12ths". It is simply a way of estimating the water level for intermediate times between high and low tide.

It assumes that the water level has the normal sinusoidal height vs. time curve.

To understand it, just look at a normal clock, imagine that high water is at noon and low water is at 6PM. The change in the height of the tip of the hour hand is also sinusoidal and is a pretty good approximation of the water level. Ignore the side to side horizontal movement of the tip of the hour hand and look only at the vertical motion, and you can easily see that the height of the hour hand changes more rapidly around the 3PM point and has very little vertical movement around the Noon and 6PM points.

A reasonable estimate of change in height of the water is that during the first hour after high tide, it falls 1/12th of the distance to low water. 2/12ths in the 2nd hour. 3/12th in both the 3rd and 4th hours, 2/12th in the 5th hour, and only 1/12th drop in the last hour before low tide.

A GPS with tide info will usually show the graphics that Charlie alludes to. Just make sure you are using the correct tide for the particular site.