Tide direction and speed

[Talbot]

Lots of good programmes available for tidal height, and there are some good programmes for ocean currents. However, are there any resources (pref free) for tidal direction and strength?

[GordMay]

The Tidal Rule of 12 states that, in the first hour after low tide the water level will rise by one twelfth of the range, in the second hour two twelfths, and so on according to the sequence - 1:2:3:3:2:1.
Hence the greatest Tidal Flow (strongest current) will occur during the middle* 2 hours of the run.
There are about 12 Hours, 25 minutes between high tides, so tidal flow reverses every 6 Hours 12 ½ Minutes.

* The greatest tidal current occurs in the third & fourth hours of the approximately six-hour half-cycle.

HTH,
Gord May

[alaskadog]

Hi Talbot, I don't mean to side track your thread but my tangential comment isn't worth a thread of its own. So I apologize for butting in here. But I have a somewhat academic question for Gord or for anyone else who may know. And I'm just throwing in this to complicate matters a bit and perhaps to exercise my gray matter somewhat (at least the part that good rum hasn't killed I have read several times about the Rule of 12 and how it works. The area where I am presently on the Gulf of Mexico has only two tides a day, except on very rare occassions when there are four and those are essentially not noticeable. Does anyone know if, and if so how, the RULE would apply here. The area of two daily tides apparently extends from just west of Panama City, FL to over in Texas somehere but not as far south as the Mexican gulf.

[Talbot]

rule of 12 is a rule of thumb only and IMHO should only be used if there is no other data avaialble. Whilst generally it is not significantly incorrect, there are places where using it could get you into trouble, especially where the area is ruled by tides from two different positions (e.g. western solent where tide from the channel is sometimes in opposition to that from Southampton.)


Tide harmonic programmes take these areas into account, although the calculation becomes rather more complex that at a more normal port.

[GordMay]

Joe:
A “tidal day” generally has a duration of 24 hours and 50 minutes, and contains one complete tidal cycle (sinusoidal in shape),with two highs and two lows. This erxplains why successive high tides are about 50 minutes later each day.

Ocean tides have complex spatial patterns, due to the complicated shapes and topographies of the different ocean basins. In general, however, ocean tides at any spot usually consist of a mixture of semidiurnal and diurnal tides. The basic pattern may be distorted by the effects of landmasses, the complicated shapes & topography’s of ocean basins, constrained waterways, friction, the Coriolis effect, or other factors.

There are three basic types of tidal patterns.
Semi-Diurnal: Most areas have two highs and two lows a day. If the two highs and the lows are each respectively about the same amplitude, and is called a semidiurnal (semi-daily) tide .
Mixed: When the highs and the lows each respectively differ in height (where a single low tide follows two high tides) the pattern is called a mixed tide (Pacific basin).
Diurnal: Some areas, such as the Gulf of Mexico, have a diurnal tide with only one high and one low tide each day (and the range is often small).

As the sea level rises and falls, it generates a tidal current that flows horizontally. Tidal currents caused by the dropping water level (as the tide "goes out") are called ebb currents. The rising tide generates flood currents. The rule of twelve normally predicts the higher velocities will occur in the middle two, of six hours in a semi-diurnal pattern. With a diurnal pattern, in the Gulf, you would expect the highest velocities in the middle four hours of the 12 hour pattern.

As previously indicated, these are approximations only.

Hope this helps,
Gord May

[alaskadog]

Gord and Talbot: As usual you guys are really on top of most subjects that I am just barely learning. I appreciate your response and input. Here on the Gulf where I now am, the tidal flux is minimal, especially compared to places like the upper east coast of the USA and Great Britain or the Gulf of Alaska where 15 to 20 foot tides are not uncommon. My interest in tidal currents and their predictions in my area is only partially due to boating interests; the other part has more to do with my attempts to catch fish on hook and line. It seems that they feed more actively in a tidal current than they do in slack tides or tides with little fluctation. For example, you may as well fish in your bathtub as in Mobile Bay on a neap tide. The fish and the little critters they feed on seem to be quite sensitive to even the smallest currents and as the currents get stronger, they seem to bite even better (at least up to a point). And Gord, the last part of your remarks may offer me the most help of being a better fisher. "You would expect the highest velocities in the middle four hours of the 12 hour pattern". I really will try to give that a test. As they say, "the proof is in the pudding", er, ah "the fish & chips".