Republishing this important article from 2014 on our local tides. If you don’t have time to read it all, just skip to How might this impact my swim? It’s important to know.
From Bernard Lynch & Ian Venner
This note is intended for those new and not so new to coastal swimming and simplifies some of the calculations and facts which you may well expect to see. Much of it is specific to Myrtleville.
The tidal streams (coastal currents) are the most important part of the tides you need to understand as a swimmer. Most OW swimmers average between 2-3km/hr. Tidal streams near the shore in the harbour can run at between 0.5 to 1.0 km/hr – so they can make a very big difference to your swim. The speed of the tidal stream varies during each High Water/ Low Water (HW/LW) tidal cycle, and also varies between spring (very high/very low) and neap (not very high/not very low) tides. Spring tides occur a day after a full moon and recede over a period of one week to a neap.
The spring/neap maximum flow rate can vary from 0.5 to 1.5km/hr. The flow rate within a 6 hour tide range will be three times as strong at its fastest (in the middle of the six hours) as in the first and last hour. This is explained by the Rule of Twelfths.
Rule of Twelfths
The level of water does not rise or fall at a constant rate throughout the 6 hour duration of a rising or falling tide. The amount by which it will do so can be estimated mentally by means of the following rough guide:
- 1st hour rise or fall = 1/12 of Range
- 2nd hour rise or fall = 2/12 of Range
- 3rd hour rise or fall = 3/12 of Range
- 4th hour rise or fall = 3/12 of Range
- 5th hour rise or fall = 2/12 of Range
- 6th hour rise or fall = 1/12 of Range
This impacts tidal speed too – the fastest speed being in the middle of the period between HW and LW. In the table below, assuming a 12 foot rise and fall in the tidal height (typical for Cork Harbour) and a maximum tidal flow rate of 1.5km/hr, you can see the how fast the tide moves and rises/falls.
|Time after HW||Twelfths||Change in Depth in that hour||Avg tidal flowkm/h|
What else influences tides?
Other factors can influence tidal heights and flow rates. Low pressure will increase tidal height (think of it as less atmospheric pressure pushing down on the water). Lots of recent rain will increase the strength of the ebb, especially out of Cork Harbour and Fountainstown too (it’s just more water trying to get out).
How might this impact my swim?
So what does all this mean for swimmers? Be conscious of the time of HW – not because the beach might be more or less sandy, but because it should influence where/how you swim. Swim against the prevailing tidal flow, so that if you get tired you will have the benefit of it on the way back. For example, you might consider a swim from Myrtleville to Church Bay and back on the third hour of a falling tide (tide against you going to Church Bay and with you coming back). You should not consider doing it on the third hour of a rising tide (tide with you going to Church Bay and against you coming back).
In reality, the tidal flows between Myrtleville Beach, the Dutchman and Bunny’s are fairly limited. There is a little more effect to consider if going Myrtleville/Fountainstown and particularly at Bunny’s point where the push or pull can be significant on the 3rd and 4th hour. If needed, there are plenty of escape routes onto the rocks between Myrtleville and Fountainstown – weather dependent. Going to Church Bay, however, puts you into much stronger tidal flows and you really need to plan your timings. Generally you would not plan a swim past the Dutchman unless the tide was ebbing (going out) for the return leg. Equally, anything further afield needs careful planning with someone who knows the area and tides.
So what direction does the tide flow?
The diagram below broadly illustrates the direction of flow of the tides around the Myrtleville area. Tides, especially close inshore, are subject to back-eddies and counter-currents, and there are a few of these to be found in the area shown.
To check tides, either purchase a tide table or click on this link.
You should always know the tide before you swim. Swim Safe.