TIDES AND SAFETY – THE RULE OF TWELFTHS

Text: Alan Bulmer   Lead image: Paul Smith

One of the most important things to understand before venturing onto the rocks, flats or in a boat to go fishing is tidal movement. Many anglers, especially those with no marine background or those new to the sport, assume that the tide comes in and goes out at a constant rate. This is definitely not the case and it is very easy to get into trouble if you do not understand the rate at which the tide moves. The media over the past two weeks has reported several incidents of either cars or people being stranded on the flats by the incoming tide.

How did this happen?

rule-of-twelfths-2

Essentially the tide run starts slowly and builds to a maximum halfway between high and low tide before smoothly decreasing to zero again when it reaches either full or dead low tide. This is shown in the graph above.

What probably happened with the strandings is that the vehicles moved onto the flats at low tide. For the first couple of hours of the incoming tide the water remained a reasonable distance from the vehicles. Then the water suddenly started to move in more quickly so the owners headed back to the vehicles to move them to higher ground.  My guess is that unfortunately one the vehicles had sunk slightly into the sand, or would not start, and it became bogged down. The maximum tidal flow occurs in a two hour band and this probably started about the time the vehicles got stuck and tried to pull each other out. There is no margin for error with the incoming tide and as the saying goes “time and tide wait for no man” so they were caught with the rising water and were powerless to escape. They should have moved the vehicles to higher ground much earlier, before the water started to flood in.  The same principles apply to the individual who got isolated by the incoming tide.

Sailors use a rule of thumb, called the “Rule of Twelfths” to estimate the height of the tide at any time, given only the time and height of high and low water. Obviously it is critical to know the heights when navigating a boat in shallow water, and when launching or retrieving boats on slipways on a tidal shore. Shore based anglers should also learn this rule as it is invaluable in helping understand when it is safe to get onto or off the flats and rocks.

rule-of-twelfths

The rule assumes that the difference between successive high tide and low tide is six hours. In the six hours between tides the rule says 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 the the sequence 1:2:3:3:2:1.

Put another way the tide will rise by roughly 8% of the total range in the first hour, 17% of the range in the second, 25% in the third and fourth hours, 17% in the fifth and 8% in the final hour. I’ve summarised the tide movement and the cumulative effect in the table below to hopefully make it easier to understand.

The scary thing from an angling perspective is that 50% of the incoming tide flows in during a two hour band, namely between the start of the third hour and the start of the fifth hour. Be warned.

rule-of-twelves-table

My advice is to get off the rocks or flats before the tide really starts to rush in. Move early to higher ground to stay safe. If you get stranded it is going to get ugly very quickly and you are potentially putting your life at risk.

In harbours the tide height will be roughly the same if you venture out at the same stage of the tide but if there is a massive difference between high and low tides (i.e. Spring tides) the current flow will obviously be faster with a greater difference as more water has to move in and out of the harbour in same time period during Spring tide cycles. Be aware of the increase in current flow when there are Spring tides as it can make crossing channels that appeared to be easy with neap tides absolutely treacherous.

Sometimes fish feed more aggressively when the current is flowing more strongly as it is easier for them to catch baitfish disoriented or concentrated by the increased current so targeting Spring tides can be a worthwhile tactic. Just take precautions, especially if fishing at dusk. I’d probably take an inflatable lifejacket, EPIRB, cell phone, torch and a change of clothes (stored in a waterproof pack) if I was fishing at dusk. The last thing that you want to do is get stranded in the dark.

One thing that I often do when fishing new areas is take a stick out on the flats with me when I venture out two hours before low. I poke the stick in the sand at the waters edge when I first arrive. This is my reference point. When the incoming tide starts to get near the stick then I exit the flat. That way I know I can get off safely. It is a simple tactic that works well.

Another thing to be aware of is that the tide can come in much faster than it goes out. Several years ago a friend and I walked out to a sandbar about a hour before low tide. The water depth in the channel that we crossed to reach the sandbar was calf deep. We fished for roughly an hour after low tide and when we came to cross the channel back it was over knee deep. If we had not been watching the incoming tide carefully we could have found ourselves wading back in waist deep water and strong current. Not ideal when you are wearing thigh waders. The message here is to always stray on the side of caution and treat the sea with respect.

 

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