Saturday, 12 May 2012

Know your tides and shoreline ~ the power of the sea!

~ This is my second article on water safety ~

When the tide comes in the sea churns though this area between the rocks.  It is a pleasant place to walk when the tide is out, but at this point there's no walking to be had, only rock climbing:


Even though we live close to the beach I like to know exactly what the tide is doing before setting off for a walk.  Depending on where the tide is, walks will be possible in some places and not in others.

The links below provide useful information about the sea and tides.  The one I use most often is the Tide graph on the Ocean Fun website.
Tides vary not only in time from one day to the next but also in their depth.  The degree of variation can be most surprising.  The tide charts listed above show these well.  They may not be exactly accurate for your area though.  We have found that ours is about fifteen minutes out, so we expect the lowest point of the tide to occur that much earlier.  We have also noticed that within that half hour there can be numbers of unexpected and restless surges, so it's a time to be especially careful and not take anything for granted.  Tide charts are great, but nothing takes the place of knowing your own stretch of shoreline well.  

Swells and surges may be influenced by wind, rain or the height of the tides themselves.  Sometimes it's hard to know what it is.  Swells turn into waves as they reach shore, and these are not even.  It's not unusual for two waves to come at once creating big, unexpected surges.

If you're climbing amongst seaweed, as Rewi was when this photo was taken, it's important to be well clear of fast-moving or rising water as it can be very hard to get out of when it's tugged about by the tide.  Rewi could have climbed up if he had had to. 


Rocks surrounded by water are likely to be more dangerous than those which are not, as water can wash around from both sides, creating a syncopated watery drag.  We both had an anxious five or so minutes when Rewi was exploring an unfamiliar rock further along our beach: he was out amongst heavy seaweed above his knees when the tide suddenly got deeper and, just when the water had ebbed from one side and it looked as if he could make his way back to the beach, it surged around from the other side. The seaweed really did have him strapped in around his legs.  The time before he got free probably seemed much longer than it was!  

Those of us who like to spend time climbing around rocks and rock pools need to be aware of all this so that we can stay safe.  I have a rule not to go clambering around near rocks by myself, much as I would like to: if I slipped or tripped and injured myself badly I might not be able to get help, and those sorts of things can happen to anyone.  It almost happened to me, when I skipped backwards from an unexpected wave, and tripped on a rock behind me and fell heavily.  Ouch!  Yes, I was on my own, so it was fortunate that I was able to get myself home and patched up.
  
These people needed rescuing and had very narrow escapes:

Big swells formed at sea by stormy weather batter the coastline.  They can shift immense amounts of sand, scouring it away from beaches and their rocks.  The rocky area pictured below can be fairly sandy and I've got used to seeing it that way:


But after a storm it can look very different.  That's Rewi standing on the far rocks sizing up the amount of water and seaweed.  His stick came in very handy for feeling down to the sandy bottom - appearances can be deceptive! 


Nearly halfway there:


But from the safety of my high vantage point I could see a big surge coming!  "Hurry!", I called out stupidly, which is the worst thing to do in this situation because if you lose your footing you're in very much greater danger!  Going slowly, even if the water comes in fairly deep, you have a better chance of staying steady and on your feet:


He had more sense than me and stood still!


Woow - deep and then deeper!


Then the water receded and Rewi waded out of the water laughing...  And I jumped down from my rock without getting my feet wet!

Here are a couple of images of these same rocks from high above, getting what I call the Deep Suds Wash from heavy seas at high tide:



These seas are immensely powerful, ripping heavy ropes of seaweed from their hold around the rocks:

The attachments that seaweeds use to fasten onto rocks are appropriately called holdfasts.  These can be really large as you can see by the size of my shadow on the rock next to this one:


Here you can see the firmness of the grip with which this one has held onto its rock: a sliver of rock and many small barnacles have come away with it:


Ropes of seaweed pile up on the beaches:


If we have also had rain, storm water and river water turn the tide brown, a mixture of silt and tannin.  Here are two photographs taken of the same spot a few days apart.  In the first one the water is clear, as it usually is:


And after heavy rain, you can see the difference:


The colouration is probably harmless, but the official advice is that it's best not to gather shellfish for three days after such weather.  After that they should be fine.

High tide coupled with heavy seas can lift logs as if they are weightless.  This is the only way this large log could have got up onto that rock.  I gave it a heave to test its stability.  I have fairly good upper body strength but I couldn't even wiggle it!


Some weeks later I saw that the sea had floated it off again.  As before it was immovable!


Any beach is always changing, even moment by moment.  For me this is fascinating.  I enjoy the stormy seas as well as the pools when they are tranquil:


And the turbulent seas that fill 'my' anemones with sand...


...also wash them clean again:


More information about tides is available here:  

Wikipedia says that:
Tides (from low-German 'tiet' = 'time') are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotation of the Earth. 
NASA has published wonderful and useful information on the subject.  
My favourite graphic can be seen here:


As requested by NASA's website, credit for this item goes to:
NASA/Goddard Space Flight Center Scientific Visualization Studio

You can find NASA's own page giving this and another longer graphic:
My first article about water safety is:
My later article as linked to below can be read as a cautionary tale!
You can find my other articles about exploring the beach and its rock pools via the link below: 

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