Thursday, April 21, 2011

Diurnal dilema

All my life I've lived fairly close to a coast.  My childhood holidays were spent twisting knee-deep while hunting for pipis at low tide and recovering from being dumped by waves at high tide.  I've snorkeled, swum, sailed, strolled, deck-chaired and liloed at coastlines in the Pacific Ocean, the North Atlantic Ocean, the North and Celtic Seas, on both sides of the Tasman Sea and at several spots around the Mediterranean Sea. Given that I have the rockpool-loving gene, I've also spent a fair amount of time waiting for low tide so I can peer into exposed watery worlds. Above are some other people I share that gene with.  A year ago we holidayed together at Waiwera and explored the coastline there.

So I thought I knew a wee bit about the highs and lows of coastal life.  Then I moved here to the edge of the Indian Ocean and learned something new.  It turns out that every coast I had ever been to had a trait in common, but that trait does not exist here. 

We discovered this fact after falling in love with the reefs and rockpools that jut out from various points along Mandurah's coastline.

After several perplexingly unsuccessful return trips to explore them, we looked up tide charts online.  Initially we thought a drunkard had plotted them as the scant and random predictions made no sense to us.  The truth is that the regular daily pattern of tide in/tide out/tide in/tide out that we've always known simply does not apply here. This coastline is one of the few in the world to have diurnal tides, or one cycle of high and low tides a day, whereas all the coasts I'd ever lived at or visited before have semidiurnal tides, or two cycles per day. 

Who knew?  Lots of people it seems, just not me (chart above from here). I was feeling a bit of a dunce until I met various locals who thought I was telling porkies with my crazytalk about places with two tide cycles a day.

I had hoped to take our two visitors to explore this coast's rockpools.  Sadly the tides are against us at the moment as low tide is currently falling in the early hours of the morning.  The shorter visitor is quite tempted by the idea of a torchlit midnight rockpooling expedition, but I'm just pleased there are plenty of other options here for making our own fun.

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