Getting dumped is never easy

This week I heard about some seasoned swimmers getting injured in the surf zone due to dumping waves. Fortunately, all were able to walk away, and we wish them well in their recovery! I've been there, royally dumped so badly that I questioned my life choices. Hearing about these swimmers, and my own memory of those dumping waves, raised in me the question of why? I sought to find out more and uncovered a real gem of knowledge that I hadn't paid attention to before.

What happened?

What I know is that the seasoned swimmers swam on consecutive days at different locations on the Gold Coast. When I looked up the ocean measures for those days (I use seabreeze.com.au), I could see the swell was not too big, the wind not too strong and I knew they weren't in the water at the low or high tide marks. The only thing that did I notice was that the tidal movement was bigger than usual, meaning the high tide level was higher and the low tide level was lower. I wondered if this could be a clue.

Bad timing.

I pondered if the tidal movement could actually have played a part in this and put it to an ocean swimming group. I struck gold when one of the members commented, “The middle 2 hours of a six hour tide change moves the fastest”. When I looked at the probable times the swimmers were in the water, it seemed to fit.

Talk about an Aha! moment.

Once I read that comment, I wondered why it had never occurred to me before? If a body of water needs to move further in the same amount of time, of course it is going to move faster. It may even result in a “double wave that dumps” ie as a smaller faster wave catches up with the typical wave (for the swell and wind), then the overall size & speed of the upper wave crest could increase right at the point of the wave breaking – hence dumpers.

If I'd known I might have understood what to look for with tidal charts

As I mentioned, I didn’t know this before and I wondered if you did? In case it’s not something you’re familiar with, I'll add a screenshot of the tidal chart I looked at for your interest.

In purple I’ve outlined the fastest sections (middle 2 hours) for the big tidal 6 hour change (Friday) and again for a smaller 6 hour tidal change (next Tuesday night). The purple lines for Friday are longer than Tuesday night's lines indicating that the water had to move further on Friday.

The yellow dots on the Friday graph indicate the peak high tide and peak low tides for that particular flow. It's worth adding that dumpers can also appear on these points of tidal change.

Take home message

Knowing that the water moves fastest in the middle 2 hours of any 6 hour tide (especially king tides) won't change how the water behaves but it may change the way you swim. Pay particular attention when returning to the shore as the waves approach from behind you.