Saturday, 6 January 2018

The state of the estuary

Sea state- the condition of the surface of the water. Technically, there's all sorts of physics going on in the 'proper' definition (wiki is yer man) but the main thing to know is they've made a simple version easy enough for a quick estimation for any ship's log. Or any birders' notebook.

You'll hear it daily in the shipping forecast or in the more accurate for landlubbers with 'scopes inshore waters forecast, or the more accurate for birders with 'bins or stuck in an estuary coastal waters forecast.

Rough example (pun intended) If 'coastal' is 'moderate', then inshore might be 'moderate to rough' and shipping almost certainly 'rough to very rough'.

Swell- a tad difficult to grasp. It's not really meant for the waves on an estuary. They're formed by local wind conditions. Technically, swell is surface gravity waves made by large-scale weather systems;a movement of large bodies of water. Think of those regular waves coming in on a beach on a calm day. They might be small, but they still might form and ride for a hundred metres or more. That's swell. Here on the estuary most waves dissipate after three or four metres. But the wind picks that wave up a half metre higher? That's sea state.

Each of those two measurements can be estimated, via the Douglas sea scale.

For a birder the sea state on the estuary is based on guesstimating wave heights, and you need to get your eye in. On the seawall shoreline of the estuary, do you ever see a wave breaking of around a metre? Not really, sometimes mid-estuary, maybe. And that would only be 'slight'. Most of the time less than half that ('smooth') or less.

Swell- well those waves don't often roll past you for more than a hundred metres, they're short wavelength, and a 'low wave' is anything under two metres in length, so the swell in the Medway is never really scores much more than a 'one' on the scale- waves low and short. The scale goes to nine.

So, even when I think it's nasty out on the estuary, it really isn't. My scores most frequently used (usually calculated in Great Crested Grebe neck lengths) are:

   0 - flat - glassy calm
   1 - to 10 cm high - rippled calm
   2 - 10 to 50 cm high - smooth
   3 - 50 cm to 125 cm - slight and
   4 - 125 cm to 250 cm - moderate.

with swell being just either

   0 - no swell
   1 - short/average.

The scales are worth a look at. A really easy way of noting your seawatching conditions.Top scores possible? 'Phenomenal and confused' Actually, in this day and age, that should be called Top Trumps. Wow wouldn't that be something to see folks? A great wall of water, greatest wall of water you could see.

Now, the point is this. If we can work all this out, you can bet your bottom dollar birds are working this out, not on the same scales, not consciously, but they do, easily, and they respond to it. Why no self-respecting Kittiwake is ever going to stay in the Medway for long.

Hopefully all of use for one or two readers for their own notebooks, hopefully a few might have a go at reading conditions; it all helps in understanding why we only see certain species, in certain areas, in certain conditions.

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