Monday, 4 July 2016

Day 29 - Interesting Coastal Snippet

When I was a boy I used to spend summer holidays with my grandparents who lived in Bexhill-On-The-Sea (due to it's older demographics Bexhill is affectionately known as 'God's waiting room') on the south coast. Sand was in short supply on the beach. Like much of the Sussex coast it's mostly shingle. These were holidays when I was on my own a lot. I didn't mind. I used to love sorting through the stones hunting for fossils and sea glass. 

A few years ago I read a book called 'The Pebbles on the Beach' by Clarence Ellis. A wonderful exploration of the geology of shingled shores. 




From this book I learnt some stuff about wave movements that I thought I would share.

There are three important terms to remember when watching waves.

Swash
The swash is the rushing water driven up the beach after the wave has broken.

Backwash
The backwash is the opposite. It's the seawater returning to the sea after the swash has reached it's end.

Fetch
This is the stretch of wind-swept water over which waves travel. The longer the fetch the bigger the waves and the longer the swash and backwash.


If the slope of the beach is gentle then the decrease of speed of the swash will be slower and so it will travel further up the beach. If the slope is steep then the backwash will be faster. On a shingle beach the backwash will drag material down so that the slope becomes gentler. The stronger swash will then push that same material up the beach and make the slope less steep.

If you watch waves on an open beach then you'll notice that the waves are not parallel with the shore line. This is because the prevailing wind rarely blows at ninety degrees to the coast so waves come in at an angle. The swash will travel at the angle of the wave. However, the backwash travels back in a straight line. This creates something called 'longshore drift'. It describes the movement of sand and pebbles on the beach.









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