Focus SW France » Trench dynamics explained

Swell being focused along the edges of the continental shelf © 2009 - msw

So how do deep-water trenches work? Resulting in bigger swell on Hossegor's beaches than the surrounding area? People always talk of swell being 'focused', which is true, but also a slight generalisation. Contrary to popular opinion the trench does not funnel swell down the middle of the trench straight to the beach as a hosepipe would. Rather the swell is focused towards the edge of the trench in a form of convex refraction. Unless the swell direction and the trench line up perfectly (impossible as the trench is not straight) the energy will increasingly focus on one side of the trench, resulting in an uneven distribution on the beach. Hence on reason why one part of Hossegor could be 2ft whilst the other is 5ft. Currents refract off the trench creating odd circular sand deposits which help to create fantastic sandbars which in turn give rise to the hollow spitting beachbreaks we have all come to love and fear.

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