St Ninian's tombolo beach: refraction or diffraction?

 

I thought tombolo beaches were due to diffraction round an object. It turns out that this includes refraction. The ends of the waves closest to the island slow down in the shallower water near the island by refraction. This changes the direction of the wave so they bend inwards on both sides and meet behind the island. The slower water is able to pick up sediment which is then deposited to form the tombolo beach.