The north-west wind was whipping big waves through the gap into the outer harbour at Silloth. They are plane waves with wavefronts at 90 degrees to the entrance walls. If you look on the left hand side of the picture, those plane waves have bent through 90 degrees and travelled along the inner wall. This is diffraction. I teach that the gap has to be the size of the wavelength for it to be really effective but the gap here is two or three times the wavelength.
