We stayed at Foyers on the south side of Loch Ness. This photograph was taken looking north up the look from the mouth of the River Foyers. Note the shingle bank.
The exact end of the river is shown here - a gap between two shingle banks. Waves were being driven up the loch by the wind. They were straight wave fronts - what we call plane waves. When they went through the gap, they became semi-circular.
When plane waves go through a gap and become semi-circular, it's called diffraction. It is supposed to be most noticeable when the gap is the same size as the wavelength. Here the gap was much bigger than the wavelength. This could actually be an example of refraction. The water would be much shallower on the edges near the shingle so the water would move slower due to greater friction from the bottom. Those edges of the wave would go slower and get left behind, which could also account for the semi-circular shape.
