Interference of Ocean Waves

While beach walking on Block Island, I saw regularly-spaced rows of rocks. Interference of ocean waves from different sources offers one possible explanation. The diagram below illustrates the concept.

Two sets of wavefronts impinge upon the beach. In this simple example, the waves are identical and are impinging at symmetrical angles. Lines of constructive interference (dotted lines) move straight toward the beach. Rocks would be pushed into piles, which would accumulate in areas of destructive interference. In more general cases, there still would be lines of constructive and destructive interference, although they might not impinge upon the beach at right angles.

To apply some reasonable values to the above example, we consider piles of rocks which are 4 m apart created by waves of wavelength 2 m. In the drawing below, the two right triangles share a common hypotenuse, shown in red.

Of course, this analysis is not valid unless we can identify sources for the waves impinging on the beach at different angles. Unfortunately, my travel plans did not include the hours necessary to witness waves at high tide over a period of days. 🙂

But we can speculate. Does the shape of the beach produce reflections which converge at the location of the rock piles? Does the ferry traffic less than a mile offshore produce wakes which interfere with naturally-occurring waves?

One thing is for certain: Physics was not on the minds of the many harbor seals using Block Island as a waypoint along their migration route. ❤️