Why do wind waves tend to straighten a coastline by focusing erosion on headlands?

Answer 1

While in deep water, a wave traveling toward the coast from directly off shore has a straight crest in map view. The wave will feel bottom first in the shallower water off the headlands, while off the embayments it is still traveling in the deeper water. This slows the advance of the wave crest toward the headlands while it continues to speed on toward the embayments. This difference in velocity converts the map view trend of the wave crest from a straight line to a curve that increasingly resembles the shape of the shoreline as it gets closer to land. Wave energy is expended perpendicular to the orientation of the crestline. Thus, when the wave breaks, its energy is focused on the headlands and spread out along the embayments. Over time, the headlands are eroded back toward the mainland, while deposition in the low energy embayments builds those areas toward the water body.