Why do open-ocean waves become smaller after entering an isolated gulf?

In the context of a gulf, open-ocean waves typically do not penetrate into relatively isolated areas. If they do enter, they become significantly reduced in size.1

This reduction occurs because gulfs are often protected by their shape or a narrow mouth, which limits the entry of large ocean swells. Once inside, the wave environment is dominated by smaller, local waves that are generated by the specific size and conditions of the gulf itself. While diffraction—the spreading of waves around obstacles—is a fundamental wave phenomenon, the isolation provided by the geography of a gulf acts as a barrier that prevents the full energy of open-ocean waves from propagating into the interior. Consequently, the wave energy is dissipated or blocked, resulting in the calmer water conditions that make many gulfs suitable for harbors and seaports.12