How do lagoons form along coastlines?

Lagoons are areas of shallow, quiet water separated from the open sea by a barrier. They generally form through two primary processes:1

• Barrier Island Formation: On low-lying coasts, waves build up sand or shingle barriers where the coastal slope is flatter than the equilibrium profile required by constructive swells. As sea levels rise, the land behind these barriers becomes flooded, creating a lagoon. These barriers can also form when material carried alongshore develops into a spit or a bay-mouth barrier.23
• Coral Reef Formation: In tropical environments, lagoons are formed by coral reefs. These structures, such as atolls, grow around subsiding volcanic peaks or along coastlines, creating a protected, circular or irregular body of water.14

The persistence of a lagoon depends on the barrier maintaining its position above sea level and the rate of sedimentation, which eventually fills the basin. While lagoons are distinct from estuaries, the two features often overlap when barrier islands restrict the exchange between river and marine waters.13456