How do barrier islands create lagoons between the coast and the sea?

Barrier islands create lagoons by acting as protective barriers that separate a stretch of coastal water from the open sea. These islands form on low-lying coasts where there is an adequate supply of sand and the coastal slope is flatter than the equilibrium profile required by constructive wave swells.123

The formation process involves several key factors:

• Wave Action: Waves deposit material to build up these barriers. If the land behind the developing barrier is low, it becomes flooded—often facilitated by a slow rise in sea level—creating a sheltered body of water.3
• Sedimentation: Once the barrier is established, the water behind it becomes quiet, allowing for the deposition of fine-grained sediments transported by rivers.24
• Structural Integrity: The barrier island must maintain its elevation above the sea to preserve the lagoon. These features can also develop from spits that extend across a bay, effectively enclosing the water.3

While the barrier protects the lagoon from direct ocean energy, the lagoon maintains access to the sea through inlets that allow for the exchange of tidal waters.14