If a breakwater is built parallel to the mainland beach, longshore drift may pile up behind the breakwater, forming a sand bridge to the island. Such a sand bridge is called a tombolo. Sea walls and revetments are constructed parallel to sea cliffs to prevent wave erosion. Both types of barriers absorb the wave energy that would otherwise be expended on the cliff. Coastal Environments - Deltas A. Deltas form where sediment supply predominates over wave energy.
Deltas are typical on coastlines with large rivers.
In fact, all major rivers will typically build deltas at their mouths. There is a typical and clear association between passive margins, wide coastal plains, and large drainage basins. This produces large rivers and yields large sediment supply. Examples of important deltas around the world. This delta is the classic example of a delta which was scientifically studied by the ancient Greeks.