Coastline

A coastline is the interface between land and sea, defined as the dynamic zone where terrestrial and marine processes interact. It is not a fixed boundary; it shifts with tides, weather, sea level, sediment supply, and human activity. The coastline includes the shoreline and the nearshore zone up to where coastal processes dominate. It often features beaches, cliffs, dunes, estuaries, deltas, mangrove swamps, salt marshes, and coral or rocky shores.

Coastal forms result from sediment supply and energy conditions. High-energy coasts tend to be rocky with cliffs and ledges, while low-energy coasts are often sandy with beaches and barrier systems. Estuaries and deltas occur where rivers deliver freshwater and sediments to the sea; coral reefs and mangroves provide complex habitats.

Key processes include coastal erosion, where waves wear away land; deposition, where sediment accumulates; longshore transport moving sand along the coast; and coastal accretion or subsidence with sea-level changes. Human actions such as dam construction, coastal development, and beach nourishment can alter sediment balance and shoreline position. The concept of the coastline length is scale-dependent; measuring it yields the coastline paradox.

Coasts support rich ecosystems and human activity. They host fish and invertebrates, breeding grounds for birds, and nurseries for many species. Economically, coasts provide fishing, tourism, ports, and transportation but are vulnerable to storms, erosion, and sea-level rise. Protection and management strategies range from hard engineering structures to soft approaches like dune restoration and managed retreat.