Waves and coasts

Coastal processes are the most dynamic and so the most destructive. Coasts can erode in one season and gain sand in another. Most change is made by breaking waves; storm and tsunami waves can change a coast far more in a short time.

Coastal landforms also depend on the shape of the land and sea floor, and on whether the coast is emerging (advancing seaward) or submerging (retreating landward). Two kinds are considered, assuming a steady sea level:

• High, rocky coasts: rivers look drowned, the coastline is highly indented, and hills drop sharply into the sea. Erosion dominates. India's west coast is a high, rocky, retreating coast.
• Low, sedimentary coasts: rivers extend by building coastal plains and deltas; the coast is smooth, with lagoons, tidal creeks, marshes and swamps. Deposition dominates. India's east coast is of this kind.

Erosional landforms:

• Sea cliffs: steep, from a few metres to 30 m or more, formed as waves pound the hillsides.
• Wave-cut platforms / terraces: flat or gently sloping rock floors left in front of a retreating cliff; above average wave height they are wave-cut terraces. Material carried offshore can build a wave-built terrace in front.
• Sea caves: hollows at the base of a cliff, widened by waves and smashed debris; their roofs collapse and the cliff retreats.
• Sea stacks: resistant remnants of a cliff left standing as small islands off the shore. They too are temporary.

Depositional landforms:

• Beaches: mostly sand, from rivers or wave erosion; shingle beaches have pebbles and cobbles. Beaches are temporary: a wide sandy beach may shrink to a strip of pebbles in another season. Wind piles sand behind the beach into dunes, often long ridges parallel to the coast.
• Off-shore bars: ridges of sand and shingle parallel to the coast, under water. When built up above the water, they are barrier bars.
• Spits: barrier bars attached to a headland or one end of a bay.
• Lagoons: when bars and spits close off a bay mouth, leaving only a small opening. Lagoons slowly fill with sediment and become swamps, then coastal plains.

Natural coastal defence: off-shore bars take the first blow of storms and tsunamis, then barriers, beaches, dunes and mangroves. Disturbing the coast's 'sediment budget' or cutting mangroves leaves settlements to take the first strike.

Example (new): removing sand from a beach for building, year after year, can starve the bars and dunes that protect a fishing village, so the next cyclone's waves reach the houses directly.