Waves and Currents

Why Coasts Change So Fast

Coastal processes are among the most dynamic and destructive on Earth. Along a single stretch of coast, one season may bring erosion and the next, deposition. Most of this change is driven by waves. When a wave breaks, water is hurled onto the shore with great force while sediments on the sea bottom get churned violently. The constant impact of breaking waves reshapes coasts drastically. Storm waves and tsunami waves can accomplish in a short time what normal waves take years to do. As the wave environment changes, so does the intensity of the force with which waves strike the shore.

The generating forces behind waves and currents are covered in the chapter on movements in ocean waters. Beyond wave action itself, coastal landforms depend on two things: the configuration of the land and sea floor, and whether the coast is advancing seaward (emerging) or retreating landward (submerging). Assuming sea level stays constant, two types of coasts are used to explain the evolution of coastal landforms:

• High, rocky coasts — submerged coasts
• Low, smooth, gently sloping sedimentary coasts — emerged coasts

High Rocky Coasts

Along high rocky coasts, rivers appear drowned and the coastline is highly irregular. The coast is deeply indented, with water extending into the land where glacial valleys (fjords) are present. Hill sides drop off sharply into the water. Initially, shores show no depositional landforms — erosion features dominate.

Waves break with great force against the land, shaping the hill sides into cliffs. With constant pounding, the cliffs recede, leaving a wave-cut platform in front of the sea cliff. Waves gradually minimise the irregularities along the shore. Material that falls off the sea cliffs and is removed gradually breaks into smaller fragments, rolls to roundness, and gets deposited offshore. After considerable cliff development and retreat, once the coastline turns somewhat smooth, further addition of material to this offshore deposit builds a wave-built terrace in front of the wave-cut terrace.

As erosion continues, a good supply of material becomes available to longshore currents and waves. They deposit this material as:

• Beaches along the shore
• Bars — long ridges of sand and/or shingle parallel to the coast, in the nearshore zone

Bars are submerged features. When bars show up above water, they are called barrier bars. A barrier bar that gets keyed up to the headland of a bay is called a spit. When barrier bars and spits form at the mouth of a bay and block it, a lagoon forms. Lagoons gradually get filled up by sediments from the land, giving rise to a coastal plain.

Low Sedimentary Coasts

Along low sedimentary coasts, rivers appear to extend their length by building coastal plains and deltas. The coastline appears smooth, with occasional incursions of water in the form of lagoons and tidal creeks. The land slopes gently into the water, and marshes and swamps may abound. Depositional features dominate.

When waves break over a gently sloping sedimentary coast, the bottom sediments get churned and move readily, building bars, barrier bars, spits and lagoons. Lagoons eventually turn into swamps, which subsequently turn into coastal plains. The maintenance of these depositional features depends on a steady supply of materials. Storm and tsunami waves cause drastic changes regardless of sediment supply. Large rivers that bring lots of sediments build deltas along low sedimentary coasts.

In India, the west coast is a high rocky retreating coast where erosional forms dominate. The east coast is a low sedimentary coast where depositional forms dominate.

Erosional Landforms: Cliffs, Terraces, Caves and Stacks

Wave-cut cliffs and terraces are two forms usually found where erosion is the dominant shore process. Almost all sea cliffs are steep and may range from a few metres to 30 metres or even more. At the foot of such cliffs there may be a flat or gently sloping platform covered by rock debris derived from the sea cliff behind. Such platforms occurring at elevations above the average height of waves are called wave-cut terraces.

The lashing of waves against the base of the cliff, along with rock debris smashed against the cliff, creates hollows. These hollows get widened and deepened to form sea caves. The roofs of caves collapse and the sea cliffs recede further inland. Retreat of the cliff may leave some remnants of rock standing isolated as small islands just off the shore. These resistant masses of rock, originally parts of a cliff or hill, are called sea stacks.

Sea stacks are temporary. Eventually, coastal hills and cliffs disappear because of wave erosion, giving rise to narrow coastal plains. These may get covered up by alluvium from the land behind, or by shingle or sand to form a wide beach.

Depositional Landforms: Beaches, Dunes, Bars, Barriers and Spits …