WAVES AND CURRENTS
Coastal processes are the most dynamic and hence most destructive. Some of the changes along the coasts take place very fast. At one place, there can be erosion in one season and deposition in another. Most of the changes along the coasts are accomplished by waves.
When waves break, the water is thrown with great force onto the shore, and simultaneously, there is a great churning of sediments on the sea bottom. The constant impact of breaking waves drastically affects the coasts. Storm waves and tsunami waves can cause far-reaching changes in a short period of time than normal breaking waves.
As the wave environment changes, the intensity of the force of breaking waves changes. Other than the action of waves, the coastal landforms depend upon:
(i) the configuration of land and seafloor;
(ii) whether the coast is advancing (emerging) seaward or retreating (submerging) landward.
Assuming sea level to be constant, two types of coasts are considered to explain the concept of evolution of coastal landforms:
(i) high, rocky coasts (submerged coasts);
(ii) low, smooth, and gently sloping sedimentary coasts (emerged coasts).
HIGH ROCKY COASTS
Along the high rocky coasts, the rivers appear to have been drowned with the highly irregular coastline. The coastline appears highly indented with the extension of water into the land where glacial valleys (fjords) are present. The hillsides drop off sharply into the water. Shores do not show any depositional landforms initially.
Erosion features dominate. Along high rocky coasts, waves break with great force against the land shaping the hillsides into cliffs. With constant pounding by waves, the cliffs recede leaving a wave-cut platform in front of the sea cliff. Waves gradually minimize the irregularities along the shore.
The materials which fall off, and are removed from the sea cliffs, gradually break into smaller fragments, and roll to roundness will get deposited offshore. After a considerable period of cliff development and retreat when the coastline turns somewhat smooth, with the addition of some more material to this deposit offshore, a wave-built terrace would develop in front of the wave-cut terrace.
As the erosion along the coast takes place a good supply of material becomes available to longshore currents and waves to deposit them as beaches along the shore and as bars (long ridges of sand and/or shingle parallel to the coast) in the nearshore zone.
Bars are submerged features and when bars show up above the water, they are called barrier bars. The barrier bar which 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.
The lagoons would gradually get filled up by sediments from the land giving rise to a coastal plain.
LOW SEDIMENTARY COASTS
Along low sedimentary coasts, the 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. Marshes and swamps may abound along the coasts.
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 would eventually turn into a swamp which would subsequently turn into a coastal plain.
The maintenance of these depositional features depends upon the steady supply of materials. Storm and tsunami waves cause drastic changes irrespective of the supply of sediments. Large rivers which bring lots of sediments build deltas along low sedimentary coasts.
The west coast of our country is a high rocky retreating coast. Erosional forms dominate on the west coast. The east coast of India is a low sedimentary coast. Depositional forms dominate on the east coast.
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 m to 30 m 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 is called wave-cut terrace. The lashing of waves against the base of the cliff and the rock debris that gets smashed against the cliff along with lashing waves create hollows and these hollows get widened and deepened to form sea caves.
The roofs of caves collapse and the sea cliffs recede further inland. The retreat of the cliff may leave some remnants of rock standing isolated as small islands just off the shore. Such resistant masses of rock, originally parts of a cliff or hill are called sea stacks.
Like all other features, sea stacks are also temporary and eventually, coastal hills and cliffs will disappear because of wave erosion giving rise to narrow coastal plains, and with the onrush of deposits from over the land behind may get covered up by alluvium or may get covered up by shingle or sand to form a wide beach.
FAQ
1.Which is not an erosional feature of sea waves?
Options:
a. Cliff
b. Beach
c. Sea Caves
Answer: Beach is not an erosional feature of sea waves. It is a depositional feature of sea waves.
