Wind, Wave & Coastal Landforms + Exercises

6.11 Wind & Currents — Coastal Sculptors

Coastal processes are the most dynamic and hence the most destructive of all. Some changes along the coasts take place very fast — in one season there may be erosion at a particular spot and in the next, deposition. 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 there is simultaneous 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 — far greater than normal breaking waves. As the wave environment changes, the intensity of breaking waves changes too.

Apart from the action of waves, coastal landforms depend on (i) the configuration of the land and the sea floor; and (ii) whether the coast is advancing (emerging) seaward or retreating (submerging) landward. Assuming the sea level to be constant, two principal types of coasts are recognised: (i) high, rocky coasts (submerged coasts) and (ii) low, smooth and gently sloping sedimentary coasts (emerged coasts).

High Rocky Coasts

Along high rocky coasts the rivers appear to have been drowned, with highly irregular coastlines. The coastline appears highly indented with extensions of water into the land where glacial valleys (fjords) are present. The hill sides 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 hill sides into cliffs. With constant pounding by waves, the cliffs recede, leaving a wave-cut platform in front of the sea cliff. Waves gradually minimise the irregularities along the shore. The materials that fall off and are removed from the sea cliffs gradually break into smaller fragments, roll to roundness, and get deposited offshore. After a long period of cliff retreat, when the coastline turns somewhat smooth and more material accumulates offshore, a wave-built terrace develops in front of the wave-cut terrace. As erosion progresses, a good supply of sediment becomes available for longshore currents and waves to deposit as beaches along the shore and as bars (long ridges of sand and/or shingle parallel to the coast) in the near-shore zone. Bars are submerged features; when they show 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 across the mouth of a bay and block it, a lagoon forms. Lagoons gradually get filled by sediment 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. Marshes and swamps may abound. Depositional features dominate.

When waves break over a gently sloping sedimentary coast, the bottom sediments are churned and move readily, building bars, barrier bars, spits and lagoons. Lagoons turn into swamps and then into coastal plains. The maintenance of these features depends on a steady supply of sediment. Storm and tsunami waves cause drastic changes irrespective of supply. Large rivers with heavy sediment loads build deltas along low sedimentary coasts.

6.12 Erosional Coastal Landforms

Cliffs, Terraces, Sea Caves & 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 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 cliff behind — such platforms occurring at elevations above the average wave height are called wave-cut terraces.

The lashing of waves against the base of the cliff and the rock debris smashed against the cliff create hollows; the hollows widen and deepen to form sea caves. The roofs of caves collapse and the sea cliffs recede further inland. When two caves on the opposite sides of a headland meet, a sea arch forms; when the arch finally collapses, the seaward portion is left as an isolated pillar of rock — a sea stack^?. Such resistant masses of rock, originally part of a cliff or hill, stand isolated as small islands just off the shore. Like all other features, sea stacks are also temporary and eventually coastal hills and cliffs disappear because of wave erosion, giving rise to narrow coastal plains; with onrush of deposits from over the land behind, these may get covered by alluvium or by shingle/sand to form a wide beach.

6.13 Depositional Coastal Landforms

Beaches and Coastal Dunes

Beaches are characteristic of shorelines dominated by deposition, but may occur as patches even along rugged shores. Most of the sediment making up beaches comes from land — carried by streams and rivers — or from wave erosion. Beaches are temporary features: a sandy beach that appears so permanent in one season may shrink to a narrow strip of coarse pebbles in another. Most beaches are made of sand-sized material; shingle beaches contain unusually small pebbles and even cobbles. Just behind the beach, sand lifted and winnowed from the beach surface is deposited as sand dunes. Coastal dunes forming long ridges parallel to the coastline are very common along low sedimentary coasts.

Bars, Barriers, Spits, Tombolos & Lagoons

A ridge of sand and shingle formed in the sea in the off-shore zone, lying approximately parallel to the coast, is called an off-shore bar. An off-shore bar exposed by further addition of sand is called a barrier bar. Off-shore bars and barriers commonly form across the mouth of a river or the entrance of a bay. When such a barrier bar gets keyed up to one end of the bay, it is called a spit. Spits may also develop attached to headlands or hills. The barriers, bars and spits at the mouth of a bay gradually extend, leaving only a small opening into the sea, and the bay eventually develops into a lagoon. The lagoon then gradually fills up by sediment from the land or from the beach itself (aided by wind), and a broad coastal plain replaces the lagoon. Where a spit grows long enough to connect an offshore island to the mainland, the resulting bridge of sand is called a tombolo^?.

Submarine Canyons

Submarine canyons are deep, V-shaped valleys cut into continental shelves and slopes. They serve as conduits along which sediment is funnelled from the coast to the deep ocean floor. Some are extensions of major rivers (e.g., the Hudson Submarine Canyon off New York); others are independent features cut by turbidity currents. They form an important link between land-derived sediment and the deep-sea fan deposits.

6.14 Winds — The Master Sculptor of Hot Deserts

Wind is one of the two dominant agents in hot deserts. Desert floors get heated up too much and too quickly because they are dry and barren. The heated floors heat up the air directly above them and result in upward movement of hot lighter air with turbulence; any obstructions in the air's path set up eddies, whirlwinds, updrafts and downdrafts. Winds also move along the desert floors with great speed, and obstructions create more turbulence. There are also storm winds which are very destructive.

Winds cause deflation, abrasion and impact. Deflation includes lifting and removal of dust and smaller particles from the surface of rocks. In the transportation process, sand and silt act as effective tools to abrade the land surface. The impact is simply the sheer force of momentum that occurs when sand is blown into or against a rock surface — similar to a sand-blasting operation. This wind action creates a number of interesting erosional and depositional features.

However, many features of deserts owe their formation to mass wasting and running water as sheet floods. Though rain is scarce in deserts, when it comes it comes torrentially in a short period. Desert rocks devoid of vegetation, exposed to mechanical and chemical weathering due to drastic diurnal temperature changes, decay faster, and torrential rains help in removing the weathered materials easily. The wind moves fine materials and general mass erosion is accomplished mainly through sheet floods or sheet wash. Stream channels in desert areas are broad, smooth and indefinite and flow only briefly after rains.

6.15 Erosional Wind Landforms

Pediments & Pediplains

Landscape evolution in deserts is primarily concerned with the formation and extension of pediments. Pediments are gently inclined rocky floors close to the mountains at their foot, with or without a thin cover of debris. Such rocky floors form through the erosion of the mountain front through a combination of lateral erosion by streams and sheet flooding.

Erosion starts along the steep margins of the landmass or along the steep sides of tectonically controlled steep incision features. Once pediments form with a steep wash slope followed by a cliff or free face above, the steep wash slope and free face retreat backwards. This method of erosion is termed parallel retreat of slopes through backwasting. So, through parallel retreat of slopes, the pediments extend backwards at the expense of the mountain front, and gradually the mountain is reduced — leaving an inselberg, which is a remnant of the mountain. That is how the high relief in desert areas is reduced to low featureless plains called pediplains.

Playas

Plains are the most prominent landforms in deserts. In basins with mountains and hills around and along, drainage is towards the centre of the basin; due to gradual deposition of sediment from basin margins, a nearly level plain forms at the centre. In times of sufficient water, this plain is covered by a shallow water body. Such shallow lakes are called playas, where water is retained only for a short duration due to evaporation. Playas often contain good deposits of salts; a playa plain covered with salts is called alkali flats.

Deflation Hollows & Caves

Weathered mantle from over rocks or bare soil is blown out by persistent movement of wind currents in one direction. This process creates shallow depressions called deflation hollows. Deflation also creates numerous small pits or cavities over rock surfaces. Rock faces suffering impact and abrasion of wind-borne sand develop shallow depressions called blow-outs; some blow-outs become deeper and wider — fit to be called caves.

Mushroom, Table & Pedestal Rocks; Yardangs & Ventifacts

Many rock outcrops in the deserts, easily susceptible to wind deflation and abrasion, are worn out quickly leaving some remnants of resistant rocks polished beautifully into mushroom shapes — a slender stalk and a broad rounded pear-shaped cap above. Sometimes the top is broad like a table top (table rock); sometimes the remnants stand out like pedestals. Asymmetrical wind-streamlined ridges aligned parallel to the prevailing wind are called yardangs^?. Pebbles polished and faceted by wind-blown sand into smooth, sharp-edged stones are called ventifacts^? — diagnostic of wind abrasion.

6.16 Depositional Wind Landforms — Sand Dunes & Loess

Wind is a good sorting agent. Depending on the velocity of wind, different sizes of grains move along the floor by rolling or saltation, or are carried in suspension; in this transportation process the materials get sorted. When the wind slows or begins to die down, the grains begin to settle depending on their sizes and critical velocities. So, in depositional landforms made by wind, good sorting of grains can be found. Since wind is everywhere, with a good source of sand and constant wind directions, depositional features in arid regions can develop almost anywhere.

Five Major Sand-Dune Types

Dry hot deserts are good places for sand-dune formation. Obstacles to initiate dune formation are equally important. There can be a great variety of dune forms.

When sand is plentiful, regular dune shapes coalesce and lose their individual character. Most desert dunes shift, but a few get stabilised — especially near human habitations. Beyond dunes, fine wind-blown silt may settle far from its source as a yellowish loose deposit called loess. The Loess Plateau of north-central China — built up by silt blown from the Gobi Desert — is the world's most famous example.

6.17 NCERT EXERCISES — Full Model Answers

6.18 Summary & Key Terms