Lanforms By Sea Waves
Significant coastal features formed due to marine erosion by sea waves and other currents and solution processes include cliffs, coves, caves, indented coastline, stacks, chimneys, arch, inlets, wave-cut platforms etc.
Steep rocky coast rising almost vertically above sea water is called sea cliff which is very precipitous with overhanging crest. The steepness of true vertical cliffs depends on variations of lithology and geological structure and relative rate of sub-aerial weathering and erosion of cliff face and crest and marine erosion of cliff base.
If marine erosion at the base of cliff is much faster than the sub-aerial weathering of cliff face and crest, overhanging cliff with steep vertical face is formed. On the other hand, if the sub-aerial processes dominate over marine processes the verticality of cliff disappears and the cliff loses its true cliff character.
True cliffs are generally formed where bedrocks are affected by low rate of sub-aerial weathering and massmovement viz. limestones, chalk, horizontally bedded sandstones, massively jointed igneous rocks and metamorphic rocks.
In fact, the morphology of sea cliffs is determined by:
(i) The influences of bedrocks lithology and structure, and
(ii) Balance between marine and sub-aerial erosional processes.
The formation of sea cliff begins with the erosion of coastal rocks through the mechanisms of hydraulic actions and abrasion by breaker waves (swash or surf currents). This results in the formation of notch and the coast becomes vertical.
There is gradual extension of notch landward due to continuous wave attack with the result the crest of the cliff overhangs the notch. If the notch at the base of the cliff is extended landward to such an extent that the support to the cliff crest is weakened the overhanging head of the cliff breaks and falls down resulting into gradual recession of the cliffs landward.
The rate of cliff recession varies both in space and time depending on the following conditions:
(i) Rock lithology and structure,
(ii) Susceptibility to chemical erosion, mass movement and sub-aerial erosion,
(iii) Cliff height,
(iv) Orientation of the coast,
(v) Wave energy,
(vi) Offshore topography, and
(vii) Rate of removal of debris from the cliff base by the backwash or the undertow currents etc.
ii. Wave-Cut Platform:
Rock-cut flat surfaces in front of cliffs are called wave-cut platforms or simply shore platforms (fig. 20.6) which are slightly concave upward. The origin and development of wave-cut platforms is related to cliff recession. These are also called wave-cut benches. Shore platforms are formed where cliff recession is active due to powerful bombardment of cliff base by up-rushing breaker waves and effective removal of eroded materials by backwash (undertow currents).
The forms of wave-cut platforms depend on geological factors. Extensive platforms are developed where the rocks are least resistant to wave erosion. In other words, thinly bedded and densely jointed, and horizontally disposed rocks with strike parallel to the coastline are more vigorously eroded by up-rushing breaker waves and thus are associated with extensive shore platforms.
On the other hand, narrow and steeper platforms with high mean elevation are developed over resistant rocks. As regards the processes and mechanism of the development of wave-cut platforms, quarrying and plucking by large and high-energy storm waves and water-level weathering are effective marine processes of shore platform development.
iii. Sea Caves and Associated Features:
Sea caves are formed along the coast due to gradual erosion of weak and strongly jointed rocks by up-rushing breaker waves (surf currents). The joints are widened into large cavities and hollows which are further enlarged due to gradual wave erosion into well-developed coastal caves. Sea caves are more frequently formed in carbonate rocks (mainly limestones and chalks) because they are eroded more by solution processes. It may be pointed out that sea caves are not permanent features as they are very often destroyed by up-rushing high-energy storm waves.
When the caves are enlarged to such an extent that their roofs become remarkably thin, they ultimately collapse and fall and the debris are removed by powerful backwash and thus resultant long narrow inlets are called ‘geo’ in Scottland. Sometimes, the air in the cave is compressed by uprushing powerful storm waves and finding no other route to escape it breaks open the roof of the cave and appears with great force making unique whistling.
Such holes are called natural chimneys or blow holes or gloup. “The nameMow hole refers to the fact that during storms spray is forcibly blown into the air each time a breaker surges through the cave beneath”. When caves are formed on opposite sides of the seaward projecting headland, a natural arch is formed due to coalescence of two caves (fig. 20.7).
It may be mentioned that natural archs are not permanent coastal features because the roof, after becoming very thin, collapses and thus the seaward part of the arch stands detached from the coast. Such isolated remnant of headland projecting well above sea level is called stack (fig. 20.8).
This is also called as chimney rock. Stacks are also called needles, columns, pillars, skerries etc. The Old Man of Hoy (137 m high) in the Orkney Island of British Isles is widely quoted example of sea stack. The elliptical hollows formed in the coastal area alternated by hard and weak rocks are called coves or mini bays (fig. 20.9).
Nearly all of the aforesaid coastal erosional features are found along the western and eastern coasts of Peninsular India. The author noticed the examples of cliffs, wave-cut platforms, caves, arches, tidal inlets, chimneys etc. along the eastern coast in the environs of Visakhapatnam.
Such features are frequently observed on the western Indian coast mainly between Mumbai and Mangalore.
The eroded materials are transported by sea waves in different manner but the transportational work of sea waves varies significantly from other agents of erosion and transportation. For example, the backwash, or undertow currents (moving from the coast and beach towards the sea) pick up the eroded materials and transport them seaward but the up-rushing breaker waves or surf currents pick up these materials and bring them again to the coast and beaches.
Thus, the transportation of materials takes place from coastland towards sea and from sea towards the/coast. When oblique waves strike the coast, longshore currents are generated. These longshore currents transport the materials parallel to the shoreline. The materials involved in the transportation by sea waves include sands, silts, gravels, pebbles, cobbles and sometime boulders.
When there is equilibrium between incoming supplies of sediments by up-rushing breaker waves and removal of sediments by backwash or undertow currents on the wave-cut platforms, a profile of equilibrium is achieved.
If the wave- cut rock platform is characterized by steep slope towards the oceanic slope, the destructive waves become very active and thus resultant powerful backwash removes the materials from the landward side so that the slope of the platform is lessened.
On the other hand, if the slope of the wave-cut platform is less steep, constructive waves become more effective as they favour sedimentation and beach deposition on the landward side so that the slope of the platform becomes steeper.
Significant depositional landforms developed by sea waves include sea beaches, bars and barriers, offshore and longshore bars, spits, hooks, loops, connecting bars, looped bars, tombolo, Barrier Island, tidal inlets, winged headlands, progradation, wave-built platforms etc.
Besides, mangroove swamps, sabkha and delta are also included in the category of depositional coastal landforms, though deltas are formed due to deposition of sediments brought by the rivers.
Temporary or short-lived deposits of marine sediments consisting of sands, shingles, cobbles etc. on the sea shore are called beaches. Beaches are deposited by breaker waves between high and low tide water. Beaches are infact wedge-shaped sediment deposits on sea shore. In width beaches vary from a few metres to several kilometres. Beaches are generally formed when sea is calm and winds are of low velocity.
Beach materials consist of fine to coarse sands, shingles (pebbles), cobbles and boulders. The major sources of the supply of beach materials are erosion of headlands and cliffs, sediments brought by the rivers and nallas at their mouths, mass wasting and mass-movement (landslides and slumping) of cliffs, scouring of the offshore zone by storm waves, erosion of pre-existing beaches etc.
The significant beaches developed on the west coasts of India include Juhu beach (Mumbai coast), Colba, Kalangut, Anjana etc, along Goa coast, Koblam beach along Kerala coast etc. Marina beach on Tamil Nadu coast (Chennai), Vishakhapatnain beach on Andhra coast and Puri beach at Puri along Orissa coast etc. are important beaches developed along eastern coast of India.
Beaches are generally classified on the basis of beach materials into:
(1) Sand beach (sand grains ranging in size between 0.5 to 2mm),
(2) Shingle beach (composed of pebbles ranging in size from 2 to 100mm), and
(3) Boulder beach (more than 100mm in diameter).
ii. Bars and Barriers and Associated Features:
The ridges, embankments or mounds of sands formed by sedimentation through sea waves parallel to the shoreline are called bars. The larger forms of bars are called barriers. The formation of bars and barriers starts with the development of shoals due to deposition of sands.
These shoals grow in height by addition of sediments until they appear above sea-level. Bars and barriers may be formed near the coast or away from the coast, parallel to the coastline or transverse to the coast. There are different forms of sand bars and barriers. If the bars are formed in such a way that they are parallel to the coast but are not attached to the land, they are called offshore or longshore bars.
If the sand bars are formed in such a way that their one end is attached to the land while the other end projects or opens out towards the sea, they are called spits. A few spits have been reported from the eastern and western coasts of India.
For examples, 50 km long spit in the mouth of Chilka lake (Orissa coast), 16 km long spit near Kalinagapatnam, a well-developed spit growing at the rate of 12 km per century to the east of Kakinada Bay, 60 km long spit to the east of Pulicat lake-all along the east coast; 22 km and 55 km long two spits enclosing the Vembanad Lake and converging at the port of Cochin on the east coast of India. Rameshwaram spit projecting seaward from Tamil Nadu coast is very important spit which is so stabilized that it bears human settlements.
High-energy storm waves very often modify the shape of spits by bending them towards the coast.
The curved spits assume the shape of hook and thus such spits are called hooked spits or simply hooks (fig. 20.12). Hooks are stabilized when there is equilibrium between constructive and destructive waves.
When the opposing currents become more dominant than the littoral currents, the spits are bent to such an extent that they are attached to the mainland (coast) and thus form complete loop which encloses sea water in the form of lagoons. Such form of a spit is called loop (fig. 20.13). When such loop is formed around and an island, it is called looped bar (fig. 20.13).
Connecting bars are formed when bars are so extended that they either join two headlands or two islands (fig. 20.13). Connecting bars are variously named on the basis of their shapes and forms. For example, a bar connecting two headlands is called connecting bar while a bar becomes tombolo when it connects the mainland with an island or connects a headland with the island.
Thus, a tombolo acts as a bridge between the coast and an island. A few examples of tombolo are observed along the western coast of India between Ratangiri and Malvan. When bars of pebbles and cobbles are formed on either side of a headland, such headland is called a winged headland.
Lagoons are formed when the coves or bays are completely enclosed by bars. Chilka lake and pulicat lake on the east coast of India are examples of lagoons.
iii. Coastal Wetlands:
Flat and rolling marshy lands developed in the coastal areas of humid tropics are called coastal wetlands, which are generally formed behind spits or bars. There is absence of reliefs and sea water remains stagnant in these wetlands. Sediments are fine and water is saline. The floral environment is dominated by mangrooves. Such wetlands are found extensively in the coastal zones of West Bengal where the mangrooves of the wetlands are known as Sundarban.
Depositional coastal areas having flat surface in the dry tropical zones are called sabkhas which are flat but barren coastal lands. Sabkhas have developed in the coastal zones of UAR (Egypt), UAE, Mexico, Baja of California (USA) etc. Sabkhas are also called as saltflats.