Coastal Landforms: Erosional and Depositional

2 years ago

Atiqur Rahman

10 minutes

Waves, currents, tides and wind play a significant role in marine erosion. However, other controlling factors influence the erosion rate in coastal areas (Thornbury, 1997). These are:

a) Kind and durability of rocks along the shore

b) Structural controls such as fractures and joints

c) Tidal range

d) Openness of shore to wave attack

e) Depth of water offshore

f) Configuration of coastline

g) Presence or absence of protective beach

h) Abundance and size of abrasive tools

i) Stability of sea level

Five major processes contributing to coastal erosion are corrosion, corrosion or abrasion, attrition, hydraulic action and shock pressure of breaking waves.

Coastal Erosional Landforms

The erosive action of waves, winds and currents on the coasts form distinctive topographical features.

Cliff

In some places, the seaward limit of the coast is marked by a nick or scarp commonly known as a cliff. Cliffs are steep (with angles of more than 40º, sometimes even vertical and sometimes hanging), and coastal slopes cut into rock formations (Bird, 2007).

The subaerial erosion of the cliff face and wave erosion at the cliff base recedes the cliff backwards. The coastline of the world, more than three-quarters are rocky and cliff-faced.

The formation of cliffs is strongly influenced by the geological structure of the coast and its response to weathering. In areas of resistant rocks, the cliffs are often vertical or steep. In areas where less resistant formations are cut back as cliffs, there is the presence of irregular rocky shores or smooth shore platforms that are exposed at low tide.

Wave Cut Platform

Wave-cut platforms or Shore platforms border many cliffs and extend across the intertidal zone. Whereas the term wave cut platform is mainly restricted to geomorphic features shaped entirely by the hydraulic action of waves on soft rocks, such as clay shore platforms are formed due to abrasion by waves containing sand or gravel, tidal scour and weathering.

The presence of a shore platform reflects the stability of the coast over a long period maintaining the same level of the sea. In case of a change in sea level, one can observe the presence of marine terraces that are elevated wave-cut benches marking the former high sea level (Thornbury,1997).

Caves

As the cliff recedes, coastal erosional processes of weathering and erosion penetrate the zone of weakness, such as faults, joints or outcrops of less resistant rocks. These processes cut clefts and crevices that further develop into caves and blow holes or deep, narrow inlets with stacks.

Marine erosion thus has acted on the zones of weakness or segments of weaker rocks in Scottie’s island of Staffa Fingal’s Cave is 20 metres high and 70 metres long in columns of tertiary basalt. The coast has originated as subterranean solution caverns in the southern Corsica caves of Bonifacio on the limestone.

Blowholes

The Blowholes develop in a cave when the hydraulic action of incoming waves and the compression of trapped air puncture the cave’s roof with the water and spray driven up through it as fountains of spray.

Kiama in New South Wales and Quobba in Australia are examples of blow holes occurring in elongated caves cut along joints in basalt. Blowholes formed in layered carboniferous sandstone in Mayo in Northwest Ireland are known as puffing holes.

Natural Arches

Natural Arch is formed when powerful wave action excavates a cave along joints and weak fractures on a cliffed coast on a headland. The wave action cuts through the caves on both sides of the headland producing this spectacular geomorphic feature.

On the Normandy coast of France Porte d’ Aval is a natural arch on a cliff of Heartchalk. Sometimes elongated natural arches may form tunnels. One such example is the tunnel on the north coast of Cornwall Merlin’s Cave (Bird,2008).

Stacks

Dissection of headlands on a cliffed coast often isolates stacks either when a natural arch collapses or when a transverse inlet is cut along a zone of weakness through the headland. Finally, they may be reduced by erosion and continue to be undercut till they collapse, leaving only a basement platform. Examples are Twelve Apostles near Port Campbell, Australia. ( Bird, 2008)

Coastal Depositional Landforms

Beaches

The beach is an accumulation of loose unconsolidated sediment, a temporary veneer of rock debris ranging in size from very fine sand to pebbles, cobbles and occasionally boulders. Nearly 40 per cent of the world’s coastline consists of unconsolidated sand and gravel deposits.

The beaches may extend for several kilometres or may be patchy. They may be almost straight or gently curved; some may be short and include sharply curved pocket beaches in bays or coves between rocky headlands. Some are exposed to open seas, and some are sheltered in bays.

The movement of sediments of sand and other particles by waves and currents from one sector to another change the shape of the beach. The movement of longshore current and longshore drift when waves arrive at an angle to the shore, the movement of longshore current and longshore drift changes the beach morphology, and when the sediments are moved to and fro between the beach and nearshore zone as waves break parallel to the coastline also helps in shaping the beach.

The beach accretion mainly occurs during relatively quiet seas,, and beach thickness is most significantly altered during storms. In coasts with high wave energy, large particles can be moved, and beaches tend to be steep relative to coastal areas where low wave energy defines the presence of only fine material.

In the dynamic coastal system, the beach reflects an equilibrium state between an input of material by swash and removal by backwash (Holden, 2008). The underlying structural rock formations of Granite, basalt, shale, conglomerate and coral define the type, colour and texture of various beaches.

A prograding beach shows net accretion and receives more sediments from different sources than losing it on shore, offshore or alongshore. The transverse profile of a prograding beach is convex or in the form of a terrace ending in a seaward slope. Longshore drifting of sand and shingle may lead to some beaches prograde. For example., Cardigan Bay on the west coast of Wales, Streaky Bay, South Australia and the Danish Island of Kyholm.

Spits

The spits are beaches that are built above high tide level, diverging from the coast and ending in one or more landward hooks or recurves (Schwartz, 1972).

The dominant pattern of wave action shapes the outlines of the spits, and they grow in the direction of longshore drift by waves arriving at an oblique angle to the shore. An example of a straight longshore spit is Orfordness, on the East coast of England, and a recurved spit is seen at Hurst Castle, Hampshire (Bird, 2008).

Evans (1942) defines it as a ridge or embankment of sediment attached to land at one end and terminating in open water at the other. The axis of the spit usually extends in a straight line parallel to the coast, but when currents are deflected landward or tides are strong, the growth of a spit may be deflected landward, with the resulting creation of a recurved spit or hook.

Several stages of hook development may produce a compound recurved spit or compound hook. Spit may be attached at both ends to the headland to form a winged headland. (Thornbury, 1997)

A cuspate spit is formed if two spits converge offshore or a simple or compound spit recurves until it becomes attached to shore at both ends. Growth of successive cuspate spit is farther and farther seaward will cause prograding of the shoreline and seaward extension, thus forming what is known as a cuspate foreland. Examples – Cape Henry, Virginia and Cape Kennedy (Canaveral), Florida. (Thornbury, 1997)

Bars

In a generic sense, the term bar includes various types of submerged or emerged embankments of sand and gravel built on the sea floor by waves and currents. They may be emergent at low tides. They extend parallel to the coastline. Between a bar and a coastline and between nearshore bars, there are elongated troughs where the water is deeper.

Bars may be exposed at low tide and submerged during high tide, creating intervening troughs that contain lagoons as the tide falls. Most bars are sandy with pebbles and shells. Migrating bars become asymmetrical with steep advancing slopes. During calm weather and quiet seas with effective constructive swash, bars move closer to the shore and depict a flat profile.

Barrier Beaches and Barrier Chains

Barrier beaches consist of elongated landforms and sand ridges parallel to the shore, which rise slightly above high tide. They are formed by depositing beach material offshore or across the mouths of inlets or embayments.

In the case of gently sloping coasts, waves may break at a considerable distance from the shore, thereby enabling churning waters to build submerged offshore bars running parallel to the coast.

These bars gradually grow and emerge above sea level, stand as a barrier between the open sea and an earlier coastline, and often result in coastal progradation.

In due course of time, they form barrier beaches and possibly large barrier islands. The barrier island has three distinct zones – a sandy beach, a sand dune and a shallow lagoon or swamp between the barrier island and the mainland. A series of these features extending for a considerable distance is known as a barrier chain.

In the low-relief coastlines, barrier islands dominate the Atlantic and Gulf coasts from New York to Texas; the ocean coasts of South Africa and Eastern South America are the most common features.

Coastal Landforms: Erosional and Depositional