11 COASTAL ZONE AND COASTAL PROCESSES

Benidhar Deshmukh

1. Objectives

• Discuss coastal zone and its importance
• Describe the processes governing coastal configuration
• Elaborate on erosional and depositional processes and the resultant features
• Elucidate the basis of classification of coasts and characteristics of Indian coasts Discuss the measures used for coastal protection

2. Concept Map

3. Coastal Zone

In this section, we shall get introduced to some terms commonly used while discussing coast. What comes to our mind immediately at the mention of the word ‘coast’ is the beautiful scene with blue green water constantly changing its levels and sandy beaches. The level of the sea falls at low tide and rises at high tide. The line of contact between the land and water is called shoreline that is the actual margin of the land and sea fluctuating  from moment to moment with waves and tides. The contact zone between the land and the sea is termed as ‘coast’ or ‘shore’. The terms coast and shore are generally used interchangeably, however, there is a difference between the two. While the term shore or shore zone is used for part of the land–sea interface between the extreme low and extreme high tide levels, the term coast is used to indicate a strip of land of indefinite width that may include the shore and some areas extending seaward and landward of the shore. Shore is further divided into several zones i.e. offshore, nearshore, foreshore and backshore as shown in Figure 1. Offshore is the zone that extends to the shallow bottom seaward of the breaking waves. Nearshore is the zone between low tide level and breaker zone. Foreshore is the intertidal sloping zones between the low tide level and the limit of high tide level or storm wave effects. The zone landward of the foreshore that is inundated only during large storms and includes beaches, cliffs or dunes is called backshore. There are other terms which have been used in this section however, we shall get introduced to these terms later in the chapter.

Figure 1: Generalised profile of a coast showing different zones.

The term coastal zone refers to a broad area that includes the portion of the sea where physical, biological and biogeochemical processes are directly affected by land, and also the portion of the land that influences the sea. Coastal zones are very important because they:

are highly dynamic in nature due to constant interactions of hydrosphere, lithosphere, atmosphere and also biosphere that continuously shape it

support important ecosystems such as seagrass, mangroves and coral reef ecosystems that are known for their very high ecological diversity and as a nursery ground for a variety of organisms including fishes contribute substantially to the global economy

are known as the areas of high population density and growth witness intense anthropogenic activities leading to changes in landuse/ landcover, and receive high influx of land based pollution and dissolved nutrients.

India has a long coastline of ~7500 km including that of its island territories. However, in addition to natural processes, anthropogenic activities like rapid urbanization, tourism development, discharge of waste effluents, municipal sewage, over exploitation of coastal resources are exerting increasing pressure on coastal zones of India (SAC Report, 2012).

4. Processes Governing Coastal Configuration

The term ‘coastal process’ refers to all the activities taking place at the coast. Coastal processes are effective in the narrow vertical zone near sea level and hence are responsible in shaping landforms within the zone limited by extreme tide limits. We shall study about the coastal processes in the next section.

Present day configuration of coasts is a result of the processes that are in action today but also the processes which were active in the past. Coast is shaped by several individual processes such as tidal, wave, subaerial, biological processes, which work simultaneously or in some sequence. Configurations of coasts is not constant rather it is highly dynamic and is constantly changing according to the nature of the material and the energy changes. According to an estimate of the coastal processes by Inman and Nordstrom (1971), of the world’s coasts about 45% are wave eroded, 36% glaciated, 11.7% wave-deposited, and the rest have formed mainly due to river, wind or biological activities. It suggests that wave process is dominant among other processes. Along the Indian coasts too, wave action is important. Let us discuss major processes.

4.1 Waves:

The seawater is constantly on motion due to waves, tides and currents. Waves are considered as the major source of energy in majority of the coasts and hence most important agent in shaping up coasts. Waves are undulations of sea water, and have well defined crests (i.e. highest point of the wave) and troughs (i.e. lowest point of the wave). It is better to define some terms here which will be used in forthcoming paragraphs. Difference between the crest and trough is called wave height and the distance between two crests or troughs is called wavelength. And, the time between two successive wave crests to pass a fixed point is called wave period or wave period of time.

4.1.1 Formation of waves: Waves are produced by the action of wind. Wind disturbs waterbody as it exerts frictional drags on the surface-water particles of more or less flat body of water (Figure 2a). It results in setting up small orbital motions of water particles in  the water with the largest ones near the surface and decreasing with depth (Figure 2b). Wind pressing on the back of a developing wave makes it steeper and gradually the waves grow bigger. When wind blows strongly for many hours, it increases height of the waves. Size of waves is governed by the following three factors:

wind velocity, duration of the wind, and

the fetch (the distance of open water surface over which the wave-generating wind blows).

When the wind is strong, wave steepness (the ratio of wave height to wavelength) increases and when it is not strong, the wave steepness decreases. If the wind is stronger it will blow longer and when the fetch is greater, the wave will be more powerful, e.g. storm waves. So, length of the fetch is the limiting factor in wave development.

Figure 2: (a) and (b) show initial stages of formation of waves; and (c) wave modification near coast. Waves have circular movement away from coasts that turn to elliptical due to frictional drag when they approach coast.

4.1.2 Wave modification near coast: Waves are generated in open Ocean and travel to coast where they break against the land. Wavelength, velocity of the waves and the wave period are the three closely related parameters that modify waves. When waves reach near coast, shallowing of water depth and irregularities of the shoreline result in change in form (dimensions) and mechanics of the waves (Figure 2c). The waves slow down but the wave period remains same. Wavelength is shortened but the wave height increases and these result in increase in wave steepness. The wave steepness keeps increasing until it becomes so steep that it breaks. The zone where the waves break is called breaker zone or surf zone. In the breaker zone, the wave force is translated up the beach due to which the water is thrown up the beach (i.e. swash). The water which drains down the beach under the influence of gravity is called backwash, which is either in a sheet flow (undertow) or as a rip-current (i.e. a localised concentration of backwash).

The initial movement of water in a wave is circular however, when waves move towards coast, water moves forward on the crest and backward in the trough changing the circular form to elliptical. Wave steepness is an important factor responsible for making waves either constructive or destructive. When most of the swash soaks into the beach with very little backwash, the waves are called constructive waves or spilling waves (Figure 3a). This happens on the gently sloping beaches where waves of long wavelength and low height approach and the ellipse becomes horizontal. When high waves of short wavelength break on steeply sloping beaches, the water plunges forward into the trough resulting in very powerful backswash that can carry material down the beach. These types of waves are called destructive waves or plunging waves (Figure 3b).

Figure 3: Waves gets modified on beaches and change to (a) constructive or spilling waves on gently sloping beaches, and to (b) destructive waves or plunging waves on steeply sloping beaches.

There are three types of changes that may occur to wave direction viz. wave reflection, wave refraction and wave diffraction. When waves approach a uniformly sloping coast at right angles to the shore, they break simultaneously and all the waves strike the shore in straight parallel lines. When the breaking waves hit a cliff or seawall, water in motion impinges against the structure and the waves are reflected. When waves approach coast, they are also subjected to refraction and or diffraction. Waves refract when waves approach shoreline (consisting of headlands and bays) at an angle and each wave impinges on the shallow sea floor before the rest of the wave. This results in slowing down of the section of the wave opposite the headlands but the section of the wave facing the bay moves shoreward at higher velocity that is still at some distance away from the shore. This relative difference in velocity leads to progressive bending of wave crest parallel to the shoreline. This is known as wave refraction. It causes the wave crest to converge on the headlands and diverge on the bay (Figure 4a). This results in high breaking waves and intense erosion at headlands and quieter waves at the bays thus producing cliffs on the headlands and beaches in bays. When waves approach a barrier obliquely, it results in diffraction of waves leading to shadow zone in the lee of the barrier. Waves also diffract when a segment of wave crest passes in river mouth or bay (Figure 4b). The currents generated within bays and harbours by diffracted waves erode and transport sediment until the shoreline fits the diffraction pattern.

Figure 4: (a) A relationship between wave refraction and diffraction is observable at the shoreline, and (b) wave diffraction as seen in the form of parallel wave crests in the North Cinque island, A&N (Source: https://zoom.earth).

4.2 Tides

Tidal processes are found more dominant on the coasts characterised by estuaries or other embayments. We have studied that shoreline does not remain constant and keeps changing with change in the level of the sea. The rhythmic periodic rise and fall of the sea water is called tide, which is caused by gravitational pull by moon and the Sun on Ocean. Rise of the sea water is called flood and is fall is called ebb. Effect of the Moon is more powerful than that of the Sun because Moon is comparatively much closer. Tide causes the sea level to move vertically by several meters daily. The gradual rise to the highest level is termed high tide and fall to the lowest level is called low tide. The difference between the level of the low and high tide on a given location is called tidal height or amplitude or range, which may vary from few meters to several meters e.g. over 15m in Bay of Fundy, Canada to less than 2m in southwestern coast of India. Variation in tidal height in different locations on the Earth is due to the different forms and depths of the ocean basins and the rotation of the Earth. The difference is more in shallow seas particularly in narrow channels and straits. Following four types of coasts are identified based on the tidal range, i.e. megatidal (tidal range >6m),

macrotidal (tidal range between 4 and 6m), mesotidal (tidal range between 2 to 4m), and microtidal (tidal range <2m).

Following three factors affect tidal movement:

• change in the declination of the Moon and the Sun,
• distance from the Earth to the Sun and the Moon, and
• position of moon and the Sun in relation to each other and to the Earth.

Different locations on the Earth’s surface are attracted by moon with varying strength because of the changing distance between these locations and the position of the Moon. It is also due to eccentricity of the orbit of the Moon round the Earth. When the Moon is closer to Earth, tides are bigger. When the Earth, Moon and the Sun are in a straight line then high tide (spring tide) is produced. When the Sun and the Moon are at right angles from the Earth then low tide (neap tide) is produced.

• Tides are divided into following two types according to the period of rise and fall:
• Diurnal – tides occur once a day during the lunar day e.g. in Northern Pacific and Indian Ocean
• Semi diurnal – tides recur twice during the lunar day e.g. in Atlantic Ocean.

The coasts with large tidal range such as the Gulf of Kachchh and Gulf of Cambay are generally characterised by the presence of extensive mudflats with mangroves and salt marshes. In the coasts with lower tidal range, relatively smaller areas are subjected to daily wave action such as the south-western coasts of India.

Tidal range and topography of the coast determine velocity of tidal currents, which dominate the water movements particularly in bays, estuaries and tidal channels (called creeks). This results in tide dominated landforms. In small tidal inlets, water discharge is determined by the tidal prism (i.e. the volume of water between the high and low tide surfaces) whereas in the large tidal creeks and estuaries, the tide progresses as a wave. In estuaries, flow is of two types:tidal currents – are the most important physical process

residual currents – are chemical or diffusive processes due to the differences in the density of fresh water and salt water.

4.3 Currents

Besides, tides and waves, currents are also responsible for coastal modification. Currents are mass movements (vertical or horizontal) of ocean water (both surface and deep) in a continuous flow. Currents are largely generated due to surface winds but can also be created by tides, difference in the density of water (i.e. temperature and salinity gradients), gravity and Earth’s rotation.

There may be several kinds of currents. There could be either surface currents (the ones found in the upper 400 meters) or deep water currents (found below 400 meters). Surface water currents make up about 10% of the ocean and deep water currents make up about 90% of the ocean. The speed of surface currents is greatest closer to the ocean’s surface and decreases at about 100 meters below the surface. While surface currents are generally caused by the wind because of the friction generated by its movement over the water and forcing the water to move in a spiral pattern and thus creating gyres. The gyres move clockwise in the northern hemisphere whereas they move counter-clockwise in the southern hemisphere. Gravity also plays a role in the movement of surface currents. In the areas where water meets land, or water is warmer, or where two currents converge, mounds in the water form. Water moves downslope on the mounds under the influence of gravity and thus resulting in creation of currents. The Coriolis force also plays a role in the movement of surface currents and deflects them, further aiding in the creation of their circular pattern. Major current systems typically flow clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere, in circular patterns that often trace the shorelines.

Currents normally move in a specific direction and aid significantly in the circulation of the Earth’s moisture, the resultant weather, and water pollution. Ocean currents are the wind induced major ocean circulations in open Ocean. They are major control on the supply of offshore energy influencing wave dimensions, supply of sediments and also biotic factor in coasts. There are many types of currents; however we shall get introduced to the coastal currents. Water flows in a definite direction as a current near coast. Following two broad types of currents are recognised near coasts:

a cell-circulation system consisting of currents flowing parallel to the coast and rip currents, and longshore currents generated by waves flowing at an angle to the coast.

A rip current is a narrow current of water that is generated by movement of water returning back to the sea from beach. It is caused by shape of the shoreline and the current runs perpendicular to beach (Figure 5a). Rip currents move along the surface of the water and not underneath the water surface. They are capable of moving coarse sediments from coast. Rip currents may occur in the areas such as gap in a reef or low area on a sandbar where there is strong longshore variability in wave breaking due to presence of jetties, sandbars, crossing wave trains.

Generally, rip currents are narrow however, they may be wider and faster where there are large and powerful breaking waves. Height of shoreward moving waves varies in direction to the shore causing longshore flow of water from the location of the highest breakers to that of lowest breakers. Further, presence of troughs in the surf zone parallel to the shore  facilitates the longshore movement of water and the water reaching the lowest breakers return to sea as strong currents thus creating the rip currents. The water returned to sea through rip currents is moved back shoreward again completing the circulation. This circulation of longshore and rip currents is influenced by bottom topography.

Figure 5: (a) Showing formation of rip currents due to difference in wave setup; and

• (b) longshore currents as a result of waves hitting the coast at low angles thus drifting the sediments in direction of its movement i.e. northeast in the image shown here (Source: https://zoom.earth).

The speed at which waves approach a coast depends on bathymetry. When a wave approaches a beach, it does not reach the beach perfectly parallel to it. Different segments of the wave reach the beach at different time interval causing slowing down of the segments. This results in bending of the wave causing it to conform to the general shape of the coast. Waves coming oblique to the beach also create longshore currents (Figure 5b). Incoming waves intercept the retreat of the water that has been carried up the beach by a previous wave. This results in more longitudinal movement of water along the beach between the line of breakers and the shore. It causes the beach material to gradually shift laterally. This type of longitudinal wave may move a considerable amount of sediment parallel to the coast. When the breakers of the incoming oblique waves are of surging type, sediments may be pushed up the beach and when the waves are of plunging type, sediments may be eroded off the beach and carried away longitudinal to the coast. These longitudinal currents move sediments from headlands or river mouths to downcurrent bays and deposit them there.

Currents are also generated due to tides when they ebb or flow. Tidal currents are strong enough to move sediments. In estuaries particularly, the ebb tides are stronger due to reinforced outflow of river water. Tidal currents are considered more effective in shaping up the bathymetry rather than modifying beaches.

4.4 Other Processes

Besides, waves, tides and currents, there are several other oceanographic, terrestrial and subaerial processes which are responsible in shaping up the coasts such as ocean currents, wind action, biological processes, fluvial processes, hillslope processes, sea ice, frost action, etc. We shall discuss briefly about these processes here.

• (i) Wind Processes: We have discussed earlier about how wind away from the coast is important for generation of waves. Waves associated with storm also influence coasts. And, the locally generated wind waves are also significant. Winds are also generated locally on coasts that may have its effect on the coasts. One example is the sea breeze that blows onshore (landward) in afternoon when land is hotter than the seawater due to differential heating of land and sea. At night, the breeze blows seaward when land is cooler than the seawater. We have read about geological action of wind and know that wind plays a major role in the formation of dunes. It is responsible for development of dunes on coasts by sorting sediments, transporting and depositing them.

• (ii) Fluvial Processes: We have read about geological action of rivers and know that river water plays a major role in the formation of several landforms. There are several landforms such as deltas that develop at the mouth of the rivers. River action is clearly visible on the deltaic coasts. Since, rivers bring fresh water and sediment, their role is important, particularly in estuaries and deltas. The sediment brought by the rivers are an important addition to the coastal sediment budget which are significant for shifting of beaches and dunes. In deltas, the sediments brought by rivers are generally deposited at the mouth of the river which are then redistributed by coastal processes whereas in the case of estuaries, it acts as a sink for both fluvial and nearshore sediments.

• (iii) Biological Processes: Plants and animals are also the agents that play important role in shaping up coasts. Biological processes are dominant on many coasts as plants trap sediments and also help in binding them e.g. on coastal dunes, mangrove coasts, algal mats, etc. In case of dunes, vegetation binds sand and on muddy coasts, algae, mangrove and marsh vegetation trap sediments. Another very good example is the coral reefs, which are the landforms generated by almost entirely by organisms. The reef framework is built by the corals and is contributed by other organisms that graze, bore and contribute sediments for infilling and binding the reef. They help building new lands on the coasts. Coral reefs are very important due to their ecological and economic importance. Another most influential factor is human, who can drastically modify a coast. Impact of human activities on coasts is now well documented.

• (iv) Hill Slope and Glacial Processes: Besides wind action, there are other subaerial processes that can be important in certain coastal settings such as hill slope particularly on rocky coasts. Nature of slope and the material it consists of could vary greatly from one

place to another hence affecting the generation and movement of materials. Similarly, processes associated with ice and frost may also affect coastal configurations. However, these actions are restricted to high latitude coasts.

5. Coastal Processes and their Distinctive Features

In this section we will study about the erosion, transport and deposition mechanisms of these processes and also the distinct features they develop.

5.1 Erosional Processes and Resulting Landforms

Although, erosion on coasts is a natural process, the rate of erosion is accelerated by tsunamis, storm surges and also human activities. Erosion on coasts depends on several factors such as:nature of coast nature of rock presence of joints and fissures in the rocks presence of rock articles with waves chemical action of seawater, and also wave strength.

Erosion is prominent in coasts where waves are not obstructed and dash directly whereas in shallow low-lying coastal plain, waves are obstructed before reaching shore thus resulting in reduced impact of waves. When waves dash against the shore, rock fragments present with waves also dash against the coastal rocks and wear them off. With the presence of dissolved materials seawater becomes better solvent thus enhancing chemical action on the coastal rocks. It is obvious that erosive action is more when wave strength is more.

Erosion on coasts takes place by the following mechanisms:

• (i) Corrosion: It is process of chemical weathering by which coastal rocks e.g. limestone rocks change/ degrade into more chemically-stable form, such as its oxide, hydroxide, or sulfide due to pH of seawater. The soluble rocks (or parts of the rocks) are dissolved. The rate of reaction is further increased by wave action which continuously removes the reacted material. This process is not considered very significant because its influence is restricted mainly to limestones or other similar rocks.

• (ii) Corrasion or Abrasion: It is the mechanical process and is considered as the most important process of erosion. When waves pound coasts, the rock fragments, sand, pebbles and boulders, present in the coasts act as tools of erosion as they break pieces of the coastal rocks and transport the broken pieces towards the sea. The forward and backward movement of sea water with these tools of erosion also cause corrasion of rock

face and the floor. Abrasion is the actual physical break-up of cohesive rock similar to quarrying, in which sea cuts into unconsolidated materials and pulls away particles/ pieces.

• (iii) Attrition: It is the mechanical process in which waves cause rock fragments to collide with each other and progressively grinding and chipping each other into smaller, smoother and rounder pieces. The rock fragments become very small that they are easily transported towards the sea by rip currents.

• (iv) Hydraulic Action: The coastal rocks are eroded even without the presence of rock fragments in the seawater. Waves erode the rocks when they continuously strike against them and the impact is powerful enough to break and disintegrate the rocks.

• (v) Shock pressure of breaking waves: When waves strike against the coastal rocks, they exert enormous pressure particularly on the rocks joints and cracks. Hence, the air trapped inside the rocks becomes compressed. With retreat of the waves, the pressure suddenly falls and the air expands. It causes stress in the rocks resulting into weakening of the rocks surrounding the air pocket leading to their exploding and breaking along the joints and cracks. Besides the mechanical process, solution activity also increases in the soluble rocks. In the process of solution, acids contained in the sea water dissolves some types of rock such as chalk or limestone.

There are several features that develop due to erosion. Some of the common features are described here:

(i) Cliff and Notch

Rocky coast vertically rising above sea water is called cliff (Figure 6). Steepness of the cliff is determined by types of rocks, inclination of the rock strata, geological structure and relative rate of cliff face and erosion of base. Softer sedimentary rocks are comparatively easily eroded than hard and resistant rocks such as igneous and metamorphic rocks. Bedding planes and joints are the weaker zones along which the rocks break easily. If the rock strata and the joints both are inclined towards the sea then the rocks easily break. Degree of steepness of the rock strata also enhances the rate of erosion. If the rate of erosion of the cliff base is faster than that of the cliff face then overhanging cliff with steep vertical face is formed whereas if the rate of erosion of the cliff face is faster than that of the cliff base then verticality of the cliff disappears. When the waves erode at the base of the cliff, the feature thus developed is called notch (Figure 6).

(ii) Wave-cut and Wave-built Terraces

Base of the coastal rocks are eroded due to hydraulic action and a notch develops near the high water mark. This undercutting process causes the notch to become big and the upper parts to collapse over time. Such a resultant feature is called wave-cut cliff (Figure 6). With continuous wave erosion and retreat of the cliff a shallow shelf called wave-cut terrace/bench/platform or shore platform develops, which is a flat erosional surface in front of the wave-cut cliff (Figure 6). Much of the materials eroded by waves is moved seaward and is used to develop an underwater delta like feature called wave-built terrace/platform (Figure 6).

(ii) Headland

In the coasts with alternating soft and hard rocks, parts of the coast containing softer rocks are eroded fast due to differential erosion. This results in a point of land consisting harder rocks that’s extends into open water. The remnant feature is called headland (Figure 7).

Figure 7: Donapaula headland, Goa.

(Source:http://bhuvan.nrsc.gov.in/globe/3d.php#).

(iii) Cave, Blow hole, Arch and Stack

Cave is formed due to continuous wave erosion of the coastal rocks. When the waves strike the caves it may develop a hole in the roof through which seawater sprays. It is called blow hole. At some places, a bridge like feature called arch is formed due to wave erosion on both sides of headland. With continuous erosion over time the arch collapses causing separation of one end of the arch from the mainland. Such remnant isolated small island like feature occurring just offshore is called stack.

5.2 Mechanism of Transportation

The eroded materials (e.g. sands, silts, gravels, pebbles, cobbles and even boulders) are transported by waves and currents. The eroded materials are transported seaward by backwash and undertow currents that move from coast to seaward. These materials are again picked up by breaker waves or surf currents that bring the materials back to coast. So, the material is transported from coast to seaward and from sea to coastward. Majority of the transportation takes place by movement at the bottom and the surface is continuously modified. Longshore currents also transport the materials but parallel to the shoreline.

Transportation takes place by one or more of the five transport mechanisms i.e. suspension, rolling, sliding, saltation and solution as in rivers. The materials are sorted by the seawater as it carries and transports them. The finest particles are carried away farthest and the coarser particles are left behind on the beach. The materials in solution are carried farther off either by reacting among themselves or by organisms, animals and plants that extract some of the dissolved matter to build their shells or tissues.

5.3 Depositional Processes and Resulting Landforms

The transported materials are deposited either in shallow waters near the shorelines in continental shelf and slope or in deep seas on the floor of the deep oceans. The common features developed due to depositional processes on coasts include beaches, spits, bars and barriers, lagoons, coastal dunes, mudflats, tidal marshes, wave built terrace/platforms, etc.

(i) Beach

The striking coastal depositional feature is the beach that occupies edge of a shoreline (Figure 8-10). Beach is a product of sediment deposition due to reduced wave energy. The sediments are derived from different sources such as erosion of cliff rocks, or brought by rivers and/or picked up by waves from the sea floor and moved shoreward. Beaches

are formed in areas where either cliffs is either absent or not in direct contact with the wave cut platforms. Beach is a gently sloping feature and in profile it slopes towards sea but the gradient changes at different places. Shape of beaches could be either straight or concave. Most of the world’s beaches are formed by sand deposition. Such beaches generally appear dazzling white. However, in the areas of high energy the beach material could be pebbles or even boulders.

(ii) Spit and Tombolo

Spit is a ridge of deposited sediments that extends from land to sea cutting off a portion of water from the open sea and is generally elongated in shape (Figure 8a). It generally extends as a continuation of a beach in the direction of sediment transport due to longshore drift and is composed primarily of sand. Spit could be of different sizes and may take various shapes. A growing spit that connects the mainland with an island or an island with another island is called tombolo (Figure 8b). This is formed by deposition of sediment (primarily sand) due to wave refraction around island

Figure 8: (a) Beach (B), spit (S), barrier island (Bi), mangrove swamp (M) (Source: http://bhuvan.nrsc.gov.in/globe/3d.php#); and (b) tombolo connecting the tied island located in lower centre with the North Cinque island in A&N. The white linear features bordering the islands are beaches (Source: https://zoom.earth).

(iii) Bars and Barriers

Bar is a submerged depositional mound or ridge, which is generally composed of sand and gravel. Formation of a bar depends on currents and supply of sediment. At some places, due to local geology a long, narrow island composed of sand and gravel is formed parallel to the shoreline. Such a feature is called barrier island (Figure 8a). It is separated from the mainland by a lagoon, tidal flat or salt marsh. Barrier island and coral reefs may also create a lagoon, which is a quiet water area behind a barrier island or reef. In some cases, spits may extend right across the small bays or mouth of rivers forming a lagoon.

(iv) Tidal Flat and Salt Marsh

Tidal flat is not actually flat rather a very gently sloping sandy or muddy area that is exposed during low tide and inundated with water during high tide. Based on composition of the materials it is either called sand flat or mud flat (Figure 9). Salt marshes are the flat wet tracts which are located in the upper intertidal area. They are marshy area that are flooded by seawater during high tide. Tidal waters enter tidal flats through tidal inlets, which are also known as creek (Figure 9).

Figure 9: Mangrove swamp (M), sandy (S) and muddy tidal flats (Mf), and creek (C) (also known as tidal inlet) as seen in Pirotan island, Gulf of Kachchh. (Source: https://zoom.earth).

(v) Mangrove Swamp

Another feature built by biological activities is the mangrove swamps (Figure 8 & 9). Mangroves are evergreen halophytic vegetation that grow on soft muddy bottom on tropical coasts. Mangrove plants can cope up with high salinity and anaerobic soil conditions and have developed characteristic adaptation such as vivipary i.e. breathing roots.

(vi) Coral Reefs

The biological structures built primarily by corals and algae along with a number of organisms are called coral reefs. Corals can grow in specific environment and hence are found in the tropics. Coral organism called polyp has exoskeleton of calcium carbonate. Corals grow in colonies and together build huge structures over a period of time. In India, coral reefs occur in a variety of settings at different places i.e. Gulf of Kachchh, Gulf of Mannar and Palk Bay, off Malvan coast in Maharashtra, Lakshadweep and Andaman & Nicobars. There are primarily three types of coral reefs:

fringing reef – that grow next to land

barrier reef – found at some distance away from the coast and separated by a shallow lagoon

atoll – near circular or elliptical reefs that are characterised by the presence of a central lagoon (Figure 10).

Figure 10: An atoll type of coral reef in Lakshadweep with characteristic central lagoon. The island visible is the Chetlat island. The while line surrounding the island is beach. (Source: https://zoom.earth).

(vii) Coastal Dunes

Dunes are generally found at the back of beaches. Like the dunes developed in deserts they are wind deposits varying in size and morphology. They are best developed in the areas of wide beaches with abundant supply of find sand and presence of low vegetation at a short distance back of the beach along with strong wind. Coastal dunes may be of several types as in the desertic dunes such as parabolic dunes, barchans, transverse, longitudinal dunes, etc.

(viii) Delta

The area where a river meets see is called estuary. The sediments brought by river may get deposited at the mouth of the river where it enters sea. Shape of such deposits generally resembles greek letter delta ‘Δ’ hence such features are called delta (Figure 11).

Figure 11: Delta formed on River Krishna (Source: http://bhuvan.nrsc.gov.in/globe/3d.php#).

6. Types of Coasts

Coasts can be classified based on its relation with sea level (such as emergent or submergent) or by the nature of material such as rocky or non-rocky coasts. Non-rocky coasts can be further classified as sandy, muddy.

6.1 Basis of Classification

Several classifications have been proposed for classifying coasts based on numerous factors however, no classification has found universal acceptance. We shall discuss here some of the parameters on which classification of coasts have been proposed.

(i) On the basis of Sea Level Fluctuations with respect to Land

Classification by D.W. Johnson (1919) is one of the commonly used which is based on the sea level fluctuations in relation to the land and categorised shorelines into following four types:

• (a) Shorelines of submergence – these are formed by partial submergence of land due to rise in sea level or subsidence of the land and are characterised by features such as cliffs, wave-cut platforms, caves, headlands, offshore islands, drowned valleys, deep embayments, etc.
• (b) Shorelines of emergence – these are formed by upliftment of land or lowering of sea level and are characterised by features such as relatively straight coasts of low relief, marine terraces, offshore bars, lagoons, etc.
• (c) Neutral shorelines -these are having characteristics which are independent of submergence or emergence e.g. delta, volcanic shorelines, coral reefs.
• (d) Compound shorelines – these shorelines include more than one type mentioned above e.g. marine terraces and drowned valleys.

(ii) On the basis of Shoreline Maturity and the Nature of Agency

Although, Johnson’s classification is simple and genetic in nature, there are several limitations of it such as emphasises on sea level change; many shorelines have undergone both submergence and emergence due to Pleistocene glaciations and interglaciations; and nearly all shorelines are compound. Considering the limitations, Shepard (1937, 1948, 1963) proposed two fold classification which is based on shoreline maturity and the nature of agency:

• (a) Primary or youthful coasts – these are formed primarily due to nonmarine terrestrial agencies and include subtypes resulting due to erosion on land and drowned subsequently; deposition on land by river, glaciers, wind, vegetation, etc.; volcanic activity; and diastrophism.
• (b) Secondary or mature coasts – these are formed mainly due to marine processes and include subtypes resulting due to marine erosion; marine deposition; and biological activities.

(iii) On the basis of Energy Environments

A more useful classification has been proposed by J.L. Davies on the basis of energy environments. He recognised following types of environments:

• (a) Storm wave environments – they are characterised by frequent destructive storm breakers
• (b) Swell environments – they are characterised by flat constructive waves

(c) Protected environments – they are characterised by low energy conditions. (iv) On the basis of Plate Tectonics

Another useful classification was proposed by Inman and Nordstrom in 1971 in which three categories of coasts are recognised based on plate tectonics:

(a)  Collision coasts – they are active margin of a continent

(b)  Trailing-edge coasts – they are passive side of a continental plate

(c)  Marginal sea coasts – they are separated from open Ocean by island arcs

These are the first order features on which second and third order features are superimposed producing local coastal morphology. Second order features are upto 100m long, 10m wide and 1m high above the ocean floor developed on first order features. Examples include deltas, coastal dunes, barrier beaches and lagoons. Third order features are smaller ones such as beach face and longshore bars.

6.2 Classification of Indian Coasts

• According to Ahmad (1972), Indian coasts can be categorised into following seven types based on their morphological attributes:
• (i)   Andaman and Nicobar type probably represents promontories of the Arakan-Yoma system and is characterised by coral reefs
• (ii)    Deltaic coasts type comprises major deltas of Ganga, Mahanadi, Godavari and Cauvery rivers and is subject to modification by strong monsoonic currents and cyclonic storms
• (iii)   Depositional plain type is a product mainly of fluvial deposition covering eastward sloping coastal plain from Bay of Bengal to Cape Comorin except the beaches affected by deltaic deposition.
• (iv)  Malabar type is submergent in nature and having undergone recession in recent times and characterised by high relief and irregular terrain
• (v)  Maharashtra type is submergent in nature caused by faulting having an irregular outline with steep rocky prominences and embayments
• (vi)    Gujarat type is a coast with shallow tectonic depressions characterised by gully erosion fronted by estuarine shore mud flats
• (vii)    Kathiawar coast emergence (such as estuaries and inlets) type is compound in nature and represents features of both barriers and coastal dunes) and submergence (such as cliffs,

7. Coastal Protection

Erosion of coastal areas is a significant problem in many regions around the globe. To protect coasts and coastal structures from erosion several types of structures such as sea wall, breakwater, groin (also called groyne) and jetties are used which act as sediment trap.

Sea walls are generally used in the areas experiencing strong wave attack, which is an embankement of concrete, boulders or other materials (Figure 12a). The seawalls either have concave face to deflect the force of the waves or sloping ramp that helps to dissipate wave energy. An offshore wall constructed parallel to shoreline to break waves is called breakwater (Figure 12b). It is generally used to protect a harbour from waves and currents.

Groin or groyne is a short wall constructed with concrete or wood perpendicular to shoreline from coast to low water level to trap sand and build-up a beach (Figure 12c). There is generally a series of walls that prevent erosion by trapping the sediments in the direction of advancement of longshore drift. Sand accumulates on the up-current side of the groin in relation to the longshore current. Jetties are long walls extending from shore at the mouths of harbours (Figure 12d). They are constructed of boulders to protect entrance of harbour from waves and current erosion and/or filling with sand.

Figure 12: Different types of structures built to protect coasts from erosion: (a) sea walls (off Mundra port, Gujarat), (b) breakwater (off Gangavaram, Vishakhapatnam port, A.P.), (c) groins (near Gopalpur port, Odisha), and (d) jetties (off Vishakhapatnam port). (Source: a, b & c- https://zoom.earth; d-http://bhuvan.nrsc.gov.in/globe/3d.php#).

8. Summary

In this lecture we learnt about: Coastal zone

Processes responsible for shaping up coasts Erosional processes and resulting landforms

Depositional processes and resulting landforms

Types of coasts and characteristics of Indian coasts Coastal protection measures.

9. References

• Ahmad, E. (1972) Coastal geomorphology of India. Orient Longman, Bombay, 222p. Inman, D.L. and C.E. Nordstrom (1971) On the tectonic and morphologic classification of coasts, Journal of Geology 79(1):1-21.
• Johnson, D.W. (1919) Shore processes and shoreline development. Wiley, New York, 584p.
• SAC Report (2012) Coastal zones of India, accessed from http://www.moef.nic.in/sites/default/files/Coastal_Zones_of_India.pdf.
• Shepard, F.P. (1937) Revised classification of marine shorelines. Journal of Geology, 45(6):602-624.
• Shepard, F.P. (1948) Submarine Geology. Harper & Row Publ, New York.Shepard, F.P. (1976) Coastal classification and changing coastlines. Geoscience and Man, 14: 53-64.

Suggested Readings

• Ahmad, E. (1972) Coastal geomorphology of India. Orient Longman, Bombay, 222p. Inman, D.L. and C.E. Nordstrom (1971) On the tectonic and morphologic classification of coasts, Journal of Geology 79(1):1-21.
• Johnson, D.W. (1919) Shore processes and shoreline development. Wiley, New York, 584p.
• SAC Report (2012) Coastal zones of India, accessed from http://www.moef.nic.in/sites/default/files/Coastal_Zones_of_India.pdf.
• Shepard, F.P. (1937) Revised classification of marine shorelines. Journal of Geology, 45(6):602-624.
• Shepard, F.P. (1948) Submarine Geology. Harper & Row Publ, New York.
• Shepard, F.P. (1976) Coastal classification and changing coastlines. Geoscience and Man, 14: 53-64.