Lagoon

Topic: Landforms and their Evolution: Rivers, Groundwater, Glaciers, Waves and Wind · NCERT: Class 11, Ch 6 "Landforms and their Evolution"

Meaning

A lagoon is a shallow body of salt or brackish water (water saltier than river water but less salty than sea water). A bar, barrier or spit cuts it off from the sea, but one or more narrow openings called inlets still connect it to the sea.

Lagoons are one stage in the life of a depositional coast: bay → lagoon → swamp → coastal plain. Large Indian lagoons such as Chilika and Pulicat appear often in exams. They are important wetlands, and they come up in questions on siltation, Ramsar sites and coastal protection.

Explanation

How a lagoon forms

• Cause: plenty of sand and gentle waves
• Lagoons form on a depositional coast, where waves and currents drop more material than they remove.

• Such coasts are usually low sedimentary coasts. These are gently sloping shores of loose sand and mud, where rivers bring in a lot of sediment.

• Process: a sand wall grows across a bay

• Longshore drift moves sand along the beach in a zig-zag, because waves hit the shore at an angle.
• Where the coast changes direction, the sand settles in calmer, deeper water. It builds a spit, which is a ridge attached to the land at one end [8].

• Elsewhere, an underwater offshore bar keeps collecting sand. It rises above the water as a barrier bar, often across a river mouth or the entrance of a bay.

• Result: a sheltered pool of water

• The bar, barrier or spit almost closes the bay.
• The trapped water behind it becomes a lagoon. Inlets still join it to the sea.

The life cycle: bay to coastal plain

• Stage 1: Bars and spits grow across the mouth of a bay.
• Stage 2: The bay is almost closed off and becomes a lagoon.
• Stage 3: Rivers bring silt from the land, and wind and waves wash sand in from the beach. The lagoon slowly fills and turns into a swamp (marsh).
• Stage 4: The swamp dries out and becomes a flat coastal plain.
• Link to Davis: this follows W.M. Davis's cycle of erosion, in which a landscape passes through youth, maturity and old age.
• Youth: the open bay
• Maturity: the closed-off lagoon
• Old age: the filled-in coastal plain

Parts of a lagoon system

• Barrier: the sand wall on the sea side. It can be:
• a spit, attached to the land at one end
• a barrier bar, an offshore bar that has grown above the water

• a barrier island, a larger, wider barrier with dunes. These form mainly from waves and longshore currents, where there is plenty of sediment and wave energy is fairly low [7][9].

• Inlet(s): narrow openings through the barrier. Sea water enters and leaves through them.

• The lagoon water: brackish, because river fresh water mixes with sea water coming in through the inlets.
• Kayal: the Malayalam word for the backwater lagoons that lie behind sand barriers on the Kerala coast.

Conditions a lagoon needs

• A large sediment supply, usually from big rivers.
• Low to moderate wave energy, so the sand settles instead of being washed away.
• A gently sloping coast with an indented bay or river mouth.
• A sediment budget in balance. This is the balance between sand brought to a coast and sand taken away. If too little sand arrives, the barrier erodes. If too much silt arrives, the lagoon fills up too fast.

Where it occurs

India: mainly the east coast

• The east coast is a low sedimentary coast. It has big rivers, a wide continental shelf and northward longshore drift, so large spits, barrier islands and lagoons form there.

Lagoon	State	Separated from sea by	Key facts
Chilika	Odisha (mostly Puri district, part in Ganjam district)	A long sandy ridge (barrier spit)	India's largest brackish-water lagoon. Water spread is 1,165 sq km in the monsoon and 906 sq km in summer. Salinity changes a lot by season and by part of the lake [2]
Pulicat	Andhra Pradesh, extending into Tamil Nadu	Sriharikota barrier island	About 50 km long and 5–16 km wide. Lies on the Coromandel Coast [4]

• Chilika as a Ramsar site (a wetland listed as internationally important under the Ramsar Convention, 1971):
• Designated on 1 October 1981, with an area of 1,16,500 ha [2].

• Supports breeding, wintering and staging of 33 waterbird species and holds 118 fish species [2].

• Sriharikota Island (Andhra Pradesh) is a long, narrow barrier island. It is home to the Satish Dhawan Space Centre, India's satellite launch site [4].

India: Kerala on the west coast

• Most of the west coast is high, rocky and eroding. The exception is Kerala, which has kayals behind sand barriers.

Kayal	State	Separated from sea by	Key facts
Vembanad	Kerala	Barrier island or foredune ridge shaped by waves and shore currents	Largest brackish, humid tropical wetland ecosystem. Fed by 10 rivers. Ramsar area 1,51,250 ha [6]
Ashtamudi	Kerala	Barrier	Second-largest estuarine system in Kerala. Has mangroves and 57 bird species, 6 of them migratory. Ramsar area 61,400 ha [5][6]

World

• Orbetello lagoon, Italy (Tuscany coast, Tyrrhenian Sea) lies between the two tombolos that join Monte Argentario to the mainland.

Don't confuse with

• Spit: a spit is the sand ridge attached to land at one end. A lagoon is the water body trapped behind such a ridge.
• Tombolo: a bar that joins an island to the mainland or to another island [8]. It does not close off a bay, but two tombolos can enclose a lagoon between them, as at Orbetello.
• Estuary: a river mouth that stays wide open to the sea. A lagoon is almost cut off from the sea by a barrier and joins it only through narrow inlets.
• Swamp / coastal plain: these are later stages. A lagoon fills with silt and becomes a swamp (marsh), then a dry coastal plain. They are not the same landform.

Prelims Hooks

• Order of lagoon infilling: bay → lagoon → swamp → coastal plain.
• Chilika (Odisha) is India's largest brackish-water lagoon. It became a Ramsar site on 1 October 1981 [2].
• Chilika was put on the Montreux Record (the Ramsar list of threatened sites) on 16 June 1993 because of siltation. It was removed on 11 November 2002 after restoration [2].
• Pulicat Lake (Andhra Pradesh–Tamil Nadu) is separated from the Bay of Bengal by Sriharikota barrier island, home to the Satish Dhawan Space Centre [4].
• Trap: both Vembanad and Ashtamudi are kayals in Kerala. Vembanad is fed by 10 rivers and has the larger Ramsar area (1,51,250 ha against 61,400 ha) [5][6].
• Trap: lagoons are a depositional landform of low sedimentary coasts, not an erosional landform like cliffs or sea caves.

Mains Points

• Siltation and lagoon management (GS-I, GS-III):
• Lagoons fill with silt naturally over time (Stage 3 of the cycle).
• Human activity can speed this up and turn a living wetland into swamp.
• Chilika is the model case: siltation put it on the Montreux Record in 1993, and restoration took it off in 2002 [2].

• The same lessons apply to Pulicat, Vembanad and Ashtamudi, which are under pressure from fisheries, tourism and reclamation.

• Sediment budget and barrier erosion (GS-III, disasters):

• Dams trap river sediment, sand mining removes beach sand, and harbour breakwaters block longshore drift.
• With less sand arriving, the bars and barriers that protect lagoons erode, so storm and tsunami waves reach the land with full force.
• About 33.6% of India's mainland coastline was eroding during 1990–2018 (NCCR, Ministry of Earth Sciences) [3].

• Responses: sediment bypassing at harbours, controls on beach mining, mangrove and dune restoration, and shoreline-change maps in Coastal Zone Management Plans.

• Why lagoons are found mainly on the east coast (GS-I, distribution):

• The east coast has large rivers (Mahanadi, Godavari, Krishna, Kaveri), a wide continental shelf and northward longshore drift. These give it deltas, spits, barrier islands and lagoons like Chilika and Pulicat.
• The steep, rocky west coast has few such features. The exception is the barrier-lagoon kayals of Kerala.

Related concepts

• Beach
• Offshore bar
• Barrier bar
• Spit
• Tombolo
• Sediment budget

Read more

• Coastal deposition: beaches, bars, spits, lagoons and the sediment budget