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

Detail

Where deposition happens: low sedimentary coasts

• Depositional coast: a shore where waves and currents drop more material than they remove. Here deposition is the main process, not erosion.
• Low sedimentary coast: a gently sloping coast made of loose sand and mud, usually where rivers bring in a lot of sediment. Such coasts have:
• wide beaches and coastal dunes
• bars, barriers, spits and lagoons

• deltas, marshes and swamps at river mouths

• High rocky coast: a steep, hard coast where waves mostly erode and form cliffs, wave-cut platforms and sea caves. Deposition there is limited to small beaches inside bays.

• India:

Feature	West coast	East coast
Main type	High rocky, retreating coast (in parts)	Low sedimentary coast
Main process	Erosion, with some beaches and kayals in Kerala	Deposition by large rivers and waves
Typical landforms	Cliffs, small bay-head beaches; Kerala kayals behind barriers	Deltas (Mahanadi, Godavari, Krishna, Kaveri), spits, barrier islands, lagoons
Examples	Konkan cliffs; Vembanad, Ashtamudi	Kakinada spit, Chilika, Pulicat

• Longshore drift (waves hit the shore at an angle, so sand moves along the beach in a zig-zag) carries the sand that builds spits and barriers. It keeps adding, removing and re-depositing sand, so these features change all the time [7].

Beaches

• Beach: a temporary deposit of sand or pebbles along a shore where deposition is the main process.
• Where the sediment comes from:
• most of it is brought by rivers and streams

• some comes from wave erosion of nearby headlands and cliffs

• Why "temporary": waves keep moving the material.

• Strong storm or monsoon waves remove the fine sand.
• A wide sandy beach may then shrink to a narrow strip of pebbles in another season.

• Calm-season waves bring the sand back.

• Types by material:

Type	Material	Where it forms
Sandy beach	Fine to coarse sand	Low sedimentary coasts with large river supply (most of India's east coast)
Shingle beach	Very small pebbles to cobbles (stones bigger than pebbles)	High-energy coasts, near rocky headlands

• Coastal dunes: onshore wind lifts dry sand from the back of the beach and piles it into low ridges.
• They lie behind the beach, parallel to the shore.
• They are common on low sedimentary coasts.
• They form a second natural wall behind the beach.

Offshore bars and barrier bars

• Offshore bar: an underwater ridge of sand and shingle, lying in the sea roughly parallel to the coast.

• Waves break on it before they reach the beach, so it takes in part of their energy.

• Barrier bar: an offshore bar that keeps receiving sand and grows above the water.

• It often forms across a river mouth or the entrance of a bay.
• A larger, wider barrier with dunes is called a barrier island. Barrier islands form mainly from waves and longshore currents. They are usually sandy and form where there is plenty of sediment and wave energy is fairly low [7][9].
• Indian example: Sriharikota Island (Andhra Pradesh) is a long, narrow barrier island. It separates Pulicat Lake from the Bay of Bengal and is home to the Satish Dhawan Space Centre, India's satellite launch site [4].

Spits

• Spit: a bar or barrier that is attached at one end to the land (one end of a bay or a headland). The other end lies free in the sea.
• How it forms:
• Longshore drift carries sand along the coast.
• Where the coast changes direction (a bay or river mouth), the drift keeps going straight into open water.
• The water there is deeper and calmer, so sand settles and the ridge grows forward in the direction of the drift [8].

• Waves often bend the free end towards the land, making a hooked spit.

• Indian example (NCERT Fig. 6.13): the Kakinada spit at the Godavari delta, Andhra Pradesh, on the Bay of Bengal. The Godavari supplies the sand, and northward longshore drift shapes it into a spit that shelters Kakinada Bay.

Tombolo

• Tombolo: a bar that joins an island to the mainland, or one island to another [8].

• Waves bend around the island and meet behind it. The water there is calm, so sand settles until a sand neck joins the island to the land [8].

• World example: Monte Argentario, Italy (Tuscany coast, Tyrrhenian Sea). It is linked to the mainland by two tombolos, with a lagoon (Orbetello) between them.

From bay to lagoon to coastal plain

• Lagoon: a shallow body of salt or brackish water that is cut off from the sea by a bar, barrier or spit, but still connected to it by one or more narrow openings (inlets).
• Brackish water: water that is saltier than river water but less salty than sea water.
• Sequence (one landform cycle on a depositional coast):
• 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 from the land and wind-blown or wave-washed sand from the beach slowly fill the lagoon. It turns into a swamp (marsh).

• Stage 4: The swamp dries and becomes a flat coastal plain.

• This follows the Davisian idea (W.M. Davis's cycle of erosion: a landscape passes through youth, maturity and old age). On a depositional coast, an open bay ("youth") is closed off as a lagoon ("maturity"), filled in and smoothed into a coastal plain ("old age").

Indian lagoons

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]
Vembanad (kayal)	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 (kayal)	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]

• Kayal: the Malayalam word for the backwater lagoons that lie behind sand barriers on the Kerala coast.
• Chilika and the Ramsar Convention:
• Ramsar site: a wetland listed as internationally important under the Ramsar Convention (1971).
• Chilika was designated on 1 October 1981, with an area of 1,16,500 ha [2].
• It supports breeding, wintering and staging of 33 waterbird species and holds 118 fish species [2].
• Montreux Record: the Ramsar list of sites whose ecology is under threat.
  • Chilika was added on 16 June 1993 because of siltation (silt filling up the lagoon), which was speeding up Stage 3 of the cycle above [2].
  • It was removed on 11 November 2002 after successful restoration [2].

The sediment budget

• Sediment budget: the balance between the sand and silt brought to a stretch of coast (by rivers, cliff erosion and longshore drift) and the sand taken away (by waves, currents and wind).
• Supply more than loss: beaches widen and spits grow (an accreting coast).

• Supply less than loss: beaches shrink and the shoreline moves inland (an eroding coast).

• Layers of natural coastal defence, from sea to land: 1. Offshore bars: first line; waves break here 2. Barriers and barrier islands 3. Beaches 4. Dunes 5. Mangroves: salt-tolerant trees whose roots slow waves and hold mud

• How the budget is upset:

• Dams: they trap river sediment upstream, so less sand reaches the delta and beach.
• Sand mining: it removes sand directly from beaches and river beds.
• Harbours, breakwaters and jetties: they block longshore drift. Sand piles up on one side and the beach on the other side is starved and erodes.

• Mangrove destruction: it removes the last living barrier.

• Result:

• Bars, barriers, beaches and dunes erode away.
• Storm and tsunami waves reach the land with full force.

• Settlements take the first strike. (See natural-hazards-disasters for the 2004 Indian Ocean tsunami and evidence that mangroves reduced damage.)

• India's erosion data:

• The National Centre for Coastal Research (NCCR), an attached office of the Ministry of Earth Sciences, mapped 6,907.18 km of the Indian mainland shoreline [3].
• From 1990 to 2018, about 33.6% of the coastline was eroding to some degree [3].
• Causes listed by the Ministry [3]:
  • more frequent cyclones
  • sea-level rise
  • human activities: harbour construction, beach mining and dams

Prelims Hooks

• Spit: a bar attached to land at one end. Tombolo: a bar joining an island to the mainland. Barrier bar: an offshore bar that has grown above sea level.
• Kakinada spit is at the Godavari delta in Andhra Pradesh (NCERT Fig. 6.13).
• Sriharikota is a barrier island that separates Pulicat Lake from the Bay of Bengal. It is home to the Satish Dhawan Space Centre [4].
• Chilika: India's largest brackish-water lagoon, in Odisha. It became a Ramsar site in 1981. It was on the Montreux Record from 1993 to 2002 because of siltation [2].
• Vembanad is fed by 10 rivers and is larger as a Ramsar site (1,51,250 ha) than Ashtamudi (61,400 ha). Both are kayals in Kerala [5][6].
• Order of natural coastal defences from sea to land: offshore bar → barrier → beach → dune → mangrove.
• Monte Argentario (Italy) is joined to the mainland by two tombolos.
• Sequence of lagoon infilling: bay → lagoon → swamp → coastal plain.
• Trap: beaches are temporary landforms. The same beach can be sandy in one season and pebbly in another.
• NCCR (1990–2018): about 33.6% of India's mainland coastline is eroding [3].

Mains Points

• Sediment budget and coastal erosion (GS-III, disasters and environment):
• Dams, sand mining and harbour breakwaters cut the sand supply, so bars, beaches and dunes erode.
• About 33.6% of India's coast was eroding during 1990–2018 [3].

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

• Natural barriers versus hard structures:

• Bars, barrier islands, dunes and mangroves are layered, self-repairing defences against cyclones and tsunamis.
• Seawalls protect only one stretch, and often speed up erosion on the next stretch down the coast.

• The 2004 tsunami showed that this "soft" protection works.

• Lagoon management (GS-I and GS-III):

• Lagoons naturally fill with silt over time.
• Human activity can make this faster.
• Chilika is the model case of restoration: it was on the Montreux Record in 1993 and was removed in 2002 [2].

• The lesson applies to Pulicat, Vembanad and Ashtamudi, which are under pressure from fisheries, tourism and reclamation.

• Distribution pattern: the east coast has big rivers, a wide continental shelf and northward longshore drift. That is why it has India's large deltas, spits and lagoons. The west coast is steep and has fewer such features, except for the barrier-lagoon kayals of Kerala.