Coastal processes

1. The coast as a dynamic system

A coast is a transition zone shaped by marine processes, terrestrial sediment supply, geological structure and biological activity. Its form depends on rock resistance, coastal slope, wave exposure, tidal range and the availability of sediment. The shoreline is not a permanently fixed boundary: its observed position changes with tides, storms and seasons. Long-term shoreline change therefore requires comparison of consistent indicators over sufficiently long periods.

A useful organising concept is the sediment budget: the balance between sediment entering, being stored within and leaving a coastal segment. Rivers, eroding cliffs, offshore deposits and wind may supply material. Longshore currents, offshore transport, dredging and sand extraction may remove it. A persistent deficit commonly causes erosion, whereas a surplus supports accretion, although local engineering and geology also matter.

A littoral cell is a coastal sediment-transport compartment with sources, pathways and sinks. Headlands, estuaries or submarine canyons may partly bound such cells. Because neighbouring beaches can share sediment, protecting one location may increase erosion elsewhere. Shoreline retreat must also be distinguished from temporary inundation: the former is landward displacement of the shoreline, while the latter is flooding that may subsequently recede.

• Principal controls: geology, waves, tides, currents, sediment supply and relative sea level.
• Prelims distinction: coastal erosion removes material; coastal flooding covers land with water.

2. Waves, tides and coastal sediment transport

Wind-generated wave characteristics depend principally on wind speed, duration and fetch, the uninterrupted distance over which wind blows. In deep water, water particles move in approximately circular orbits that diminish with depth. As waves enter shallow water, seabed interaction reduces their speed and wavelength; their height commonly increases through shoaling until they become unstable and break. The uprush on a beach is swash, and the returning flow is backwash.

Wave refraction is the bending of wave crests caused by differences in propagation speed over varying depths. The shallower part of a wave slows first. Along an irregular coast this usually concentrates wave energy around headlands and spreads it across bays, encouraging headland erosion and deposition in sheltered areas. Local seabed configuration can modify this pattern.

When waves break obliquely, swash carries sediment diagonally up the beach while gravity-driven backwash returns it more directly downslope. Together with wave-generated longshore currents, this produces alongshore sediment transport. Cross-shore transport redistributes material between beaches and offshore bars. Steep storm waves commonly move sand offshore; gentler conditions may return some of it. Tidal currents are particularly important in estuaries and inlets, whereas rip currents are narrow seaward flows, not tides.

• Spring tides occur around new and full moon; neap tides occur around the first and third quarters.
• Constructive and destructive waves are useful textbook categories, but actual beach response also depends on sediment size, slope and antecedent conditions.

3. Erosion and its characteristic landforms

Marine erosion operates through hydraulic action, abrasion and solution. Hydraulic action involves wave impact and pressure changes, including compression of air in rock joints. Abrasion occurs when transported sand and pebbles scour the coast. Solution is chemical dissolution, especially important in susceptible rocks such as limestone. Attrition differs from abrasion: transported fragments collide with one another and become smaller and more rounded.

Repeated wave attack can excavate a notch near the foot of a cliff. Undercutting and weathering weaken the overlying rock, which may collapse through rockfall or other mass movements. Cliff retreat can leave a gently sloping wave-cut platform, often exposed at low tide. Its development reflects wave action, rock structure, weathering and sea-level history rather than a single process.

On jointed headlands, waves can enlarge weaknesses into caves. A cave may extend through a headland to form an arch; roof collapse can leave an isolated stack, later reduced to a stump. This is a common explanatory sequence, not a compulsory evolution for every rocky coast. Alternating resistant and weak rocks can produce headlands and bays through differential erosion. Beaches within bays may subsequently dissipate wave energy and limit further erosion.

• Rock structure matters: joints, faults, bedding and lithology influence the position and rate of erosion.
• Marine erosion and subaerial weathering commonly act together; cliffs are not shaped by waves alone.

4. Deposition, coastal ecosystems and sea-level change

Deposition occurs when transporting agents lose the capacity to carry sediment. Beaches are accumulations of sand, gravel or other loose material within the active coastal zone. A spit is a narrow depositional ridge attached to land at one end, commonly extending across an inlet where longshore transport continues beyond a change in coastline direction. Its distal end may curve because of changing wave directions and currents.

A baymouth bar extends across a bay entrance, while a tombolo connects an island to the mainland or another island. Barrier islands are elongated sedimentary bodies separated from the mainland by lagoons or marshes. Lagoons are shallow coastal water bodies partly or largely isolated from the open sea. Tidal flats form where fine sediment accumulates under suitable low-energy conditions; vegetation can promote sediment trapping in salt marshes and mangroves.

Wind can transfer dry beach sand inland to form coastal dunes, whose vegetation helps stabilise sediment. Dunes also provide sand reserves during storms. Emergent coasts reflect relative sea-level fall and may display raised beaches or marine terraces. Submergent coasts reflect relative sea-level rise: rias are drowned river valleys, while fjords are drowned glacial valleys. Relative sea level can rise because the ocean rises, the land subsides, or both.

• A delta forms where river-borne sediment accumulation builds land at a river mouth; not every river mouth develops a delta.
• An estuary is a coastal water body with marine connection and mixing of seawater with land-derived freshwater.

5. Indian coastal hazards and management

India’s coasts illustrate contrasting process regimes. Strong tidal currents influence the Gulfs of Khambhat and Kachchh, while major east-coast rivers support extensive deltaic environments. Monsoon-driven changes in waves and currents cause pronounced seasonal beach fluctuations. Tropical cyclones add storm surge and energetic waves; surge is an abnormal meteorological rise in sea level, distinct from astronomical tide. Tsunamis are also distinct, usually originating from sudden displacement of the seabed.

Human activities can disrupt sediment budgets. Dams may reduce downstream sediment delivery; river and beach sand mining remove material; harbours and breakwaters can intercept longshore transport. Groundwater extraction and sediment compaction can intensify relative sea-level rise through subsidence. The National Centre for Coastal Research assessed about 6,907 km of mainland coastline for 1990–2016 and classified 33.6 per cent as eroding, illustrating that shoreline behaviour varies substantially along India’s coast.

Management combines hazard mapping, development setbacks, ecosystem conservation and selective engineering. Seawalls protect assets locally but can increase reflection and toe scour. Groynes trap alongshore-moving sediment but may starve downdrift beaches. Beach nourishment replenishes sand but requires suitable material and periodic renewal. Coastal Regulation Zone notifications under the Environment (Protection) Act, 1986 regulate coastal activities. Integrated coastal management should evaluate the whole sediment cell and favour long-term risk reduction over isolated structures.

• Soft approaches include dune restoration, wetland protection and beach nourishment.
• Hard structures should be assessed for effects on neighbouring shorelines, habitats and fishing access.

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