COASTAL EROSION

1.What is Coastal erosion

Coastal erosion is the wearing away and redistributing of solid elements of the shoreline as well as sediment, normally such normal forces as waves, tidal and littoral currents, and deflation

The developmental activities have put tremendous pressure on coastal lines and 20% of the Indian population lives on that coastal lines

Industrial cities as well as thickly populated cities are coastal cities in India like Mumbai, Chennai, Vishakapattanam, Kolkata, and Kochi

1.1.The usual problems along the Indian coast which need to be intervened by engineering activities are

1. Silting up of entrance channels

2. Closing river mouth

3. Flooding during a storm surge

4. Sand bar formation near mouth, inlets, rivers, and estuaries, and erosion of the coast

A coastline is a complex series of interlinked physical systems in which offshore and onshore are involved

Erosion occurs when material being, for depositing somewhere else exceeds the rate of supply finally resulting in the landward shifting of the shoreline

The coastal sediments, together with those arising from inland erosion and transported seaward by rivers are redistributed along the coast providing material for dunes, beaches, marshes, and reefs

Sand can be moved to another beach, to the deeper ocean bottom into an ocean trench, or onto the landslide of the dune

The removal of the sand from the sand-sharing system can cause permanent changes in beach shape and structure

Waves are the main reason for coastal erosion, they form in the middle of the ocean and will reach the coast

Waves bring enormous energy to the coast that is dissipated through the wave breaking, generation of currents, water level changes and movement of sediments, turbulence and heat

1.2.Wave energy is the result of the three factors

1. The speed of the wind blowing over the surface of the sea

2. The length of the fetch (distance of the sea over which wind blowing)

3. The length of the time the wind has been blowing for

Incident waves vary spatially and temporally, with their properties changing with movement over the bottom

Waves are the major factor in determining the geometry and composition of the beaches. The action of waves dictates the process of removal and addition of material/ sediment on the coast

1.3.Accretion: Coastal sediments returns to the visible part of the coast through the process called accretion

The two processes of accretion and erosion play a major role in defining coastal geography

The shoreline changes induced by coastal erosion and accretion are natural processes that take place over some time scale

2.History of coastal erosion in India

• Kerala is the state which is worst affected by coastal erosion in India, in the original assessment in the 1960s, about  57% of the coastline was identified as vulnerable
• Erosion of the coastal land in the state has been experienced for a long time, records show first anti-erosion works were done in the 19^th century
• According to the assessment made in the 1980s, 85% of the Kerala coastline was in the grip of erosion
• Karnataka and Maharashtra are also get affected badly by the coastal erosion

3.Causes of coastal erosion

• The shoreline or coastline, the boundary between land and sea, keeps on changing its shape and position continuously due to dynamic environmental activities
• Developmental projects are made in coastal areas, placing great pressure on them, and leading to diverse coastal hazards like soil erosion, seawater intrusion, coral bleaching, shoreline change etc.
• The causes of erosion are either natural or man-made, sometimes it is a combination of both, natural and man-made factors. While the combination of both is a relentless process that is often impossible to resist and the latter is often due to ill-planned activities and certainly be contained or reversed
• Coastal areas are given vital importance because people settlements have increased along coastal lines as well as development activities

4.Natural causes

• The natural factors influencing coastline erosion are winds, waves, tides, near-shore currents, storms, sea level rise etc
• The combined action of different processes on the coastline like waves and tides maintain the stability of the shoreline
• Sediment supply to a section of beach is reduced to littoral drift/ sea level rise or constant impact of waves which can cause severe erosion
• The increased gradient in transport rate in the direction of the net transport
• Natural variation in the supply of sediments to the coastline from the river can affect the erosion of the coastline
• An increasing sea level will promote shoreline setbacks, this setback is higher in the littoral coasts consisting of finer sediments as compared to coasts consisting of coarser sediments
• Subsidence is a regional phenomenon that lowers the surface area in the specific region
• Catastrophic events like severe storms, tidal surges, and cyclones cause the sea level to rise to abnormal heights and cause severe erosion

5.Man induced erosion

• Building houses through land reclamation or within dune areas has a long-term impact on coastal processes and sediment stability
• Harbours meant to provide safe mooring and navigation for the calling vessels, have shore perpendicular/inclined solid quays and breakwaters, which obstruct the long-shore transport of sand cause accretion on the up-drift side, and erosion down drift
• Sand removal above replenishable quantities from the coast upsets the longshore sand transport budget and can result in erosion down drift
• Groynes and jetties and other structures on the coast/ shoreline interfere with long-shore and sand transport and can result in erosion when these are ill-designed, Groynes protect a part of the shoreline by blocking littoral transport through the accumulation of the littoral sediments on the upstream side of the groynes, This causes a deficit in the littoral drift budget and this has negative consequences on the downstream as the erosion problem shifts to the downstream areas
• Structures like seawalls,  bulkheads, breakwaters etc. have side effects in terms of erosion of adjacent areas
• The mining of sand/ gravel along beaches and in the surf zone will cause erosion by depleting the shore of its sediment resources
• The maintenance of dredging of harbours, navigational channels, and tidal inlets causes loss of sand from the littoral zone and the sand is dumped into the deep sea
• Coral mining and other means of spoiling the protective coral reefs will also cause coastal erosion and beach degradation
• Vegetation is important for maintaining/ improving the sediment slope stability and consolidating the sediments by trapping the sediments

6.Coastal Protection measures

Coastal protection measures moderate the long-term average erosion rate of shoreline change from natural or man-made causes

Reduced erosion reciprocates the buffer(longer )  zone between land and sea

Protection works to prevent erosion should be on a long-term basis and must be planned to suit the particular site conditions based on thorough field investigation and available data which require observations over an extended period

7.Non-structural measures

Some measures are called as a Soft solution, some of these are

1. Artificial nourishment of beaches

2. Coastal vegetation such as mangrove and palm plantation

3. Sand bypassing at tidal inlets

4. Dune construction/rehabilitation

8.Structural measures

The structural measures used for coastal erosion prevention include seawalls, revetment, off-shore, breakwaters  and artificial headland