(GIST OF YOJANA) Coastal Erosion
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. Although the erosion of the coastal land has been experienced in the State for a long time and records show anti-erosion works having been constructed even in the nineteenth century. An assessment made in the late 1980s indicated that almost 85% length of Kerala’s coastline was in the grip of erosion. Later, it was found that Karnataka and Maharashtra were also affected badly by sea erosion. The problem in other states was found to be in patches/coastal pockets depending on various factors. The first anti-sea erosion measure in Puducherry was initiated by the French in the early 1920s wall when a 1.75 km long retaining wall was constructed along the urban coastline in Puducherry.
- Natural factors influencing coastal erosion are waves, winds, tides, near-shore currents, storms, sea level rise, etc. The combined action of different processes on the coastline like waves and tides maintains the stability of the shoreline. If for any reason, the sediment supply to a section of beach is reduced due to littoral drift/sea level rise or constant impact of waves, it can cause severe erosion.
- Another important factor here is an increasing gradient in transport rate in the direction of the net transport, e.g. consider the gradient in the wave conditions due to certain relief features or bathymetric conditions.
Also, the natural variation in the supply of sediments to the coastline from the river can affect the erosion of the coastline. Another major factor promoting coastal erosion is the sea level rise. 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.
- Another factor is the phenomenon of subsidence. Subsidence is a regional phenomenon that lowers the
sur ace area in a specific region. It impacts the coastline in a way similar to sea level rise, however, the rate may vary as per the factor causing this subsidence.
Most of the human-induced erosion is due to human interventions in the natural transportation process as well as in the sediment load of the rivers. Human activity may be enumerated as Coastal defence structures, river regulation works, dredging aggregate extraction/ sand mining, oil/gas exploration (in the form of long-term subsidence), and ports/harbours that impact sediment transport.
Coastal activities can also directly or indirectly result in beach erosion. The following are some examples:
- Building houses via land reclamation or within sand 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, often have shore-perpendicular/inclined solid quays and breakwaters, which obstruct the long-shore transport of sand and 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.
- 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 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. This disturbs the dynamic sediment equilibrium of the coast and promotes erosion to re-establish the equilibrium.
- Coral mining and other means of spoiling the protective coral reefs will also cause coastal erosion and beach degradation. The production of carbonate sand stops due to the killing of the corals and the protective function of the reef disappears.
- Vegetation is important for maintaining/improving the sediment slope stability and consolidating the sediments by trapping the sediments. The removal of dune vegetation and mangroves due to man interventions causes exposure of the low-energy shorelines to the increased energy and reduced sediment stability. This further promotes erosion of coastal zone.
- The Non-structural measures aim at the dissipation of the wave energy by mirroring the natural forces and maintaining the natural topography of the coast. These measures are also called soft solutions. Some of these are:
- Artificial nourishment of beaches
- Coastal vegetation such as mangrove and palm plantation
- Sand bypassing at tidal inlets
- Dune reconstruction/rehabilitation
- Before opting for the hard structures, nonstructural measures like adaptation to natural coastal
processes (by using large setback distances, relocating vital structures, etc.) and moderation of coastal erosion (by stabilising coastal slopes, tripping the waves, etc.) should be used.
- These measures have limitations. While artificial nourishment of beaches is complicated and costly, mangrove plantation is possible only in marshy land and in semi-tropical or tropical conditions. Some of these measures are:
- The structural measures, also known as the hard structural/engineering measures use physical structures constructed near the coast to prevent or restrict water from reaching the potential damage areas. These solutions influence the coastal processes to stop/reduce the rate of coastal erosion.
- The structural measures used for coastal erosion prevention include
seawalls, revetment, off-shore breakwaters, groins/groynes/spurs, offshore reefs, and artificial headland.
- Out of the above measures, seawall is popular and generally used in almost all maritime States in varying proportions.
In view of aforesaid aspects and also to optimize the long-term positive impact of soft solutions, many combinations of soft and hard solutions can be selected. These combinations act as interim hard structures and some of the common approaches of combinations are:
- Combining beach nourishment with artificial headlands/groynes.
- Revegetation with temporary offshore breakwaters/artificial reefs is commonly used.
- Using a combination of beach nourishment and groynes/artificial headlands promotes the trapping of
the downdrift movement of the sediment, thus reducing downdrift erosion. This also reduces the frequency of re-nourishment.
- We realise that coastal erosion is an extensive and multi-dimensional problem for a vast country like ours.
- Efforts are being made to counter the menace of coastal erosion and to protect our coasts, using both the traditional approaches (using hard structures like a seawall, etc) and also using the new, innovative soft measures like dune rehabilitation.
- The protection works are prioritised, planned, and designed as per the graveness and extent of the problem.