The Indian coastline spreads over 7,500 km and is endowed with a wide variety of ecosystems, rich habitats and biodiversity. The coastal environment plays a vital role in the nation’s economy by virtue of its resources. These areas, however, are also facing complex issues due to increasing human population, urbanisation and accelerated developmental activities. Of the several threats faced by India’s coast, concern related to coastal erosion reigns high. The coast comprises multifaceted processes that are interlinked by many physical systems—both offshore and onshore. Coastal erosion is a physical process as it entails the removal of beach sand that is either returned to the sea or redistributed on the adjacent coast. It finally results in the landward shifting of shoreline. Coastal accretion is another process, opposite to erosion that enhances beach growth and seaward shifting of shoreline. The processes of accretion, erosion and sediment transport play a major role in defining coastal geography.
The shoreline changes induced by coastal erosion and accretion are natural processes that take place due to waves, tides, near shore currents, etc. In a temporal scale, these processes may occur in response to both small-scale events, such as storms, regular wave action, tides and winds and large-scale events such as glaciation, orogenic cycles or tectonic activities that cause coastal land subsidence or emergence.
Measures for controlling beach erosion depend on local conditions of shore and beach, coastal climate and sediment transport. Continuous monitoring and study of coastal processes world over have yielded considerable experience in various coastal protection measures.
The causes of erosion can be broadly classified into natural or human induced and in many cases are an outcome of both. The natural processes are relentless, often impossible to repel. However, the human induced processes arise from improper planned activities that can certainly be contained, or even reversed, such as interference in the natural transportation process as well as in the sediment load from rivers. Effects of climate change, sea level rise and other long-term causes for the erosion are still unaccounted for.
Structural control measures include 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. There are many such hard structural coastal protection measures—seawall, revetment, off-shore breakwater, groynes/spurs, offshore-reefs, artificial headland, etc. Seawall is, however, the most popular and used in almost all maritime states in varying proportions.
The non-structural measures aim at dissipation of wave energy by mirroring the natural forces and maintaining the natural topography of the coast—also termed as soft solutions. Some of these are artificial nourishment of beaches, coastal vegetation such as mangrove and palm plantation, sand bypassing at tidal inlets and dune reconstruction/rehabilitation.
A combination of structural and non-structural measures helps in better efficacy and efficiency providing synergetic outcomes with an environmentally and economically acceptable coastal protection system. Some of the common approaches of combinations are, combining beach nourishment with artificial headlands/groynes and re-vegetation with temporary offshore breakwaters/artificial reefs.
Coastal Protection in India
The first measure to counter coastal erosion was initiated by the French in Puducherry in the early 1920s. A 75 km long retaining wall was constructed along the urban coastline. Data, however, reveals that Kerala is the worst affected with a 1960 assessment pointing to at least 57 per cent of its coastline being vulnerable. A later assessment made in the late 1980s pointed out that nearly 85 per cent of Kerala’s coast is being eroded (CWC, 2016). The sea walls constructed as an immediate relief measure now show detrimental effects on adjacent coastlines. According to the Central Water Commission’s Report, 2016, on the shoreline change about 40 per cent of India’s mainland shoreline is affected by erosion. It further observes that the coastal region of the state of Maharashtra was affected more by erosion; of about 60 per cent of its total coastline followed by Andhra Pradesh of about 53 per cent of its coastal boundaries (Fig. 1).
NIOT and Sustainable Coastal Protection
The National Institute of Ocean Technology (NIOT), Chennai, the technical arm of the Ministry of Earth Sciences (MoES) through its Coastal and Environmental Engineering (CEE) and Offshore Structure (OS) group has carried out many coastal engineering and related activities along the Indian coast. The works include the design of coastal engineering structures through scientific data collection and numerical modelling, port harbour and engineering activities and sustainable coastal protection works. NIOT through its pilot projects and demonstration has successfully implemented sustainable solutions at Kadalur Periyakuppam and at Puducherry. The institute has been focusing on implementing sustainable solutions for coastal erosion. The solutions offered are soft as well as a combination of soft and hard solutions in order to ensure that there is minimal impact on the environment and the coastal community, enabling stakeholder participation in the process.
The Kadalur fishing villages, about 60 km south of Chennai near Kalpakkam, have been suffering from heavy erosive action on the beach of over 60 m between 2003 and 2013 (Sivakholundu, et al., 2013). The fisheries department of Tamil Nadu had installed several structures to be used as fish landing centre along the coastline, which were destroyed due to the erosion. Built-up areas nearby were in a dilapidated condition and hanging precariously. NIOT’s studies concluded that erosion is aggravated during the cyclones, causing sudden encroachment of the sea, prompting an unstable coastline.
Long term analysis of satellite images by NIOT indicates that the coastline of Kadalur Periyakuppam had been stable till 2003. After the 2004 Tsunami event the coastline started to undergo rapid changes. Following the effect of Thane cyclone in 2011 and Nilam cyclone in 2012, this shoreline has been subjected to severe erosion. Extensive bathymetry, oceanography and hydrographic surveys were undertaken. Monthly shoreline changes were measured through beach profiling surveys conducted each month.
Numerical modelling using MIKE 21 SW, LITPACK and DELFT 3D were carried out for wave transformation, sediment transport and various combinations of segmented submerged dyke for determination of the best combination of dyke segment and gap.
The solution proposed was an off-shore submerged dyke which was to be made of large sand-filled geosynthetic tubes (3.5 m x 200 m). The principle is to shift the severe wave breaking during cyclones to deeper waters so that the energy of waves is reduced by the time it approaches the shoreline. A submerged dyke configuration was selected as it is capable of dissipating the energy of the larger waves rather than completely blocking it. The reduced wave results in reduced scour near the beach. This method avoids the use of boulders for construction and has very little interference with alongshore movement of sediment. Interference with the alongshore sand transport may result in accretion / erosion issues which is transferred to coastlines beyond the affected zones. The offshore submerged dyke has yielded better results and the beach was found to be growing (Fig. 2).
The coastlines of Puducherry and the neighbouring Tamil Nadu have suffered from severe coastal erosion due to natural and anthropogenic activities. Initially, sand bypassing was carried out by harbour authorities to prevent down drift erosion and to maintain the channel free from siltation. Later, discontinuing sand bypassing due to various technical reasons, led to erosion along the Puducherry city. The governments of Puducherry and Tamil Nadu resorted to short term measures to protect the coast from erosion. The seawalls and groynes constructed by the Puducherry at various time to mitigate the problem of erosion and protect the coastal infrastructure, actually shifted the erosion further north. A need for an integrated long-term solution was felt by the authorities and stakeholders in order to restore the beach. The government of Puducherry then sought consultation from NIOT. Assessing various long term shoreline changes using satellite data and process based measurements during various seasons, a detailed shoreline management plan was prepared.
While analysing alternatives for beach restoration, the Puducherry port department informed that about 50,000 cubic m of dredged sand was available near the harbour. A pilot experiment was then suggested and dry beach nourishment was carried out along a 500 m coastline, north of the Puducherry harbour. Since, the nourishment was carried out during the southwest monsoon, no threat of siltation at the harbour mouth due to the northerly drift was perceived. The sand nourishment resulted in gaining of a 60 m beach near the Puducherry Light House and Puducherry New Pier. The performance of beach nourishment was monitored at regular intervals during the northeast monsoon. Due to southerly near shore currents, the beach material moved to south. The observations from this study showed that the beach nourishment sand movement was contained within the breakwater and new pier clearly indicating that seasonal variation of near shore currents is responsible for oscillation in beach width. Hence, beach nourishment requires sand retaining structures to contain excessive loss and movement.
After the success of experimental beach nourishment assessment of 18 km long coastline in Puducherry showed that it was considerably modified from its natural state of equilibrium with the addition of various coastal protections measures (seawalls, groynes) during last 15 years. The development of conceptual design alternatives requires the understanding of coastal processes and prediction of near shore wave climate. Hence, process based measurements including water levels and near shore currents were undertaken during two predominant seasons and numerical models were used to study the shoreline response for various configurations of retaining structures like groyne field, tapering groynes, off-shore reef, near-shore reef and a combination of these systems. Based on these results a northern near-shore reef and southern offshore reef with beach nourishment was proposed. The studies indicated an immediate requirement of 0.5 million cubic m of sand for dry beach nourishment. Various configurations of offshore retaining structures are being studied that will increase the life of the nourished beach and minimise the effect of erosion on the north side. The proposed hybrid solution (beach nourishment and two reefs) not only protects the coast but also restores the lost beach (Fig. 3).
The northern reef has been implemented by MoES and the formation of beach was already noticed in just three months after the construction was started. The southern reef along with beach nourishment will be implemented by the Puducherry Government and this will help the city to get back its beach.
Coastal erosion in India has become a widespread issue in the last few decades that needs to be protected for the welfare of the coastal economy and livelihoods. Control measures however, should not only combat the erosion but also impact the environment minimally. NIOT has taken a lead in sustainable shore protection methods and has successfully demonstrated shoreline friendly measures. The Institute has thus increased the scope for more sustainable solutions elsewhere in the country where conventional methods have not brought any results and performed poorly. NIOT is extensively exploring many sustainable options to manage coastal erosion to take a massive change over the existing methods at par with universal standards. The knowledge and technical gap in understanding also needs to be addressed as states engage in erosion controlling schemes without any expansive and targeted scientific study. Integrated coastal management approach is therefore necessary and periodic training for all the stakeholders—the academic, scientific and engineering community are required for adapting guidelines for coastal protection measures.
Central Water Commission (CWC), 2016. Status report on coastal protection and development in India, Coastal Erosion Directorate, CWC, Pune.
Sivakholundu, K. M., R. Vijaya, A.S. Kiran, and T. Abhishek, 2014. Short term morphological evolution of sandy beach and possible mitigation: a case study off Kadalur Periyakuppam. Indian Journal of Marine Sciences, 43(07):1297-1305.