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Seashore spanning 55-hectares in coastal Maharashtra submerged: Study

Over last three decades, this coastal ecosystem 10 times the size of Wankhede Stadium including mangroves, creeklets, mudflats, beach areas eroded and inundated.

Seashore spanning 55-hectares in coastal Maharashtra submerged: Study
SHARES

Researchers using satellite images have identified the submergence of a 55 hectare (ha) area - nearly ten times the size of Mumbai’s Wankhede Stadium (which has an area of 5.4 ha) -- near Devghar in Raigad district. It clearly indicates a worrying sign - that of coastal inundation and extreme shoreline erosion.

The Srushti Conservation Foundation (SCF), a non-profit organisation based in Pune, conducted the study close to the mouth of Bankot Creek, which has a 1.5-kilometer-long beach area. Preliminary data show that between 1990 and 2022, there was a total loss of around 55 ha of coastal ecosystems, including mangrove, creeklets, mudflats, and sandy coasts, and that almost 300 metres of shore area was eroded.

In order to produce a thorough report to assist policymakers on the subject of coastal inundation and land degradation, SCF is continuing its overall study on coastal inundation by developing satellite datasets all along the Maharashtra coastline. Last year, SCF released its assessment of the shrinking width of creeks and waterways along the Mumbai Metropolitan Region and succession of mangroves over 60 sq.km of agricultural land along Karanja Creek.

The latest study was prompted by information shared by Devghar residents on how this beach had been steadily eroding since the 1990s. The researchers ran a preliminary analysis to determine the degree of erosion since the 1990s and assembled Landsat (satellite) datasets using the Google Earth Engine to look into the claim.

It was observed that the satellite-derived shoreline (unsupervised classification-based waterbody boundary) had now shifted 300-500 meters towards land. Apart from this it was clear that mangrove and Casuarina plantation was also not able to withstand the loss of sediment and eventually they got uprooted,” said Dr Deepak Apte, Managing Director, SCF.

The impact on the study site

At present, the Casuarina plantation (Coastal She-oak or Sura in Marathi) extends along the north-south belt on Devghar Beach. However, the beach-side plantation was found to be getting destroyed due to continuous loss of sediment resulting in the erosion of the shore and also change in sediment profile from a clay-silty dominated seascape to coarse sand dominated seascape, the analysis read.

The satellite images further showed that mangroves were distributed unevenly in this patch. In the central part of this patch, dead mangroves were observed on a large scale. In the peripheral areas of this patch, mangroves were dominated by the bushy and sparsely distributed Avicennia marina (gray mangrove). The inner areas were dominated by Rhizophora mucronata (Asiatic mangrove). In exceptional cases, Aegiceras corniculatum (black mangrove) could be observed along with Acanthus ilicifolius (Sea Holly).

In 2020, the landfall of Cyclone Nisarga close to the coast of Devghar, saw the death of trees, shrubs, and (approximately 5 ha) of mangrove species like Avicennia marina. “Some of these patches are now recovering but large tracts of mangrove are already on the verge of death, most probably due to a shift in sediment structure from muddy to sandy nature,” said Apte.

Lessons from the study site 

The shoreline is changing steadily in several parts of the state and there is a need for a systematic assessment of the same, the study suggested. For example, in many areas, mangroves are expanding in agriculture lands due to saltwater ingress while in many areas, loss of sediment causing large scale erosion.

Climate change related sea-level rise will further worsen the situation. The Government should commission a study to seek solutions to deal with the changing seascape. We are currently doing ad hoc interventions which will only worsen the situation,” Apte said, adding that for example, promotion of kharland bunds which essentially kills mangroves is a favorite but ill-conceived remedy as a universal solution. “It will not only erode our natural barrier, it will facilitate erosion in the long run.

SCF’s studies on impact of Cyclone Nisarga on mangrove and non-mangrove areas points to the value of Mangrove during storm surges and cyclones. “Unregulated construction of seawalls is another case where the problem of erosion is shifted from one place to another. These may be solutions as a short-term remedy at extremely eroding shores, but it cannot and should not be the only solution,” said Apte.

Recently, National Institute of Ocean Technology did extensive analysis of shore erosion suggesting 25.5% of Maharashtra shoreline is eroded (From 1990-2018). Sediment movement depends on several factors and thus issues of erosion require holistic understanding before making any engineering interventions, the study suggested.

Threats to the study site

The area of Devghar beach is in close proximity to the Bankot creek, which has been witnessing sand mining. Though the impact of sand mining is unclear at this point of time on the erosion at Devghar region, unregulated sand mining is facilitating erosion in several areas along the Maharashtra coastline. For example, various stretches between Alibaug and Mandwa have witnessed illegal sand mining eroding beaches completely.

While on one side sea level is increasing and on the other side, sand mining on beaches is flattening natural shore gradient facilitating coastal inundation. In case of sand mining in the creeks, these are getting deeper and deeper facilitating erosion and  large-scale salt-water inundation in nearby areas as it increases its water carrying capacity during high tides and storm surges. Though deepening of creeks is necessary and essential in many cases to facilitate storm water drainage especially in the rainy season, it is very site specific and needs to be done in a very careful manner,” said Apte.

In case of Devghar area, increase of erosion coincides well with the construction of bridge and related activities, the study found. A detailed analysis with higher resolution satellite imagery is expected to throw further light along with local people’s perception. 

Preliminary solutions suggested by the study

Climate change facilitated increase in the sea level and coastal inundation will put coastal assets, investments and communities at risk over next few decades. Thus, the State should focus on developing Shore Management Policy. Use of cutting-edge science will be vital in developing such a policy.

Launch community-based climate resilience, adaptation and awareness programme.

Scientific management across creek areas to maintain optimal depth and water level

Reviewing the Kharland Bund Policy as construction of bunds to prevent salt water ingress will only shift the problem from one place to another and it is facilitating large scale mangrove mortality.

Review efficacy of sea walls and its impact on shore erosion in nearby areas

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