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India deploys coastal tube defense against tidal surges

Source: Thomson Reuters Foundation - Tue, 11 Dec 2012 16:48 GMT
Author: Manipadma Jena
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PENTHA, India (AlertNet) - The blackened wood stumps of a breakwater stand forlornly on Pentha beach, symbols of failed attempts to lessen the effects of high tides and storm surges.

In the past two years alone, huge waves have destroyed two 7-metre high (23-feet) embankments and eroded 20 hectares (49 acres) of farmland in this village in Odisha state on India’s eastern coast, threatening homes and agricultural land.

As the region’s climate changes, what were six distinct seasons have become just two, summer and monsoon, said 48-year-old rice farmer Bhanu Senapati. Extreme weather, tidal surges and the increasing salt contamination of the land is making agriculture even harder.

“In saline soil, (rice) saplings can only survive up to eight days. If rain fails for even one week due to increasingly erratic monsoons, the soil dries out and a particular worm also cuts the roots,” Senapati said.

But the ruined breakwater on Pentha’s beach will soon give way to a new coastal defence system – a 675-metre geotextile tube (or geotube) embedded in the shore to form an embankment that will help protect the community and its neighbouring villages from erosion caused by shifting tides and storm surges.

According to a 2011 assessment by the Institute of Ocean Management at Chennai’s Anna University, more than a third of Odisha’s coastline is prone to erosion, and eight percent is vulnerable to severe erosion, including Gahirmatha beach, which is the world’s second largest nesting ground for the endangered Olive Ridley sea turtle.  

Sarat Chandra Tripathy, a 61-year old farmer and part-time priest in Pentha, has witnessed the dramatic effects of coastal erosion during his lifetime. He remembers walking to the coast from the village for annual religious ceremonies when he was a child.

“We would only return by sundown. The sea was around 3 km away from the village. Today, it is knocking 50 metres from our doors,” Tripathy said.

R. Sundaravadivelu, a professor in the department of ocean engineering at the Indian Institute of Technology Madras (IIT-M) in Chennai, which is providing technical advice for the geotube project, explained that storm surges due to cyclones occur about every two years on the Odisha coast and average almost 8 metres (26 feet).

Cyclones have wreaked devastation in Odisha numerous times in living memory. By far the worst was the cyclone of 1999, which killed 10,000 people across the state. The storm surge was more than 11 metres (36 feet) and uprooted what little remained of the mangroves standing between the encroaching tides and Pentha, penetrating the Mahanadi river system and causing serious flooding inland, according to Sundaravadivelu.

“Since the 1999 super-cyclone, every year there are at least two coastal depressions when the sea is very disturbed, and every other year it rushes in fiercely towards the village,” said Tripathy.

“We are fearful whenever the sea surges,” he added. “In extreme cases we take some food, cash and important papers and leave the village for safety.”

According to Sundaravadivelu, changes in the circulation of the sea are contributing to coastal erosion in some areas and but also to deposition of sand embankments in other areas. A 2011 study by the Geological Survey of India found that these periods of deposition and erosion may be related to climatic change and rising sea levels as well as human activity.

The Pentha geotube will be the second in India, following the installation of a kilometre-long tube on West Bengal’s Digha coast in 2008. Sundaravadivelu explained that past embankments in the village were built of earth and poorly designed. The geotube that IIT-M has designed for Pentha is planned as a durable line of defense that is suited specifically to the width of the affected beach and the height of highest tidal surge in the location.

The tube will be constructed of one or more sediment-filled sleeves of geotextile fabric placed in a line along a trench on the beach running parallel to the shoreline, about 0.6 meters (2 feet) above mean sea level, and then covered with sand and natural beach vegetation.  

DISSIPATING STORM SURGES

The geotube embankment is designed to dissipate the force of storm surges, allowing other embankments to deflect the weakened tidal surge. At Pentha, a “bio-shield” of mangroves and similar species will act as a second line of defence behind the new embankment, along with an existing mud and stone wall beyond it.

The new embankment is a pilot scheme initiated in 2009 by the federal environment and forests ministry as part of a statewide Integrated Coastal Zone Management Project (ICZMP). With 2.27 billion Indian rupees ($41 million) of funding from the World Bank, the program aims eventually to benefit 235 coastal villages with a total of 400,000 residents.

There have been concerns locally that the government has not been spending the 220 million Indian rupees ($4 million) of funding for the Pentha geotube project quickly enough, leaving villagers to plug breaches in their existing embankment with sandbags. But Ajit Kumar Pattnaik, project director of the ICZMP, insisted that a large amount of field research had been carried out over the past 18 months in order to design the new embankment, and that the bidding process for its construction was now underway.

Access roads to Pentha village for heavy machinery are now being laid, and an office building for the geo-tube installation and maintenance project has been completed.

Kailash Dash, head of the Regional Centre for Development Cooperation, an non-governmental organisation that has been training the local community to manage risks from climate disasters, said that the geotube was needed in part because of a lack of environmental awareness among the cluster of villages around Pentha, which have let a natural protective wall of mangroves vanish gradually from the shore.

“Community capacity needs to be built, awareness generated on climate change and disasters, and (local people) should be involved in programmes from their inception to get the feeling of ownership which is indispensible for success,” said Dash.

While the geotube project has been welcomed, some people question whether so-called bio-shields like mangroves might be a more economically viable weapon against sea erosion in developing countries.

The Odisha government has planted over 6,500 hectares (16,000 acres) of the trees in four vulnerable coastal districts over the last five years.

But the ICZMP’s Pattnaik argues that the severe erosion and increasing storm surges in the area mean that a bio-shield alone is not a feasible option.

“Considering the life and property under threat, the investment (in the geo-tube) is justified,” he said. “Unless prevention measures are taken, Pentha village would be severely affected and more than 16,000 hectares (40,000 acres) of paddy field would be negatively impacted.”

Manipadma Jena is an environmental journalist based in India. She can be contacted on manipadma.jena@gmail.com

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