Dams across rivers also reason for coastal erosion: Draft shoreline management plan for TN

CHENNAI: Even as the artificial structures such as groynes and ports that cover 16% of the TN’s coast contribute to the changing beaches by stimulating erosion and accretion, the draft shoreline management plan (SMP) for Tamil Nadu, which is yet to be released, pointed out that dams built elsewhere inland also a contributor.

Preethi Sekar, a scientist attached to National Centre for Coastal Research (NCCR), explained that sediments transported by rivers to the sea were important for preventing sea erosion.

“For many years, wind and sea current flowed south to north along the coast and the current would carry sand to the north. “When sediments (sands) from the rivers are stopped due to dams or other obstructions, eroded sand could not be replaced,” she explained to the officials of Revenue department and Greater Chennai Corporation during a workshop on coastal management, on Thursday.

Protection structures

Moreover, artificial structures that are constructed as a protection against sea erosion and intrusion, along with ports and jetties, play a role in creation of new beaches and erosion of existing beaches.

Protection structures constructed to protect one village causes erosion or accretion in nearby villages. Tamil Nadu and Puducherry each have 80 and 6 tidal inlets (including river mouths) from where sediments enter the sea.

As per the SMP, the southern side of all the artificial structures have witnessed accretion (or expansion of beach) and the northern side of such structures witnessed erosion. This is also due to south to north movement of wind and sea current. NCCR is preparing the SMP to guide authorities on where to construct artificial structures and where not to.

“If wave energy is high, natural erosion will occur. Structures should not be proposed in such areas. Moreover, when proposing a structure, rise in sea level should be considered. Sediment budgeting should be done to understand its availability,” she added.

Meanwhile, out of 991 km of the Tamil Nadu coast, 16% is covered by protection structures or ports and harbours. In Chennai district, 25 km of the total 44 km (57%) of the coast is covered by structures. Similarly, around 48% of the total coastal length of 69 km in Kanniyakumari is covered by artificial structures.

Marina Beach

Terming the Marina Beach as an artificial beach, Preethi explained that the beach got bigger only after the construction of Chennai Port. On the other hand, Bharathiyar Nagar, located on the northern side of the Port, is undergoing rapid erosion.

However, the erosion rate on the northern side of Ennore Port is lesser when compared to Chennai Port since the sand dredged for deepening the Port was dumped at the northern side. Between 2000 and 2006, the presence of sand has only decreased to 1.05 million cubic metres from 3.15 million cubic metres. This also made the accretion rate slower on the southern side.

The SMP recommends authorities to, instead of constructing groynes, choose nature-based solution consisting of beach nourishment using dredged sand from Kosasthalaiyar river mouth and Tiruvottiyur harbour to prevent further erosion in Bharathiyar Nagar. The plan will also mandate shoreline change analysis at a radial distance of 6 km before and after the construction of the port and suitable mitigation strategies should be implemented.

“The possibility of utilising capital and maintenance dredging as nourishment for mitigating erosion should be explored before seeking structural intervention. The effect and impact of adjacent structures should be taken into consideration,” the draft said.

Shoreline status

According to the draft, around 42.7% of the Tamil Nadu coast is eroding and 33.6% of the coast is stable. However, 23.8% is in the accretion region.

Among the districts, Ramanathapuram, Chengalpattu and Kanniyakumari have the longest coasts under erosion with 95 km, 49 km and 42 km respectively. Accretion is higher in Ramanathapuram, Nagapattinam, and Thoothukudi with 38.12 km, 38.08 km and 36.34 km of coasts in the districts gaining sediments. In Chennai, 17.64 km of coast is eroding and 6.26 km of coasts is under accretion. Around 20 km is stable.

NCCR also identified 69 erosional stretches along the State’s coast with 10% of the stretches under high erosion and 19% under moderate erosion. Remaining 71% is under low erosion zones. It may be noted that coasts that lose 5 metres per year can be categorised as high erosion zones.

Under the SMP, erosion and accretion spots along the Tamil Nadu coast will be identified apart from providing recommendations to prevent erosion. Without the approved plan, the State government could not construct any structure to prevent erosion.

NCCR has identified 22 specific erosion hotspots in Tamil Nadu, which has already lost around 1,802 hectares of the coast. Some of the high erosion hotspots are in Tiruvallur and Kancheepuram.

Various TN government departments proposed 177 projects on the shoreline to prevent erosion. However, draft SMP has approved 29 projects, and 86 projects have been approved with some modifications. The NCCR has requested the government to reconsider 62 projects apart from recommending additional 22 projects in high erosion spots.

NCCR’s recommendations

• Conduct long-term and short-term analysis of the shoreline at the proposed project locations
• Analyse geo-morphological setting, coastal orientation and the existing structures
• Study coastal processes such as waves, tides, currents and others, and nearshore processes including beach profiles
• Analyse sediment sources, sinks, sediment budget
• Coastal management strategy should predominantly be nature-based solution or a hybrid solution; in case no other alternatives are feasible, then hard solutions may be implemented after detailed studies, considering its impact.

Identified by NCCR

42.7% of TN coast eroding

69 erosional stretches along TN

· 10% - high erosion

· 19% - moderate erosion

· 71% - low erosion

(Losing 5 metres/year – high erosion zones)

· Specific erosion hotspots in TN – 22

· State already lost around 1,802 hectares of the coast

· High erosion hotspots – Tiruvallur, Kancheepuram

Chennai

17.64 km eroding

6.26 km accretion

EROSION, ACCRETION IN DISTRICT COASTLINE

Covered by artificial structures

• 991 km of TN coast, 16% covered by protection structures, ports, harbours
• Chennai – 44 km, 25 km (57%) covered by structures
• Kanniyakumari – 69 km, around 48% covered by artificial structures