30 Dec 2019  |   05:00am IST

Goa CZMP: lessons from the December 2004 tsunami

Antonio Mascarenhas

Fifteen years ago, on December 26, 2004, a powerful tsunami pounded the coast of Tamil Nadu. The administration was left dumbfounded, and the scientific community in particular had to scramble for answers. Tsunami waves rose 6.5m above sea level and induced flooding from 31 to 862m from the swash water line. The unexpected oceanographic event took a heavy toll; 8700 coastal dwellers perished, damage to property was enormous, and economic loss was Rs 3242 crores. Coastal geomorphic features and associated vegetation was also impacted. The tsunami that lasted just 10 minutes profoundly transformed the face of Tamil Nadu coast.

The powerful oceanographic episode left a poignant imprint along the coast of Tamil Nadu: (1) Impact on/response of coastal sand dunes: Sand dunes withstood virulent waves; frontal edifices were damaged, breached, and eroded. Over topping and over wash was noticed.2) Impact on/response of vegetation: Dune plants and creepers were uprooted and removed. Frontal casuarina forests were the only portions attacked, bent, and stripped of its leaves by wave up-rush; large parts remained intact. 3) Impact on sea front structures: Loss of life and property was identified only along the frontal strip. A village strip of around 2 km witnessed stone-walled houses razed to rubble; around 6000 dwellers perished. The tsunami over topped and shattered the sea walls. (4) Impact on/response of wetlands: Permanent saline water bodies were created on former low-lying flood prone low wetlands. Lowlands served as flood bays that accommodated excess water.

As a member of the NIO team, this author confirms that coastal geomorphic features such as sand dunes and dense casuarina forests played a defensive role in the wake of the tsunami. High dunes with vegetation and with steep seaward gradients neutralised wave up-rush. More importantly, thickly vegetated stretches displayed an exceptional resilience by dissipating high waves. Only the frontal tree strip ranging up to 25m were twisted, bent and stripped of leaf cover. Only a few trees that faced the open ocean were uprooted. All the beach front houses disappeared in totality.

Can such a dreadful scenario be applied to our own coast of Goa? And considering that the CZMP is in the making, and with all the haphazard activity happening, has a thought been given to what the ocean can do if coastal processes are not respected? Consider this: On December 11, 2019, Candolim beach was invaded by marine water leading to panic among beach shack users and tourists. On November 21, 2019, Sernabatim beach was likewise overshot by sea water wherein several shacks turned into islands. A rise in the sea level also occurred on December 3, 2017, January 3, 2018 and on May 2, 2018. These episodes flooded several beaches for a day. Although water reached the base of dunes, erosive activity was nil, and the overall impact on the morphology of sandy beaches was minimal.

Similarly, Morjm beach was also occupied by sea water during high tide in December 2013, a day when wind speed was minimal and sea was calm; several shacks were damaged. Comparatively, a somewhat different situation was noted on August 13, 2018 at Miramar. Strong winds forced the ensuing high waves to overshoot the beach and break at the base of dunes; erosion of dune base was evident at several places; thick carpet of ipomoea creepers was effective against wave attack and arrested sand loss, a large breach (gap) created on the frontal dune due to excessive footfalls facilitated marine water to advance some 50 metres on to the beach; influx of marine water lasted for an hour.

Major dynamics that govern the advance of marine water on the beach are attributed to: wind speed and direction, tidal height, wave height, beach width, beach level, dune elevation and morphology, and the state of dune vegetation. Thus, the fact that the sea transgressed the beach, even during fair weather in January, implies that the level of the beach is/was lower than the level of the sea. By analogy, if the level of the beach was higher, if the volume of sand was larger, and if the dune was tall, well preserved, with dense vegetation, the geological stability of the pioneer dune is ensured. Thus, if the intrinsic buffer capacity of the beach-dune system is functional, marine incursions on to the beach during calm weather are unlikely.

Coastal dwellers of Goa can rest assured that chances of a tsunami and storm surges are remote. Also, cyclones in the Arabian Sea tend to move away from rather than towards our coast. But we need to note that marine transgressions on our beaches seem to be more frequent during recent years. These events have surprisingly occurred in fair weather when the sea ought to have been calm. These incursions are presently of major concern. Loss of beach sand due to human actions is a major cause. That is why this author reiterates once again that the beach is the property of the ocean. A beach is where the ocean withdraws. Any obstacle as a shack or a wall in the way of an incoming wave is a recipe for disaster; the tsunami has shown it all. That is why the CZMP needs to propose a robust beach-dune system free of obstacles, but with wide sandy beaches with significant elevation and backed by high stable dunes with luxuriant vegetation.

(Dr Antonio Mascarenhas is a former Scientist, NIO, Goa)


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