Acharyya, A., B. Sengupta, and R. N. Ghosh
Geological Survey of India, Calcutta, India
The sea resort of Digha (21° 37′ N: 87° 31′ E) on the east coast of India runs the risk of natural disasters due to frequent development of atmospheric depressions in the bay of Bengal. A 3.7 km sea-wall (brick/concrete/boulder embankment) was built long back to protect the ENE-WSW coastline of this sea resort from erosion vis-à-vis wave action.
An unusual tidal surge struck the Digha coast on 20 August 1997 causing extensive damage to the seawall, various structures within CRZ-II and natural landscape. The Geological Survey of India (GSI) studied the cause and effect of this natural calamity. A deep depression was formed in the northern Bay of Bengal and the eye of anticlockwise circulation of wind from east to west became incremental with the attainment of minimum atmospheric pressure at the coast. The maximum wind speed was at 8:30 in the morning of 20 August 1997, which was second lunar day next to the full moon, and the highest spring tide was anticipated at 10:00. Besides, this was a period of autumn equinoctial tides when maximum tide raising forces were expected. This highest spring tide coupled with the easterly high velocity wind and prevailing longshore current of the sea towards ENE resulted in abnormal tidal surges for nearly one hour on 20 August 1997 (around 10:00). The sea at Digha otherwise attains an average of highest wave-crest height ranging between 3.5 m and 4.0 m above msl. The plunging breakers were reported to have risen that day to a wave crest height of about 6.4 m above msl and they overtopped the seawall upper level having 5.4 m of average Reference Level. About 378 million litres of seawater spilled over the wall and returned to the sea in just one hour’s time by breaking the wall in the weak zones and adjoining structures. Geological parameters, measured in the natural setting of beach-coastline dunes, revealed marked changes in the magnitude of beach dimensions; lowering of beach front dune top by 2.46 m and erosion of the beach surface by 0.65 m constitute the direct fall out of this tidal surge.
The GSI recommended both short- and long-term mitigation measures to minimise effects of such natural disasters. These included, interalia, a design of an ideal seawall with three consecutive foundation piles and one seaside sheet pile; lined drains to ease the surface runoff especially in case of overtopping by surges; provision for landslide filter drain and seaside geotextile apron, and systematic network of sewerage.
As of now, the largely damaged part of the sea-wall has been newly laid by a 28.0 m wide concrete ramp with 2.44 m deep sheet pile in the seaward side. Lined surface drain and impermeable filter drain of clay have been constructed behind the seawall. Raising of vertical height of the sea-wall and laying of sewerage network have also commenced. In addition, the proposal of laying geotextile mat in the unrestored part of the wall is on the anvil.