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Natural Disasters Issues

Cyclones

The east coast of India is prone to incidences of cyclones. Records of incidence of cyclonic storms and severe cyclonic storms in Orissa section of the Bay of Bengal of India Meteorological Department (1877 to 1980) give the average month wise numbers as follows:

January

0 (0)

July

36(7)

 

April-May

8(7)

February

0 (0)

August

29(4)

 

June-September 

118(22)

March

0(0)

September

29(8)

 

October-December

29(14)

April

0(0)

October

20(9)

 

Annual

155(43)

May

8(7)

November

9(5)

 

 

 

June

24(3)

December

0(0)

 

 

 

 

The probable maximum storm surge in metres that expected is as follows:
Contai – 12.5                      Sagar – 11.5                        Moore Island -  8.5

Earthquakes

There are records of a number of moderate magnitude (M 4 to 5.5) earthquakes  in the coastal areas of West Bengal. A few of them are : 11.10.1937 (22.6/88.4) with intensity X, 9.12.1852 (22.4/88.4) with magnitude 5.7, 23.1.1860 (21.8/87.8) with magnitude 5.0, 26.12.1906 (22.6/88.4) with magnitude 5.0, 15.4.1964 (21.7/87.7) Contai earthquake with magnitude 5, 27.4.1993 (22.0/88.0) with magnitude 4.2 and 12.6.1993 (21.8/89.7) with magnitude 5.7. Depth of focus of thee earthquake events varies from a few km to 60 km. The maximum intensity reached around the mouth of Ganga has been VIII in MM scale.

The Indian Bureau of Standard has divided the whole country into 4 zones (Zone II to V according to the expected earthquake hazard potential of the country – zone II having minimum vulnerability and zone V having maximum hazard potential. According to this classification, coastal areas of West Bengal east of the mouth of Ganga fall in zone IV having expected earthquake intensity of VII in MSK scale. The coastal areas lying west of mouth of Ganga fall in zone III having expected earthquake intensity of VII in MSK scale. Therefore, it is clear that the coastal areas of West Bengal are vulnerable to face damages by earthquake of moderate magnitude

Tsunamis

A major and devastating tsunami affected parts of coastal areas of Andaman & Nicobar Islands and coastal states of Andhra Pradesh, Tamil Nadu and Kerala on December 26, 2004. The tsunami was generated because of an earthquake (M 9.1) in the Bay of Bengal. The earthquake and the attendant tsunami originated due to westward thrust given to the water column of the ocean by a landmass (Myanmar, Andaman and Sunda micro-plate) measuring 1200 km by 150 km that moved along the inclined plane (subduction zone) over the Antarctic and Indo-Australian plate) for a distance of 15 m gaining maximum net vertical height of 6 m in the ocean floor in a time span of 500 second. The maximum run up elevation of the tsunami reached in Tamil Nadu coast was 5.2 m and the maximum lateral inundation had been 800 m in Nagapatnam. In Andaman Island the maximum run up elevation and the maximum lateral inundation had been 7 m and 2000 m respectively. There is no evidence to show that the tsunami waves reached the coastal zone of West Bengal. The zone of rupture along with the displacement that took place is an active one and it continues to the north covered by recent bay sediments up to the mouth of Ganges-Brahmaputra River. It may not be possible with the present state of our knowledge to say definitely about the return period of tsunamis that may be generated in the Bay of Bengal in near future due to active tectonism along the Sumatra-Andaman subduction zone and also their magnitude. However, keeping in mind the tectonic set up in the Bay Bengal with an active subduction zone running from Indonesia to Andaman and Nicobar Islands and further north, it is not improbable that a tsunami may be generated if there is a rupture in the sea bed along the subduction zone in the near future. The experience of the 2004 tsunami brought out that although the occurrence of a tsunami is a low frequency but high magnitude natural event bringing in unprecedented disaster, there is hardly any provision in current coastal zone management plan to mitigate the threats posed in the coastal states of India especially those bordering the Bay of Bengal and islands in the Bay. Therefore it is imperative that while formulating the ICZMP of West Bengal, investigations on run up distance and altitude and mitigation measures including mechanism to receive early warnings related to probable tsunami that may be generated in the Bay of Bengal in the future are adequately provided for.