Geography

Kolkata is located in eastern India at 22°33′N, 88°20′E in the Ganges Delta at an elevation ranging between 1.5 m (5 ft) to 9 m (30 ft).^[27] It is spread linearly along the banks of the River Hooghly in a north-south direction. Much of the city was originally a vast wetland, reclaimed over the decades to accommodate the city's burgeoning population.^[28] The remaining wetland, known as East Calcutta Wetlands has been designated a "wetland of international importance" under the Ramsar Convention.^[29]

Like the most of the Indo-Gangetic plains, the predominant soil type is alluvial. Quaternary sediments consisting of clay, silt, various grades of sand and gravel underlie the city. These sediments are sandwiched between two clay beds, the lower one at depths between 250 m (820 ft) and 650 m (2,133 ft) and the upper one ranging between 10 m (33 ft) and 40 m (131 ft) in thickness.^[30] According to the Bureau of Indian Standards, the town falls under seismic zone-III, in a scale of I to V (in order of increasing proneness to earthquakes)^[31] while the wind and cyclone zoning is "very high damage risk", according to UNDP report.^[31]