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Damage Zone

The strong winds, heavy rains and large storm surges associated with tropical cyclones are the factors that eventually lead to loss of life and property.

Wind Zone

The damages produced by winds are extensive and cover areas occasionally greater than the areas of heavy rains and storm surges which are in general localized in nature. The impact of the passage of the cyclone eye, directly over a place is quite different from that of a cyclone that does not hit the place directly. Parts of structures that were loosened or weakened by the winds from one direction are subsequently severely damaged or blown down when hit upon by the strong winds from the opposite direction. Coastal roads/locations are vulnerable to damage from inundation/waves run-up. The most Damage photograph Damage caused by very severe cyclonic storm ‘Nargis’ detrimental hazards to roadways are uprooted trees, power poles and lines, and debris falling on roads and blocking them. This becomes a serious problem when winds reach 80 Kts (149 Kmph) or more.

Cyclone Intensity and Damages

Intensity Damage Wind Speed in Kms./hour
Depression and Deep Depression Minor damage to loose and unsecured structures 31 to 61
Cyclonic Storm (CS) MDamage to thatched huts. Breaking of tree branches causing minor damage to power and communication lines 62 to 87
Severe Cyclonic Storm Extensive damage to thatched roofs and huts. Minor damage to power and communication lines due to uprooting of large avenue trees. Flooding of escape routes. 88 to 117
Very Severe Cyclonic Storm Extensive damage to kutcha houses. Partial disruption of power and communication line. Minor disruption of rail and road traffic. Potential threat from flying debris. Flooding of escape routes. 118 to 166
Extremely Severe Cyclonic Storm Extensive damage to kutcha houses. Some damage to old buildings. Large-scale disruption of power and communication lines. Disruption of rail and road traffic due to extensive flooding 167 to 221
Super Severe Cyclonic Storm Extensive structural damage to residential and industrial buildings. Total disruption of communication and power supply. Extensive damage to bridges causing large-scale disruption of rail and road traffic. Largescale flooding and inundation of sea water. Air full of flying debris. 222 & above

Storm Surge

Storm surge is the single major cause of devastation from tropical storms .Though, the deaths and destruction are caused directly by the winds in a tropical cyclone. winds also lead to massive piling of sea water in the form of what is known as storm surge that lead to sudden inundation and flooding of coastal regions. The worst devastation is caused when the peak surge occurs at the time of high tide. As the leading edge of the storm surge crashes against the coastline, the speed of the surge places great stress on the walls. The debris like uprooted trees, fences and parts of broken houses, act as battering rams and cause further damage. The sand and gravel carried by the moving currents at the bottom of the surge can cause sand papering action of the foundations. The huge volume of water can cause such pressure difference that the house “floats” and once the house is lifted from the foundations, water enters the structure that eventually collapses. Risk of storm surges is prominent at few locations only on the Indian coast.

Classifications of Storm Surge Zones are given below:

Storm Surge Scale

Storm Surge Zones Height of Storm Surge in Meters
Very High Risk Zone (VHRZ) 5 and above
High Risk Zone (HRZ) 3 to 5
Moderate Risk Zone (MRZ) 1.5 to 3

Risk Zones identified by ESSO

Storm Surge Zones Risk Height of Storm Surge in meters
Coastal Areas and Offshore Islands of Bengal VHRZ 10 to 13
East Coast of India between Para deep & Balsore VHRZ 5 to 7
Andhra coast between Bapatala & Kakinada holding estuaries of two major rivers Krishna and Godavari VHRZ 5 to 7
Gujarat along the west coast of India MRZ 1 to 3
Coastal Areas and Offshore Islands of Bengal VHRZ 10 to 13

Flood Zone Costal and Non-Costal

Flood risk is significant due to heavy to very heavy rain fall in cyclones causing flash floods. Rains associated with cyclones are another source of damage. Unabated rains give rise to unprecedented floods. Rainwater on the top of storm surge may add to the fury of the storm. It creates problems in post cyclone relief operations. Rainfall is generally very heavy and spread over a large area thus leading to excessive amount of water, which leads to flooding.
Cyclone induced Flood prone districts are identified in the IIBI report in coastal and non-coastal districts up to 100 kms. from the coast which is realistic. Most cyclones loose their intensity (both wind speeds and high precipitation) after hitting the coast there are exceptions of one or two cyclones maintaining strength over long distance inland.

Monsoon &Tropical Cyclones in Indian Subcontinent

India, Pakistan, Bangladesh, Sri Lanka, Nepal& Bhutanform Indian Subcontinent spread between latitudes N 50 and N 370. In addition there arethree oceans viz. Arabian Sea, Bay of Bengal and Indian Ocean. Tropic of Cancer passes through the center of India. Sun is at its Zenith at 900 on the Tropic Cancer in summer&receives maximum solar energy. The land gets heated in day and also cools down in nightquickly compared to what takes place on the sea.This differential heating leads to formation of a larger Convection Cell, leading to a four months monsoon season from June to September. But the distribution of rainfall is not uniform over the land since movement clouds is affected by mountains e.g. Mahabaleshwar (hill station in Sahyadri mountain) receives 200 inches of rain, Panchagani (hill station at lower height) 20 Kms from Mahabaleshwar receives 70 inches of rain while on the plains in town Wai (another 25 Kms) the rain fall is scanty 15 inches. Some areas receive heavy rainfall while states of Rajasthan, Kutch district are deserts. The El Nino affects monsoon adversely as was observed during the worst El Nino years of 1987-88 & 1997-98with drought like situation.

Tropical cyclones are formed as the sea water is heated by solar energy. They are normally formed between latitudes N 50& N 100. The distribution of cyclonesas observed from about 114 years data (up to 1991) of Indian Meteorological Department (IMD) reveals that:

East Coast of India receives on an average one cyclone every year.

Bangladesh (which has a small coast compared East Coast of India) is hit by a large no. of cyclones.

Very few cyclones hit West Coast of India except the coast of Gujarat State.

Similarly very few cyclones hit Pakistan and Sri Lanka.

Cyclones in the Indian Subcontinent are normally of moderate intensity barring a few super Cyclones.

Cyclones do not occur during the monsoon.

The peak season for Cyclones is October/May.

East coast of India, Gujarat State and Bangladesh are considered for a study of Risk of Cyclone viz.high winds, Storm Surge and flooding.

Tropical Cyclone Scale

IMD Wind Speeds Categorisation

Category Sign Wind Speed in Kms./hour
Depression D 31 to 49
Deep Depression DD 50 to 61
Cyclonic Storm CS 62 to 87
Severe Cyclonic Storm SCS 88 to 117
Very Severe Cyclonic Storm VSCS 118 to 166
Extremely Severe Cyclonic Storm ESCS 167 to 221
Super Severe Cyclonic Storm SuCS 222 & above

Saffir Simpson Scale for Cyclones Hurricanes

Category Wind Speed in Kms/hour
1 119 to 153
2 154 to 177
3 178 to 208
4 209 to 251
5 252 and above

Munich Re Hazard Zones

Category Peak Wind Speeds in Kms./hour
No Hazard Less than 76
Zone 0 76 to 141
Zone 1 142 to 184
Zone 2 185 to 212
Zone 3 213 to 251
Zone 4 251 to 299
Zone 5 300 and above

Dvorak's Scale

T Number one Minute wind speed in Kms./hour
1.0 – 1.5 46
2 40
2.5 65
3 83
3.5 102
4 120
4.5 143
5 167
5.5 189
6 213
6.5 235
7 260
7.5 287
8 315
8.5 343

Radial Maximum Distance from the center of the Tropical Cyclones

Arabian Sea

Type of Storm Windspeed 63 kmh .Radial Distance in Km Windspeed 92 kmh .Radial Distance in Kms Windspeed 118 kmh .Radial Distance in Kms
Cyclonic Storm 197.95
Severe Cyclonic Storm 197.95 144.3
Very Severe and Extremely Severe Cyclonic Storm 247.9 131.35 79.55
Super Cyclonic Storm 234.95 133.2 85.1

Bay of Bengal

Type of Storm Windspeed 63 kmh .Radial Distance in Km Windspeed 92 kmh .Radial Distance in Kms Windspeed 118 kmh .Radial Distance in Kms
Cyclonic Storm 127.65
Severe Cyclonic Storm 133.2 79.55
Very Severe and Extremely Severe Cyclonic Storm 223.85 94.35 51.8
Super Cyclonic Storm 271.95 155.4 92.5