9.1 Multi-Hazard Risk Concept
The commonly encountered hazards are:
a) Earthquake,
b) Cyclone,
PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS
c) Wind storm,
d) Floods,
e) Landslides,
f) Liquefaction of soils,
g) Extreme winds,
h) Cloud bursts, and
j) Failure of slopes.
A study of the earthquake, wind/cyclone, and flood hazard maps of India indicate that there are several areas in the country which run the risk of being affected by more than one of these hazards.
Further there may be instances where one hazard may cause occurrence or accentuation of another hazard such as landslides may be triggered/accelerated by earthquakes and wind storms and floods by the cyclones.
It is important to study and examine the possibility of occurrence of multiple hazards, as applicable to an area. However, it is not economically viable to design all the structures for multiple hazards. The special structures such as nuclear power plants, and life line structures such as hospitals and emergency rescue shelters may be designed for multiple hazards. For such special structures, site specific data have to be collected and the design be carried out based on the accepted levels of risk. The factors that have to be considered in determining this risk are:
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The severity of the hazard characterized by M.M. (or M.S.K.) intensity in the case of earthquake; the duration and velocity of wind in the storms; and unprotected or protected situation of flood prone areas; and
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The frequency of occurrence of the severe hazards.
Till such time that risk evaluation procedures are formalized, the special structures may be designed for multiple hazards using the historical data, that can be obtained for a given site and the available Code for loads already covered. The designer may have to
consider the loads due to any one of the hazards individually or in combination as appropriate.
9.2 Multi-Hazard Prone Areas
The criteria adopted for identifying multi-hazard prone areas may be as follows:
a) Earthquake and flood risk prone — Districts which have seismic zone of Intensity 7 or more and also flood prone unprotected or protected area. Earthquake and flood can occur separately or simultaneously.
b) Cyclone and flood risk prone — Districts which have cyclone and flood prone areas. Here floods can occur separately from cyclones, but simultaneous also along with possibility of storm surge too.
c) Earthquake, cyclone and flood risk prone — Districts which have earthquake zone of intensity 7 or more, cyclone prone as well as flood prone (protected or unprotected) areas. Here the three hazards can occur separately and also simultaneously as in (a) and (b) but earthquake and cyclone will be assumed to occur separately only.
d) Earthquake and cyclone risk prone — Districts which have earthquake zone of intensity 7 or more and prone to cyclone hazard too. The two will be assumed to occur separately.
Based on the approach given above, the districts with multi-hazard risk are given in Annex P.
9.3 Use of the List of the District with Multi-Hazard Risk
The list provides some ready information for use of the authorities involved in the task of disaster mitigation, preparedness and preventive action. This information gives the districts which are prone to high risk for more than one hazard. This information will be useful in establishing the need for developing housing design to resist such multi-hazard situation.