Vulnerability and Risk Assessment for Flood Hazard

For population vulnerability assessment real time scenario was calculated to show the temporal distribution of population. The main objective to assess vulnerability of population is to get a fair estimate of the population who might get affected if an event happened. Preparedness and mitigation plan can be prepared in advance based on the result of population vulnerability. Population vulnerability of a particular place also changes in different time periods in a day. Thus it plays a significant role in risk assessment for any type of hazard.  In this study, population vulnerability assessment has been carried for different time periods. Population within the buildings for the study area were estimated for different time periods of a day i.e. morning, day, evening and night. People within a particular building in different period depend on the different use type of that building.  From the figures it can be interpreted that the buildings are more vulnerable during morning time and night time in residential areas, whereas in commercial, institutions, and other building use are having concentration during the day time. Vulnerable population occupying the buildings have also been identified. This group includes children below the age of 14 years, females and elders above the age of 60 years. In the study area, it is observed that there is a considerable vulnerable population in the very high vulnerable zones. About 70 % of the buildings are being occupied by the vulnerable population. This is more prominent in the heart of the city. It may be noted here, that population characteristics and population vulnerability assessment for all the hazards is considered as same.

For building vulnerability assessment for Guwahati several parameter were taken into consideration and weightages and ranks were assigned to them based on the type of hazard as well as condition of the building. Characteristics of buildings found in the city are described as follows:

Building Structure Type

Concrete buildings are found in commercial areas, most of the Assam type of houses found in residential areas and brick masonry building types are found  spreading out all over the city.

Building Roof Material Type
CGI roof type dominates the area. In areas of concrete structure, cement roof are common.

Building Height
Buildings with three and more stories are dominating wards of Paltan Bazar, Dispur, Ulubari and Sarania. Two to three stories buildings are found in major pockets of the city. The city is dominated by one story buildings.

Building Age
Guwahati city is an old urban area. Within the core area of the city houses are more than 15 years. New built-up areas are being added on the fringes. Recently constructed built-up areas are in Sunsaligrant, Madghoria, Ramshahil, Udalbakragaon and Barsapara. Within the core area buildings with less than 5 years are also present. 

Building Walls:
The city is dominated by buildings having concrete walls. In hilly areas, buildings are of rural structure with wooden or bamboo mixes with mud, sand or lime

Vulnerability assessment related to flood hazard was based on the parameters namely, building wall material, distance from river/stream/drains, number of floors, return period of floods, protecting walls and water tanks. For flood hazard, wall material of the building will be assumed to be the most important factor. In case of a flood event if the building wall material is concrete then there will be less chance for buildings to be washed away. Therefore, the maximum weightage 10 is assigned to the wall material of the building. Again, distance of a building far away from river/stream/drains will also affect the vulnerability of the building.

Weightage assigned to this indicator is 8. However, this aspect will be taken into account in risk assessment, where the vulnerability and hazard factors are combined. The number of floors of a building is also an important parameter for building vulnerability assessment in case of flood hazard. Thus the number of floors is considered to the next important factor and assigned a weightage of 6. The return period of floods, presence of protecting wall and water tank are the three parameters which will be considered in flood vulnerability assessment. The assigned weightages are 5, 3 and 2 respectively. The building vulnerability assessment in case of flood hazard was calculated using some formula.