Hazards in Dhaka City

During the last couple of decades Dhaka city is one of the fastest growing metropolises in the world. The urban development seems to lack in planning and the problem of urbanization are intensified by high population growth and rural urban migration. Again, Bangladesh suffers from natural calamities every now and then and the urbanization of metropolitan Dhaka is also affected by it. The existing utility services are not capable of coping with the increased demand of the urbanization process. Faulty planning, Lack of coordination among various agencies and overall disregard to environment issues affecting Dhaka include floods, which are associated with river water overflow and rain water stagnation, earthquakes and tornadoes.

Flood Hazard:

Flood is a type of hydro-meteorological phenomenon and it is very hazardous as it can cause devastating losses in property and human lives. Fast and unplanned development aggravates the consequences of flooding by increasing the monetary costs of losses. These kinds of losses are especially high in urban areas which would be possible to ease with proper planning and management of the urban structures.

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Geographical Information Systems (GIS) based modeling and Remote Sensing (RS) techniques can help by supplying maps and techniques as assistance to make early warning for risk areas. In this case study, different criteria have been analyzed that have potential impact on the amount of devastation, such as the elevation of the areas, flooding depth, building density, terrain slope, soil type, land use types etc.

Based on different factors analysis, results are visualized with the help of GIS and RS presentation and dissemination techniques.

In addition, the impacts of different factors on flooding itself are also discussed. Finally, a flood prediction map for Dhaka City Corporation (DCC) in Bangladesh was prepared using the Multi-Criteria Evaluation (MCE) method with particular focus on the different criteria that influence the flooding in Dhaka city.

Water Logging:

Water logging is a very common problem in Dhaka city during the monsoon period as well as all the year round. Even a little rain causes serious problems in some parts of the city on every occasion and can create water logging situation for several days which in turn causes huge infrastructural damages by reducing the amount of accessible assets and supplies in the city. This situation normally occurs due to unplanned infrastructure development, insufficient or limited drainage systems, blockage of the drainages, lack of consciousness of these problems by the inhabitants, unplanned build-up of areas within the city, filling up the canals, lakes and water bodies in areas which are mainly acquired by housing companies and brick burning industries within the city.

The Dhaka city is usually flooded as a result of rainfalls and the high water level of surrounding river systems. For instance, 1998 flooding was the most catastrophic in the flooding history of Bangladesh, where Dhaka city was affected by heavy rainfall and the high water levels of the river system. Unplanned urbanization is increasing water logging.

Earthquakes:

The city can be severely affected by earthquakes as it has already happened in the past. Intensities of VI, VII, and VIII have already been registered in the city. The national building code of Bangladesh (BNBC 1993) includes three seismic zones, and Dhaka is located on the second one with a zoning coefficient equivalent to the 15% of the gravity (z=0.15g).

According to the results of the Earthquake Disaster Risk Index (EDRI) project, major earthquake risk is associated with the high vulnerability of the building stock due to both poor materials and poor construction processes.

Tornados:

The two transitional periods between southwest and northeast monsoons over the Indian sub-continent are characterized by local severe storms. The transitional periods are usually referred to as pre-monsoon (March-May), and post-monsoon (October- November). It is the pre-monsoon period when most of the abnormal rainfall or drought conditions frequently occur in different parts of Bangladesh. Also there are severe local seasonal storms, popularly known as nor?westers (kalbaishakhi). Severe nor?westers are generally associated with tornadoes. Tornadoes are embedded within a mother thundercloud, and moves along the direction of the squall of the mother storm.

The frequency of devastating nor?westers usually reaches the maximum in April, while a few occur in May, and the minimum in March. Nor?westers and tornadoes are more frequent in the afternoon. Table 3 shows some of the devastating nor?westers and tornadoes that hit Bangladesh. Nor?westers may occur in late February due to early withdrawal of winter from Bangladesh, Bihar, West Bengal, Assam, and adjoining areas. The occasional occurrence of nor?westers in early June is due to the delay in the onset of the southwest monsoon over the region (Karmakar, 1989). Table: Some of the devastating westers and tornadoes