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Application of GIS in flood hazard mapping: A case study of Gangetic West Bengal, India

Joy Sanyal and Xi Xi Lu
Department of Geography, National University of Singapore
Tel: +(65)-68748465
Email: g0202381@nus.edu.sg

1. Introduction:
The deltaic part of West Bengal state is traditionally identified as a flood prone area of India. Most of the flood management strategies in this region have been geared towards ‘preventing’ flood by an attempt to contain the river. Very little attention is paid on formulating rational land use planning to reduce flood induced disaster. Preparation of a comprehensive flood hazard map for this state would be the one of most crucial steps for implementing non-structural remedial measures. This paper attempts to synthesize the relevant database in a spatial framework to evolve a flood hazard map for Gangetic West Bengal. Geographical Information System (GIS) is extensively used to assemble information from different maps and digital elevation models. Census data and other relevant statistical abstract about the available infrastructure facilities have also been used to make the hazard map more oriented to need of the local inhabitants.

2. Study Area:
Three major river basins of the southern West Bengal, namely Bhagirathi-Hoogly, Jalangi and Churni have been selected for the current study. These three rivers are the distributaries of Ganga River (Refer Figure 1). Eastern part of the district Bardhaman, Murshidabad, Hoogly, Howrah, Western part of North 24-Pargana and most of the Nadia and Kolkata constitute the administrative entity of these three river basins. Due to unavailability of information in and around Kolkata urban mass district Kolkata, Howrah and North 24 Pargana have been excluded from the current study. In the rest of our discussion we shall collectively designate these three river basins as Gangetic West Bengal. The current study area is typically identified as moribund delta. In this section of the delta the rivers are decaying and the land building process has entirely ceased. The interfluves of the numerous distributaries are ill drained (Spate, 1965). This situation ultimately led to stagnation of water and development of palaeo channels locally known as bills. The overall geomorphology of the study area depicts a degenerating fluvial system.

3. Flood Hazard Mapping from different Scale Perspective:
Mapping flood hazard is not a new endeavour in the developed countries of the world. Federal Emergency Management Agency (FEMA) is one of the most active and well known in this sphere. However, a closer look reveals that these hazard maps are very data intensive in nature and primarily depends upon very high resolution terrain data. In the current state of technology and resources possessed by India preparation of this kind of hazard maps is not feasible. Islam et al (2000a) formulated methodology to prepare flood hazard map for data poor Bangladesh. Later, efforts have been made to integrate population density in the flood hazard maps in order to create land development priority maps (Islam, 2002).

In the current study the issue of flood hazard mapping has been addressed from the perspective of different mapping scale in a GIS environment. The present study has been done in regional and sub-regional scale. In both occasion administrative units have been selected as the unit of investigation. A flood hazard map based on administrative units is particularly handy for the planners and administrators for formulating remedial strategy. It also makes the process of resource allocation simple resulting in a smooth and effective implementation of the adopted flood management strategy. The aim of regional study is to broadly identify the high hazard area in the three river basins. A regional study eventually leads to identification of the higher hazard zone. A more detailed and high resolution study of this zone optimizes resource allocation and saves time.

3.1 The regional study

3.1.1 Choice of variable:
For the regional study a total of 69 development blocks been analyzed. Mean area of the blocks is 205.42Sq. Km.

Therefore, the hazard map is not expected to depict any finer detail. In the regional scale 4 factors have been taken into consideration for developing the composite flood hazard index. Each of the factors has been assigned different weightage to quantify the severity of hazard. Frequency of flood occurrence in last 10 years has been considered as the measure of flood proneness of a particular block. The varibable is named as ‘flood-prone’. This variable suites with the current frame work of investigation as the flood record is collected in development block basis by the Irrigation and Waterways Department. To quantify the economic assets under potential flood threat population density of the development blocks has been chosen as another important variable.

Population density figures have been collected from Census of India, 2001. Later in this study this variable will be identified as ‘pop-den’.

One of the main components of flood mitigation strategy is fast evacuation of the affected community. Good network of all-weather roads is an essential prerequisite of safe and effective evacuation during severe floods. Availability of surfaced road per Sq. Km. in each of the development blocks is considered to quantify the ease of movement. Source of this data was the District Statistical Handbook of West Bengal, 1998.This variable was assigned the name as ‘evacuation’. As mentioned earlier, any Access to safe drinking water has been considered as the key factor to prevent this post flood hazard like out break of a water borne disease. To quantify this aspect of intangible hazard we devised another variable named ‘epidemic’. This variable measures percentage of villages having no access to safe drinking water to the total number of villages in each of the development blocks.

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