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3.1.2. Preparation of data:
Boundary of the river basins namely; Bhagirathi-Hoogly, Jalangi and Churni has been delineated from a 1:500,000 map prepared by Geological Circle, State Water Investigation Directorate, Water Investigation and Development Department of Government of West Bengal. The map has been transformed into soft copy. It has been georeferenced using GPS control points collected during a field survey in the area. Since GPS satellites use WGS 84 as their reference ellipsoid the rectified map has been defined in that particular reference system using ArcCatalog component of ArcInfo. Using onscreen digitization method all development blocks have been digitized in a single polygon shape file from this rectified image. Each polygon has been assigned a unique ID in the attribute table so that composite hazard index can be joined to the GIS data base using the common unique ID.

For preparing a composite index an additive model has been adopted. It is recognized that the principle of assigning weightage to the variables is very crucial in this entire process of hazard mapping. To make the variables suitable for a composite index they have been made unit free by dividing each series by their corresponding means. This process has an edge over the standard process of x as the standard deviation of the standardized series is not transformed into 1; mean of the standardized series becomes 1. Thus, it helps in retaining the inherent heterogeneity of the data. In this paper the process of assigning weightages to the flood hazard indicators is primarily knowledge based. The variable ‘flood-prone’ has been attached high importance because where the risk of inundation is very low other variables do not contribute anything to the element of flood hazard. Indicators representing a high level of dispersion across the development blocks have been given more weightages and vise versa. The weightage scheme has been implemented in three steps. First, all the 4 variables have been standardized and named as st_epdm, st_evcn, st_popden and st_fldprn. Then an intermediate hazard index has been computed by the following formula:

Intermediate Index = [st_epdm ×1 + st_evcn × (-1.2) + st_popden ×1.4]

A negative weightage is assigned to ‘st_evcn’ as it has an inverse relationship with flood hazard.

A scheme of progressive weightage has been adopted for the variable ‘flood-prone’ so that the hazard curve becomes progressively steeper at the higher values of ‘flood-prone’. On the basis of this assumption the final weightage scheme has been developed as follows:

Flood Hazard = [Intermediate Index + st_fldprn × 0] if ‘flood-prone’ = 0

= [Intermediate Index + st_fldprn × .25] if 1 ≤‘flood-prone’< 3

= [Intermediate Index + st_fldprn × 1.5] if ‘flood-prone’ = 3

= [Intermediate Index + st_fldprn × 2.5] if ‘flood-prone’ = 4

= [Intermediate Index + st_fldprn × 4.0] if ‘flood-prone’ = 5

= [Intermediate Index + st_fldprn × 5.5] if ‘flood-prone’ = 6

The above weightage scheme clearly displays that very low or 0 weightage have been applied at very low ‘flood-prone’ value to prevent the non-flood prone blocks from getting a higher flood hazard index on the basis of other factors. On the other hand, weightages have been increased at an exponential rate with higher risk of flood occurrence. After the final flood hazard index was devised it has been represented in a graduated clour map using ArcMap. Hazard values have been divided into 4 classes on the basis of 3 quartiles measurements. 1st, 2nd and 3rd quartiles of the hazard index values are .454, 2.53 and 8.11 respectively. The classification scheme is summarized in Table 1.

Table 1. Classification of composite flood hazard index values into qualitative hazard intensity classes

Index Value Ranges	Number of Blocks in Class	Flood Hazard Category
-5.16 – 0.45	18	Low
0.46 – 2.53	17	Medium
2.54 – 8.11	17	High
8.12 – 15.37	17	Very High

Figure: 1 A relief map of Gangetic West Bengal showing major river basins. The elevation is derived from Global 30 Arc Second Elevation (GTOP30) of United States Geological Survey

Figure: 2 Regional flood hazard map of Gangetic West Bengal. Inset showing location of blocks over the three river basins.
Association of Figure 2 with Figure 1 reveals that in Bhagirathi and Jalangi Basin the high hazard zone roughly corresponds with the low lying area. In the extreme south two blocks of Hoogly District fall in high hazard category by virtue of their proximity to Kolkata urban mass and consequent high population density. The overall picture shows that majority of the development blocks in Murshidabad district and the adjoining blocks in Nadia district need greater attention as far as flood management measures are concerned.

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