Flood hazard map and land development priority map
Developed using NOAA AVHRR and GIS data
Flood-affected Frequency
Flood-affected frequency within the event of 1988, 1995 and 1998 were estimated by using the images of 18 September 1988, 31 October 1995 and 18 September 1998. The terminology of flood-affected frequency is determined for each pixel as the ratio of the number of NOAA images within the three flood events of 1988, 1995 and 1998, showing inundation to the total number of cloud free NOAA images available near to the peak flood time. The concept of the different degree of flood-affected frequency is shown in Figure 2. Inundated area that did not appear in any of the above mentioned three images (i.e. 8 in Fig.2) were considered to be a non-hazard area and that which appeared in a single image (5, 6 or 7 in Fig. 2) was considered to be a low hazard area. The common inundated area that appeared in two images (2 or 3 or 4 in Fig. 2) and that which appeared in all three images (1 in Fig. 2) were considered to be medium and high hazard areas, respectively. Flood-affected frequency within the three events was categorized as non-hazard, low, medium and high according to their hazardous hit number of the floods.
Figure 2: Schematic concept of flood-affected frequency for the flood events of 1988, 1995 and 1998
Flood Depth
Flood depths were classified as shallow, medium and deep by using the maximum likelihood method of supervised classification for the same images. Training areas for shallow, medium and deep flood were selected on NOAA AVHRR images of 18 September 1988, 31 October 1995 and 18 September 1998 according to the difference in colors and gray scales for different categories of depth. To measure the flood depth using remote sensing imagery is very difficult. The differences among the depths were understood after superimposing the NOAA images onto the digital elevation image of Bangladesh. Furthermore, the ranking for flood depth was categorized as no-flooding, shallow, medium and deep flood. Therefore, three flood depth maps were constructed for three flood events of 1988, 1995 and 1998 by using above-mentioned images, respectively.
Hazard Rank Assessment Through GIS for Flood Hazard Map
Flood hazard ranks were estimated based on a weighted score for land cover, physiography and geological data for each pixel of the land area of Bangladesh. A weighted score was estimated by
Weightedscore=0.0 X A + 1.0 X B + 3.0 X C + 5.0 X D (1)
where A, B, C and D represent the occupied area percentage by non-hazardous area, low, medium and high flood-affected frequency, respectively, for each category of land cover categories (9 categories), physiographic divisions (31 divisions) and geologic divisions (28 divisions) (Islam & Sado 2000b ), when flood-affected frequency was considered to be a hydraulic factor. Similarly, A, B, C and D represent the occupied area percentage by non-flooded area, shallow, medium and deep flood, respectively, for each category of above mentioned GIS component, when flood depth was considered to be a hydraulic factor. The coefficients of 0.0, 1.0, 3.0 and 5.0 for A-D in equation (1) were used to describe the weight for flood damage. The acquired area percentage by non-hazardous, low, medium and high damaged areas only for the land cover categories are shown in Table 1 with calculated weighted score. Points for the categories of land cover were estimated on the basis of linear interpolation between 0 and 100, where 0 corresponds to the lowest (0) and 100 to the highest (230.21 in Table 1) weighted score. To quantify the flood hazard, the three rankings for flood damage (HR 1~3) were obtained from the allocated point. Hazard ranks were fixed according to the corresponding value of the points: points 0-33 corresponded to hazard rank 1, 33-66 rank 2 and 66-100 rank 3, as shown in Table 1. Similarly hazard ranks were determined using the same algorithm for geologic divisions and physiographic divisions, using flood-affected frequency. Different four categories of flood depth were estimated independently for the images of 18 September 1988, 31 October 1995 and 18 September 1998. Hazard ranks were determined for each event using the above-mentioned algorithm for physiographic divisions, geologic divisions and land cover categories, using flood depth.
Table 1: Flood hazard ranks for land cover categories by using flood-affected frequency (Cat.: land cover category)
| Cat. | A | B | C | D | Score | Points | HR |
| 1 | 36.93 | 28.49 | 22.01 | 12.57 | 157.38 | 68.36 | 3 |
| 2 | 29.88 | 19.22 | 21.73 | 29.17 | 230.21 | 100.00 | 3 |
| 3 | 28.29 | 23.08 | 26.17 | 22.46 | 213.89 | 92.91 | 3 |
| 4 | 55.84 | 23.14 | 12.70 | 8.32 | 102.82 | 44.66 | 2 |
| 5 | 47.54 | 26.05 | 14.58 | 11.83 | 128.93 | 56.01 | 2 |
| 6 | 60.93 | 22.54 | 13.46 | 3.07 | 78.29 | 34.01 | 2 |
| 7 | 77.11 | 12.59 | 5.55 | 4.75 | 52.99 | 23.02 | 1 |
| 8 | 42.96 | 33.66 | 14.03 | 9.35 | 122.52 | 53.22 | 2 |
| 9 | 16.98 | 30.67 | 34.84 | 17.51 | 222.76 | 96.76 | 3 |
Development of Flood Hazard Map
Flood hazard maps were constructed by considering the interactive effect of flood damages onto the land cover categories, physiographic and geological divisions. The concept of the ranking matrix in three dimensional multiplication modes is shown in Figure 3.
Flood hazard map using flood-affected frequency as hydraulic factor:
Flood-affected frequency consisted of four classes: non-hazardous, low, medium and high damaged areas. Hazard ranks were considered from 1 to 27 after combining the hazard ranks of land cover categories (HR 1-3), physiographic divisions (HR 1-3) and geologic divisions (HR 1-3) simultaneously, using the ranking matrix of three dimensional multiplication modes (Fig. 3).
Flood hazard map using flood depth as hydraulic factor:
Flood depths consisted of four classes: non-flooded area, shallow, medium and deep flood. Initially three different hazard maps were developed for three above mentioned images by considering the interactive effect of land cover categories, physiographic divisions and geologic divisions onto the flood depth, using the ranking matrix (Fig. 3). Each hazard map consisted of hazard ranks from 1 to 27. Finally, hazard map of 1988 event for the flood depth was selected as a hazard map among the three hazard maps, because this hazard map shows the deviation in the marginal distribution toward higher ranks compared with other two hazard maps.
Figure 3: Concept of the ranking matrix
