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27 May 2020 | Blog

Mapping floods in Bangladesh caused by Cyclone Amphan to support humanitarian response

Mir Abdul Matin & Kabir Uddin

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An aerial image of typical flood inundation in Bangladesh. The murky waters reflect flood inundation areas and rural settlements and fields covered by floodwaters. (Photo: Kabir Uddin/ICIMOD)

Cyclone Amphan made landfall in Bangladesh and eastern India on 20 May 2020. The massive tropical cyclone resulted in the loss of at least 96 lives and caused extensive damage to infrastructure. To make matters worse, communities in the southwest coastal districts of Bangladesh are now facing floods from the subsequent heavy rains and tidal surges while having to adhere to social distancing norms during the ongoing COVID-19 pandemic. The flooding is interrupting the return to normal life for millions of people who were relocated to community shelters (despite fears of an outbreak of COVID-19) after the cyclone struck.

Flood mapping for response

At the moment, humanitarian agencies and the Government of Bangladesh are conducting impact assessments to prepare an effective response strategy. Flood inundation maps are helpful for such assessments. ICIMOD has worked on using synthetic-aperture radar (SAR) satellite remote sensing to provide near real-time inundation maps for supporting flood response, such as during the 2019 Bangladesh floods.

ICIMOD prepared flood inundation maps for Bangladesh by analysing satellite imagery from the Copernicus Sentinel-1 satellite and Sentinel Asia. These maps provide a synoptic overview of the extent of inundation caused by the floods. They can aid disaster management agencies in prioritizing relief and rescue activities in the worst affected areas. To distinguish between perennial waterbodies and the flooding caused by Cyclone Amphan, an image from 16 May was used as the base situation while another from 22 May was used for assessing the flooding situation (see Figures 1 and 2).

Pre/post flooding: Sentinel-1 imageries from 16 May 2020 and 22 May 2020
Figure 1. Pre/post flooding: Sentinel-1 imageries from 16 May 2020 and 22 May 2020 provide a synoptic overview of the extent of the floods due to the catastrophic Cyclone Amphan. The blue sections indicate flood-inundated areas and perennial water bodies. (Source: Copernicus Open Access Hub/Sentinel-1)


An RGB representation (VH on 16 May as a red band, VH on 22 May as a green band, and VH/VV on 22 May as a blue band) of Sentinel-1
Figure 2. An RGB representation (VH on 16 May as a red band, VH on 22 May as a green band, and VH/VV on 22 May as a blue band) of Sentinel-1 imagery indicates that the red areas are inundated by stormwater, blue areas indicate pre-existing waterbodies (before the cyclone), and yellowish-green areas are other land areas. (Source: Copernicus Open Access Hub/ Sentinel-1)
Inundation situation

Most of the flooding happened in three districts: Satkhira, Khulna, and Bagerhat. Some flooding also happened in parts of Gopalganj and Barisal. District-wise inundation areas are shown in Figure 3 and Table 1.

Sentinel-1 based flood inundation map
Figure 3. Sentinel-1 based flood inundation map on 22 May 2020 for the Cyclone Amphan affected area. (Source: ICIMOD/Sentinel-1)

Table 1. Area of sub-districts inundated by flooding caused by Cyclone Amphan (Source: ICIMOD/Sentinel-1)

District Sub-district Total area (ha) Flood-inundated area (ha) Perennial waterbodies (ha) Other land (ha)
Bagerhat Bagerhat S. 26,863 5,129 161 21,573
Bagerhat Kachua 11,580 1,727 4 9,848
Bagerhat Morrelganj 42,033 13,355 119 28,482
Bagerhat Rampal 31,497 15,128 270 16,087
Barisal Bakerganj 35,633 7,345 407 27,882
Barisal Barisal S. 24,336 2,641 852 20,842
Bhola Burhanuddin 26,652 3,464 187 23,001
Bhola Tazumuddin 6,722 2,373 55 4,292
Borgona Betagi 16,509 2,534 118 13,845
Gopalgonj Kotalipara 36,369 4,207 275 31,887
Gopalgonj Tungipara 16,601 4,017 345 12,239
Jessore Chaugachha 26,651 2,723 387 23,541
Jessore Sharsha 33,261 4,088 525 28,648
Jhalakati Jhalakati S. 18,873 2,375 52 16,446
Jhalakati Kathalia 14,321 2,653 18 11,633
Jhalakati Nalchity 22,266 3,864 119 18,284
Jhalakati Rajapur 15,226 4,511 13 10,702
Jhenaidah Jhenaidaha S. 46,090 4,872 150 41,067
Khulna Dumuria 45,332 5,103 2,698 37,531
Khulna Paikgachha 35,129 19,199 868 14,959
Lakshmipur Ramgati 35,845 3,881 4,544 23,297
Patuakhali Bauphal 43,483 6,621 383 36,437
Patuakhali Mirzaganj 14,699 2,248 51 12,390
Pirojpur Bhandaria 15,542 3,402 37 12,081
Pirojpur Kawkhali 8,961 2,681 5 6,274
Pirojpur Mathbaria 33,432 6,286 110 27,036
Pirojpur Nazirpur 22,026 3,946 281 17,800
Pirojpur Pirojpur S. 25,275 6,067 203 18,988
Shatkhira Assasuni 27,395 9,736 129 17,498
Shatkhira Debhata 17,197 8,709 73 8,390
Shatkhira Kaliganj 44,164 17,854 198 26,113
Shatkhira Satkhira S. 37,120 6,329 1,530 29,245
Shatkhira Shyamnagar 153,921 20,790 2,308 130,823
Shatkhira Tala 33,233 4,613 1,750 26,870

*Inundated areas estimated on the basis of the 22 May Sentinel 1 images; can vary on other days (Source: ICIMOD/Sentinel-1)



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