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  • Writer's pictureArpit Shah

Determining Flooding caused by Super Cyclone Amphan

Updated: Aug 22, 2021

The deadly tropical super cyclone Amphan battered everything that came in the path of its 150 km/hr+ winds - trees, houses, vehicles, electric poles and telephone lines. The situation is still far from normal - the army was requested to help out for relief and restoration work. With an estimated storm surge (rise in sea water) of 5m and rainfall over 200mm, Amphan left several regions, especially coastal areas, inundated.

Using satellite imagery, it is possible to map flooded areas: I have overlaid the crisis image dated 22nd May 2020 midnight (roughly 24 hours after the cyclone had passed) on the archive image dated 23rd March 2020 to observe the flooding effect. The result is below (red color depicts the extent of floods).

(Note: I have attributed the low backscatter found in the crisis image - colored in red - to the presence of floods. Low backscatter can be attributed to presence of water which reflect away the radar signal. However, this may not be the only reason for it - other changes in ground topography / properties can also result in low backscatter. The darker the red - the more the accumulation of water on the ground.)

Prepared using ESA's Copernicus Sentinel 1A GRD imagery. Much thanks to EO-College, RUS Copernicus and OpenStreetMap for the training material. The exercise is only meant to be informative and the results are indicative in nature.

As you would observe in the map extent, large swathes of West Bengal and regions of coastal Orissa have been flooded. If you would observe closely, the flood trail of Amphan closely mimics the route which the cyclone took i.e. landfall around the western edge of the Sunderbans and going north from there, towards Kolkata (not part of the map extent).

Another thing to note is the striking accuracy of the Indian Meteorological Department's forecast. It is very difficult to forecast even daily weather patterns and to get the cyclone event path so right speaks volumes about their technical prowess.

One anomaly which I could spot is the relative 'lack' of red zones / flooded areas over the Sunderbans. Afterall, they bore the extreme brunt of the cyclone. There are two possible reasons I could think of - 1) the delta has high surface water content even on a normal day and the floods didn't make much of an impact in the satellite reading (backscatter) and/or 2) low tide during the time of cyclone landfall which resulted in less flooding.

What else could you spot or interpret?

Still reeling from Coronavirus, Amphan brings more devastation and misery to the nation. Let's hope that Eastern India is better prepared to deal with such calamities in the future because these are sure to strike again.

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