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  • Writer's pictureArpit Shah

Mapping the extent of Flooding after Amphan Cyclone using Remote Sensing

Updated: Jun 13, 2023

The deadly Super Cyclonic Storm - Amphan - battered everything that came in the path of its 150 km/hr.+ winds on 21st / 22nd May 2020 - be it trees, houses, vehicles, electric poles or telephone lines. The situation is still far from normal (as on May 24th 2020) - the army has been directed to aid in rescue, relief and restoration work. With an estimated Storm Surge (rise in water level over and above the tidal levels) of 5m and rainfall over 200mm, Amphan left a destructive trail - inundating several regions, especially coastal areas in its path.


Using Satellite imagery-based Remote Sensing, it is possible to determine the flooded areas: In the output below, 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 extent of flooding.


(Wherever there is a significant backscatter differential between the two satellite images - the pixels have been color-coded in red, implying floods. The differential is due to the fact that reflectance reading is much lower in the crisis image when compared to the same pixels in the archive image - Water reflects the radar waves away from the receiver on the satellite image resulting in a lower backscatter reading in the crisis image when compared to the archive image. That being said, do note that backscatter differential may not necessarily be attributed to the presence of water alone - significant changes in ground topography can also cause it. That being said, given our awareness of the crisis image i.e. the cyclone's aftermath, it is reasonable for us to conclude that the lower reflectance readings across the affected areas lying on the path of Amphan is primarily due to the presence of water i.e. flooding.)


The darker the red pixels - the greater the deviation in backscatter, implying more severe flooding.

Output of Detecting Flooding from Satellite Imagery (Sentinel-1) - Post Cyclone Amphan (as on 22nd May 2020)
Figure 1: Output of Detecting Flooding from Satellite Imagery (Sentinel-1 GRD) - Post Cyclone Amphan (as on 22nd May 2020)

Much thanks to EO-College, RUS Copernicus and OpenStreetMap for the training material.


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


One must also appreciate 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 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 Sundarbans. Afterall, they bore the worst brunt of the cyclone. There are two possible reasons I could think of - 1) the delta has high surface water presence even on a normal day and the floods didn't make much of an impact to the satellite reflectance readings (backscatter differential was not significant) and/or 2) low tide during the time of Amphan's 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.
 

May 2023 Update: A less known, but a deadly health implication post-Cyclonic activity


As many in India would know, our East Coast is a hotbed for cyclones & tropical storms. For those curious, here is the explanation. While rescue, relief and rehabilitation is the standard course of action whenever this region is impacted, an equally important facet is disease-prevention. Flooding results in the creation of stagnant pools of water which becomes a breeding ground for malaria-causing mosquitoes as well as the origin of other water-borne diseases. One major long-term health risk, which is ignored and which I became aware of today arises from Asbestos overexposure.

Serpentine Asbestos
Figure 2: Serpentine Asbestos. Source: Wikimedia Commons

Asbestos? Yes, it is the same fibrous mineral which has excellent insulating properties. I came to know this material during my early teens & fondly remember posing an innocent question to my private tutor - 'Can we travel to the sun wearing an Asbestos suit?' He had a big grin on his face and ever since, he used to routinely mock me in front of the entire batch of students - 'Look, here is the boy who thinks he can travel to the sun wearing Asbestos!' Those were happy days!


But it is only today that I realized that Asbestos, in certain conditions, is highly toxic to humans. Once its cancer-causing properties were identified, its usage in home insulation - in roofs, walls, concrete etc. - has significantly reduced, despite it being an inexpensive material. To elaborate, over-exposure to Asbestos causes Mesothelioma - a deadly cancer that affects the lungs, abdomen and heart).


Storm-ravaged Old Farm House
Figure 3: Storm-ravaged Old Farm House. Source: Photo by Tneil Abt on Unsplash

How is Asbestos connected to Amphan / Cyclones / Floods? Old buildings?


Asbestos' microscopic fibers become exposed and airborne when old buildings are ravaged by cyclonic storms. These fibers, particularly when inhaled, pose a significant health threat to rescue & relief personnel, residents and whosoever is in the vicinity. The longer the exposure, the riskier it is. Because of lack of awareness, reducing the exposure to it using appropriate category of masks isn't an obvious priority in the aftermath of a cyclone.


Much thanks to Outreach Coordinator of The Mesothelioma Center - Elvira Jiménez - for reaching out and sharing this valuable information. She specified:

"There is no known safe level of asbestos exposure. Because of this health threat, it is highly recommended that flood-damaged asbestos-containing material be repaired, enclosed, encapsulated or safely removed"

Below is the list of common Household materials that typically contain Asbestos:

Common Household Materials containing Asbestos
Figure 4: Common Household Materials containing Asbestos. Source: https://www.asbestos.com/asbestos/natural-disasters/

The Mesothelioma Centre has created a detailed guide which would assist homeowners and first responders with useful information on how to minimize the exposure to asbestos in the aftermath of a natural disaster - be it a cyclone or a wildfire. It can be accessed here.


Do spread the word! It is not just the immediate aftermath of the cyclone which has to be considered, its long-term health implications should also be controlled and awareness goes a long way in doing that.

 

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Regards,

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