Mapping Floods in the wake of Super Cyclone Amphan using Radar Remote Sensing
- Arpit Shah
- May 25, 2020
- 4 min read
Updated: 3 days ago
The devastating Super Cyclonic Storm - Amphan - battered everything in the path of its 150+ km/hr winds on 20-21 May 2020 - men, material, and entire settlements. As of today (24 May 2020), the situation in the aftermath remains grim. The Indian Army has been mobilised to support rescue, relief, and restoration efforts. With an estimated storm surge (the abnormal rise in sea level over and above the tidal levels) of five metres along low-lying coastal areas and rainfall exceeding 200 mm inland, Amphan left behind a catastrophic trail, inundating vast stretches of land.
Using SAR (Radar) Remote Sensing, it is possible to determine flood-affected areas by performing Change Detection between pre- and post-disaster satellite imagery. Water, being a specular reflector, reflects microwaves away from the satellite. Land features, in contrast, usually exhibit double-bounce reflection, sending part of the microwave energy back to the sensor- this returned energy is known as Backscatter. This stark difference makes flood mapping possible: areas with high backscatter before the disaster and negligible backscatter after the disaster generally indicate inundation.
To derive the output in Figure 1, I used the Crisis image acquired on 22 May 2020 (about 24 hours after Amphan dissipated) and compared it with an Archive image from 23 March 2020. A change-detection algorithm was applied to identify flooded areas—shown in red (darker shade = greater severity).
Much thanks to EO-College, RUS Copernicus and OpenStreetMap for the training.
Large swathes of coastal Odisha and West Bengal appear flooded, mirroring the cyclone’s path. This outcome, though tragic, is unsurprising.
However, it is important to note that a reduction in backscatter may occur due to reasons other than flooding—for example, topographic or structural changes caused by strong winds. Disoriented vegetation, for instance, can alter backscatter behaviour. That said, water drastically reduces backscatter, whereas disturbed vegetation reduces it only partially. Thus, the darkest red pixels—signifying maximum contrast—are very likely to represent flooded terrain.
One initially puzzling observation is the relative lack of red pixels over the Sundarbans. Given that this is the world’s largest contiguous mangrove forest and directly in Amphan’s path, one might expect a uniformly red output. But the explanation became clear: being a river delta, the Sundarbans are perennially wet—especially during high tide. As a result, floodwater did not significantly alter the already low backscatter in the Archive image.
Still reeling from the COVID-19 pandemic, East India faced yet another blow from Super Cyclone Amphan. One hopes the region remains resilient, as tropical cyclones will undoubtedly strike again. I must also acknowledge the remarkable accuracy of the Indian Meteorological Department's cyclone forecasts. Predicting daily weather is difficult; predicting a cyclone’s exact path with such precision is extraordinary.
Is there anything else you observe from the map? Let me know. Let me know.
May 2023 Update
While rescue, relief, and rehabilitation are the usual priorities after a cyclone, an equally important—yet often overlooked—aspect is disease prevention. Flooding creates stagnant pools of water, which become breeding grounds for malarial mosquitoes and water-borne diseases. One major long-term health risk I was made aware of today, prompting this update, involves overexposure to Asbestos.
Asbestos—the same fibrous mineral once widely used for insulation— fascinated me as a child. I vividly remember asking my private tutor, “Can we travel to the Sun wearing an asbestos spacesuit?” and distinctly remember the big grin on his face and the sarcastic jibes that followed - 'Look, here is the boy who thinks he can travel to the Sun wearing Asbestos!'😅
However, once its cancer-causing properties became known, especially its link to Mesothelioma (a deadly cancer affecting the lungs, abdomen, and heart), asbestos was banned worldwide for household use.
But how is asbestos-related cancer connected to cyclones?
When old buildings containing asbestos are ravaged by cyclonic winds, microscopic asbestos fibres get exposed and become airborne. These fibres are easily inhaled by residents, relief personnel, and by anyone nearby—often without any awareness of the risk. The longer the exposure, the greater the danger.
Due to lack of awareness, personal protective equipment is rarely prioritised during post-disaster cleanup, leaving many people vulnerable.
Much thanks to Elvira Jiménez, Outreach Coordinator of The Mesothelioma Center, for reaching out and sharing this critical information. She highlighted:
There is no known safe level of asbestos exposure. Flood-damaged asbestos-containing materials must be repaired, enclosed, encapsulated, or safely removed.
Below is a list of common household materials that often contain asbestos:
The Mesothelioma Centre has created an excellent guide to help homeowners and first responders reduce their exposure to asbestos in the aftermath of natural disasters—whether cyclones or wildfires.
Spread the word!
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