The devastating Super Cyclonic Storm - Amphan - battered everything that came in the path of its 150+ km/hr winds on 20th and 21st May 2020 - be it men or material. As of today (May 24th 2020), the situation in the aftermath continues to be grim - the Indian Army has been mobilized to assist in rescue, relief and restoration efforts. With an estimated Storm Surge (the abnormal rise in sea level over and above the tidal levels) of 5 metres in the low lying areas around the coast and recorded rainfall of over 200 millimeters further inland, Amphan left a catastrophic trail besides inundating large tracts of land.
Using SAR (Radar) Remote Sensing, it is possible to determine the areas that have been flooded by performing Change Detection between pre and post-disaster Satellite Imagery datasets. The fundamental aspect is that Water, by virtue of being a specular reflector, reflects the microwaves emitted from the satellite in a direction away from it, unlike Land features, typically double-bounce reflectors, reflect a portion of the microwaves back towards the satellite - this reflected energy is technically known as Backscatter as the waves return in the same direction from which they had been emitted. This high contrast in reflectance behaviour is what helps an analyst to delineate those pixels that could have been flooded i.e. those pixels which have received sizeable backscatter pre-disaster and negligible backscatter post-disaster.
To derive my output as depicted in Figure 1 below, I overlaid the Crisis image acquired on 22nd May 2020 (roughly 24 hours after Amphan had dissipated) on the Archive image acquired on 23rd March 2020 and subsequently performed Change Detection to identify the areas affected by floods (red pixels - darker the shade, greater the severity of flooding).
Much thanks to EO-College, RUS Copernicus and OpenStreetMap for the training.
One can observe that large swathes of coastal Odisha and West Bengal to the north have been flooded - this mimics the route of Amphan cyclone and seems to be a natural fallout to the disaster.
I must state that a reduction in Backscatter can be caused by factors other than the presence of Water - for example, changes in Surface Topography such as disorientation of Vegetation by winds (a definite possibility during cyclonic weather) can also cause it. That being said, Water virtually eliminates the possibility of Backscatter whereas disoriented vegetation could potentially only reduce the backscatter and hence, the dark red pixels at the very least (signifying the maximum difference in Backscatter between Archive and Crisis images) are highly likely to be Flooded.
An occurrence that I initially felt very strange about was the relative lack of red pixels over the Sundarbans. The world's largest contiguous mangrove forest bore the brunt of the Super Cyclonic Storm and I felt it should have been flooded beyond recognition - was expecting the output to be completely shaded in red in this region. However, I realized soon enough what could be the inhibiting factor - by virtue of being a River delta, the Sunderbans are perennially drenched, and particularly so during high tides. Hence, the excess water from flooding didn't make much of an impact to the Backscatter readings which were already very low over this region in the Archive image.
Still reeling from the Coronavirus pandemic, Super Cyclone Amphan brings more devastation and misery to India. One hopes that East India in particular remains resilient as tropical cyclones are sure to strike again as they have done in the past. I must also take this opportunity to highlight and appreciate the striking accuracy of the Indian Meteorological Department's Cyclone Path forecasts. It is very difficult to forecast even daily weather let alone a ravaging storm and to predict the latter's path so precisely speaks volumes about the country's top meteorological institute's technical prowess.
Is there anything else that you could observe from the output? Let me know.
May 2023 Update
While rescue, relief and rehabilitation forms the standard course of action in the aftermath of a cyclone, an equally important but potentially overlooked facet is the prevention of disease. Flooding results in the creation of stagnant pools of water which becomes a breeding ground for malarial mosquitoes and a hotbed of water-borne diseases. One major long-term health risk that I was made aware of today, which brought me to update this post, arises from overexposure to Asbestos.
Asbestos? Yes, it is the same fibrous mineral which has excellent insulating properties. I was fascinated by this material as a kid and fondly remember posing an innocent question to my private tutor in front of the whole class - Can we travel to the Sun wearing an Asbestos spacesuit? I distinctly remember the big grin on his face and the sarcastic jibes that followed and were persistent for an eternity - Look, here is the boy who thinks he can travel to the Sun wearing Asbestos!'
It so happens that over-exposure to Asbestos causes Mesothelioma - a deadly cancer that affects the lungs, abdomen and heart. Once the cancer-causing property of this rather inexpensive material was identified, its usage in home insulation - roofs, walls, concrete etc. was banned globally.
But how is Asbestos-induced Cancer connected to Cyclones?
When old buildings which have Asbestos in them are ravaged by Cyclonic storms, microscopic fibers become exposed and airborne.
These floating Asbestos microfibers are invariably inhaled, posing a significant health risk to residents, relief personnel and anyone who come into its contact. The longer the exposure, the riskier it is. Because of lack of awareness, reducing the exposure to these fibres using personal protective equipment isn't given a priority in the cyclone's aftermath.
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 commonly-used household materials that typically contain Asbestos-
The Mesothelioma Centre has created this detailed guide which would assist homeowners and first responders with useful information on how to minimize their exposure to asbestos in the aftermath of a natural disaster - be it a cyclone or a wildfire. Spread the word!
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