Quantifying the Chobham Common Burnt Area with Sentinel-2 images

Many thanks to Gail for this write up of this image acquisition

The Incident

On Friday 7 August 2020 a wildfire began to burn on Chobham Common an area of heathland in Surrey. The wildfire required multiple fire crews to fight the flames and the incident halted the Rose Ladies Series Grand Final at Wentworth Golf Club (BBC News, 2020). The wildfire continued to burn and then smoulder for the next four days until it was announced on 11 August 2020 that the fire was “under control” with the incident officially closed on 17 August 2020 (SurreyLive, 2020).

Sourcing Sentinel 2 pre-fire image

The Copernicus Sentinel-2 satellite data provided an excellent insight into the amount of heathland burned during the Chobham Common wildfire. The last pre-fire cloud free Sentinel-2 image was acquired 30 July 2020 at 11:06am (Figure 1A) and shows the undisturbed heathland vegetation north of the M3. There was other Sentinel-2 data acquired on 04 August 2020 and 06 August 2020 but unfortunately the Chobham Common area was not visible due to cloud cover. Sentinel-2A and -2B data consist of 13 spectral bands. Figure 1A shows 30 July true colour composite which uses spectral bands 4 (red), 3 (green) and 2 (blue) to create a pre-fire image.

Figure 1 (a) Pre-fire true colour image, (b) post-fire image of Chobham Common fire

Burned area images during the wildfire event

Figure 1B shows the first Sentinel-2 true colour image during the wildfire acquired 9 August 2020 at 11:06am (2 days into the wildfire event). The burnt area can be clearly seen with the black to dark grey tones and the burn scar perimeter has been digitised to provide a quantification of total area burned at ~69 hectares.

A false colour composite was also produced using spectral bands 8 (near infrared), 4 (red), 3 (green) assigned to the red, green and blue colour channels (Figure 2). The burn scar area can clearly be delineated in dark grey tones from the surrounding healthy vegetation visualised in red tones. Healthy vegetation reflects near infrared radiation which in this colour composite has been assigned to the red channel. Whereas the red and green wavelengths (bands 4 and 3) assigned to the green and blue channels are absorbed by healthy vegetation as part of the photosynthesis process. The buildings in the landscape are depicted in cyan blue tones and show how close they are relative to the burned area.  

Figure 2. False colour composite of the Chobham Common burn scar

The wildfire was under control on the 11 August and ideally Sentinel-2 data covering Chobham Common on that day would have been very useful to compare with the 9 August image. Unfortunately the swath of the 11 August 2020 13:41 Sentinel-2A image acquisition just missed the Chobham Common area and image acquisitions thereafter on 14 and 16 August 2020 have been covered by cloud. A Landsat 8 image acquired on 12 August 2020 shows no change in burned area extent since the Sentinel-2 9 August image. Therefore, the 69 hectares measured on 9 August 2020 looks to be a good overall quantification of burned area for this incident. It is worth noting this figure is 133 hectares lower than the estimated area burned of 202 hectares (500 acres) headlined on 17 August 2020 when the incident was closed (SurreyLive, 2020).

References

BBC News (2020) Chobham Common wildfire evacuation warning [Online]. BBC. Available from: https://www.bbc.co.uk/news/uk-england-surrey-53700404 (Accessed 19 August 2020).

SurreyLive (2020) Chobham Common incident closed 10 days after wildfire broke out [Online]. SurreyLive. Available from https://www.getsurrey.co.uk/news/surrey-news/chobham-common-incident-closed-10-18784980 (Accessed 19 August 2020).

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