Wildfire and Debris Flow Analysis in Southern California using ENVI

Summary

This project had the primary goal of measuring the recovery of the Thomas Fire burn scar in Santa Barbara County from January to June 2018. The secondary goal of this project was to measure the impact area of the subsequent debris flows following the containment of the Thomas fire in January 2018. This project was created in collaboration with partner Serena Morgan, a CU boulder student.

Background

The Thomas Fire

The Thomas fire was the largest wildfire in California history at the time when it burned in December 2017 to January 2018. It burned 440 square miles in Santa Barbara County. The fire was ignited near Santa Paula, California, due to power lines coming in contact during high winds. (Ventura County Fire Department, 2019)

January 8 Debris Flows

Debris flow risk is highly elevated following wildfires for several reasons, mainly the lack of vegetation holding soil in place and hydrophobic soils due to ash layers. Following the wildfire, in the early morning hours of January 8, 2018, a cold front brought heavy rains to the foothills of Santa Barbara. Catastrophic debris flows came cascading down the creek beds into the community of Montecito. 23 people were killed, and 408 homes were damaged, with dozens destroyed completely (Cal EOS News, 2018).

Study Area

Santa Barbara county is located 100 miles up the coast from Los Angeles. The Thomas fire was ignited by power lines near Santa Paula, California. The Thomas Fire burned up the coast of California, eventually reaching Montecito and Santa Barbara.

Data Collection

Sentinel 2

Short wave infrared (SWIR) composite (Bands 12, 8a, and 4) for January to June were obtained from the Copernicus Browser (EoPortal). A SWIR composite image represents burned areas well, as burn scars give off high reflectance in the short wave infrared (Kampen, M, n.d.)

Pre Burn SWIR Composite (November 18, 2017)

Analysis

ENVI ROIs

Using the remote sensing software ENVI, a maximum likelihood supervised classification was used on the SWIR composite images to identify areas that were burned. Regions of Interest (square polygons) were created for each month (January to June), which trained the classifier to identify burned areas. The recovery of the Thomas Fire burn scar can then be tracked by multiplying the pixel count by the spatial resolution.

In the following images, the left column is the raw SWIR composite images. In the right column, orange areas images represent burned areas identified by the classification. Black areas are identified as not burned or recovered.

January SWIR composite

January Post Classification

February SWIR Composite

February Post Classification

March SWIR Composite

March Post Classification

May SWIR Composite

May Post Classification

June SWIR Composite

June Post Classification

Area Burned and Precipitation

As time progresses, the burn scar area decreases as vegetation recovers. This trend continues until May, when the burn area seems to grow. In March, there was 7 inches of precipitation in Santa Barbara, which aided in the growth of vegetation. From April to June, there was little precipitation, leading to dry and dead vegetation. Because dead vegetation gives off high reflectance in SWIR, a false increase in burn area is reported between May and June (County of Santa Barbara).

Results: Burn Area

January: 1026.87 km²

February: 1012.41 km²

March: 689.91 km²

April: 506.53 km²

May: 453.021 km²

June: 612.769 km²

Part 2: Debris Flow Analysis

Using true color 10m imagery from the Copernicus Browser (above) and an existing debris flow impact area map from DigitalGlobe (right), an approximate impact area was determined (Karklis, et al, 2018).

Analysis

Impact Area

Using ENVI, polygons were drawn to outline the debris flow impact area. The an approximate measurement of the area impacted could be determined.

Results: Debris Flow Impact Area

Cold Springs: 1.4419 km²

San Ysidro: 1.0674 km²

Romero: 0.7243 km²

Buena Vista: 0.5203 km²

Total Area: 3.7539 km²

Conclusion

Using Sentinel 2 imagery and ENVI software, an analysis of the Thomas fire recovery and the subsequent debris flow could be conducted. Using SWIR composite imagery and ENVI software, the recovery of the Thomas Fire burn scar could be tracked. From January to May, the burn scar shrunk in size as vegetation recovered with ample precipitation. In June, the opposite trend was observed, as precipitation declined significantly and vegetation dried, representing a false increase in the burn scar area.

The debris flows of January 2018 resulted in dignificant loss of life and property. Using Sentinel 2 true color imagery, the impact area could be mapped using ENVI software. Cold Springs creek had the largest impact area of 1.4 km². The total impact area is estimated to be 3.75 km².

Sources

Kampen, M. (n.d.). Mapping forest fire progression with Sentinel-2 and Sentinel-1. Sentinel Hub. https://custom-scripts.sentinel-hub.com/custom-scripts/data-fusion/s2_s1_forest_fire_progression/

(2018, November 7). Montecito Mudslides Anniversary, Reflections Through Images. Cal EOS News. Retrieved April 5, 2024, from https://news.caloes.ca.gov/montecito-mudslides-anniversary-reflections-through-images/

Karklis, L., Tierney, L., & Meko, T. (2018, November 9). Before and after the mudslides in Montecito. Washington Post. Retrieved April 5, 2024, from https://www.washingtonpost.com/graphics/2018/national/montecito-before-after/

"Historical Rainfall & Reservoir Information." County of Santa Barbara, www.countyofsb.org/2256/Historical-Rainfall-Reservoir-Informatio. Accessed 16 Apr. 2024.

"Copernicus: Sentinel-2." EoPortal, 14 Jun. 2012, www.eoportal.org/satellite-missions/copernicus-sentinel-2#msi-multispectral-imager. Accessed 16 Apr. 2024.

Copernicus Browser, browser.dataspace.copernicus.eu/.

Ventura County Fire Department, 13 Mar. 2019, vcfd.org/news/vcfd-determines-cause-of-the-thomas-fire/. Accessed 5 Apr. 2024.

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