In Detail: Mudslides and Associated Flood Detection Using Sentinel-1 Data

Abstract: 

Floods and landslides are the first and fourth most recurrent disasters around the world (Petley, 2012). There are several examples of downstream flooding caused by massive mudslides where rapid mapping is an indispensable tool for supporting disaster management activities by civil protection authorities.

From July 2014, the Copernicus programme of the European Union has provided Sentinel-1 data with full world coverage, allowing easy access to Big SAR Data. This allows the exploration of new applications to strengthen the hazards monitoring and disaster mitigation activities.

This UN-SPIDER Recommended Practice emphasizes the use of SAR data during and after a disaster crisis, since optimum atmospheric conditions for optical satellite images are not always available. In the example provided here, it is used to map the mudflow following the dam collapse that occured on 25 January 2019 at Brumadinho, Brazil.

The methodology can be applied for massive mudslides and large areas of sludge-water. This proposed method is focused on extracting the most significant amount of backscattering information linked to the surficial roughness of the massive mudslide using the magnitude information of SAR image. To achieve this task, it uses change detection techniques (log- ratio) and principal component analysis (PCA).

Requirements: 

Data requirements:

- For applying this methodology, it is necessary to use SAR images in Level-0 (equivalent to RAW product type) or Level-1 (equivalent to SLC product type). According to Big SAR Data available in the Copernicus service platform, the following Sentinel-1 acquisition modes can be used: Stripmap (SM), Interferometric Wide swath (IW) and Extra-Wide swath (EW).

-For the ortho-rectification process, a Digital Elevation Model is necessary, which is automatically downloaded by SNAP software.

Software requirements:

For this recommended practice, SNAP is necessary. The software is freely available online for Windows 32 and 64-bit versions and Mac OS X; the current version is 6.0.0.

Similarly, we used QGIS software available for free for Windows 32 and 64-bit versions and Mac OS X; the current version is 3.6.3

Skills requirements:

Basic understanding about SAR theory and SAR image processing is required.

Hardware requirements:

The minimum computing capacity specifications recommended are the following:

-300GB of free disk space

-8GB of RAM

-Dual core processor (Intel i7)

Strengths and Limitations: 

Strengths:

This method is recommended when the flooded areas cannot be detected by the traditional techniques due to ground moisture conditions or the large volume of sediments involved during a flood. The strength of this method lies in the digital treatment techniques, which allow the most information about backscattering mechanisms linked to the surficial roughness of the massive mudslide to be extracted. Thus, it is possible to detect spatial patterns of the disaster undetectable by an optical image.

This recommended practice is a useful tool for rapid mapping to support the emergency response during a disaster crisis by civil protection authorities.

Limitations:

This method can be applied only in massive mudslides and associated major flooding. Similarly, to apply this recommended practice only SAR images with two polarizations minimum can be used.

Workflow: 

Workflow Recommended Practice: Mudslides and Associated Flood

Bibliography: 

-J. Carlos Santamarina, Luis A. Torres-Cruz and Robert C. Bachus, (2019).  Why coal ash and tailings dam disasters occur, Science 364 (6440), 526-528. DOI: 10.1126/science.aax1927

- David Petley, (2012). Global patterns of loss of life from landslides, Geology ; 40 (10),  927–930. DOI: https://doi.org/10.1130/G33217.1

- First look at Planet imagery of the Brumadinho (Brazil) dam collapse. https://kokoalberti.com/articles/first-look-at-planet-imagery-brumadinho-dam-collapse/

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