Snow avalanche detection and mapping in multitemporal and multiorbital radar images from TerraSAR-X and Sentinel-1

Abstract

Abstract. Snow avalanches can endanger people and infrastructure, especially in densely populated mountainous regions. In Switzerland, the public is informed by an avalanche bulletin issued twice a day during winter which is based on weather information and snow and avalanche reports from a network of observers. During bad weather, however, information about avalanches that have occurred can be scarce or even be missing completely. To assess the potential of weather-independent radar satellites, we compared manual and automatic change detection avalanche mapping results from high-resolution TerraSAR-X (TSX) stripmap images and medium-resolution Sentinel-1 (S1) interferometric wide-swath images for a study site in central Switzerland. The TSX results were also compared to available mapping results from high-resolution SPOT-6 optical satellite images. We found that avalanche outlines from TSX and S1 agree well with each other. Cutoff thresholds of mapped avalanche areas were found with 500 m2 for TSX and 2000 m2 for S1. S1 provides a much higher spatial and temporal coverage and allows for mapping of the entire Alps at least every 6 d with freely available acquisitions. With costly SPOT-6 images the Alps can even be covered in a single day at meter resolution, at least for clear-sky conditions. For the SPOT-6 and TSX mapping results, we found a fair agreement, but the temporal information from radar change detection allows for a better separation of overlapping avalanches. Still, the total mapped avalanche area differed by at least a factor of 3 because with radar mainly the avalanche deposition zone was detected, whereas the release zone was very visible already in SPOT-6 data. With automatic avalanche mapping we detected around 70 % of manually mapped new avalanches, at least when the number of old avalanches is low. To further improve the radar mapping capabilities, we combined S1 images from multiple orbits and polarizations and obtained a notable enhancement of resolution and speckle reduction such that the obtained mapping results are almost comparable to the single-orbit TSX change detection results. In a multiorbital S1 mosaic covering all of Switzerland, we manually counted 7361 new avalanches which occurred during an extreme avalanche period around 4 January 2018.

Backscatter Characteristics of Snow Avalanches for Mapping With Local Resolution Weighting

“…2) TerraSAR-X backscatter images were available. For a few cases of avalanches in that study area for which we were uncertain whether the pattern detected in S1 data corresponds to an avalanche, we considered high resolution TerraSAR-X backscatter-change RGB composites [7] to support our decision from manual S1 mapping. The used S1 images were defined according to set 1 in Table I and are listed in Table II.…”

“…2) Counting of avalanches in the entire Swiss Alps for the 4 th of January 2018: The avalanche event of the 4 th of January 2018 had been analyzed by [7]. They had created a difference image based on an unweighted arithmetic average of overlapping orbits.…”

“…A more wide-ranged, weatherindependent and systematic avalanche mapping could significantly improve future avalanche predictions by providing more comprehensive statistical analyses. Many recent works have shown the suitability of Sentinel-1 (S1) SAR backscatter images for this task (e.g., [3]- [7]). Because the manual mapping All authors are with the Institute of Environmental Engineering, ETH Zurich, 8093-Zurich, Switzerland.…”

“…The visibility of snow avalanches in SAR images is caused by a change in the radar backscatter signal due to changing snow properties. Compared to a smooth, undisturbed snow surface, avalanches roughen the surface at least in the deposition zone [7], [16]. According to Ulaby et al (2014) scattering from very small surface roughness is predominantly (forward) directed whereas with increased roughness more energy is transferred to omnidirectional scattering [17], [18].…”

“…Larger incidence angles, favored by LRW, provide not only a higher spatial resolution, but it has also been qualitatively argued that larger incidence angles should lead to a stronger brightness of avalanches [7]. Therefore, LRW should enhance the avalanche detection because it weights image parts of larger incidence angle more strongly.…”