Where are the avalanches? Rapid mapping of a large snow avalanche period with optical satellites

Abstract

Accurate and timely information on avalanche occurrence are key for avalanche warning, crisis 10 management and avalanche documentation. Today such information is mainly available at isolated locations provided by observers in the field. The achieved reliability considering accuracy, completeness and reliability of the reported avalanche events is limited. In this study we present the spatial continuous mapping of a large avalanche period in January 2018 covering the majority of the Swiss Alps (12'500 km 2 ). 15We tested different satellite sensors available for rapid mapping during a first avalanche period. Based on these experiences, we tasked SPOT6/7 data for data acquisition to cover the second, much larger avalanche period. We manually mapped the outlines of 18'737 individual avalanche events, applying image enhancement techniques to analyze regions in cast shadow as well as brightly illuminated ones. 20The resulting dataset of mapped avalanche outlines, having a unique completeness and reliability, is evaluated to produce maps of avalanche occurrence and avalanche size. We validated the mapping of the avalanche outlines using photographs acquired from helicopters just after the avalanche period.This study demonstrates the applicability of optical, very high spatial resolution satellite data to map an 25 exceptional avalanche period with very high completeness, accuracy and reliability over a large region.

Snow Avalanche Detection and Mapping in single, multitemporal, and multiorbital Radar Images from TerraSAR-X and Sentinel-1

“…Regardless of the advantages of radar change detection, the spatial resolution of optical sensors is better compared to radar sensors of the same nominal resolution because the intrinsically coherent SAR imaging method makes radar speckle unavoidable and requires spatial or temporal averaging. Furthermore, the resolution of TSX and S1 is not good enough to recognize flow structures of the avalanche surface which are well visible in the optical SPOT-6 images (Bühler et al, 2019).…”

“…The second TSX image was acquired on 2018-01-11, one day before the second S1 image (2018-01-12). On the day in between, the avalanche activity was very low ( To analyze the second avalanche event, the SLF ordered optical SPOT-6 images which were acquired with the single-pass multi-strip collection mode through which the most of the Swiss alps (300×40 km 2 ) could be imaged in a single day (2018-01-24) at a resolution of 1.5 m. These images were visually searched for avalanches by an expert (Bühler et al, 2019). For comparison we also analyzed TSX data from 2018-02-02, acquired 9 days later.…”

“…With the temporal information from radar change detection the 125 avalanches detected with SPOT-6 but also with TSX (Table 5c) can be further classified into 27 new, 38 unsure, and 6 old avalanches. The remaining 54 avalanches could not be uniquely classified, because they cover areas which were differentiated into multiple different classes by radar whereas such a temporal classification is difficult with single SPOT6 images (Bühler et al, 2019). Figure 7 shows a subset of the SPOT-6 images and visualizes the manually mapped avalanches.…”

“…Unfortunately, low visibility due to heavy snow fall and valleys and ski resorts closed due to high avalanche activity can lead to incomplete or missing avalanche occurrence information. In such situations, as happened in Switzerland in January 2018 and 2019, avalanches can be mapped manually in optical airborne images (Bühler et al, 2009;Korzeniowska et al, 2017) or satellite images which have to be tasked in rapid mapping mode (Scott, 2009;Lato et al, 2012;Bühler et al, 2019). The resulting avalanche outlines can then be used to update avalanche databases which are of great value for hazard mapping and mitigation measure planning (Rudolf-Miklau et al, 2014).…”

Answers to Markus Eckerstorfer (Reviewer #1)

“…Providing more examples would go beyond this papers scope but they are several additional examples provided within the example key that is delivered with the dataset that is available on request on Envidat (Hafner and Bühler 2019).…”