Abstract
Human assets in Alpine regions are prone to gravitational natural hazards such as rock fall, shallow landslides and avalanches. Forests make up a substantial share in that landscape and can mitigate those hazards. Management of avalanche protection forests must cope with avalanches potentially released in forest gaps, which can damage downslope forests. The Swiss guidelines “Sustainability and success monitoring in protection forests” prescribe forest-gap extents in slope-line direction critical to the release of avalanches in forested areas. This article proposes a topography-informed morphology approach (TIMA) to automate the detection of critical gaps based on a digital terrain model and a canopy height model (CHM) derived from airborne LiDAR-data. TIMA uses complementary information about topography to probe forest gaps computed from the CHM with templates meeting critical-gap extents adjusted to local topography. The method was applied to a test site in Klosters-Serneus (Switzerland). The comparison of a critical-gap map with the results of a field assessment at 19 sample locations resulted in 84% overall accuracy. Moreover, plausibility of gap detection could be improved by including linear features forest roads and torrent channels in TIMA to account for decoupled snow layer resulting from abrupt breaks on the hillslope. If the TIMA concept can be successfully applied to the case of avalanches, this would encourage its use in assessing other gravitational natural hazard processes.
Highlights
We propose a mathematical morphology approach to automate the detection of critical gaps in an avalanche protection forest based on a high-resolution digital terrain model (DTM) and canopy height model (CHM)
We have learned the following when applying the topography-informed morphology approach (TIMA) to the 30 hectare study area in Klosters-Serneus (Switzerland): 1. Single trees delineated on the CHM are a convenient means to identify forests effective against the release of snow avalanches based on forest parameter thresholds
TIMA uses morphological opening to probe forest gaps with a template representing a critical-gap extent. Since this template must adapt to the local topography to produce plausible results, space was discretized into a set of topography classes and corresponding critical-gap templates
Summary
Populated Alpine regions as encountered in Austria, France, Italy and Switzerland are prone to gravitational natural hazard processes such as rockfall, shallow landslides and avalanches as the consequence of the steep topographical relief. The increasing availability of high-resolution digital terrain- and canopy height models (DTM, CHM), such as those processed from airborne LiDAR data, facilitates the digital characterization of topography and forest structure This allows for the automation of the critical-gap identification-task. The application of TIMA to a test area in Klosters-Serneus (Switzerland) aims at (1) exploring the sensitivity of detected critical gaps on the parameters that specify forest gaps, (2) assessing the critical-gap map accuracy based on a sample set of locations assessed in the field according to the Swiss guidelines, and (3) exploring the plausibility of critical-gap maps after adding linear features decoupling snow layers on hillslopes. We will discuss the results and present our conclusions in the corresponding sections
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