GIS documents are of great interest for a quick and low-cost determination of areas endangered by rockfalls. They allow

  • the detection of potential instabilities from steep slopes and cliff areas, and

  • the preliminary estimation of potential run-out areas by means of a so-called cone method.

After the presentation of the tools used to compute these areas, the paper focuses on two methods developed for a preliminary hazard mapping, one at regional scale and the other at local scale.

Mittels GIS-Dokumenten können von Steinschlag bedrohte Gebiete schnell und kostenguenstig ermittelt werden. Die sogenannte Kegelmethode erlaubt die Erkennung potentiell instabiler Zonen von Steilhangen und Felswanden sowie eine erste Einschatzung der möglichen Reichweiten. Nach der Vorstellung der zur Berechnung der bedrohten Gebiete benötigten Hilfsmittel stellt der Artikel zwei Methoden zur vorlaufigen Gefahrenkartierung in lokalem und regionalem Maßstab vor. Les SIG sont très interessants pour une determination rapide et peu onereuse des zones affectees par des chutes de blocs. Ils permettent une detection des zones potentiellement instables et une estimation sommaire des zones de propagation des blocs par une methode dite des cônes. Après la description des outils utilises pour definir ces zones, l'article expose deux methodologies elaborees pour un zonage preliminaire du danger, l'une à l'echelle regionale et l'autre à l'echelle locale.

Introduction

Authorities of mountainous regions often need a quick and low-cost determination of areas endangered by rockfalls. The objectives of this first rough delineation are the early detection of conflicts between land use and rockfall hazard and consequently the identification of zones of the territory where detailed and expensive investigations are required. Nowadays the increasing availability of geographic information system (GIS) data, such as digital elevation model (DEM), topographic vector map, etc. makes the analyses on large areas easier and cheaper using simple models.

Rockfall Fahrböschung and shadow angle

For the preliminary estimation of maximum rockfall reach, several authors (Heim, 1932; Scheidegger, 1973; Onofri & Candian, 1979; Evans & Hungr, 1993) suggest a simple approach that models rockfalls as the sliding or rolling of a mass on a sloping surface with an average friction angle jp (Fig. 1). From energy considerations, this means that a block starting from a source will travel down the slope and stop at the intersection point of the topography with a so-called energy line drawn from the source point and making (Figure in full paper) an angle jp with horizontal. An alternative approach uses a shadow angle keyed to the apex of the talus slope (Fig. 2). It assumes that the kinetic energy acquired in the initial fall is largely lost in the first impacts near the top of the slope.

GIS based cone method
Detection of potential rock instabilities

The determination of potentially unstable rock areas (block sources) depends on the available data and documents: field observations, register of events, air photographs, geological and topographic maps. When GIS data are available, as it is the case in Switzerland, a quick and preliminary delineation of potential instabilities is possible.

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