Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
ROCK FALL HAZARD MAPPING AND RUN OUT SIMULATION ‐ A CASE STUDY
FROM BOLONIA BAY, SOUTHERN SPAIN
N. Höbig (1), A. Braun (2), C. Grützner (1), T. Fernández‐Steeger (2) and K. Reicherter (1)
(1) Institute of Neotectonics and Natural Hazards, RWTH Aachen University, Lochnerstr. 4‐20, 52056 Aachen. GERMANY.
(2) Department of Engineering Geology and Hydrogeology, RWTH Aachen University, Lochnerstr. 4‐20, 52056 Aachen, GERMANY.
nicole.hoebig@rwth‐aachen.de
Abstract: For a case study on rock fall simulation and hazard mapping the rock fall site around the San Bartolomé mountain ridge in Southern
Spain was investigated. The major aim was to analyse the potential of rock falls and the risk of the inhabitants of the villages Betis and El
Chaparral. Thus rock fall debris were mapped in detail to analyse them with regard to possible trigger mechanisms. Different empirical and
numerical runout simulations were computed and resulted in three possibilities for rock movement. According to trigger mechanisms and
predicted runout distances rock fall hazard maps were created and risk analysis was undertaken.
Key words: rock falls, runout analysis, trigger mechanism, risk assessment
INTRODUCTION
A rock fall analysis for hazard mapping in the Bolonia Bay
Region close to Tarifa (Spain) was undertaken (Fig. 1). The
NE and SW‐side of the San Bartolomé mountain ridge
were investigated and mapped in detail to identify the
hazardous areas. The deposits of large‐scaled rock fall
blocks and debris were mapped and runout distances
were determined based on GIS analysis. Since housing
areas are located beneath the source zone and large
blocks may be found close to houses and settlements,
there is a risk for inhabitants to be victim of rock falls and
damage of houses or infrastructure.
Fig. 1: Overview of the San Bartolomé with the two main
investigation areas.
GEOLOGICAL SETTING
The Bolonia Bay is part of the Flysch zone of the Betic
Cordillera. Steep rock walls, which consist of quartzitic
Aljibe sandstone shape the NE‐ and SW‐flank of the San
Bartolomé mountain ridge. This turbiditic Miocene rock is
intensively weathered and shows a distinct, perpendicular
shaped joint pattern. The joint system causes large
boulders, which are tilted or rotated out of the rock wall
as a consequence of gravitational mass movements or
possible earthquakes (Silva et al., 2009) (Fig. 2).
The deposits are widespread beneath the source zone
and reach runout distances of up to 2 kilometers and sizes
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from 2 m 3 up to 350 m 3 . For more than 300 blocks data
on size, shape, type of rock and location were collected.
Fig. 2: Intensely joined and weathered source zone with
rotating boulders on the N‐side of the San Bartolomé.
RUNOUT ANALYSIS
As a first step runout analysis was performed using
empirical models. Most empirical models for rock fall
analysis are based on relationships between
topographical factors and runout distances. Two
important models are the "Fahrböschung" or "angle of
reach", introduced 1932 by Heim and the "shadow angle"
by Evans and Hungr (1993). The principle of both models
is shown in figure 3. The identification of minimum angles
can serve as an approximate runout distance prediction.
In different case studies minimum values for the angle of
reach and the shadow angle were postulated. Evans and
Hungr (1993) observed a lower limit for the angle of reach
of 28,43° and a minimum shadow angle of 27,5°. Dorren
(2003) proposed a range of 22‐30° for the minimum
shadow angle.
The angle of reach and the shadow angle were calculated
for the rock fall deposits on the west flank of the San
Bartolomé mountain ridge using the geographical
information system ArcGIS (ESRI).
Compared to the proposed minimum angles from the
case studies, the models fit quiet well for some areas
close to the rock fall source zone (green background in
Fig. 4). In areas with large travel distances (more than 300