Open Session - SWISS GEOSCIENCE MEETINGs

2 2
Symposium 9: Natural Hazards and Risks
Figure 3. Map of wind fields in Switzerland.
.12
A method for risk analysis related to lahars and floods –
a case study at Nevado del Tolima volcano, Colombia
Künzler Matthias*, Huggel Christian*, Ramírez Juan Manuel**
*Department of Geography, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich (mkuenzler@access.unizh.ch, christian.huggel@
geo.uzh.ch)
**Swiss Agency for Development and Cooperation, Bogotá, Colombia
The glacier-covered Nevado del Tolima located in the Colombian Cordillera Central (Figure 1) is an active volcano with potential
lahar hazards similar to those on Nevado del Ruiz (Thouret et al., 1995). For effective disaster prevention a risk analysis
is an important tool. We present here a methodology that allows for a relatively rapid assessment of the risk based on a
first-order analysis of lahar and rainfall related flood hazards, and vulnerability. The methodology is performed for five villages
in the Combeima Valley and the regional capital Ibagué (≈450,000 inhabitants).
Firstly, the models LAHARZ for lahars and HEC-RAS for floods were applied to generate hazard maps. Lahar scenarios are
based on melting of 0.5, 1, 5, and 15 m of glacier ice due to volcanic activity, resulting in lahar volumes of 0.5, 1, 5, and 15
million m 3 . For flood modelling, design floods with a return period of 10 and 100 years were calculated. A second step involves
the analysis of different vulnerabilities. Physical vulnerability is operationalised by market values of dwelling parcels and
population density, whereas social vulnerability is expressed by population age and poverty. Thirdly, hazard H and vulnerably
V values are both transformed into a scale from 0 to 1 and subsequently multiplied following the risk equation R = H * V
(Varnes, 1984). The results are qualitatively risk values per parcel level and quantitative damage estimates. Figure 2 schematically
illustrates the risk analysis concept. The left side refers to the vulnerability and the right side to the hazard, respectively.
Grey parallelograms are GIS layers.
Whereas flood hazard has limited effects on population and infrastructure, the impact of lahars is more serious. An assumed
lahar volume of 15 million m 3 may lead to lahar heights of 19 m and a maximum horizontal cross section of ca. 450 m. Over
20,000 people may be affected.