V33A-07
October 2005 Debris Flows at Panabaj, Guatemala: Hazard Assessment
M.F. Sheridan1 (
[email protected]), C. Connor2, L. Connor2, A. Stinton, O.R. Galacia4, and G. Barrios4 1. Center for Geohazards Studies, University at Buffalo, Buffalo, NY 14260 2. Department of Geology, University of South Florida, Tampa, FL 33620 3. Department of Geology, University at Buffalo, Buffalo, NY 14260 4. CONRED, Guatemala City, 01013, Guatemala Background In October, 2005, tropical storm Stan caused heavy precipitation throughout much of Guatemala. In the community of Panabaj, Santiago Atitlán, a landslide of pyroclastic material originating high on the slopes of Tolimán volcano buried much of the community, leaving approximately 400 people dead. Current estimates by the Coordinadora Nacional para la Reducción de Desastres (CONRED) suggest that at least 2,600 people from the community of Panabaj, Santiago Atitlán have been displaced by the debris flows.
Because the temporary housing for people displaced by the debris flows is located in an area that is geologically and morphologically similar to the area inundated by flows in October, 2005, this area may be potentially inundated by debris flows as well. In addition to the thousands of people living in temporary shelters, many hundreds of people are currently reoccupying land adjacent to or on the October, 2005 debris flows. Thus a large fraction of the surviving Panabaj community appears to remain at risk from future debris flows.
Mass movement on Tolimán volcano resulted in the generation of a moderate size debris flow (360,000 m3 of sediment plus water) that descended the volcano rapidly, bifurcated into two stream valleys high on the flanks of the volcano, and continued to descend both channels until these flows reached the alluvial fan near the shores of Lake Atitlán as two distinct flow lobes..
Methodology We used differential GPS (Global Positioning System) to outline the boundaries of the debris flows, plot flow directions, estimate variation in flow thicknesses, and to determine flow volumes. Flow vectors were plotted using stoss-side accumulations against obstacles. Flow velocities were determined by superelevation on curves.
Results
After bifurcating into two flows high on the flanks of the volcano, about 65% of the flow (by volume) descended the western channel, forming the Western flow. Approximately one kilometer above the alluvial fan, this channel descends steep topography, with a slope of 11.5°. This average slope gradually decreases down the channel, reaching only 5.3° just above the alluvial fan. In contrast, the Eastern channel is not a normal drainage on the volcano. A debris flow moved into this channel only because the capacity of the western channel was exceeded.. Average slopes on the Eastern channel are up to 16.7°. Also of note, this channel thalweg steepens dramatically to 12.8° just above the alluvial fan. This steeper slope increased the debris flow velocity just before it reached the populated alluvial fan.
Flow velocities in channelized sections were estimated by superelevation at bends at two locations for each of the two flow branches. In measured cross sectional areas between 2 144 and 160 m the calculated velocities ranged from 8.3-10.6 3 -1 ms-1 yielding fluxes between 1280 and 1680 m s . The fluxes for the two flows are surprisingly similar.
The planimetric area inundated by the Western flow is approximately 180,000 m2 and the area inundated by the Eastern debris flow is 77,000 m2. On reaching the gentlysloping (2.8º) depositional fan where the village of Panabaj is located, the flows thinned to 0.5-3.0 m and spread laterally as a broad sheet flow bounded by distinct flow fronts of 0.30-0.6 m height.
Although thin, the flows had sufficient power to sweep away most of the concrete block houses in their paths. Based on observations of high water marks preserved on buildings, up to 40% of the flow by volume consisted of water and fine grained sediments that have been dewatered from the deposit during and since deposition.
Recommendations Immediately devote significant resources to public outreach and education. Encourage relocation of much of the Panabaj community off of the alluvial fan. This area is simply too dangerous for the government or similar entities to encourage development.
•Given that some individuals will continue to live on the alluvial fan, develop strategies for mitigating hazards. These include construction of levees and/or retainment ponds to mitigate hazards from small-volume flows. •Monitoring should include rainfall intensity, infiltration, and seismic measurements. A lahar monitoring system may be useful, but will not obviate the major impacts of lahars because of the short travel-time of the lahars down the slopes of the volcano. Acknowledgements We are grateful for field support provided by Juan Skinner and Saskia Carusi (OXFAM GB). Jennifer Connor assisted in GPS mapping.
