HR: 1400h
AN: V33A-07    [Abstracts]
TI: October 2005 Debris Flows at Panabaj, Guatemala:Hazard Assessment
AU: * Sheridan, M F
EM: mfs@geology.buffalo.edu
AF: Center for Geohazards Studies, 876 Natural Science Complex University at Buffalo, Buffalo, NY 14260, United States
AU: Connor, C
EM: cconnor@cas.usf.edu
AF: Department of Geology, University of South Florida 4202 E. Fowler Ave, Tampa, FL 33620, United States
AU: Connor, L
EM: ljconnor@gmail.com
AF: Department of Geology, University of South Florida 4202 E. Fowler Ave, Tampa, FL 33620, United States
AU: Stinton, A
EM: astinton@geology.buffalo.edu
AF: Department of Geology, 876 Natural Science Complex University at Buffalo, Buffalo, NY 14260, United States
AU: Galacia, O R
EM: ogalacia@conred.org.gt
AF: CONRED, Avenida Hinacapie 21-72, Guatemala, 01013, Guatemala
AU: Barrios, G
EM: gbarrios@conred.org.gt
AF: CONRED, Avenida Hinacapie 21-72, Guatemala, 01013, Guatemala
AB: 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. We used differential GPS (Global Positioning System) to outline the boundaries of the debris flows, to estimate variation in flow thicknesses, and to determine their volumes. 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 Lago de Atitlán. 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, average slopes on the Eastern channel are up to 16.7°. Also, this channel thalweg steepens dramatically to 12.8° just above the 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 144 and 160 m2 the calculated velocities ranged from 8.3-10.6 ms-1 yielding fluxes between 1280 and 1680 m³s^{- 1}. 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 gently-sloping (2.8°) 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.
DE: 8488 Volcanic hazards and risks
DE: 9360 South America
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly