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Publication

Sheridan, M.F., Bonnard, C., Carreno, R., Siebe, C., Strauch, W., Navarro, M., Calero, J.C., and Trujilo, N.B., 1999, 30 October 1998 Rock Fall / Avalanche and Breakout Flow of Casita Volcano, Nicaragua, Triggered by Hurricane Mitch, Landslide News, no. 12, pp. 2-4.

Abstracts

American Geophysical Union, San Francisco, December 1999

A Hazard Model for the 30 October, 1998 Debris Avalanche and Lahar at Casita Volcano, Nicaragua

Michael F. Sheridan¹, Bernard Hubbard¹, Wilfried Strauch² and Benjamin van Wyk de Vries³

1. Department of Geology, 876 Natural Sciences, SUNY at Buffalo, USA,
2. INETER, Managua, Nicaragua
3. Université Blaise Pascal, Magmas et Volcans, (UMR 6524), O.P.G.C., Clermont-Ferrand,              France(vanwyk@opgc.univ-bpclermont.fr)

On October 30, 1998 a disastrous avalanche and lahar occurred on the south flank of Casita Volcano. This event took place during the period when rainfall from Hurricane Mitch reached its maximum of nearly 500 mm per day. The source zone for the avalanche was a strongly altered and brecciated andesite lava cliff near the summit. An initial slab of approximately 2.0 x 10⁵ m³ disintegrated and slid for about 30 m down a steep face, breaking into large blocks that continued to avalanche for another 3 km. A lahar that originated within the avalanche deposits rapidly spread across the terrain on the depositional apron surrounding the volcano. It eroded the soil and left a thin (40-60 cm) deposit of gravel, sand and silt in its wake. Huge boulders of dark lava averaging 2 m diameter, but reaching as much as 7 m in length, are conspicuous in the lahar deposits. Everything in the path of this debris flow was destroyed, including two medium-sized towns. According to official reports, the incident killed between 1560 and 1680 people, displaced hundreds more, destroyed several settlements, and the distal hyperconcentrated stream-flow disrupted the Pan American Highway at numerous bridges.

The source of the avalanche was a landslide from a cliff of highly fractured andesite lava that had previously failed several times. Zones of altered cristobalite, 1 to 2 cm thick, bound cracks in this rock; the matrix is altered to an undetermined smectite. Beneath this lava at the base of the cliff is a 3-m thick horizon of red clay (Fe-montmorillonite) that represents a hydrothermally altered scoria horizon. Below the altered scoria is a 2 to 3 m thick dark lava. Remnants of several previous avalanche deposits crop out above these units in the source region exhumed by the 1998 events.

The extent of the avalanche and lahar, mapped on a classified SPOT image of the volcano, has a surface area of about 1.2 x 10⁷ m². Assuming an average deposit thickness of about 0.5 m for the deposit, its volume would be 6.0 x 10⁶ m³. This yields a 30-fold bulking ratio for the lahar. Deep dissection of the red clay horizon and the underlying lava are evident in the upper reaches of the devastated zone, supporting this calculation. In fact, undercutting and headward erosion of the dark lava in the main flood channel is the probable source for the large boulders that caused such destruction in the two villages overwhelmed by the lahar.

To test its use in hazard assessment, the computer code FLOW3D traced the path of the avalanche, including overbank distributaries, and calculated its velocity history. The avalanche only traveled about 3 km and it alone would not have caused property destruction or death. LAHARZ developed by Iverson, et al.(1998) modeled the area covered by the lahar and computed the cross sectional area of its peak flow. A model volume of 1.0 x 10⁷ m³ and a starting point from the edge of the energy cone described by FLOW3D gives the best fit to the mapped deposits. This model has a planimetric area of 8.5 x 10⁶ m² which compares well with the actual mapped lahar. A cross section of the peak flow gives flow depths of 5 and 2 m, respectively at the devastated towns of Rolando Rodriguez and El Porvenir. This compares with field observations of flood damage to trees at 2 to 3 m height above the general land surface. Computer simulations such as these could be useful in designating lahar hazard zones for volcanoes such as Casita, which have not been studied in detail, but which have a geologic record of avalanches and debris flows.