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Catastrophic precipitation‐triggered lahar at Casita volcano, Nicaragua: occurrence, bulking and transformation
Author(s) -
Scott Kevin M.,
Vallance James W.,
Kerle Norman,
Luis Macías Jose,
Strauch Wilfried,
Devoli Graziella
Publication year - 2005
Publication title -
earth surface processes and landforms
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.294
H-Index - 127
eISSN - 1096-9837
pISSN - 0197-9337
DOI - 10.1002/esp.1127
Subject(s) - lahar , debris , geology , volcano , mudflow , stratovolcano , geomorphology , phreatic , precipitation , hydrology (agriculture) , rockslide , sediment , landslide , geochemistry , pyroclastic rock , groundwater , geotechnical engineering , geography , oceanography , meteorology , aquifer
A catastrophic lahar began on 30 October 1998, as hurricane precipitation triggered a small ank collapse of Casita volcano, a complex and probably dormant stratovolcano. The initial rockslide‐debris avalanche evolved on the ank to yield a watery debris ood with a sediment concentration less than 60 per cent by volume at the base of the volcano. Within 2·5 km, however, the watery ow entrained (bulked) enough sediment to transform entirely to a debris ow. The debris ow, 6 km downstream and 1·2 km wide and 3 to 6 m deep, killed 2500 people, nearly the entire populations of the communities of El Porvenir and Rolando Rodriguez. These ‘new towns’ were developed in a prehistoric lahar pathway: at least three ows of similar size since 8330 14 C years bp are documented by stratigraphy in the same 30‐degree sector. Travel time between perception of the ow and destruction of the towns was only 2·5–3·0 minutes. The evolution of the ow wave occurred with hydraulic continuity and without pause or any extraordinary addition of water. The precipitation trigger of the Casita lahar emphasizes the need, in volcano hazard assessments, for including the potential for non‐eruption‐related collapse lahars with the more predictable potential of their syneruption analogues. The ow behaviour emphasizes that volcano collapses can yield not only volcanic debris avalanches with restricted runouts, but also mobile lahars that enlarge by bulking as they ow. Volumes and hence inundation areas of collapse‐runout lahars can increase greatly beyond their sources: the volume of the Casita lahar bulked to at least 2·6 times the contributing volume of the ank collapse and 4·2 times that of the debris ood. At least 78 per cent of the debris ow matrix (sediment < −1·0Φ; 2 mm) was entrained during ow. Copyright © 2004 John Wiley & Sons, Ltd.