A catastrophic lahar began on 30 October 1998, as hurricane precipitation triggered a small ﬂank collapse of Casita volcano, a complex and probably dormant stratovolcano. The initial rockslide‐debris avalanche evolved on the ﬂank to yield a watery debris ﬂood with a sediment concentration less than 60 per cent by volume at the base of the volcano. Within 2·5 km, however, the watery ﬂow entrained (bulked) enough sediment to transform entirely to a debris ﬂow. The debris ﬂow, 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 ﬂows of similar size since 8330 14C years bp are documented by stratigraphy in the same 30‐degree sector. Travel time between perception of the ﬂow and destruction of the towns was only 2·5–3·0 minutes. The evolution of the ﬂow 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 ﬂow 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 ﬂow. 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 ﬂank collapse and 4·2 times that of the debris ﬂood. At least 78 per cent of the debris ﬂow matrix (sediment < −1·0Φ; 2 mm) was entrained during ﬂow. Copyright © 2004 John Wiley & Sons, Ltd.
|Title||Catastrophic precipitation-triggered lahar at Casita volcano, Nicaragua: Occurrence, bulking and transformation|
|Authors||K. M. Scott, J. W. Vallance, N. Kerle, J.L. Macias, W. Strauch, G. Devoli|
|Publication Subtype||Journal Article|
|Series Title||Earth Surface Processes and Landforms|
|Record Source||USGS Publications Warehouse|
|USGS Organization||Volcano Hazards Program|