Harry J. R. Keys and Paul M. Green

Abstract:
A moderately large lahar caused by a collapse of a tephra dam over the outlet of Crater Lake occurred 11 years and 4 months after the situation that led to it was first recognized. This allowed sufficient time for the considerable period needed by emergency managers and the Government to learn from scientists about the hazard, assess risks in pragmatic and robust ways, debate and progressively mitigate them. Early involvement of a wide range of agencies and the media was encouraged, enabling transfer of knowledge, development of interagency relationships and provided fora for a mitigation strategy to evolve in the face of sometimes controversial mitigation options. Decisions about options were based primarily on maximizing public safety and long-term utility in a valley subject to frequent, sometimes large, past and future lahars. These decisions, which had a significant political component also considered option suitability in a national park setting. A best-practice alarm system was installed based on acoustic flow monitors. A training levee structure was built to protect a highway and other vulnerable infrastructure and a river system near-by, and a key highway bridge raised and strengthened. Response agencies each prepared their own emergency plans which were then integrated and practiced over 3-4 years, incorporating an increasing readiness triggered by a Warning Level system linking to rising lake levels. Execution of the plans progressed smoothly on 18 March. The outcome showed that community resilience to lahars was achieved by adopting long-term solutions, rather than short-term ones, and carefully considering cultural, social, and technological perspectives, especially in a national park setting. Decision-making procedures that give weight to impartial scientific information, as well as values and legislation, are required for effective disaster mitigation, including an acceptance of an appropriate degree of residual risk.

Keywords: Ruapehu, Crater Lake, lahar, mitigation, warning systems