Lahars as major geological hazards

  • Neall V. E. 
Symposium 113 Geological Hazards And The Environment

Summary

Major loss of life caused by lahars (volcanic mudflows) in historical times has been largely restricted to the Circum-Pacific region and more particularly to Japan (>11,650 killed), Indonesia (>9,300 killed) and Central America (>1,300 killed). In addition to such losses of life, widespread damage may occur to buildings, bridges, communication networks and arable land. A review of the causal mechanisms of lahars, flow behaviour and protective measures, with selected case histories, is therefore appropriate to an understanding of this major geological hazard.

The potentially most destructive lahars are those involving sudden release of very large quantities of water from crater lakes or from subglacial lakes. The Icelandic jökulhlaups, although not strictly lahars, give some idea of the huge discharges of water that can be released — ephemeral maximum discharge rates have been estimated up to 100,000 m3/sec, or temporarily equivalent to the flow of the River Amazon. Other potentially destructive lahars are those resulting from pyroclastic flows becoming admixed with running or ponded waters.

Of more common but less devastating occurrence are lahars generated by heavy rainfall on the slopes of volcanoes, more particularly on recently ejected pyroclastics. Historical lahar disasters of this type occur most frequently in tropical regions.

Other initiating mechanisms include melting of snow and ice directly accompanying eruptions, earthquake triggered collapse, phreatic explosions and directed blasts. Historical lahars generated by these mechanisms have not been responsible for any considerable loss of life, with the exception of the Shimbara Catastrophe in Japan where a lahar entered the sea producing tsunamis.

Upon initiation of a lahar, mud, sand and gravel combine with available water to form a high bulk density (>1,400 kg/m3) flow. In some lahars the flow behaviour may approximate to a Newtonian liquid, whilst in others a high concentration Non-Newtonian liquid is formed with the capability of transporting very large clasts which may each weigh over 200 tonnes. The formation of a laminar boundary layer at the base of the flow is responsible for a low friction factor that enables some lahars to travel very large distances (>100 km). It also explains how lahar deposits often overlie completely undisturbed yet easliy erodible materials. This boundary layer can often be identified in many lahar deposits by a fine-grained layer at the base. The continuous phase of such lahars exhibits strength which retards the sinking of boulders and is responsible for the unsupported framework and poor sorting of lahar deposits.

Protective measures against loss of life and damage to property are discussed with particular reference to case histories in Indonesia and New Zealand. Indonesian measures have included siphoning water from the crater lake of Mt. Kelut, effective warning systems, and preparation of maps showing regions that may be destroyed by lahars. In New Zealand, two principal centres of Post-glacial lahar activity are Mt. Ruapehu and Mt. Egmont. Since 1861 A.D. eight lahar episodes have been generated from the crater-lake on Mt. Ruapehu, the 1953 lahar being responsible for the ”Tangiwai Disaster”, when 151 persons were killed. Existing and future protective measures against Mt. Ruapehu lahars are discussed. Mt. Egmont has a long record of Post-glacial lahar activity. The causal mechanism of some Egmont lahars has been heavy rains, but the existence of a former crater lake in the summit area cannot be discounted. Based on detailed geological, pedological and botanical investigations a geological hazards map of the Mt. Egmont region has been prepared.

Les Lahars; Importants Risques Géologiques

Résumé

C’est essentiellement autour de l’Océan Pacifique que les lahars (coulées de boue d’origine volcanique) ont causé de nombreuses pertes de vies humaines au cours des temps historiques: plus de 11 650 morts au Japon, plus de 9 300 en Indonésie et plus de 1 300 en Amérique centrale. Outre ces pertes de vies humaines, de graves dégâts peuvent affecter les constructions, les ponts, les voies de communication et la terre arable. Pour mieux connaître ce risque géologique important, il paraît judicieux de passer en revue les processus générateurs de lahars, le comportement des écoulements et les mesures de protection appropriées, en choisissant des exemples typiques.

Les lahars qui peuvent être les plus destructeurs sont ceux qui libèrent de très grandes quantités d’eau venant de lacs de cratère ou de lacs sous-glaciaires. Les jökulhlaups d’Islande (qui ne sont pas strictement des lahars) donnent une idée des énormes quantités d’eau qui peuvent être libérées: pendant un temps assez bref, le débit maximal a été évalué à plus de 100 000 m3 ce qui équivant au débit instantané de l’Amazone. Les lahars résultat du mélange de coulées pyroclastiques avec des eaux courantes ou stagnantes peuvent être aussi très destructeurs.

Plus communs mais moins dévastateurs sont les lahars dus à d’importantes pluies sur les pentes des volcans, plus particulièrement sur des dépots pyroclastiques récents. Pendant l’époque historique, les catastrophes de ce type affectèrent surtout les régions tropicales.

Parmi les autres processus générateurs, on peut citer la fusion de neige ou de glace due à des éruptions volcaniques, les effondrements déclenchés par des séismes, les explosions phréatiques, et les muées ardentes. Historiquements les lahars engendrés par ces processus n’ont pas été responsables de grosses pertes de vies humaines, sauf en ce qui concerne la catastrophe de Shimbara au Japon où un lahar en pénétrant dans la mer produisit des tsunamis.

Au début d’un lahar, boue, sable et graviers se mélangent à l’eau disponible pour former une coulée de grande densité moyenne (plus de 1,4). Dans certains lahars, l’écoulement est approximativement celui d’un liquide newtonien, alors que dans d’autres lahars, la formation d’une grande concentration de liquide non-newtonien peut permettre le déplacement de très gros blocs pouvant dépasser les 200 t. La formation d’une couche limite à écoulement laminaire à la base de la coulée diminue le facteur de frottement ce qui permet à certains lahars de parcourir de très grandes distances (plus de 100 km). Ceci explique aussi comment des dépôts de lahars peuvent recouvrir des formations presque intactes alors qu’elles auraient pu être facilement érodées. Cette couche limite peut souvent être identifiée dans de nombreux dépôts de lahars: elle est représentée par un horizon de base à grain fin. La phase continue de ce genre de lahars met en jeu une force qui retarde le dépôt des gros blocs; aussi les sédiments de lahars sont-ils peu structurés et peu granoclassés.

Les mesures à prendre pour préserver les vies humaines et les biens sont discutées à la lumière des exemples d’Indonésie et de Nouvelle-Zélande. En Indonésie on a siphoné l’eau du lac de cratre du M.t Kelut, installé des systèmes d’avertissement efficaces et préparé des cartes montrant les régions susceptibles d’être détruites par les lahars. En Nouvelle-Zélande, deux des principaux centres d’activité de lahars post-glaciaires sont le M.t Ruapehu et le M.t Egmont. Depuis 1861, huit manifestations lahariennes furent engendrées par le lac de cratère du M.t Ruapehu; le lahar de 1953 est cause de la «catastrophe de Tangiwai» où 151 personnes trouvèrent la mort. On discute les mesures de protection existantes ou à crèer au M.t Ruapehu. L’activité laharienne post-glaciaire du M.t Egmont est connue depuis longtemps. Certains de ses lahars sont dus à des pluies importantes mais on ne peut pas écarter l’hypothèse de l’existence d’un ancien lac de cratère. En utilisant les observations géologiques, pédologiques et botaniques, on a pu préparer une carte des risques géologiques de la région du M.t Egmont.

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Copyright information

© International Association of Engineering Geology 1976

Authors and Affiliations

  • Neall V. E. 
    • 1
  1. 1.Departement of Soil ScienceMassey UniversityPalmerston NorthNew Zealand

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