Otway, P.M.; Hodgson, K.A.; Nairn, I.A. 1995 Whakapapa skifields lahar study . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 95/39 ix, 90 p.
Abstract: Ruapehu presents a unique problem in understanding the processes of lahars formed by the ejection of Crater Lake water onto ice and snow. The present Crater Lake vent has been active for the last 2000 years with the lake apparently in existence for much of this time. All significant explosive eruptions will send lahars down the Whangaehu River, into which the lake drains. Only relatively large eruptions can eject lake water over the crater rim into other catchments, including the Whakapapa valleys to the north, in which the Whakapapa skifields are sited. The record of prehistoric and historic eruptions during the last 2000 years indicates that eruptions large enough to generate damaging lahars in the Whakapapa catchments occur at an average interval of about 30 to 50 years. The lahars produced by future eruptions of Crater Lake will be controlled by multiple variables which can counteract or reinforce each other in unpredictable ways. These include; the eruption-related parameters of explosion magnitude, depth of focus; volumes, temperatures, and partitioning of the ejected lake water and solid material; the transport mechanisms of the dispersed ejecta; external variables of wind direction and speed; and the nature and distribution of ice and snow cover. The paths and effects of the 1969 and 1975 lahars have been defined in the Whakapapa skifields. Scenario eruptions are considered of (i) the 1975 event repeated with southerly winds under both summer and winter snow conditions, and (ii) a larger eruption ejecting 50 percent of the lake volume in both summer and winter snow conditions. In all cases damage greater than the 1975 eruption results, and lahars large enough to put at risk both skifield installations and people in some of the most popular ski run areas are generated. Some relatively high risk areas are identified, and possible mitigating actions suggested such as lahar diversion protective engineering works, and the modification or initiation of snow-grooming practices. The lahar risk at Whakapapa is neither so small that it can be ignored nor so large that it necessitates abandonment of the skifields. Acceptance of the risk requires a high level of public awareness of the lahar threat, and or the actions to be taken if a lahar-generating eruption occurs. The lahar early warning system must be extended to cover all areas of the Whakapapa skifields, and be kept operating at all times. This will require upgrading of the existing system technology. (auth/ARC)
