Paterson, B.R. 1976 The effects of lahars from the 1975 April Mt Ruapehu eruption and the threat of future eruptions on the Tongariro Power Development. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 230 15 p.
Abstract: The eruption of Mt Ruapehu on 24 April 1975 have varied and widespread effects, causing damage to property, endangering lives and contaminating several rivers. Tongariro Power Development (TPD) which utilises water from several of these rivers was adversely affected by lahars from this eruption. Investigations were carried out to determine the extent of the damage and the likely effects from similar future eruptions. For this report only eruptions which produce lahars have been considered, as eruptions of ash and lava present a less serious threat to TPD. Lahars are caused by volcanic eruptions and sudden releases of lake water resulting from partial collapse of the barrier impounding the lake. As a result the Whangaehu River which is a natural outlet of Crater Lake, is the most vulnerable of all rivers draining Mt Ruapheu. Even small eruptions send surges down the Whangaehu River. Since late 1972 Western Diversion, the first phase of TPD to be completed, has been diverting water from the Whakapapa River and other streams into Lake Rotoaira. As Lahars had previously travelled down the Whakapapa River from Ruapehu eruptions as recently as 1969, it was necessary to assess the risk of contamination from similar future eruptions to safeguard the important fishery, Lake Rotoaira. In late 1972, recommendations were made to install devices that would prevent inadvertent diversion of volcanic contaminants into Western Diversion reservoirs and Lake Rotoaria. The 1975 eruption provided an opportunity to test the effectiveness of the protection measures taken and to assess the risk of damage from future eruptions to those phases of TPD still under construction. Investigations of the effects of the 1975 April eruption were carried out by several departments and much of the data collected has been made available for this report. Detailed geological data were collected by members of the NZ Geological Survey, Rotorua (Nairn et al 1975), hydrological data were provided by C E Page, Ministry of Works and Development, Wanganui (Appendix 1) and the effects of the eruption on aquatic life have been documented by E.J. Cudby, Ministry of Agriculture and Fisheries, Turangi (Appendix 2). (auth)
