Lahar history and hazard of the Tongariro River, northeastern Tongariro Volcanic Centre, New Zealand

Abstract

Lahar deposits beside the Tongariro River have been mapped and dated using andesitic and rhyolitic marker tephras. Coupling the stratigraphic record obtained with that of the Tongariro and Ruapehu ring plains has enabled reconstruction of the history of lahars along the Tongariro River. This forms the basis of a lahar hazard map for the entire catchment. Six lahar hazard zones, with assigned recurrence intervals ranging from 1 in 35 yr to 1 in >15 000 yr, have been mapped. Lahar deposits between the ages of 14.7 and 9.8 ka cover the greatest areas, while younger lahar deposits are confined to lower surfaces closer to the present river channel. Holocene lahar deposits along the Tongariro River are not as well preserved as older units, probably because the Holocene lahars were confined to a more deeply incised channel and were more readily eroded following emplacement. All recorded post‐11.85 ka lahars in the Tongariro catchment were derived from Ruapehu volcano. The Mangatoetoenui Stream has been the conduit for the greatest number of Holocene lahars and for all of the historic ones. Most of Turangi is built on a surface which has not been inundated by a lahar since c. 10 000 yr ago. However, the area identified where high risk affects the greatest population is a part of Turangi, which was inundated by a lahar less than 1850 yr ago. The infrastructures at greatest risk include the State Highway 1 bridge across the Mangatoetoenui Stream and the Rangipo Dam and power station.