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.

The 55- to 60-ka Te Whaiau Formation: a catastrophic, avalanche-induced, cohesive debris-flow deposit from Proto-Tongariro Volcano, New Zealand

“…Ring-plain studies conducted in New Zealand show that both lahar and debris-avalanche deposits form the bulk of the aprons surrounding the composite edifice of Ruapehu and the simple cone of Taranaki/Egmont (Donoghue 1991;Hodgson 1993;Cronin et al 1996Cronin et al , 1997aCronin and Neall 1997). Repeated gravitational collapse is the most common mechanism that leads to the re-establishment of the equilibrium profile of composite volcanoes (Davidson and De Silva 2000).…”

1995 Ruapehu lahars in relation to the late Holocene lahars of Whangaehu River, New Zealand

“…Because of their great magnitude, these flows pose serious risk to all bridging points of the Whangaehu River in addition to all structures, livestock, and people in low-lying areas adjacent to the channel. Lahars of this magnitude also have the potential to flood into the Upper Waikato Stream catchment and into the Tongariro River, where a greater population is potentially at risk (Cronin et al 1997b). Over the last c. 1900 years, Hodgson (1993) records seven lahars with volumes >10 7 m 3 (5-25 times that of the 1975 or 1953 lahars).…”

Depositional record of historic lahars in the upper Whangaehu Valley, Mt. Ruapehu, New Zealand: implications for trigger mechanisms, flow dynamics and lahar hazards

“…Terraces of older lahars occur in wider portions of the gorge, with narrow segments of the channel containing only the youngest deposits, and in places exposed bedrock. The unconsolidated deposits are easily remobilised and buried by frequent lahar activity and can vary dramatically during eruptive episodes where prolonged activity through the lake can result in lahars every few days (Cronin et al 1997c).…”

“…However, deposits of only four historic lahars are identified and named as the youngest members of the Onetapu Formation (1861, 1953, 1975Palmer 1991;Cronin et al 1997a;Hodgson et al 2007). The most recent historic lahars have been thoroughly studied through direct observation, or analysis of their deposits immediately after emplacement (Cronin et al 1996(Cronin et al , 1997b(Cronin et al , 1997c(Cronin et al , 1999(Cronin et al , 2000Manville et al 1998Hodgson and Manville 1999), none more so than the March 2007 Crater Lake break-out Marutani et al 2007;Carrivick et al 2008). There is, however a knowledge gap of historic lahars in proximal areas, i.e.…”