Geologists and Civil Defence staff are taking the threat of a landslide in the Hipaua thermal area above the settlement of Waihi on the shore of Lake Taupo seriously. The unstable slopes have claimed more than 200 lives since 1780.
Thursday 2nd July 2009
Taupo District Council ordered the evacuation of the Waihi settlement on the southern shore of Lake Taupo on Monday afternoon and closed a section of State Highway 41 between Tokaanu and Pukawa. The council acted after assessing known risk factors for the Hipaua thermal area which lies upslope from the village. Since 1780, there have been three large landslips in the area, causing significant loss of life. Known risk factors for another slip include earthquakes, heavy rain activity, and changes to the geology.
A swarm of several hundred earthquakes has been active near Turangi since the end of May, with a notable burst of activity scattered about the southern shore of Lake Taupo during the latter part of June. The strongest earthquakes in the swarm, very shallow magnitude 4.3 and 4.4 events which struck last Saturday morning, were the largest quakes to strike the area in more than 10 years.
Heavy rainfall has been experienced in recent days, and changes to the landscape were reported by residents of Waihi village, including the waterfall in Waihi village moving significantly, streams drying up and others beginning to flow where streams did not exist before. The council also noted, “Waihi residents are reporting anecdotally that birdlife is silent and black swans have moved from their usual area.”
Since the evacuation and road closure, council staff and scientists from GNS Science have carried out aerial and ground reconnaissance of the area, looking for signs of slippage, cracks appearing and checking the location of monitoring pegs that were installed to record land movement.
The two most recent events which occurred in 1846 and 1910 have been studied to determine the sequence of events that triggered the huge landslides.
GeoNet News, Issue 2, February 2003 reported on the 1846 event: “Some years before 1846, a landslide occurred on the side of Kakaramea, the mountain above Waihi settlement at the southern end of Lake Taupo. The slide dammed a stream, forming a lake about 600 metres above the Maori village of Te Rapa.
On 7 May 1846, the landslide dam collapsed during heavy rain. The water and debris created a mudflow that overwhelmed the village, killing the Maori chief Te Heuheu and 63 members of his tribe.”
In March 2005, the Hazardwatch website reported on the 1910 event: “Around 9 a.m. on 20 March 1910, people in the village of Waihi, on the southern shore of Lake Taupo, heard a noise like cannon fire. As they watched, a section of steaming hillside collapsed into the Waimatai Valley above the village and sped downslope. Most people escaped to higher ground, but one man was overcome by the landslide. The landslide entered the lake, creating a 3-metre high wave that crossed the bay to the far shore, nearly drowning some children. The landslide may have been caused by a hydrothermal eruption.”
Of the two, the 1910 slip was the largest. A diagram of the landslide areas superimposed on an image of the Hipaua thermal area can be found on page 7 of GeoNet News Issue 2.
Initial results from the investigations by the scientific teams were reported on the Taupo District Council website this morning. No significant signs of cracking or fallen rocks had been sighted and birdlife has returned to the area. Eleven metal pins were installed to monitor land movement yesterday, and more will be installed today.
The teams have returned to the area this morning and it is expected that a full report from GNS Science will be available at 4 o’clock this afternoon. The evacuation order expires on Friday and a decision to relax restrictions will be based on earthquake activity, expected rainfall and any signs of instability detected by the scientific teams.
In the meantime, State Highway 41 remains closed between Tokaanu and Pukawa, Waihi village is unoccupied, and boats have been excluded from Lake Taupo in an area within 600 metres of Waihi village.
[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST., and the Taupo District Council.]
The State of Emergency in the Turangi-Tongariro area was lifted at 7 p.m. on Thursday July 2nd. Residents were allowed to return to Waihi village and S.H. 41 was re-opened. Scientists found that, while there was still a significant risk of slippage in the Hipaua thermal area, there was no evidence of recent damage. (from Taupo District Council media release.)