Drivers using State Highway 41 near Tokaanu are at risk from falling rocks and slips from the Hipaua Cliffs and Omoho Valley, this week’s Regional Land Transport Committee heard.
Environment Waikato Hazards Analyst Lamorna Cooper said landslides had blocked the road twice in the last six months, forcing road users living west of Little Waihi village to find alternate routes to work in Turangi, Taupo and Tokaanu. There was no warning of the slips which closed the road.
The area was thermally altered with unstable ground and had a history of fatal slips. More than 60 people were killed in a debris fall in 1846, with more deaths in 1910 after a large slip. In 2001 a 2000 square metre slip closed the highway for three days and a larger slip in July this year closed it for four days.
More than 1700 vehicles a day used the route and it was likely to fail again, she said. The area was now altered clay with large rocks which could block culverts and take the road out, or create a landslide dam.
“The increased risk is quite considerable to people using that road and it is quite a nasty little site. There is no warning for anyone using the road and a failure is likely after rain storm events.”
She said where to put the soil was also an issue as the fill had no engineering strength. Previous dumps had been moved, as they created flooding.
Environment Waikato was liasing with Taupo District Council and intended to consult with iwi as well. It would like to encourage Transit NZ to develop a contingency plan to warn road users, develop alternate routes and identify a suitable dump site for the debris.
A geotechnical evaluation was needed. Information packs also needed to be developed for the communities in the area, schools and tourists using the road.
“People who live locally need to know the dangers of that road.”
Present measures included three yearly aerial infrared surveys of the Hipaua cliffs and geothermal hot spots to determine movement, collation of information from a flyover, historic and research documents to determine the risk and working with Taupo District Council Emergency Management Officer in monitoring individual slope failures.
Taupo District Council representative Andrew Welch said he was concerned at the potential isolation of lakeshore settlements and the 30 or 40 commuters who used the road daily. An alternate route was about 140kms to Taupo and the effect of a collapse on North Island transport would be disastrous.