How often does the Waioeka Gorge road shut and why?
SH2 through the Waioeka Gorge is vulnerable to large slips and rockfalls because of the high number of rainfall events and subsequent moisture levels, as well as the erodible geology through the area. High winds, as well as earthquakes, can also destabilise soil and rock above the road. Changing river levels can cause erosion to the road as well.
Our closure data shows 20 unplanned closures have occurred within the Gorge over the past 10 years, with an average closure period of 180 hours (7.5 days). This includes larger events such as in:
- March 2012: A series of large slips that occurred on 3rd March 2012 resulted in the displacement of approximately 80,000 cubic metres of material and SH2 through the Gorge was closed for 27 days. The costs of mitigation work were estimated at $11 million, with ongoing management of the road corridor lasting into 2013.
- July 2017: An approximately 8,000m3 rockfall event occurred on 7th July 2017 at the Son of Midway site on SH2, closing the road for seven days and restricting operations to one lane for a further 13 days. Smaller events followed over the next eight months, causing additional disruption.
- June 2018: A large slip occurred on 24th June 2018 between Oponae and Wairata resulting in the full closure of the Gorge route for six days. Following re-opening, access was restricted during the night due to safety concerns and full 24hr access was not achieved until 18th July.
How vulnerable is the road to future closures?
SH2 through the Waioeka Gorge is ranked with ‘Extreme risk’ under Waka Kotahi’s risk prioritisation assessment in the National Resilience Programme Business Case. This is the highest level of risk, and is given because of the high likelihood of rockfall, and the detrimental impact this has to connectivity for people and businesses in Tairāwhiti.
National Resilience Programme Business Case(external link)
What type of improvements is the business case investigating?
We are looking at a range of treatment packages from onsite monitoring, through to major realignment works. These packages contain hard engineering such as reprofiling slopes, creating rock chutes and installing barriers; as well as other options such as road widening, native planting programmes, and creating new storage areas for debris.
While improving the resilience of the current route is the preferred option, an alternative route is also being considered in this business case.
Investment objectives for this business case are:
- Reducing risk of long-term closures
- Reducing the frequency and duration of medium-term closures
- Improving safety by reducing the number of deaths and serious injuries through the Gorge
Through detailed investigation and multi-criteria-analysis, as well as consultation with mana whenua and stakeholders, options will be endorsed and eliminated so that a preferred package of works is put forward.
Why do slips occur?
Slopes adjacent to roads have been cut to enable construction and widening of the road. Over time, the materials that form the slope withstand heavy wind, rain and frost, and they weaken. This continues until the weight of the material above the weakened area exceeds the carrying capacity of the weathered rock/soil and a slip occurs. Further weakening occurs when water gets into the slope, due to rainfall or development above the slope. While the water may not affect the stability of the slope immediately, over time the reduction in strength and rise in pore water pressure results in failure.
What challenges does the road through the Waioeka Gorge face?
There are several methods engineers use to help prevent slips – from bolting, to meshing the slope, and constructing fences and bunds at the bottom of the slope to stop material affecting the road. Many of these however, are a challenge on SH2 in the Waioeka Gorge as the slopes are so close to the road that fences and bunds often cannot be used due to lack of space. Most measures also require maintenance over their life and are not simple ‘install and walk-away’ solutions.
The winter months are often the most challenging for the state highway, due to rainfall and icy conditions through the Waioeka Gorge. The amount of recent rainfall has resulted in extremely saturated soils which has led to a higher risk of slips and rockfall occurring.
What currently happens when there is a road closure?
The recommended alternative route is SH35 which will take approximately five hours between Opotiki and Gisborne. Motorists can also use SH2 between Gisborne and Napier and SH5 between Napier and Taupō.
The emergency response to a slip in the Waioeka Gorge is managed by Waka Kotahi’s Bay of Plenty office in collaboration with the Tairāwhiti office. Members of the emergency response team have daily (or sometimes twice daily) calls with the contractors on site, geotechnical engineers, Journey and System Managers, communications and media staff, and with staff who update the NZTA journey planner and the electronic signs on the side of the road.
How many vehicles on average use SH2 through the Waioeka Gorge?
Traffic volumes north of Gisborne are relatively low with less than 1100 vehicles per day (on average) passing through the Waioeka Gorge in 2020. Heavy vehicle volumes represent about 20% of all traffic movements through this route.
What can I do to assist the project?
We’d love to hear your experiences about previous closures through the Waioeka Gorge. You can:
- fill in the online survey(external link), which takes 5-10 minutes, or
- email connectingtairawhiti@nzta.govt.nz, or
Share your thoughts on what improvements you think would benefit the route.
