Frustration continues as Tiniroto Road closure drags on despite expert input

An independent investigation by an Australian geologist and soil scientist challenges the decision to close Tiniroto Road at the Hangaroa Bluffs.

Gisborne District Council closed the road at that point after Cyclone Gabrielle when geo-tech engineers found cracks in the Bluffs’ face.

A report by consultants LDE (Land Development & Engineering) deemed the land stability situation too dangerous for traffic to use that piece of road.

Australia-based Dr Richard Doyle — a soil scientist, geologist and geomorphologist with 38 years experience in earth sciences — offered to do an independent review of the Bluffs’ risk situation on behalf of concerned local residents.

Dr Doyle was holidaying in the district before Christmas when he took an interest in the issue and visited the site.

“I challenge the conclusion statement in the LDE report that “Given the unpredictable nature of the hazards identified, a full road closure is recommended” and “it is considered probable that large blocks and large volumes of material could reach the road with high velocity, even without significant rainfalls. Due to the potential consequences of the hazard being severe, all areas are considered either high or extreme risk, relative to the likelihood of occurrence”.

Dr Doyle said the LDE report provided the perception that the dominant road penetrating process was rockfall on all slopes of the Bluffs.

“But it appears from the site and imagery post-Gabrielle that ‘sliding’, and ‘flow’ are the dominant processes, rather than ‘falls’.

“Flows and slides are most commonly associated with wet soils and significant rainstorms/cyclones and so are far more predictable, and so are the risks of rockfalls overstated, particularly at Bluff 1 and 2?

“Is mitigation easier if the major risk is in known and predictable weather conditions?”

“The Bluffs are dominated by soft mudstones which fret and are well known to manifest as earth flows if concretion layers or firmer sandstone layers fail.

“Many earth flow features of this nature are evident in Google Earth imagery taken on 8/3/2023 a few days after Cyclone Gabrielle devastated the region with torrential rains and severe flooding.

“There are hundreds of thousands of earth and mud flows obvious in the region, highlighting it as the dominant landscape failure process associated with severe rainstorms.

“This fact very strongly highlights the link between extreme rains, excessive water flows and soil saturation as a major driver in erosion and associated risks below the Bluffs.

“Thus, significant and high-risk failures are far more likely related to severe rainstorms like Bola, Hale and Gabrielle, and far less to day-to-day meteorological conditions.”

Dr Doyle queried if that had led to an “overly pessimistic risk assessment” with a focus on rock falls in all weather conditions as compared to flows and slides in wetter conditions.

“Especially when other risk mitigation strategies could be employed such as hazard warning lights set on the road approximately 500 metre before the Bluffs which activate during and after major rainstorms.

“Indeed, the traffic flow is quite low on this road, so could the road be closed if cyclones are forecast, especially if after extended periods of preceding wet weather.

“Such simple strategies would greatly reduce the risk level to road users. Have these considerations been used in the risk assessments?”

He also asked was the use of catch-ditches and sediment bunds (gravel/soil) or low concrete barriers not been more fully explored as backup catch-mechanisms for rogue boulders or concretions sliding/rolling down the slopes.

“I note the fine gravel talus fans which form due to the pronounced fretting of the Tolaga mudstones at the Bluffs would also naturally catch and slow sliding materials, for example, like sand traps catch out of control racing cars/bikes at race tracks.

“Direct falls of large blocks on to the road are most unlikely, indeed highly improbable, due to the concave slope profiles and softer sediments at their base.”

“Flows and slides are exponentially more likely and demonstrated in the Google Earth imagery of the site done in March last year.

Dr Doyle said the three Bluffs were exposed to significant sluicing events in cyclones Gabrielle and Hale.

“Surely a significant amount of the susceptible materials have been removed from the slopes and the actual risk of flows and slides has declined.”

He asked if at Bluff 2, river works allowing a widening of the road payments been considered to mitigate risks?

“Significant volumes of material occur as two islands in the Hangaroa River at this point and these materials could be in part cleared to allow straightening of the river and hold the channel toward the right banks and thus prevent undercutting at the base of Bluff 2.”

“The islands materials could also be used to armour and build out the left bank, reducing undercutting in addition to allowing road widening.

“Contractors could work in the drier parts of the year to remove any obvious loose boulder on the slopes to reduce the exposure risks during remediation, ensuring a stop-work period occurs if significant rains occur causing soil saturation.”

*  Hangaroa resident Alex Campbell said community confidence in Gisborne District Council has been wearing thin for Hangaroa and Tiniroto residents over on-going uncertainties around the Hangaroa Bluffs.

Concrete blocks were put in place to stop traffic but some people took it upon themselves to move those blocks to one side to allow access.

Mr Campbell said a group of around 30 residents came away from a meeting last month to discuss the ongoing issue of the Bluffs with Dave Hadfield from council staff and councillor Colin Alder, very frustrated with many questions unanswered.

The overarching issue for the community meeting was progress on reinstatement of the council’s roading infrastructure,” Mr Campbell said.

“We were told an investigation process into the feasibility of a new road across the river bypassing the Hangaroa bluffs (including two new bridges) was expected to take around 18 months.

“Also, an investigation into temporarily reopening the Hangaroa Bluffs, was expected to take 8 to 10 weeks,” he said.

“When queried about opening the bluffs highway permanently the council’s response was this is not planned due to directives coming from the Government.

“Mr Hadfield informed us that a lack of approved funding is a major factor limiting current progress.

“The community’s confidence in the council is wearing very thin for what is being achieved,” Mr Campbell said.

“Day after day, week after week, month after month vital repairs to the main access roads of Tiniroto show virtually no progress.

“The community are becoming increasingly anxious as to the fragility of their roading network and its ability to even withstand small rain events while in its current state.

“We are approaching winter and much of our roading network is hanging on by not much more than a thread, and that’s a sentiment that’s widespread throughout the region.”

A copy of Dr Doyle’s report was presented to the council at the Tiniroto public meeting.

* Dr Richard Doyle BSc(Hons), MSc(Geol), PhD (Soil Sci.), CPSS (Certified Prof Soil Scientist) geologist and soil scientist.

Dr Doyle is a highly qualified geologist, geomorphologist and soil scientist with over 38 years work experience in earth sciences.

He has a BSc (Hons) in geology with a double major in physical geography (Victoria University of Wellington, NZ), an MSc in geology awarded with distinction specialising in geomorphology, erosion and soil development (Victoria University of Wellington, NZ) and a PhD in soil science (UTAS).

He is a Certified Professional Soil Scientist (CPSS) of the Australian Society of Soil Science of which he is the former state and national president.

He is a Programme Leader with the Soil CRC, an Australian Government supported national cooperative soil research centre.

Gisborne District Council was approached for comment over the Bluffs road issue.

Journeys Infrastructure Manager Dave Hadfield said the council provided a Cyclone Gabrielle update to the Tiniroto community on January 18.

“Staff discussed three bridge repair projects, Tiniroto Road alignment and the status of the Hangaroa Bluffs.

“At the meeting it was announced that a review of the original geotechnical report to close the Bluffs was being commissioned and that this would be carried out by our geotechnical advisors LDE, WSP and staff,” Mr Hadfield said.

“The goal of this is to provide a plan to safely open this section of the road and determine not only the reopening costs, but also the ongoing maintenance costs.

“Until this is known the road remains closed.

“The community was advised this would take eight to 10 weeks, so until this is received from our technical consultants, the council is not able to make any comment on the Dr Doyle report.”