By Max Frethey, Local Democracy Reporter
Almost 1100 properties have shifted out of the the slope management overlay. Photo: Max Frethey
Almost 1100 properties have been removed from Nelson City Council's slope hazard mapping after an error was discovered.
Some residents and developers previously challenged the accuracy of the maps, but experts representing the council had defended them as "robust".
A slope hazard map identifies areas which are potentially susceptible to slope instability and may be used in building and resource consent processes.
The error relates to the "smoothing out" of the slope angle data, rather than the underlying dataset itself which was derived from LiDAR (laser measuring) readings from 2008-2015.
The mistake was admitted in a joint witness statement dated 6 November between experts representing the council and a group of developers challenging the council's hazard maps.
The hazard maps form part of the council's major overhaul of its planning rules, known as Plan Change 29.
Some homeowners in the city have reported challenges in insuring their homes following the widespread flooding and landslides caused by the August 2022 storm Photo: Sara Hollyman/Nelson Weekly
The plan change aims to enable denser housing across the city while managing the risks posed by natural hazards, but the proposal has proven contentious among residents.
The council's subsequent 27 November geotechnical reply to the independent commissioners overseeing the plan change suggested they adopt the corrected mapping which would see 1098 properties removed from the slope management overlay entirely.
No new properties have been added into the overlays because the council doesn't have the scope to include previously unaffected properties.
The number of properties in the debris run-out overlays has increased by 72 because of the mapping changes, and an additional 42 are affected by both the debris and slope overlays. These properties had already been included in the council's slope hazard mapping.
The area of the Tāhunanui Slump - a slow moving, active slip - is unchanged.
Two large-scale subdivisions in Bishopdale and the Bayview area are among the properties that have been excluded from the new maps because of large-scale geotechnical engineering which occurred during their development.
Council's group manager environmental management, Mandy Bishop, said that the work developing and refining the maps and overlays is carried out by technical experts and is peer-reviewed.
"[The revised maps are] now being considered as part of the PC29 process and council has full confidence in the technical analysis provided by experts."
A group of developers had challenged the council's hazard mapping through the plan change's September hearing.
The group's counsel, resource management lawyer Nigel McFadden, had said at the time that the report underpinning the susceptibility overlays was fundamentally "flawed".
During the hearing, he urged commissioners to throw the maps out and for the council to "get it done properly".
But senior geologist Dan Chamberose, representing the council, had replied that the underlying report was "robust" and had gone through a "significant" review process.
The maps have been developed as part of the city's controversial overhaul of its planning rules Photo: Supplied/Nelson City Council
Experts representing the developer group have since been conferencing with the council's experts to try and address their concerns in the lead up to the commissioners' deliberations.
They believe that additional developed areas which have undergone geotechnical engineering, beyond the current two, should also be removed from the maps.
Their own analysis of the geomorphic methodology used in the slope hazard mapping also identified what they believed to be a wide range of "errors and inconsistencies".
But in both cases, the council's experts disagree and said that the maps are now "sufficiently accurate" for their intended purpose.
Should the maps be adopted through the plan change process, it would take another plan change to alter them.
The approach of including the hazard maps in the plan aimed to provide certainty for plan users and property owners.
During the September hearing, some residents also claimed their properties shouldn't be included in the overlays because they have recent geotechnical reports that showed their land was stable.
At a 1:5000 scale, analysing slope hazards on a property-by-property basis hasn't been pursued because of the cost and practicability of examining thousands of individual sites.
The draft hazard maps notified through the plan change process have been alleged to have been responsible for "chilling" the city's housing market by making insurance more difficult to get.
The Insurance Council rejects that assertion and has said that council maps are only one of many factors used when to determine a property's risk profile.
Almost 1100 properties have shifted out of the the slope management overlay Photo: Max Frethey
Hazard information from 2021 mapping, with minor adjustments following the August 2022 storm, which has been used to inform the plan change maps is currently included on properties' Land Information Memorandum (LIM) Reports.
The proposed hazard maps themselves aren't included in the LIMs.
"Council is in the process of ensuring we have consistent and up-to-date mapping and information for LIM purposes," Bishop said.
"This is a continual process, subject to ongoing change and refinement, and maps within [Plan Change 29] are part of that process."
The Natural Hazard Commission also directs website visitors seeking landslide hazards to Nelson council's original draft maps, which are still being finalised and have been amended through the plan change process.
LDR is local body journalism co-funded by RNZ and NZ On Air.