The advice in 2005 has been superseded in part by new science that has led to the adoption of smaller runout zone sizes in the Mount Maunganui and P\u0101p\u0101moa neighbourhoods, but the thrust was obvious \u2013 to steer clear.<\/p>\n
\u201cOnly in rare circumstances would it be prudent to violate\u201d the zone criteria, wrote two geotechnical engineers in the study into 300-plus landslides across Tauranga triggered by a big storm.<\/p>\n
\u201cBecause of this we believe that the following changes to the criteria would be appropriate: 1. Buildings are not to be located within the [zone]. Only if special measures are taken, such as construction of properly engineered structures (ie retaining walls, piled foundations and deflection bunds) can these criteria be exceeded.\u201d<\/p>\n
The old runout zones in Tauranga were calculated as \u201c4H:1V\u201d \u2013 four times the vertical height of the slope (V), extending horizontally from the base (H). The new zone in 2025 was suggested as half that \u2013 2H:1V.<\/p>\n
The study on this for the city council last year by engineering consultants WSP covered all the Pacific seaside neighbourhoods except Mauao itself. RNZ has asked the council why Mauao was not mapped, and also for comment on runout zones.<\/p>\n
\u2018Directed away from areas with an unacceptable risk\u2019<\/p>\n
Auckland Council has been sharing its country-leading work on landslides with Tauranga and the Bay of Plenty region. Tauranga was \u201cunique\u201d in how prone it was to rain-induced slips, WSP said in 2025.<\/p>\n
Mayor Mah\u00e9 Drysdale told Morning Report they were setting up an independent review into the \u201cfacts and events leading up to the landslide\u201d.<\/p>\n
Tauranga Mayor Mah\u00e9 Drysdale. Photo \/ RNZ, Calvin Samuel<\/p>\n
Auckland Council said because of that review it was inappropriate for it to answer queries about landslide risk on Mount Maunganui.<\/p>\n
But it laid out its own advances on landslide mapping that it said meant the information people could get about landslides on Land Information Memorandums, or LIMs, was more reliable, and its controls over what got built and where were better.<\/p>\n
\u201cBefore our 2025 mapping, we didn\u2019t have good regional maps showing landslide susceptibility because we didn\u2019t have the data available to create them,\u201d chief engineer Ross Roberts said in a statement.<\/p>\n
\u201cThis meant that it was more difficult to identify where activities were taking place in areas potentially susceptible to landslides.\u201d<\/p>\n
It was also difficult and complex to apply controls to existing activities.<\/p>\n
Since last year, it could statistically model the landslide susceptibility, which was feeding into plans and LIMs.<\/p>\n
\u201cThis means it can be used by people when choosing where to buy or rent a house, and by infrastructure and asset owners … Together, these mean that future development can be directed away from areas with an unacceptable risk, and existing assets can be assessed and their use modified if appropriate,\u201d Roberts said.<\/p>\n
Checks on individual sites were still sometimes needed, such as when protective structures had been built, earthworks had changed the land profile, or underlying characteristics were unusual.<\/p>\n
Several people are unaccounted for following a slip near a campsite in Mount Maunganui. Photo \/ Dean Purcell<\/p>\n
\u2018Avoid building in run-out areas\u2019<\/p>\n
Martin Brook, professor of applied geology at the University of Auckland, said understanding landslide run-out on Mauao was important because the mountain had a history of slips.<\/p>\n
\u201cUnderstanding and modelling the run-out of landslides and where the detached material is going to run out and end up and possibly inundate is really important.<\/p>\n
\u201cYou should avoid building in runout areas. That\u2019s sensible, and people would follow that advice in most parts of the Western world.\u201d<\/p>\n
But the light-detection (Lidar) tech used to detect historical landslides was not so useful for runout and he was not aware of much research on it.<\/p>\n
\u201cMost of New Zealand is under-researched from a sort of a geological standpoint.\u201d<\/p>\n
The country had woken up recently to tsunami risk and signs and advice had sprung up, but had only begun waking up to the risk of landslides since 2023, though they were the country\u2019s most deadly natural hazard, he added.<\/p>\n
\u2018Reliant on outside advice\u2019<\/p>\n
Northland geotechnical engineer David Buxton said councils nationwide had really struggled since the 1990s \u2013 when they began shedding expertise \u2013 to get their own technical advice to back their decisions on the likes of runout zones.<\/p>\n
\u201cThey\u2019re reliant on outside advice to give these things, but then they\u2019re reliant on in-house people without that depth of technical knowledge to make that decision-making, and I think it would be really good if councils were able to invest in that in-house technical ability.\u201d<\/p>\n
Buxton himself, as a consultant, benefited from the current setup, he said \u2013 however it was not just expensive, but led to patchy follow-through sometimes.<\/p>\n
Auckland Council had invested in-house and that was paying off in its response to floods, Buxton said.<\/p>\n
Prime Minister Christopher Luxon at He Maimai Aroha Community Care Centre on Monday after the Mount Maunganui landslide. Photo \/ RNZ, Nick Monro<\/p>\n
The WPS study for Tauranga last year had some gaps \u2013 for instance, it did not look at the likely size or volume of likely slips.<\/p>\n
\u201cThe factors that determine the volume and runout characteristics of the landslide, and the consequent impacts on infrastructure in proximity to the slope, could be considered when looking at specific slopes as part of site-specific studies,\u201d said the short study, that sits alongside two longer engineering reports into landslides in the city and region.<\/p>\n
\u201cThis should be carried out as part of considering the risk posed to infrastructure or property at particular sites.\u201d<\/p>\n
The landslip map showed small zones of \u201cfailure\u201d and runout at several streets, the largest of which were around Hopukiore Reserve.<\/p>\n
– RNZ <\/p>\n","protected":false},"excerpt":{"rendered":"The advice in 2005 has been superseded in part by new science that has led to the adoption…\n","protected":false},"author":2,"featured_media":254431,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[2327,24752,4763,12683,34893,78,56662,130,111,43,139,5006,69,10711,31341,21037,6046,1305,1227,131,61,6859,1530,30209],"class_list":["post-254430","post","type-post","status-publish","format-standard","has-post-thumbnail","category-new-zealand","tag-2327","tag-ago","tag-allow","tag-buildings","tag-documents","tag-in","tag-landslide","tag-more","tag-new-zealand","tag-news","tag-newzealand","tag-not","tag-nz","tag-officials","tag-pennington","tag-phil","tag-potential","tag-show","tag-tauranga","tag-than","tag-to","tag-told","tag-years","tag-zones"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/254430","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/comments?post=254430"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/254430\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media\/254431"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media?parent=254430"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/categories?post=254430"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/tags?post=254430"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}