The then EQC chief executive, David Middleton ONZM, appeared on the TV show Fair Go explaining this. In the 1990s and early 2000s, the Rodney District (which includes Muriwai) was ranked first nationally in having EQC claims rejected on the basis that houses had been built on existing unstable ground. But it stipulated that compensation can be refused if a house was constructed on unstable land. The Earthquake Commission (EQC) Act in 1993 was an important step forward for natural disaster insurance. While the Building Act 19 have improved matters, we are still dealing with section 641A’s legacy. Author providedĬoncern about the effects of section 641A was highlighted in 1986 by highly respected engineers Nick Rogers and Don Taylor in a paper published in New Zealand Engineering magazine, titled “Safe as houses”. This figure shows the extent of the 1965 landslide and where new homes were built two decades later. Councils were also absolved of any civil liability. In 1981, the Local Government Amendment Act (section 641A) allowed councils to issue building permits for houses on unstable land prone to erosion, subsidence, slippage or inundation. The timing of this new construction (denoted by the yellow arrows in the figure below) is intriguing. This held until the early 1980s, when gradual house construction began again. Soon after, it was reported a Rodney District Council engineer had stated no new houses would be built on the 1965 landslide footprint. The landslide extent is denoted in red hash in the figure above.Ī 1966 New Zealand Geographer article recorded that witnesses said the landslide moved at 90 kilometres per hour. In August 1965, following heavy rainfall, fatal landslides over 200 metres long occurred on consecutive days at the south-east end of Domain Crescent, destroying houses and killing two people. They are constructed on Kaihu sands, with some of the houses built on debris from former landslides. Houses on Domain Crescent and Motutara Road are at the foot of the escarpment, below landslide source areas. This digital elevation model shows former landslides, roads and properties in Muriwai. This uses airborne laser scanning of the land surface, which removes vegetation and exposes the land surface “geomorphology” underneath. The figure below is a digital elevation model (DEM) based on 2016 data gathered by the remote-sensing method LiDAR. These crenulations, or “embayments”, represent the headscarps (or source areas) of landslides. But if too much water is added, the castle collapses rapidly as a “flow-slide”.Ī prominent geomorphological feature of Muriwai is an escarpment of soft Pleistocene Kaihu Group dune sands that forms the crenulated ridgeline immediately west of Oaia Road. If a little water is added, a steep-sided sand castle can be built. As water content increases, however, this negative pressure drops, and the sands fail and flow.Ī good analogy is sand on a beach. Initially, this has a suction effect (negative pore pressure), whereby the water pulls the sand grains together, increasing strength. During rainfall, water starts to fill these pore spaces. The sands are weak and are poorly cemented, or completely uncemented, meaning there are “pore” spaces between the grains that are filled with air. These are geologically young (Pleistocene age, less than 2.6 million years old) and form the high country around Muriwai. Much of Muriwai, like other parts of Auckland’s west coast, is underlain by Kaihu Group sands. Slippery slopes: why the Auckland storm caused so many landslides – and what can be done about it We already have a good understanding of the soils, landscape, geomorphology and exposure to landslide hazards – as well as the history of planning decisions that allowed houses to be built on land prone to slips. Muriwai offers a case study of that equation. This can be summarised as: risk = hazard x exposure x vulnerability.
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