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Landslides caused by the June-August 2008 rainfall in Auckland and Wellington, New Zealand

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    Hancox, G.T.; Nelis, S. 2009 Landslides caused by the June-August 2008 rainfall in Auckland and Wellington, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/04 25 p.

     Abstract: The winter of 2008 was stormy and wet, with floods in many parts of New Zealand. Rainfall of 150% above normal in Auckland, and 135% higher in Wellington triggered many landslides in those areas, closing roads and damaging or destroying several houses. The rain was only moderately intense but prolonged. Most of the landslides occurred towards the end of several episodes of wet weather from June to August 2008, during which c. 50-120 mm rain fell over periods of 5 to 10 days. Major landsides occurred at Torbay, Glenfield, and Huia in Auckland, damaging several houses. These slides were initiated by rainfall, but the underlying causes resulted from building on old landslides, or previously unstable land where there had been major alterations to the ground profile, without due allowance for the underlying instability. The prolonged rainfall also triggered numerous landsides on coastal cliffs, but other factors, such as wave undercutting of cliffs and houses built along the cliff-edge, provided the underlying preconditions for the failures. A common problem is that many houses are built too close to the edge of unstable cliffs that are prone to erosion and periodic slope failures. Most of the houses affected by coastal cliff failures appear to have been built close to, or in some cases within the ‘Foreshore Yard’, a 15-25 m wide buffer zone from which cliff-top residential development in Auckland is set back from the coast. Most landslides on Auckland’s east coast cliffs were small falls of soil, regolith, and vegetation at the tops of cliffs and did little or no damage to houses. Major landslides at Achilles Point and Buckland’s Beach, which caused substantial damage, involved both regolith falls and failures on joints and faults within the underlying interbedded sandstone and siltstone bedrock. Rainfall also reactivated an old large coastal landslide at Kawakawa Bay. The failure closed the northern access road to the settlement for one month, and necessitated demolition of a house, extensive earthworks, and drainage works to stabilise the slope and make the road safe. This landslide illustrates the danger of constructing an unsupported road cut across the toe of an old landslide, and the role of prolonged rainfall in its reactivation. The 2008 landslides are a reminder that much of Auckland is at high or moderate risk from landslides. New houses or modifications to homes on coastal cliffs and other high-risk areas may need geotechnical assessments and stabilising measures to proceed. Building too close to the cliff-edge is inadvisable, and a set-back distance of at least the height of the cliff may be required. Buyers of existing homes would be wise to obtain pre-purchase geotechnical assessments to determine if there are any slope stability issues or hazards that could affect the property. Prolonged and higher than normal rainfall in Wellington triggered landslides in many places, causing substantial damage to road s and houses. However, other factors appear to have provided the preconditions for the slope failures. All of the observed landslides occurred on modified (cut) slopes which had not been stabilised or designed for long-term stability. The houses affected by landslides in Wellington in 2008 were not substantially damaged. Postfailure response mainly involved temporary evacuation of occupants, restoration of damaged services, and clearance of debris from roads. Although the affected properties have been reoccupied the failed slopes have not been permanently stabilised. All of the houses that were undermined by landslides were built too close to the tops of unsupported cuts, and they are at significant risk of further collapses in the future. Such slope failures are typical of those that occur in Wellington during heavy and prolonged rainfall. All houses in Wellington that are built too close to the tops or bottoms of steep (>~60), high cuts are at risk from landslides, not only during future rainstorms, but also during strong (MM 8 to MM10) earthquake shaking. (auth)

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