Landslides and debris flows caused by the 15-17 June 2013 rain storm in the Marahau-Motueka area, and the fatal landslide at Otuwhero Inlet

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Page, M.J. 2013 Landslides and debris flows caused by the 15-17 June 2013 rain storm in the Marahau-Motueka area, and the fatal landslide at Otuwhero Inlet. Lower Hutt, N.Z.: GNS Science. GNS Science report 2013/44 35 p.

Abstract: A rain storm in the Tasman District on the 15-17 June 2013 caused numerous landslides and debris flows in the Motueka area. One of these landslides struck a house at Otuwhero Inlet, killing the woman occupant. Another landslide in the Marahau Valley destroyed part of a dwelling, fortunately just missing a father and daughter who were sleeping in another room. Landsliding was particularly severe in a logged area of Shaggery Forest west of Motueka. Many roads were blocked by landslides and debris flows between Marahau and Kaiteriteri, in the Riwaka and Brooklyn Valleys, and along the west bank of the Motueka River as far south as Rocky River. A salmon farm at Anatoki west of Takaka was badly damaged by a debris flow. A site inspection was made of the fatal landslide site at Otuwhero Inlet. The landslide occurred at ~1 pm on Sunday 16 June after ~180 mm of rain fell in the preceding 24 hours and ~80 mm fell in the preceding 4 hours. The house was pushed several meters off its piles and collapsed, and the woman’s body was subsequently recovered from the landslide debris outside the house. The landslide occurred in an area of convergent drainage on the slope directly above the house. The head scarp of the landslide is level with an access track that cuts across the slope. The house had been built on material deposited by an earlier (paleo-) landslide. Factors contributing to the landslide were: the high intensity rainfall, the deeply-weathered Separation Point Granite lithology, steep convergent slopes, and probably the side castings deposited along the outer edge of the track during construction and maintenance. The death of the occupant was the result of the house being located at the base of slopes susceptible to landsliding, and the light-weight materials with which the house was constructed. The landslide that severely damaged the dwelling in the Marahau Valley occurred after ~290 mm rainfall in the preceding 24 hours, ~140 mm in the preceding 12 hours and ~30 mm in the preceding hour. Again the dwelling had been built at the base of steep convergent slopes underlain by Separation Point Granite, and on material deposited by an older landslide. In this case the natural toe of the slope had been excavated to increase the area for building. This steepening of the lower part of the slope reduced support for the area above and facilitated the landslide. (auth)