Landslides caused by the February 2004 rainstorms and floods in southern North Island, New Zealand

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Hancox, G.T.; Wright, K.C. 2005 Landslides caused by the February 2004 rainstorms and floods in southern North Island, New Zealand . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2005/10 32 p.

Abstract: Heavy and prolonged rainfall during the February 2004 storm caused widespread landsliding over about 16,000 km2 of southern North Island, extending across the Wanganui-Manawatu area to southern Hawke’s Bay, Wairarapa, and the greater Wellington areas. Landslide damage was more extensive, with a wider and more diverse area affected, than occurred during Cyclone Bola in 1988. The most severely damaged areas were in the Mangawhero, Whangaehu, Turakina, and Pohangina valleys. Many thousands of small to medium (3) shallow (1-2 m deep) soil and debris slides and flows occurred. There were also some larger (~1000-200,000 m3) deep-seated landslides in Tertiary mudstone. Some landslides dammed streams to form lakes. Most landslides occurred on steeper (~20-35°) grass-covered hill slopes, gullies and steep terrace edges. In some areas, numerous landslides produced extensive areas of coalescing soil slides affecting many hectares. Debris from many landslides did not reach stream channels, but remains on the slopes. Gully and river-bank failures, however, contributed considerable sediment and trees to flooded rivers, with the latter causing some bridge failures. Soil and debris flows exhibit scar length to debris runoff length ratios of 1:3 to more than 1:10 in some of the more mobile soil flows. (auth)