Reconnaissance studies of landslides caused by the July-October 2006 rainstorms in southern North Island, New Zealand

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Hancox, G.T.; Dellow, G.D.; Massey, C.I.; Perrin, N.D. 2007 Reconnaissance studies of landslides caused by the July-October 2006 rainstorms in southern North Island, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2006/26 37 p.

Abstract: Rainstorms in July, August, and October 2006 caused widespread landsliding in southern North Island. Grass-covered hill country in Wanganui, south Taranaki, and Wairarapa areas were worst affected in July, although landslides also caused considerable damage in urban areas in Wanganui, Lower Hutt, and Wellington. Ground and aerial reconnaissance by GNS GeoNet Project showed that shallow landsliding affected at least 2,500 km2 of eastern Wairarapa hill country. The total area affected by landsliding was far less extensive than during the February 2004 and March 2005 rainstorms, and the density of landsliding was possibly greater in March 2005, especially at Castlepoint, and coastal areas north of White Rock. However, landslide damage to houses was greater in 2006 and more disruptive over a longer period than in previous recent events. In the Wairarapa, steep grass-covered hill country was most affected by shallow landsliding, and many country and farm roads were temporarily blocked by slips. Areas of native bush, scrub, and exotic forest were little affected by landslides, with the latter proving to provide an effective land use option to prevent landsliding. The Rimutaka Hill Road was blocked by cut slope failures in July, but adjacent scrub and bush-covered hill slopes were generally not affected, demonstrating the value of vegetation for preventing or reducing rainfall-induced landsliding in mountain areas. Landslide distribution and density was variable across areas of similar terrain and rock type in the Wairarapa, suggesting that cells of higher intensity rainfall probably occurred in those areas most affected by landslides. The Wanganui-Taranaki hill country was more severely affected by landsliding during rainstorms in July 2006 than was the Wairarapa area. Shallow soil slides and flows extended across an area of 4000-5000 km2. Landslides closed many roads, including SH 3 and SH 4, for 2-3 weeks, houses were evacuated because of landslides at Wanganui and Hunterville, and an abutment collapse destroyed a bridge, isolating Mangamahu. Numerous slips occurred on hill country farms northeast of Wanganui and inland from Waitotora. These areas were generally worse affected by slips than in February 2004. The Mangawhero, Whangaehu, and Hunterville to Taihape areas were less affected by shallow soil slips and flows in July 2006 than in February 2004, but several deep-seated landslides (Hunterville, Utiku, and Taihape) showed creep movement in response to the prolonged and intense rainfall in July 2006. Steep, grass-covered hill country was most affected by landslides, while areas of native bush and exotic forest provided effective protection against landsliding. Poplar trees were ineffective in preventing landslides or the flow of debris. Further rain during August 2006 caused several large landslides in urban parts of Wellington and Lower Hutt, with relatively little rural landsliding such as o ccurred in July. Two landslides caused substantial damage to houses at Kelson in Lower Hutt on 7 August, and an apartment building at Oriental Bay in Wellington on 16 August. More landslide damage to houses in Wellington occurred during a rainstorm on 26 August. That event caused further slips and debris flood problems at Paekakariki temporarily blocking State Highway 1, and closing Paekakariki Hill Road for three months while repairs were carried out. Damage caused by these landslide events took several months to repair, and the final cost is likely to have exceeded 10 million dollars. Repairs to the apartment building at Oriental Bay is estimated at about $900,000, and damage to houses in Kelson, one of which had to be demolished due to a large failure in a poorly constructed fill, is likely to cost $500,000–750,000. More rainstorms hit the Wellington area in October 2006, causing debris flood damage at Paekakariki and delaying remedial works to the Hill Road. Two debris flows from an area of recently-milled pine trees at Eastbourne damaged two properties at the bottom of a steep gully. Because of the high risk of further debris flows in the area, these house sites have been abandoned. (auth)