Reconnaissance of landslide and flood damage in the Gisborne area caused by the 2005 Labour weekend storm

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Beetham, R.D.; Grant, H. 2006 Reconnaissance of landslide and flood damage in the Gisborne area caused by the 2005 Labour weekend storm. Lower Hutt, N.Z.: GNS Science. GNS Science report 2006/22 31 p.

Abstract: The Labour Weekend storm event was generated by a low pressure system that travelled southeast across the East Cape of the North Island over Friday 21 and Saturday 22 October 2005. Gisborne District was the worst affected area suffering the most damaging flooding since Cyclone Bola in 1988. The highest total rainfall was experienced in the coastal ranges between Tolaga Bay and Te Puia Springs where up to 385 mm of rain fell in 44 hours. Peak rainfall intensities of between 34.5 and 44.5 mm/hour were recorded between Tolaga Bay and Ruatoria. Provisional 48 hour rainfall return periods were estimated at 40 years for Ruatoria and Gisborne city and 85 years for the Hikuwai Valley. Peak water levels on the Waipaoa and Hikuwai rivers were within 0.6 metres of those recorded during Cyclone Bola. However, the Labour Weekend storm was of shorter duration resulting in shorter flood peaks and causing less damage to surrounding farmland and flood protection works. Floodwaters entered seven houses around Tolaga Bay, along with the Mangatuna Marae, depositing silt and debris. Crop losses due to flooding and silting on the Tolaga Bay and Poverty Bay flats were estimated at $8.4 million. State Highway 35 north of Gisborne, and some local roads, were blocked at several locations by silting and landsliding. Damage to local roads was estimated to be at least $1 million. Flood protection works damage totalled around $500,000. Aerial and ground reconnaissance of landslide and flood damage between Gisborne and Te Puia Springs was undertaken by a GeoNet response team between 7 and 10 November 2005. The most significant landsliding and erosion occurred in the Hikuwai catchment near the area of heaviest rainfall. This generally took the form of reinitiated gullying of weak Miocene mudstone in the catchment headwaters. Many new, small soil slides and flows were also triggered by the storm in both farm and forest land along the length of the district. Gullying and shallow soil flows were the largest sources of the Hikuwai River’s sediment load which was subsequently deposited on the Tolaga Bay flats. River bank slumping, caused by elevated pore pressures in saturated silty river banks as water levels rapidly receded, was observed between Gisborne and Te Puia. Damage from river bank slumping was minimal. Many erosion prone areas of farmland were planted in exotic forestry after Cyclone Bola. This has succeeded in stabilising soil and preventing numerous small landslides, particularly soil flows, which occur in many areas of pastoral farming. However, the steep, dissected headwaters of some of the district’s rivers, while planted, continue to erode and supply large amounts of sediment to rivers and streams during high intensity rainfall events. (auth)