Layer: Landslide Susceptibility (ID: 340)

Parent Layer: Ground Stability

Name: Landslide Susceptibility

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Description: <DIV STYLE="text-align:Left;"><DIV>EXECUTIVE SUMMARY GNS Science (GNS) has been commissioned by Rotorua District Council (RDC) to describe landslide hazards within the Rotorua district. The project has assessed landslides throughout the district in order to differentiate areas where the landslide hazard is similar. The report and accompanying map explain and show the variation in relative landslide susceptibility throughout the district. This information will help RDC develop policies and rules for dealing with landslide hazards in the new district plan. The landslide susceptibility map prepared using the methodology outlined in this report is in good agreement with the known landslide information in the Rotorua district. The agreement between the susceptibility model and historical landslide data provides confidence that the relative landslide susceptibility displayed on the map effectively discriminates relative landslide hazard. Two rainfall events generated multiple landslides over a 14 year period between 1996 and 2010. Analysis of rainfall data indicates that 200 mm of rainfall in 24 hours is the threshold for causing multiple landslides. Given that the rainfall events were limited in their extent in the district and a threshold of 200 mm of rain in 24 hours (approximately a 1-in-20 year return period event based on HIRDS data), to have two multiple landslide generating rainstorms (greater than 20-year return period storms) in 14 years matches rainfall data (the rainfall and landslide datasets were collected independently). The landslide damage caused by the 2004 Rotoehu earthquake is equivalent to MM8 in the epicentral area around Lakes Rotoehu and Rotoma. The 1987 Edgecumbe earthquake caused landslide damage equivalent to MM6 in the same area. In Rotorua City MM8 intensity has a return period of about 2500 years but for areas east of the city MM8 has a return period of 500 years. Thus the landslide distribution in the epicentral area of the Rotoehu earthquake is an analog for the type of landslide damage that might be expected during MM8 shaking in Rotorua city, which is expected to occur an average once every 2500 years. Large pre-existing landslides have been searched for by systematically examining vertical aerial photographs. No pre-existing large landslides have been identified in the Rotorua district. Based on the lack of evidence for large landslides in these terrains nationally, there is no large, pre-existing landslide hazard in the Rotorua district. First-time landslides not triggered by high-intensity rainfall or earthquake-shaking occur occasionally. Some of these occur in association with rainfall but there are often other contributing factors such as recent slope modification. GNS Science Consultancy Report 2010/82</DIV></DIV>

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