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GNS Science (GNS) has been commissioned by Rotorua District Council (RDC) to describe landslide hazards within the Rotorua district |
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GNS Science (GNS) has been commissioned by Rotorua District Council (RDC) to describe landslide hazards within the Rotorua district |
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<DIV STYLE="text-align:Left;"><DIV><P><SPAN>EXECUTIVE SUMMARY </SPAN></P><P><SPAN /></P><P><SPAN>GNS Science (GNS) has been commissioned by Rotorua District Council (RDC) to describe </SPAN></P><P><SPAN>landslide hazards within the Rotorua district. The project has assessed landslides throughout </SPAN></P><P><SPAN>the district in order to differentiate areas where the landslide hazard is similar. The report </SPAN></P><P><SPAN>and accompanying map explain and show the variation in relative landslide susceptibility </SPAN></P><P><SPAN>throughout the district. This information will help RDC develop policies and rules for dealing </SPAN></P><P><SPAN>with landslide hazards in the new district plan. </SPAN></P><P><SPAN /></P><P><SPAN>The landslide susceptibility map prepared using the methodology outlined in this report is in </SPAN></P><P><SPAN>good agreement with the known landslide information in the Rotorua district. The agreement </SPAN></P><P><SPAN>between the susceptibility model and historical landslide data provides confidence that the </SPAN></P><P><SPAN>relative landslide susceptibility displayed on the map effectively discriminates relative </SPAN></P><P><SPAN>landslide hazard. </SPAN></P><P><SPAN /></P><P><SPAN>Two rainfall events generated multiple landslides over a 14 year period between 1996 and </SPAN></P><P><SPAN>2010. Analysis of rainfall data indicates that 200 mm of rainfall in 24 hours is the threshold for </SPAN></P><P><SPAN>causing multiple landslides. Given that the rainfall events were limited in their extent in the </SPAN></P><P><SPAN>district and a threshold of 200 mm of rain in 24 hours (approximately a 1-in-20 year return </SPAN></P><P><SPAN>period event based on HIRDS data), to have two multiple landslide generating rainstorms </SPAN></P><P><SPAN>(greater than 20-year return period storms) in 14 years matches rainfall data (the rainfall and </SPAN></P><P><SPAN>landslide datasets were collected independently). </SPAN></P><P><SPAN /></P><P><SPAN>The landslide damage caused by the 2004 Rotoehu earthquake is equivalent to MM8 in the </SPAN></P><P><SPAN>epicentral area around Lakes Rotoehu and Rotoma. The 1987 Edgecumbe earthquake </SPAN></P><P><SPAN>caused landslide damage equivalent to MM6 in the same area. In Rotorua City MM8 </SPAN></P><P><SPAN>intensity has a return period of about 2500 years but for areas east of the city MM8 has a </SPAN></P><P><SPAN>return period of 500 years. Thus the landslide distribution in the epicentral area of the </SPAN></P><P><SPAN>Rotoehu earthquake is an analog for the type of landslide damage that might be expected </SPAN></P><P><SPAN>during MM8 shaking in Rotorua city, which is expected to occur an average once every 2500 </SPAN></P><P><SPAN>years. </SPAN></P><P><SPAN /></P><P><SPAN>Large pre-existing landslides have been searched for by systematically examining vertical </SPAN></P><P><SPAN>aerial photographs. No pre-existing large landslides have been identified in the Rotorua </SPAN></P><P><SPAN>district. Based on the lack of evidence for large landslides in these terrains nationally, there </SPAN></P><P><SPAN>is no large, pre-existing landslide hazard in the Rotorua district. </SPAN></P><P><SPAN /></P><P><SPAN>First-time landslides not triggered by high-intensity rainfall or earthquake-shaking occur </SPAN></P><P><SPAN>occasionally. Some of these occur in association with rainfall but there are often other </SPAN></P><P><SPAN>contributing factors such as recent slope modification. </SPAN></P><P><SPAN /></P><P><SPAN>GNS Science Consultancy Report 2010/82</SPAN></P></DIV></DIV> |
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DP_Operative.DBO.LandslideSusceptibility |
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["Landslide"] |
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en-NZ |
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5000000 |
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