1. Qualitative landslide risk assessment in New Zealand
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Qualitative landslide risk assessment in New Zealand
Graham Hancox
GNS Science (Lower Hutt, New Zealand)

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INTRODUCTION:
Qualitative Landslide Risk Assessments (QLR) have been used by GNS Science to assess landslide risk to proposed subdivisions on the Wairarapa Coast of North Island, NZ.
Provided as part of the geological/geotechnical information required by District Councils under the Resource Consents process.
The QLR procedure is widely used and accepted in NZ for obtaining consents for the development of new urban subdivisions. Peer review is an integral part of the process.
This presentation describes the process, using an example from one subdivision (now public information).

3. METHODOLOGY – Required Data:
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METHODOLOGY – Required Data:
Geological and geomorphic mapping of the proposed subdivision area and house sites.
Good topographic data (1 to 2 m contours), possibly on orthophoto base, with layout of subdivision and proposed house sites.
Vertical aerial photos (possibly of different ages). Recent oblique aerial photos of the site are also most helpful - and very cost effective.
Mapping of landslides, rock fall deposits, debris fans etc. for geological / historical age control and return times for landslide hazard events.

4. Location and Topographic Setting:
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Location and Topographic Setting:
Map showing topographic setting of Hiwikirikiri Subdivision on the southeast Wairarapa coast. The painting (top right) alerted South Wairarapa DC to possible landslide hazard at the site.

5. Geomorphic Features on Aerial Photos:
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Geomorphic Features on Aerial Photos:
Old landslide debris (pre-1849)
Rock fall areas
-1948 Eq
Debris Flood
-1936 Storm
Coastal Platform (c.6,500 years)
Old sea cliff (6-10,000 years)
Vertical aerial photos of Hiwikirikiri Subdivision area taken in 1944 provide the basis for determining geomorphic changes (landslides, rock falls, debris flow) in the last 60 years.

6. Landslide and rockfall features:
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Landslide and rockfall features:
Annotated aerial photo of Hiwikirikiri SD area showing mapped landslides and rockfall features, and debris fan in relation to proposed house sites.

7. Rainfall-Induced Landslide Features – 30 March 2005:
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Rainfall-Induced Landslide Features – 30 March 2005:
Annotated aerial photo of Hiwikirikiri SD area showing the soil slides/flows, and debris flood/flow on Karaka Grove fan that formed during a rainstorm on 30 March 2005.

8. RIL Features – 30 March 2005: ICL Landslide Teaching Tools
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RIL Features – 30 March 2005:
Annotated aerial photo of Hiwikirikiri SD area showing the soil slides/flows, and debris flood/flow on Karaka Grove fan that formed during a rainstorm on 30 March 2005.

9. Landslide Risk Assessment – Flow Chart:
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Landslide Risk Assessment – Flow Chart:
Qualitative landslide risk assessment method adapted from AGS (Australian Geomechanics Society) methodology.
Similar to Risk Management Standard used in NZ/Australia (AS/NZS 4360: which has been adopted by MCDEM).
AGS methodology is designed for landslides hazards, and now widely used by engineering consultants in NZ for RMA Consents applications.
The Flow Chart (left) summarises the risk assessment process – which involves several main steps…
Flow Chart for Qualitative Landslide Risk Assessment.

10. Landslide Risk Assessment – Main Steps:
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Landslide Risk Assessment – Main Steps:
(A) Define purpose and context which risk assessment will be used in (e.g. RMA Application), and the scope of work.
(B) Hazard assessment: Identify landslide hazard events that could occur in the proposed development area based on:
(i) Mapping of landslides and geomorphic features (streams, debris fans, cliffs and high steep slopes etc.).
(ii) Mapping of possible landslide collapse and runout zones, debris flow and debris flood paths etc.
(iii) Identification of triggering events for landslide hazards (e.g. rainfall, earthquakes, excavations, slope loading; storm runoff and disposal; leakage from water mains etc.).
(C) Qualitative Risk Analysis -using hazard/ risk critera and Risk Analysis Matrix…
Flow Chart for Qualitative Landslide Risk Assessment.

11. Landslide Risk Assessment -Steps:
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Landslide Risk Assessment -Steps:
Tables summarising Criteria and Risk Analysis Matrix for Qualitative Landslide Risk Assessment.

12. Risk Assessment – Criteria and Terminolgy
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Risk Assessment – Criteria and Terminolgy
(C) Qualitative Risk Analysis Terms:
Landslide Risk is defined as the combination of the likelihood and consequences a landslide hazard event affecting a site.
1 – Likelihood: Indicative probability or return period of a hazard event is expressed in words - estimated from historical and prehistoric evidence of geohazards and processes at a site.
2 – Consequences: Likely impact of a landslide hazard on a site (building damage, loss of life) if event occurs. (generally depends on landslide type, size, and speed of movement).
Risk assesment Criteria and Terminology

13. Landslide Risk Assessment Matrix:
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Landslide Risk Assessment Matrix:
(C) Qualitative Risk Analysis:
3 – Risk Analysis Matrix: The levels of risk for each landslide hazard event is estimated by cross matching Likelihood (Classes A–F) against Consequences (Classes 1–5) on the Risk Analysis Matrix (3).
4 – Risk Implications: Response to the estimated risk level is based on the possible damage and effects for each hazard event.
Note: The risk may be acceptable, or may be unacceptable unless mitigation or protective works are carried out to reduce the level of risk - as suggested by implications for different risk levels .
Risk Analysis Matrix and Implications.

14. Landslide Risk Assesment: Hiwikiriki SD
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Landslide Risk Assesment: Hiwikiriki SD ).
Example: Landslide risk assessment for proposed Hiwikirikiri subdivision.

15. Hiwikiriki Risk Asssessment – Conclusions:
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Hiwikiriki Risk Asssessment – Conclusions:
Risk from landslides, rock falls, debris floods and flows to the house sites assessed as being very low to high.
Rockfall risk (low to moderate) was rated as acceptable if measures were implemented to reduce risk at the site.
A ‘catch ditch’ and berm located on the northern side of the subdivision were recommended to reduce rock fall risk.
Drainage was suggested to control runoff from the slope (catch ditch also acts as swale drain), along with debris fences to retain soil flow material during rainstorms.
An earth bund was recommended at the western end of the subdivision to provide protection against debris flows from the nearby stream (moderate to high risk).