Quantitative Rockfall Risk Assessment in New Zealand: Christchurch Port Hills

1. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(1) Quantitative landslide risk assessment in New Zealand: Rockfallrisk in the Port Hills, Christchurch, New Zealand, following the 2010-2011 Earthquake sequence MauriMcSAVENEY GNS Science (Lower Hutt, New Zealand) e-mail: m.mcsaveney@gns.cri.nz

2. Rockfalls (boulder rolls) and cliff collapses involve different processes Require different analyses methods ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(2)

3. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(3) Rockfall risk in the Port Hills The risk assessment comprises 5 main steps: Consider the full possible range of rockfall triggers (seismic and non-seismic triggers); Choose representative events across the range and estimate the numbers of boulders they will trigger; Estimate the probability of a boulder passing a location on a slope; Estimate the probability a person is killed if they are in the way; Combine Steps 2-4 to estimate the risk for each event, and sum them to estimate the overall risk.

4. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(4) Rockfall risk in the Port Hills – Step 1 and 2 • Estimating the likely numbers of rockfalls triggered at each PGA band PGA – Peak ground acceleration Frequencies for each PGA band are estimated from the composite seismic hazard model for the next 2-year period

5. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(5) Estimating the likely numbers of rockfalls triggered at each PGA band PGA hazard curve for site HVSC (Heathcote Valley School) using seismicity model for next 2 years, using a minimum earthquake magnitude of M5. Sites class: shallow soil (NZS 1170 site class C). Expected number of fallen rocks by suburb (lines), as a function of peak ground acceleration (g).

6. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(6) Rockfall risk in the Port Hills – Step 1 and 2 • Estimating likely numbers of rockfalls triggered by “other” events (e.g. rainfall and frost) Number of rockfalls triggered by an event within the 1 -15 year band comes from the GNS Science database of number of reported (in the press) homes hit by rockfall in the Port Hills within the last 15 years. Number of rockfalls triggered by an event within the 100 -1000 year band is assumed to be comparable to those triggered by 22 February 2011 earthquake

7. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(7) Rockfall frequency magnitude (all events)

8. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(8) Rockfall risks in the Port Hills – Step 3 • Estimate the probability of a boulder passing a location on a slope Frequency of rockfalls passing a given location decreases with distance (runout) from source area – can be expressed using “shadow angles” Rockfalls triggered by 22nd Feb. 2013 earthquakes and where they came from (sources)

9. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(9) Rockfall risks in the Port Hills – Step 4 • Estimate probability that a person is killed if they are in the way of a boulder; • Estimated using statistics from 22nd February 2011 events (# of deaths and houses hit)

10. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(10) Rockfall risks in the Port Hills – Step 4 • N people killed from rockfalls (22 Feb) = 2 • N properties (home and garden) in hazard zones = 1,070 • N homes struck by boulders (22/Feb/11) = 121 • Home occupancy (estimated for daytime) (22/Feb/11) = 10% • N homes occupied (estimated) during earthquake = 107 • N homes occupied and struck by boulders = 12 • P fatality IF house hit and a person present = 2% (P1) • P home hit IF in a hazard zones & 22/Feb event occurs = 11% (P2)

11. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(11) Rockfall risk in the Port Hills – Step 4 For average home in hazard zones • P fatality IF at home & 22/2/2011 quake occurs = P1 x P2 = 2% (day time) • P fatality for average resident in a 22 Feb-type event = 6% (P3) (for a 30/70 time split between at-home and away) • P3 will be higher for those homes closer to rockfall source areas • However, nobody was killed by a lone rolling boulder in a residential area (The 5 killed by falling rocks were hit by debris avalanches)

12. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(12) Rockfall risk in the Port Hills – Step 4 • P property hit IF in hazard zones & 22/Feb event occurs (P2) = Population of suburb/divided by populated area of suburb = ~1% (all suburbs) • This can also be expressed per shadow zone per suburb • P fatality IF property hit and person present P = 50% (Hong Kong “data”) assumes person is outside and can get out of the way. NZ houses allow boulders to pass through, therefore P = 80% • Key data – how many people (in a boulder shadow zone) were home at the time of the 22 Feb EQ. This is unknown at present.

13. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(13) Rockfall risk in the Port Hills – Step 5 • Risk map: showing annual loss of life of an individual within a rockfall zone from all considered events

14. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(14) It was recommended that: • Policy makers consider what levels of risk are to be tolerated • Undertake site-specific assessments of properties where risks are intolerable, considering: • the state of where rockfalls originate above the properties • other local factors that may contribute to the risk. • Mitigation measures may be considered where intolerable risks are confirmed • If mitigation measures are deemed impractical/uneconomic then consider retreat

15. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(15) Cliff collapse The risk assessment comprised 5 main steps: • Consider the full possible range of triggers (seismic and non-seismic triggers); • Pick events that trigger collapse and estimate the numbers and volumes of the failures (landslides); • Estimate the probability of debris reaching a given distance away from the source: • Estimate the probability a person is killed if they are in the way; • Combine 2-4 to estimate the risk for each event, and sum them to estimate the overall risk.

16. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(16) Cliff collapse Needs to consider: • Debris inundation at toe of cliff • Undercutting of cliff crest

17. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(17) Cliff collapse: Step 1 • Estimated in a similar way to rockfalls • However, much better control on magnitude / frequency of landslides for a given PGA event • Frequency of “other” triggers estimated from geomorphic evidence

18. Cliff collapse: Step 2 (Redcliffs) ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(18) 22 Feb, 2011, earthquake (Redcliffs School, Redcliffs: photo by G. Hancox)

19. Cliff collapse: Step 2 ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(19) 13 June, 2011, Earthquake 22 Feb, 2011, Earthquake 13 June, 2011, Earthquake

20. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(20) Monitoring: Repeat terrestrial laser scans Change Map – loss (blue) & gain (yellow) 13 June, 2011, earthquake (Redcliffs School area)

21. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(21) Cliff collapse: Step 2 The risk assessment comprises 5 main steps: Change model between 2 March and 16 June 2011 Changes triggered by 13 June aftershock

22. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(22) Cliff collapse: Step 2 12 m • Change of the cliff profile caused by 13 June, 2011 earthquake • About 12 m (plan distance) of material lost from cliff crest • Debris at cliff toe encroaches a further 15 m into property 2 March 2011 16 June 2011 25 m 15 m Redcliffs Section 3

23. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(23) Cliff collapse: Step 2 • Frequency/magnitude distribution of landslides triggered by 13 June earthquake • Data are for Redcliffs and Peacock’s Gallop • Frequency /magnitude distribution of other EQ events also shown • Frequency of various triggering events estimated from probabilistic seismic model

24. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(24) Cliff collapse: Step 2 Peacock’s Gallop Cliff top retreat models Change models based on pre and post 13 June EQ slope models 25 m 2 March 2011 16 June 2011 Redcliffs 25 m 2 March 2011 16 June 2011 Plan views of clifftop edges

25. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(25) Cliff collapse: Step 2

26. 12 m 2 March 2011 16 June 2011 25 m 15 m Redcliffs Section 3 ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(26) Cliff collapse: Step 3 (travel distance of debris) • Estimated using simple empirical relationships e.g. volume/travel distance • Based on actual site specific data

27. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(27) Cliff collapse: Step 3 Landslides and rockfalls triggered by 13 June earthquake and how far they travelled

28. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(28) Cliff collapse: Step 3 Travel distance of landslides and rockfalls (all areas) triggered by 22 Feb and 13 June 2011 earthquakes Fahrboeschung projected from cliff crest to toe of 1) talus; and 2) rockfall

29. ICL Landslide Teaching Tools PPT-tool 4.064-1.3 (a)(29) Cliff collapse • Step 4: • Estimate the probability a person is killed if they are in the way; • Estimated using statistics from the 22 Feb EQ • 3 fatalities at cliff toe (1 in a home, 2 in gardens) • 20 homes hit by debris • P (hit) = ~50% (all areas) (P (hit) rockfall = ~1%) • Step 5: • Combine steps 2-4 to estimate the risk for each event, and sum them to estimate the overall risk. • This work was ongoing when presented in 2012