International Conference Vajont2013 - 8 October

1. DEALING WITH PREDICTED LANDSLIDE
CATASTROPHES
Tim Davies
Geological Sciences,
University of Canterbury,
New Zealand

2. Vajont
Predictable (in retrospect…Was the impact predictable?)
The event was allowed to happen
Would it be any different today?

3. 1963-2013:
What has changed?
1. The Vajont catastrophe occurred and is fairly well understood
2. Landslide physics has made some progress
3. Landslide and flood modelling are much more advanced – consequences can be
estimated if location and volume can be predicted
What has not changed?
1. Politics is a very important factor in risk management decision-making
2. Risk is a poorly-understood topic; acceptable risk is poorly defined
3. Communication between scientists and non-scientists is poor
4. Decision-makers are reluctant to acknowledge inevitable future catastrophes

4. Example 1: Kaikoura, New Zealand
State highway & rail
South Bay
Kaikoura
Goose Bay
Sediment accumulation
0.25 km3
Kaikoura Canyon
Assuming the whole 0.25
km3 of sediment can fail at
once, the resulting tsunami
has been modelled

5. Waves up to 12 m
high at the coast;
5 m at South Bay
(Walters et al,
2007).

6. This has been known about since 1999
Consequences: coseismic failure means NO WARNING - hundreds of deaths, up to $1Bn costs
The reality of the complete collapse scenario has not been investigated until now – lack of
funding
RISK (probability * consequence):
Estimated recurrence interval of this event ~ 200 years (Lewis & Barnes
1999)
Number of deaths ~ 100; giving 0.5 deaths per year

7. Acceptable risk of numbers of potential fatalities from dam failure
(Munger et al., 2009)
Dam failure:
known hazard,
known location,
known consequences;
Applicable to known future
landslide failure
Acceptable risk of 100 fatalities ~ 10-6 per year
Risk at Kaikoura is unacceptable
by 5 orders of magnitude

8. Slope
break
Example 2: Franz
Josef, NZ
Profile line
Franz Josef

9. 1 km
?
Township
Slope profile
Alpine fault
Alpine fault generates M8 earthquakes every ~ 300 years; last rupture was in 1717.
Is this a deep-seated coseismic/gravitational slope deformation?
Is there a failure surface that extends with every earthquake?
Could this slope fail as a coseismic rock avalanche (> 107 m3)?

10. Cascade rock avalanche ~ 0.7 km3; 500 y BP?
Deposit
Headscarp
Note associated
slope break

11. Round Top rock avalanche 4 x 107 m3: ~ 800 y BP
Head
scarp
Deposit
Slope
break

12. The Franz Josef slope has been through ~ 50 earthquakes since deglaciation without
collapsing, so the probability it will fail in the next earthquake is say 1%
The annual probability of an earthquake is about 1%, so the annual probability of a rock
avalanche is about 10-4
The number of people who will be killed is at least 100; so the average annual death rate
is ~ 10-2; unacceptable by a factor of about ten thousand
Acceptable risk of
100 deaths ~ 10-6 per
year

13. Example 3: Milford Sound, NZ
Iconic World Heritage tourist
location
> 600,000 day visitors per year
All within 5 m of sea level

14. There are > 20 postglacial landslide deposits > 106 m3 on the bottom of the Sound
About every 1000 years there is a coseismic landslide that causes a tsunami with
about 10 m of runup; on average this will kill about 400 people at current visitor
rates (Dykstra, 2013)
The risk to an individual tourist is < 10-6 per lifetime – this is acceptable
The risk to each live-in employee is about 10-3 per year – much higher than
acceptable under employment safety legislation
The societal risk is about 0.4 per year - ~ a million times greater than acceptable

15. Acceptable risk of 400 fatalities ~ 2 x 10-7

16. It is extremely difficult to persuade authorities to investigate such situations.
DO WE NEED:
• MORE & BETTER SCIENCE?
YES – BUT THAT’S NOT THE PROBLEM
• BETTER SCIENCE COMMUNICATION?
YES – BUT HOW TO DO IT?
Obstacles to communication
1. Time-scales.
Decision-makers – maybe 10 years
Geologists - > millennia
2. Risk tolerance. Decision-makers – unusually high
Scientists & public - lower
3. Politicians don’t understand geology; Geologists don’t understand politics
BUT – POLITICIANS CAN BE HELD ACCOUNTABLE
If comprehensible published science is ignored
by politicians, they will be held responsible for the
resulting disaster when it happens...

17. How does this apply to Milford Sound, Franz Josef, Kaikoura?
Reduction of the societal risk to acceptable levels at Milford Sound is only
possible by reducing visitor numbers by a factor of about 1 million – i.e.
abandoning the site as a visitor destination
This will be politically and economically unacceptable; there is a serious problem
A possible solution would be for the society running the risk – NZ society – to
knowingly and willingly accept the risk and its consequences.
A necessary first step is for the risk information to
be peer-reviewed and published so that it is
publically available. Then an informed society-wide
(public/government) debate can take place…
The key responsibility of the scientist is to communicate the
information on risk and consequence to the public and
decision-makers – in such a way that is is accepted and
understood. Then it might be acted on…

18. tim.davies@canterbury.ac.nz
It is also crucial for scientists to publically
correct any public misinterpretation of
science by politicians and others (including
scientists)