Canadian Dam Association Bulletin - Fall 2017 - 23

that really matter as they follow the (2080) Pareto rule. In the present portfolio
under examination, two dams account
for 99% of the intolerable risks.
4.2 Failure probabilities
For this case history, the Factors of
Safety (FoS) for stability were known
by prior studies. Stability failure probabilities were determined by using a
simplified approach (Silva et al., 2008).
This method links the factor of safety,
defined by geotechnical engineers
using classical stability methods, to
the annual probability of failure. Due
to space limitations, details of this
method will not be presented here;
they can be found in the original paper.
Note that the Silva et al. (2008) method
is one among many possible solutions
which have the advantage of being
immediate and simple to use. As a matter of fact, each portfolio could develop
their own calibrated curves or proceed
with preliminary probabilistic stability analyses. Other failure modes were
assessed using a mix of model-derived
or encoded expert based likelihood (Ang
and Tang, 1975) leading to the global
probability of failure, pf, of each dam.
For larger portfolios, if the FoS are
not known and prior analyses are
not available, the indirect hybrid
assessment method can be used.
This approach is based on a historical
approach to failure records (Oboni
and Oboni, 2013), empirical observations (Oboni et al., 2014), and a systemic failure model (Oboni and Oboni,
2016b) which uses causality analysis
in relation to design, construction,
inspection, and maintenance. These
papers have shown it is possible to
establish, a priori, a range for a portfolio with very little actual specific
data. One trades perceived lack of
precision with efficiency and good
understanding of ranges. Values of the
probability of failure of specific failure modes, for slurry and dewatered
tailings dams, have been derived
and compared (Taguchi, 2014) to the
initial estimates (Oboni and Oboni,
2013), as depicted in Figure 3.
One can expect that the world population of reasonably designed, built,
maintained, and managed dams lies
Canadian Dam Association * Fall 2017

within the blue rectangle bounded
by model uncertainty and historical
uncertainty. External symptoms such
as beach width, wet spots, good or
poor maintenance, and deformations
can be used to modify the a priori pf
determined above. A poorly designed,
built, maintained or managed dam
would lie outside of the rectangle. It
would reach failure by an ascending
trajectory towards the left.
4.3 Consequences
For large portfolios, indirect assessment of consequences can be based on
direct or indirect observations, models
(if they exist), etc. In this case history,
the potential range of consequences was
outlined for each dam and each hazard
scenario defined in Table 1. Under the
zone of influence column, consequence
classes C1 to C5 are described. Each
class is defined for every component,
including associated uncertainties.
The client had identified and classified additional aspects of consequences such as community and
public concerns, media interest, and
legal implications. Thus, environmental, human, health and safety,
as well as reputational-crisis consequences were included in the evaluation procedure (USEPA, 1999, 2003,
Barnthouse et al., 2000).
This study did not attribute a cost to
human life (Mooney, 1977, Jones-Lee,
1989, Marin, 1992, Pearce, D.W. et al.,
1995), a notion often considered repugnant and ethically unacceptable. Instead,
it looked at the willingness to pay (WTP).
This value represents the amount of
money a society is ready to spend to save
the life of citizens exposed to hazards.
In other words, health and safety was
combined with the WTP. Obviously, the
willingness to pay is strongly influenced
by cultural, religious, philosophical, and,
quite evidently, economic considerations. This amount simultaneously considers the view on risk and the response
to risk of a given society. Similar concepts were applied to environmental and
reputational crisis.
4.4 Risk ranking
The portfolio owner had established
a probability impact graph prior to

the dam risk assessment. To illustrate
the results, the risks were ranked in
descending order, and used the client
developed colouring scheme.
Figure 4 shows the portfolio sorted
by decreasing dam risks with the
initial corporate colour pattern.
Management felt overwhelmed with
four (4) dams being in the red zone
and nine (9) in the yellow zone. It
was a classic case of an overwhelming syndrome, with the owner facing
a daunting task with too much to
deal with.
Interestingly, Dams 6, 7b, 8, 11 and
15 which have the worst consequences
as will be seen in Figure 5, show up
as yellow in Figure 4. With relatively
small consequences, Dam 3 has a very
high pf, and shows up as red. These
results are rather confusing. They may
mislead the mitigative agenda, and
they illustrate the need for a better
prioritizing technique.
4.5 Tolerance prioritization
To offer the owner with a sustainable solution to dam improvement and
to solve his overwhelming syndrome,
development of a tolerance based prioritization was necessary.
As a preamble, it must be noted that
modification of a dam, for example,
reinforcement or buttressing, might
reduce the probability of failure, but
not necessarily the consequences. If a
risk can be mitigated in this manner
within the realm of credibility, it is
a tactical risk. To reduce the consequences, the system must be altered,
for example, the tailings pond not
filled to maximum capacity or the
construction of emergency catchment
area. If the system must be modified, it
is in the domain of strategic decisions.
The fifteen (15) dams in the current portfolio have estimated failure
probability, pf, covering a very wide
range, and their estimated failure
consequences in dollars vary greatly.
Dams 1, 4, 5, 6 and 11 have a low pf as
can be seen on Figure 5; they would
show below the historical range in
Figure 3. In Figure 5, Dams 7a, 7b, 8,
10 and 15 exhibit a greater pf than the
preceding group; they would be at the
lower bound of the historical range in
23



Table of Contents for the Digital Edition of Canadian Dam Association Bulletin - Fall 2017

Board of Directors
President’s Message
2018 Board of Directors Update
2017 CDA Awards
Screening Level Risk Assessment for a Portfolio of Tailings Dams
Dam Safety Review, Public Safety Workshops Hit the Road
CDA Events Photospread
Gardiner Dam 50th Anniversary
Jim’s Journey – Memoirs by Jim Gordon
CDA Launches Digital Editions of Guidelines
Invitation From Icold 2018 in Vienna
Icold Corner
Notice of the Annual General Meeting
Ouranos Press Release – 2017 Spring Floods in Quebec
CDA Young Professionals
CDA 2018 Conference
Time to Renew Your Membership
CDA News Brief
Buyers’ Guide and Trade List
Canadian Dam Association Bulletin - Fall 2017 - Intro
Canadian Dam Association Bulletin - Fall 2017 - cover1
Canadian Dam Association Bulletin - Fall 2017 - cover2
Canadian Dam Association Bulletin - Fall 2017 - 3
Canadian Dam Association Bulletin - Fall 2017 - 4
Canadian Dam Association Bulletin - Fall 2017 - 5
Canadian Dam Association Bulletin - Fall 2017 - 6
Canadian Dam Association Bulletin - Fall 2017 - Board of Directors
Canadian Dam Association Bulletin - Fall 2017 - 8
Canadian Dam Association Bulletin - Fall 2017 - President’s Message
Canadian Dam Association Bulletin - Fall 2017 - 10
Canadian Dam Association Bulletin - Fall 2017 - 11
Canadian Dam Association Bulletin - Fall 2017 - 12
Canadian Dam Association Bulletin - Fall 2017 - 13
Canadian Dam Association Bulletin - Fall 2017 - 2018 Board of Directors Update
Canadian Dam Association Bulletin - Fall 2017 - 2017 CDA Awards
Canadian Dam Association Bulletin - Fall 2017 - Screening Level Risk Assessment for a Portfolio of Tailings Dams
Canadian Dam Association Bulletin - Fall 2017 - 17
Canadian Dam Association Bulletin - Fall 2017 - 18
Canadian Dam Association Bulletin - Fall 2017 - 19
Canadian Dam Association Bulletin - Fall 2017 - 20
Canadian Dam Association Bulletin - Fall 2017 - 21
Canadian Dam Association Bulletin - Fall 2017 - 22
Canadian Dam Association Bulletin - Fall 2017 - 23
Canadian Dam Association Bulletin - Fall 2017 - 24
Canadian Dam Association Bulletin - Fall 2017 - 25
Canadian Dam Association Bulletin - Fall 2017 - 26
Canadian Dam Association Bulletin - Fall 2017 - 27
Canadian Dam Association Bulletin - Fall 2017 - 28
Canadian Dam Association Bulletin - Fall 2017 - 29
Canadian Dam Association Bulletin - Fall 2017 - Dam Safety Review, Public Safety Workshops Hit the Road
Canadian Dam Association Bulletin - Fall 2017 - 31
Canadian Dam Association Bulletin - Fall 2017 - 32
Canadian Dam Association Bulletin - Fall 2017 - 33
Canadian Dam Association Bulletin - Fall 2017 - CDA Events Photospread
Canadian Dam Association Bulletin - Fall 2017 - Gardiner Dam 50th Anniversary
Canadian Dam Association Bulletin - Fall 2017 - 36
Canadian Dam Association Bulletin - Fall 2017 - 37
Canadian Dam Association Bulletin - Fall 2017 - Jim’s Journey – Memoirs by Jim Gordon
Canadian Dam Association Bulletin - Fall 2017 - CDA Launches Digital Editions of Guidelines
Canadian Dam Association Bulletin - Fall 2017 - Invitation From Icold 2018 in Vienna
Canadian Dam Association Bulletin - Fall 2017 - Icold Corner
Canadian Dam Association Bulletin - Fall 2017 - 42
Canadian Dam Association Bulletin - Fall 2017 - Notice of the Annual General Meeting
Canadian Dam Association Bulletin - Fall 2017 - Ouranos Press Release – 2017 Spring Floods in Quebec
Canadian Dam Association Bulletin - Fall 2017 - 45
Canadian Dam Association Bulletin - Fall 2017 - CDA Young Professionals
Canadian Dam Association Bulletin - Fall 2017 - 47
Canadian Dam Association Bulletin - Fall 2017 - CDA 2018 Conference
Canadian Dam Association Bulletin - Fall 2017 - 49
Canadian Dam Association Bulletin - Fall 2017 - Time to Renew Your Membership
Canadian Dam Association Bulletin - Fall 2017 - CDA News Brief
Canadian Dam Association Bulletin - Fall 2017 - 52
Canadian Dam Association Bulletin - Fall 2017 - 53
Canadian Dam Association Bulletin - Fall 2017 - Buyers’ Guide and Trade List
Canadian Dam Association Bulletin - Fall 2017 - cover3
Canadian Dam Association Bulletin - Fall 2017 - cover4
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