Canadian Dam Association Bulletin - Summer 2018 - 24

400

P(mm)
300
200
100
0
0.99

0.9

0.5

-1

10

-2

10

-3

-4

-5

-6

10 10 10 10

Exceedance frequency
Figure 4: Empirical (circles) and synthetic (lower curve) of annual maxima of daily
precipitation, P, at Coquitlam Lake, BC (the upper curve represents an approximate
upper bound, the shaded area covers the estimate range for low-probability events).
Taken from page 195 Klemes (2011).

forecast of future conditions based
on climate-change scenarios, may
rely on forecasts of upstream changes to the drainage basin, or may
incorporate other predicted conditions." The stochastic modelling
approach is still relatively new and
not commonly used in BC or YT. As
discussed in the Introduction, dam
failures historically occur not due
to a single event such as the IDF, but
due to a combination of events. If the
frequency distribution of a gate not
opening when required or a spillway wing wall of a floodway failing
can be determined, the mechanical
failure can be incorporated into a
stochastic analysis of the dam failure. Micovic et al. (2016) indicate
that "the IDF does not carry much
relevance, no matter how accurately
it is characterized." and "the probability of the reservoir inflow tells us
little about the probability of a dam
overtopping." The authors indicate
that the analysis focus should not
be on reservoir inflows, but rather
on reservoir outflow as well as the
corresponding elevation of the reservoir water level. In the paper the
authors incorporate stochastic analysis of river flows, reservoir operating
rules, and on-demand availability
of several discharge facilities from
the reservoir.
24

8.3 Combinations of Deterministic
and Stochastic Analysis
Klemes (2011) presents a frequency
analysis methodology using daily precipitable water (W) and orographic
and synoptic storm efficiencies (Eo
and Es, respectively). He provided
an example for the Coquitlam Lake
Dam, BC where he calculated a
range of daily precipitation values
for exceedance frequency as low as
10 -5 as shown in Figure 4.

9. FINAL COMMENTS
The practise of calculating an IDF
for reservoirs in BC and YT will likely
continue as spillway design requires
the estimation of the magnitude of
low frequency flood events. It was
briefly discussed that the history of
dam safety indicates that dam failure
typically does not occur due to a low
frequency (high magnitude) flood event
that exceeds the design capacity of the
system. Rather a combination of factors,
such as a gate not opening on-demand
during a flood event, more often results
in overtopping and possible failure of
the dam. Procedures of incorporating
spillway component failure along with
extreme flood events using stochastic analysis is a promising method of
assessing the hazard on a dam.
The level of effort extended to
estimate the IDF for a dam should be

based on the dam failure consequence
and should be completed using more
than one method. A discussion of the
uncertainty with the calculation of
IDF is mandatory to produce a defensible report. PMP and PMF should be
reported as a range of values, not as
a single best estimate.
The IDF is not equivalent to the
required flow capacity of the reservoir's spillway. The reservoir's volume, shape, and operating rules will
all have an effect on the attenuation
of the IDF into the reservoir.
In most cases, statistical methods
for estimating PMP should not be used
as standalone calculations for the IDF
of a reservoir. Meteorological analysis
must also be used when determining
the range of appropriate PMP for a
reservoir's catchment.
Published government funded
regional frequency analysis of precipitation and streamflow data would
in general improve the quality and
lower the cost of the completion of
reservoir IDF studies and flood mitigation studies in the region.
Readers of this paper are encouraged
to contact the author with comments
at robert.mclean@gov.bc.ca.
■

10. REFERENCES
APEGBC. 2016. "Legislated Dam Safety
Reviews in BC." Association of Professional
Engineers and Geoscientists of British
Columbia Professional Practice Guidelines.
V3.0 October 2016.
BC Hydro. 2013. "Site C Clean Energy Project
env i ron ment a l

i mpac t

st atement."

January 23, 2013.
Brimelow, J.C. and Reuter, G.W. 2005. "Transport
of Atmospheric Moisture during Three
Extreme Rainfall Events over the Mackenzie
River Basin." Journal of Hydrometeorology:
Volume 6: 423-440.
Buttle, J.M, Allen, D.M, Caissie, D, Davison,
B, Hayashi, M, Peters, D.L, Pomeroy, J.W,
Simonovic, S, St-Hilaire, A, and Whitfield,
P.H. 2016. "Flood processes in Canada:
Regional and special aspects." CWRJ Vol. 41,
Nos. 1-2.
CDA website. www.cda.ca Accessed 27
June 2017.
Cannon, A.J. 2014. "An intercomparison of
regional and at-site rainfall extreme value

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Table of Contents for the Digital Edition of Canadian Dam Association Bulletin - Summer 2018

Board of Directors
President’s Message
Inflow Design Flood Estimation for Reservoirs in British Columbia and Yukon Territory
ICOLD 2018: Canadian Expertise on the World Stage
ICOLD Corner
CDA Will Be Meeting in Quebec City!
CDA Directors & Officers – Call for Nominations
2018 CDA Awards – Call for Nominations
Notice of Annual General Meeting
Time to Renew Your Membership
Clean Energy B.C.'s Operational Excellence Award for Corra Linn Dam
Buyers’ Guide and Trade List
Canadian Dam Association Bulletin - Summer 2018 - Intro
Canadian Dam Association Bulletin - Summer 2018 - cover1
Canadian Dam Association Bulletin - Summer 2018 - cover2
Canadian Dam Association Bulletin - Summer 2018 - 3
Canadian Dam Association Bulletin - Summer 2018 - 4
Canadian Dam Association Bulletin - Summer 2018 - 5
Canadian Dam Association Bulletin - Summer 2018 - 6
Canadian Dam Association Bulletin - Summer 2018 - Board of Directors
Canadian Dam Association Bulletin - Summer 2018 - President’s Message
Canadian Dam Association Bulletin - Summer 2018 - 9
Canadian Dam Association Bulletin - Summer 2018 - Inflow Design Flood Estimation for Reservoirs in British Columbia and Yukon Territory
Canadian Dam Association Bulletin - Summer 2018 - 11
Canadian Dam Association Bulletin - Summer 2018 - 12
Canadian Dam Association Bulletin - Summer 2018 - 13
Canadian Dam Association Bulletin - Summer 2018 - 14
Canadian Dam Association Bulletin - Summer 2018 - 15
Canadian Dam Association Bulletin - Summer 2018 - 16
Canadian Dam Association Bulletin - Summer 2018 - 17
Canadian Dam Association Bulletin - Summer 2018 - 18
Canadian Dam Association Bulletin - Summer 2018 - 19
Canadian Dam Association Bulletin - Summer 2018 - 20
Canadian Dam Association Bulletin - Summer 2018 - 21
Canadian Dam Association Bulletin - Summer 2018 - 22
Canadian Dam Association Bulletin - Summer 2018 - 23
Canadian Dam Association Bulletin - Summer 2018 - 24
Canadian Dam Association Bulletin - Summer 2018 - 25
Canadian Dam Association Bulletin - Summer 2018 - 26
Canadian Dam Association Bulletin - Summer 2018 - 27
Canadian Dam Association Bulletin - Summer 2018 - ICOLD 2018: Canadian Expertise on the World Stage
Canadian Dam Association Bulletin - Summer 2018 - ICOLD Corner
Canadian Dam Association Bulletin - Summer 2018 - CDA Will Be Meeting in Quebec City!
Canadian Dam Association Bulletin - Summer 2018 - 31
Canadian Dam Association Bulletin - Summer 2018 - 32
Canadian Dam Association Bulletin - Summer 2018 - 33
Canadian Dam Association Bulletin - Summer 2018 - CDA Directors & Officers – Call for Nominations
Canadian Dam Association Bulletin - Summer 2018 - 2018 CDA Awards – Call for Nominations
Canadian Dam Association Bulletin - Summer 2018 - Notice of Annual General Meeting
Canadian Dam Association Bulletin - Summer 2018 - Time to Renew Your Membership
Canadian Dam Association Bulletin - Summer 2018 - 38
Canadian Dam Association Bulletin - Summer 2018 - Clean Energy B.C.'s Operational Excellence Award for Corra Linn Dam
Canadian Dam Association Bulletin - Summer 2018 - 40
Canadian Dam Association Bulletin - Summer 2018 - Buyers’ Guide and Trade List
Canadian Dam Association Bulletin - Summer 2018 - 42
Canadian Dam Association Bulletin - Summer 2018 - cover3
Canadian Dam Association Bulletin - Summer 2018 - cover4
Canadian Dam Association Bulletin - Summer 2018 - outsert1
Canadian Dam Association Bulletin - Summer 2018 - outsert2
Canadian Dam Association Bulletin - Summer 2018 - outsert3
Canadian Dam Association Bulletin - Summer 2018 - outsert4
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