Canadian Dam Association Bulletin - Summer 2018 - 12

1. INTRODUCTION
The design inflow capacity for
reservoirs in British Columbia (BC)
and Yukon (YT) are typically determined using the concept of the Inflow
Design Flood (IDF). The Canadian Dam
Association (CDA) 2007 Dam Safety
Guidelines (CDA, 2013) and CDA
Technical Bulletin - Hydrotechnical
Considerations for Dam Safety (CDA,
2007) provide standards-based IDF
target values for initial consideration
and consultation between owner and
regulator for freshwater dams. The BC
Ministry of Forests, Lands and Natural
Resource Operations (FLNRO) and
YT regulators have adopted the CDA
guidelines for IDF presented in Table 1.
The BC Ministry of Energy and Mines
(MEM) Guidance Document - Health,
Safety and Reclamation Code for Mines
in British Columbia (the Code) provides
minimal IDF requirements for tailings
dams in BC and are also presented in
Table 1.
The IDF for water and tailings dams
varies according to the dam failure
consequence classification (dam classification) and is based on the design
return period specified in Table 1 and
the Probable Maximum Flood (PMF).
The IDF must be evaluated for both the
peak flow rate and total flow.
Considerable information, technical
expertise, and analyses, is required to
calculate the flood events listed in
Table 1. Calculation of the 100 year
flood can, in some cases, be defensibly completed using meteorological
and/or hydrometric data collected in
the vicinity of the reservoir catchment
area. To be scientifically defensible,
regional analysis must typically be

utilized to estimate the 975 or 1000 year
flood and PMF. CDA (2007) provides
general guidance on use of statistical
flood analysis computation of the PMF
and selection of the IDF. Engineers, who
are designing a spillway or completing
a hydrotechnical analysis as part of
a Dam Safety Review (DSR) in BC or
YT, should refer to the guideline as a
starting point for the analysis. CDA
(2007) indicates "Until the scientific
community defines safe practices, high
and extreme floods should be evaluated
with a (realistic) degree of conservatism
and flood frequency estimates should
be updated as frequently as possible,
when new information becomes available, for example after the occurrence of
a very large flood or when new advances
on climate modeling become available."
While the use of Table 1 to decide
upon a logical IDF for a dam appears
conservative, it must be recognized that
many dam failures caused by embankment overtopping can be attributed to
a combination of unfortunate occurrences. Hartford et al. (2016) indicate
"The approach to the analysis and
understanding of operational safety
of dams and reservoirs developed in
this volume starts from the observation
that the dominant risks to be managed
in dam safety derive not from unique
events [for example the PMF] but from
adverse combinations of more usual
events." Klemes (2011) indicates a similar thought "...historical evidence suggests that the high extremes occur more
due to unusual combinations of these
factors than to unusual magnitudes of
the factors themselves." Examples of
potentially disastrous combinations
of occurrences along with a large flood

that is less than the IDF include, erosion of the reservoir spillway, blockage
of the spillway with logs, failure of a
gate to open on-demand, etc.
Some generalizations with respect
to the regulation of dams have been
made by the author. A dam owner or
qualified professional engineer seeking
guidance regarding a dam topic should
speak directly with the appropriate
regulator.

2. REGULATORY
CONSIDERATIONS
The BC Ministries of FLNRO and
MEM, Yukon Environmental Socioeconomic Assessment Board (YESAB)
and Yukon Environment have adopted
the five tier dam failure consequence
classification proposed by the CDA and
in general refer to the CDA guidelines.
There are no regulator guidelines in
BC or YT for determining the IDF for
a reservoir. Several regulatory agencies in the United States provide IDF
guidelines. For example, Montana
Department of Natural Resources
and Conservation have a technical
note Analysis of Spillway Capacity in
Montana (Hydrometrics, 2008). The
hydrologist tasked with the estimation
of the IDF for a reservoir in BC and YT
is expected to follow CDA guidelines,
and utilize best industry practices and
other qualified professionals when
required, such as a meteorologist.
The guidelines produced by the
CDA are an essential component of the
dam regulatory system in BC and YT.
Without the input from CDA volunteers
from across Canada, there would be far
less dam safety guidance and regulatory ability in the region.

Table 1: IDF Target Levels for Water Dams (CDA, 2007) and Tailings Dams (BC MEM, 2016)
Dam Classification

IDF Tailings Dam2

IDF Water Dam1

Low

100yr

1/3 between 975yr and PMF

Significant

between 100yr and 1000yr

1/3 between 975yr and PMF

High

1/3 between 1000yr and PMF

1/3 between 1000yr and PMF

Very High

2/3 between 1000yr and PMF

2/3 between 1000yr and PMF

Extreme

PMF

PMF

Note: 1 - IDF target levels for initial consideration and consultation between owner and regulator, table modified from
page 17 of CDA (2007)
2 - IDF target levels are minimum requirements as specified in the Code, table modified from page 16 of MEM (2016)
IDF evaluated for instantaneous peak flow and/or volume of flood
100yr is a short form notation for the 1 in 100 year design return period flood, etc.
12

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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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