Canadian Dam Association Bulletin - Summer 2018 - 22

* Results obtained may vary significantly from one statistical distribution to another, and no reliable
method exists for the selection of
the most appropriate distribution.
This task relies entirely on the
hydrologist's judgment.
* The evaluation of the highest floods
on record often depends on the
extrapolation of the rating curve
at the station, which may be subject
to a large degree of uncertainty.
* Beyond an AEP [annual exceedance probability] in the order of
1/500 year, statistics give only an
"order of magnitude" estimate. For
the purpose of flood evaluation in
the dam safety process, extrapolation beyond the 1/1000 year flood
is discouraged.
* Evaluation of the confidence limits on the statistical estimate is
recommended.
Hosking and Wallis (1997) indicate
"Because there are numerous sources
of uncertainty about the physical
processes that give rise to observed
events, a statistical approach to the
analysis of data is often desirable."
Klemes (2011) is critical of statistical
frequency analysis for this very reason; the user typically has no understanding of the physical processes
that result in the flood event! Hosking
and Wallis (1997) indicate that statistical methods allow the quantification of uncertainty. It would appear
reasonable for a hydrologist to always
include a discussion and analysis
regarding the uncertainty of a frequency analysis.
Hosking and Wallis (1997) indicate
that regional frequency analysis, for
analysing precipitation or flood data,
can be used to reach more accurate
conclusions by using several data sets
if "the event frequencies are similar
for the different observed quantities." The region of analysis should
have a homogeneous climate and
geography. Cannon (2014) indicates
that if extreme rainfall data belong
to a homogeneous region, this implies
that the normalized series share a
common distribution and the data can
be pooled and a regional distribution
fit to the extreme rainfall data. In
22

practice of course, the precipitation
frequency and characteristics of the
group of catchments will not be completely homogeneous but it is up to the
hydrologist to design the analysis
such that the data used are acceptable.
USACE (2017) indicate that regional
frequency analysis generally follows
four steps and refer to Hosking and
Wallis (1997):
1) Screening of data.
2) Identify homogeneous regions.
3) Selection of a frequency distribution.
4) Estimate of the frequency distribution parameters.
Regional frequency analysis is
much more time consuming than
the analysis of a single dataset but
is required to defensibly arrive at
extreme events. USACE (2017) utilized regional frequency analysis to
generate 72 hour isopluvial maps for
most of Oregon and Washington with
Annual Exceedance Probabilities
(AEP) of 1 in 100, 500, 1000, 5000
and 10,000 years. The climate and
topography of BC and YT are similar to Oregon and Washington and it
therefore appears feasible to derive
such isopluvial maps for BC and YT,
despite the mountainous conditions.
Cannon (2014), in a study of regional
and at-site rainfall extreme value
analyses in southern BC, found that
regional frequency analysis resulted
in a substantial reduction in error
compared with single station analysis, especially when the single stations had small sample sizes.
Standardized regional rainfall and
flood frequency analysis, funded by
government support, should be completed across BC and YT. A steering
committee of hydrologists is required
to determine the scope and methodology of such studies.

7. STREAMFLOW SIMULATION
Watt et al. (1989) provide guidance on
the selection of flood simulation methods and discuss the roles of storage in
the catchment, channel and lakes/reservoirs; the routing of floods through
lakes and reservoirs; analysis methodologies; and the many components of
a simulation that must be understood
before modelling can begin.

Some regulatory authorities have
produced guidelines for streamflow
simulation. Dowl (2016) in a report for
the Montana Department of Natural
Resources and Conservation Dam
Safety Program, provide guidance for
the completion of a rainfall-runoff
hydrologic model.
The hydrologist completing the
study has several options with respect
to software programs to select. The
software must be appropriate for the
application and documentation of
the model setup and modelling runs
must be provided. The calibration of
a streamflow simulation model, for
an extreme flood event, is difficult
as watershed response to rainfall and
snowmelt events are not linear.
Note that the 100 year precipitation
or snowmelt event does not equal the
100 year flood event. The same is true
for other return interval frequencies.
The antecedent conditions prior to
the rainfall/snowmelt event greatly
influence the resulting flood peak
flow rate and total volume of runoff
over a specified time interval.
Several factors are at play when
considering the appropriate IDF for
a resevoir and include as a minimum
the following:
* Catchment area, slope, shape, soils,
plants, storage, depth to bedrock,
and elevation.
* Antecedent conditions (including
soil moisture, depth to groundwater, and snowpack).
* Storm intensity and duration.
Two general scenarios must be
evaluated when determining IDF for
a reservoir, a spring rain on snowpack
event, and a summer storm event.

8. FLOOD ANALYSIS METHODS
There are several combinations
of analysis methods available to the
hydrologist tasked with determining
an appropriate IDF for a reservoir.
For example, the 1 in 1000 year flood
event could be calculated by regional
analysis of hydrometric data or by
regional analysis and determination
of the 1 in 1000 year rainfall event
followed by streamflow simulation.
The IDF should be calculated using
more than one method. The average

View past issues of the CDA Bulletin online at www.naylornetwork.com/cda‑nxt/


http://www.naylornetwork.com/cda‑nxt/

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
https://www.nxtbook.com/naylor/CDAQ/CDAQ0119
https://www.nxtbook.com/naylor/CDAQ/CDAQ0418
https://www.nxtbook.com/naylor/CDAQ/CDAQ0318
https://www.nxtbook.com/naylor/CDAQ/CDAQ0218
https://www.nxtbook.com/naylor/CDAQ/CDAQ0118
https://www.nxtbook.com/naylor/CDAQ/CDAQ0417
https://www.nxtbook.com/naylor/CDAQ/CDAQ0317
https://www.nxtbook.com/naylor/CDAQ/CDAQ0217
https://www.nxtbook.com/naylor/CDAQ/CDAQ0117
https://www.nxtbook.com/naylor/CDAQ/CDAQ0416
https://www.nxtbook.com/naylor/CDAQ/CDAQ0316
https://www.nxtbook.com/naylor/CDAQ/CDAQ0216
https://www.nxtbook.com/naylor/CDAQ/CDAQ0116
https://www.nxtbook.com/naylor/CDAQ/CDAQ0415
https://www.nxtbook.com/naylor/CDAQ/CDAQ0315
https://www.nxtbook.com/naylor/CDAQ/CDAQ0215
https://www.nxtbook.com/naylor/CDAQ/CDAQ0115
https://www.nxtbookmedia.com