May 26 2014
COLLIERY DAMS TECHNICAL COMMITTEE PUBLIC INFORMATION UPDATE
RISK CLASSIFICATION OF COLLIERY DAMS REDUCED
FOLLOWING PHASE ONE WORK OF TECHNICAL COMMITTEE
Work undertaken by Golder Associates on behalf of the Technical Committee during Phase One of its mandate included detailed site testing, analysis and risk assessment of the Colliery Dams. That technical analysis indicated that there is an extremely low risk of sudden or rapid failure of the dams in an earthquake that would cause a large wave of water or debris to impact the neighbourhood below the dams.
That work has been a significant factor contributing to the replacement of the “Extreme” consequence classification of the dams by lower classifications, as described in this public update. Analysis is continuing to determine the best option to increase spillway capacity, which must be expanded to meet flow requirements in a prolonged heavy rainfall. A further report will be issued in approximately 4-6 weeks on the remediation work to be undertaken, the schedule, the status of signage in place, and the timing for release of the consultants’ reports.
· An update on the status of the Technical Committee’s work was last provided on March 24 2014. This update provides information about the physical state of the dams that has emerged through Phase One of the work of the Committee.
· The mandate of the Committee is to focus on options for remediating the existing dams
that will satisfy the requirements of the BC Dam Safety Regulation, which is administered by
Dam Safety Section (DSS) of the Ministry of Forests, Lands and Natural Resource Operations
· As part of Phase One of its work under that mandate, the Committee was tasked with
reviewing existing data and collecting additional data with the objective, among other
things, of “potentially reducing the hazard rating of the dams through better understanding
of the dam structures, failure rates and methods/probability of events that would result in
failure of the dams.” (In the Regulation, the term used for “hazard rating” is “consequence
Risk assessment approach:
· In its approach to the Phase One work, the Committee endorsed a “risk assessment”
approach to assessing the physical state of the dams (engineers Golder Associates were
selected by the Committee to undertake this work based on their high level of expertise in
this area and their recommendation that a risk assessment approach be used to develop the
most appropriate remediation options in due course).
· Why has the Committee taken this approach?
· Why has the Committee taken this approach?
o Changes to the Canadian Dam Association Dam Safety Guidelines made in October 2013
provided the Technical Committee with the opportunity to employ this approach to
evaluate the dams and their hazard rating (consequence classification), and develop the
most appropriate remediation options to achieve acceptably low public safety risk.
o This is a technical and complex undertaking, but worth understanding as it has made a
significant difference to the outcome of the Committee’s work. Without the ability to
use this approach, the Committee would have been limited to only considering
remediation options that would meet the design standards associated with dams with
“extreme consequence” ratings, which likely would require more extensive remediation.
o Risk, in this context, is a measure of the likelihood and severity of adverse consequences
from potential dam failure under various conditions, such as an earthquake or
prolonged heavy rainfall, and is usually expressed in terms of life-safety and economic
o Understanding the extent of the risk assists with the selection of remedial options to
address deficiencies in the dams in a way that will meet Dam Safety Section
requirements.1 In the case of the Colliery Dams, the priority is to ensure that any work
done on the dams is essential to meet life-safety requirements while meeting the
Committee’s other objectives as outlined at its first meeting (attached to this update).
· As part of the risk assessment process, Golder has undertaken detailed testing and analysis
of the dams over the past six months. That new information was required to support a case
to reduce the hazard ratings or consequence classifications of the dams and to establish the
appropriate level of remediation required.
· Golder therefore undertook physical site investigations, and core samples of the dams, as
well as hydrological, hydraulic, structural, geotechnical and earthquake analyses, to ensure
that the Committee had complete information.
Results of risk assessment:
· Until now, both of the dams have been classified by Dam Safety Section as “Extreme
consequence” in the event of a severe storm or earthquake, which is the highest
consequence rating under the Regulation.
· The Extreme rating was based on a previous perception that the dams were very vulnerable
to serious damage and potential sudden failure in the event of an earthquake or severe
storm, and therefore posed a serious risk to public safety.
· Golder’s extensive site investigation work and comprehensive analysis of the results and
risks associated with the dams indicates that risks related to the dams are in fact
significantly lower than previously believed.
· The dams appear to be in the same condition they were when remediation work was done
on them in 1980. Testing has shown that the concrete walls are in good condition with very
limited signs of deterioration.
· Earthquake: The results of the analysis indicate that given the current physical state of the
dams, there is an extremely low probability of a dramatic rupture in an earthquake that
would put public safety at risk. It was established that the dams are very unlikely to fail
during an earthquake in a manner that would result in loss of life. If the dams were to be
damaged by an earthquake, it would most likely be manifested through slow leaking
through cracks in the dam walls rather than a sudden failure of the walls.
· Severe storm: The results of the analysis also indicate that risks of serious damage resulting
from a severe storm are lower than previously thought. However, spillway capacity must be
addressed to reduce the risk further to the level required by dam safety standards.
· The Committee is pleased to report that, based on this new information, the Dam Safety
Section has decided to reduce the hazard rating, or consequence classification of the dams,
from their current “Extreme” rating to “High” for the Middle Dam and “Very High” for the
Lower Dam. Those new classifications will be put in place as soon as Dam Safety Section has
received and reviewed all of the detailed reports currently being completed by Golder.
What does this mean in practical terms?
· These rating reductions represent an acknowledgement by Dam Safety Section that the
dams carry significantly lower risks than an Extreme rating suggests.
· The Committee is now able to develop remediation options that will satisfy public safety
standards, consistent with these lower classifications. These include options to increase
spillway capacity at the Lower Dam, as Golder has determined that this improvement would
be of benefit in a severe storm.
· Any remaining incremental risk (due to dam failure) to the public downstream of the dams,
after remediation is in place, will fall within generally acceptable societal norms for
exposure of the public to risks (similar to that associated with the likelihood of a fatality due
to an auto accident).
· Golder’s detailed risk assessment report will be publicly available when it is finalized later
· The Committee remains engaged in the review of remediation options generated by Golder,
based on the outcome of the risk assessment and in discussions with Dam Safety Section
and expects that detailed information will be provided in this regard in the next 4-6 weeks.
· The City is also reviewing its current short term emergency mitigation plan in light of this
new information about the risk associated with the dams, recognizing that this plan was
taken into account in the risk assessment.
Mandate of Technical Committee
As described in October 21 2013 report approved by Nanaimo City Council
Focus on outcomes that only involve remediating the existing dams, using the following phased approach:
· Phase 1: Review the existing data and direct additional data collection as necessary.
· Phase 2: Develop option(s) for remediating the existing dams that will satisfy the
requirements of the Dam Safety Section of the Ministry of Forests, Lands and
· Develop plans for short-term physical alterations to the dams in 2014 to provide
the necessary time to carry out the long-term strategy (if required).
Objective of Technical Committee
As set out by Committee at its first meeting
Development of an environmentally minimally invasive, cost- and time-effective solution while satisfying required safety standards - i.e. a solution that addresses:
· The safety of downstream residents and workers;
· Dam Safety Section requirements;
· The respective objectives of the City, Snuneymuxw First Nation, the Colliery Dam
Park Preservation Society and the community;
· Environmental concerns, including fisheries habitat and ecology;
· Cost-effectiveness; and
· Having a timely permanent solution in place in 2014 if possible, but no later than
2015, with shorter term mitigation in place if required in 2014.