volume 1 - Halifax Regional Municipality

Halifax Water Integrated Resource Plan
Appendices
As noted previously, the first stage of the IRP analysis was carried out for the initial and
intermediate alternative resource plans. It explored the A, B, C (monolithic) level for all
asset systems (i.e. water, wastewater and stormwater) and all components within each
system (e.g. distribution mains).
In reality, the risk of failure has significantly different impacts depending upon which
asset system or component is under consideration. For example, the failure of a single
sewer pipe, while inconvenient, does not have a widespread or generally high impact. In
contrast, the failure of a major water transmission main will have a significant and
widespread impact on water supply. Since asset renewal resources are constrained, it is
appropriate to have a higher level of asset renewal for those assets that are “high risk”
and a lower level of renewal for those whose failure poses a “low risk”. An asset
component-by-component risk analysis was conducted to determine the acceptable
levels of asset renewal for each component. This was the second stage of the asset
renewal program analysis and was undertaken for the intermediate plans creating a
single optimized component asset renewal strategy.
The risk analysis was based on methodology originally developed by the City of
Edmonton 7 to assess the risks of failure associated with all municipal assets – not just
water, wastewater and stormwater infrastructure. The failure impact of each asset
component was assessed on the basis of best professional judgment by the study team
(Halifax Water and consultants). A high (red), medium (yellow) and low (blue) coding
scheme was used. The meaning of failure impact was understood to be “the impact of
the failure of the system component (e.g. distribution main) on the ability to meet the
asset system (e.g. water system) LOS”. Accordingly, the failure of a major WSP would
have a high impact on the ability to meet the LOS for service outages while the failure of
a single distribution main would have a relatively low impact on the LOS from a systemwide
perspective.
The failure severity is defined as “the impact of a single failure x the failure amount”.
The failure amount is in turn related to the likelihood or probability of one or more
components failing. This has most relevance to asset systems with multiple components
such as distribution mains or sewers. For example, water transmission mains have a high
failure impact. If it were determined that due to condition, age and other factors that
multiple mains have an elevated probability of failure, then the overall failure severity
would be high. Condition data are limited and hence decisions in this IRP rely on the
knowledge of asset age and an estimate based on industry norms of expected remaining
life. This was, in fact, the basis of the scoring for the failure severity for each asset
component. Two metrics derived from Table 2.2 for water assets and Table 2.4 for
wastewater and stormwater assets were applied – average weighted asset age at the
7 Edmonton Bridges the Gap – A Risk Analysis Approach to the Municipal Infrastructure
Gap, S. AbouRizk et al., Summit, Jan/Feb 2005
Revision: 2012-10-29 Integrated Resource Plan –Appendices F-17
October 31 2012 Page 196 of 272