Environmental Engineering Reference
In-Depth Information
i , clearly the probability of service inter-
ruption to A is lower under a latticed system than under a single line system.
Latticed Redundancy is more effective at reducing the probability of service
interruption than both Looped and Multiline Redundancy. This is intuitive
i ¼ P ðÞ
n . Since PF ðÞ PF
PF
the
probability of interruption under a Multiline Redundancy includes that of the
Latticed Redundancy and that of failures occurring on every line without any zone
(see diagram) having n failures: P ð I A Þ ¼P ð I A Þþ P (every line experiences failure
| no zone experiences n failures). It follows that PI
A PI ð . That is, Latticed
Redundancy is better than Multiline Redundancy. Therefore, by transitivity, Lat-
ticed Redundancy is better than Looped Redundancy.
7.3 Risk Assessment
In order to manage risk properly, a comprehensive risk assessment must be con-
ducted. In developing an asset management plan, much information must be col-
lected. It would be ideal to collect information pertaining to risk in this data
collection stage. All risks must be identi
ed. Many of the risks relating to water
provision have been identi
ed earlier in this chapter. Each of these must be iden-
ti
c utility must
also be identified. Utilities may have risks specific to their geographic location,
population size, water source, local ecology, and so on. Any special features must
also be identi
ed in the risk assessment. Additionally, any risks unique to a speci
ed. From these features, any special risks must also be documented.
Upon documentation of all standard and special risks, the costs of these risks
must be determined. When determining these costs, it is necessary to measure them
in consistent units. Empirical evidence should indicate the cost and probability of
the risk. For those risks with a stochastic element, the variance of the risk and
probability distribution must be calculated.
The next step in the risk assessment is to determine an acceptable level of risk. In
the water industry, the state or provincial level of administration or indeed a higher
national level of government generally provides standards. However, in the event
that these standards are not provided, the utility must determine the standard on its
own. Upon determination of its water standards, the utility must place a value on
the costs of not achieving these standards. This valuation will assist the utility in
determining an acceptable level of risk. It will also assist in determining the value of
risk reduction.
The
final step in the risk analysis involves the monitoring of the risk manage-
ment plan. Risk assessment needs to be continuous. As new risks are encountered,
the assessment must be updated. The effectiveness of the risk management plan
needs to be monitored as well. This will indicate whether or not the plan needs to be
revised. Finally, the risky situations need to be constantly re-evaluated. Any
anticipated changes in risk are best dealt with proactively and in anticipation rather
than reactively.
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