aggregate conditional probability of dam failure
along each failure path is calculated. The
summation of the probabilities of failure of all
paths gives the probability of dam failure. The
issue of piping is complex, and judgement coupled
with analytical method is required as a guide for
this type of risk analysis.
Consequences
Consequences include potential loss of life,
economic loss, environmental and social
damages. The inundation maps prepared from the
dam break simulations were used to estimate the
population at risks and the properties in the flood
plain. The consequences due to the dam failure
would be catastrophic.
The risk management plan followed the
guidelines of the Canadian Dam Association on
the principle that the risk should be as low as
reasonably practicable (ALARP) and the maximum
level of societal risk for life safety should be less
than 10 -3 /year for loss of one life that was not
explicitly foreseen and identified in advance of the
failure; a higher risk is considered unacceptable.
Risk Reduction Measures – Options
Evaluation
Options evaluated include: Option 1 – Do-
nothing, Option 2 - Lowering the reservoir water
level, Option 3 - Construction control and Option
4 - Structural measures using soil improvement
works.
Option 2 – Lowering the existing reservoir
water level from an elevation of 43m to 34m was
recommended for the least contractual impact
and minimum cost involved. Other merits of
Option 2 included:
- The ridge contains large amount of suitable
clay core materials which can be abstracted for
construction of the project. The need to import
clay core materials from external sources can be
avoided.
- A channel can be cut through the ridge so
that the two reservoirs can be connected. This
would allow omission of draw off works at the
right valley reservoir and extensive optimisation of
design for raising the existing dam.
However, Option 2 involved reducing the
amount of water stored in the reservoir from 22
MCM to 10 MCM which had a serious impact on
the security of the water supply system of Penang
State. The potential risk was not acceptable to the
Penang Government unless an alternative water
source as a back-up could be provided. Beris Dam
water source from Kedah state was identified as
a suitable back up source. However, charges on
raw water release from Beris Dam was imposed
by the Kedah Government. The issue was resolved
through a process of negotiation amongst the
Federal Ministry (KeTTHA), the Penang and Kedah
State Governments.
Conclusions
1.
2.
3.
Construction of a large scale water source
development project is always exposed to
some construction risks and uncertainties
that can lead to delay and cost overruns. Risk
management should be applied throughout
the construction period to minimise risk and
to achieve the best outcomes for the pro