Eurobodalla Integrated Water Cycle Management Strategy

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Eurobodalla Integrated Water Cycle Management Strategy
Examine the performance and adequacy of the barriers and management
procedures in place to minimise the supply of poor water quality
Determine the water quality parameters that frequently exceed the failure criteria
and the consequences of these failures.
The Australian Drinking Water Quality Guidelines were used as the failure criteria when
assessing the medium to long term consequences and service provision risk.
The study found that the water quality of the supply sources is influenced by weather
patterns and activities in the catchment. It concluded that whilst the combination of
management procedures and barriers are effective in reducing the supply of poor water
quality, they do not meet the current best management practice standards.
Security of Supply
Security of supply is a measurement of the reliability of the water supply headworks; in this
case the source rivers and the dam, particularly during drought periods. Supply security is
considered to be adequate when reasonable customer demands can be met on most
occasions without restrictions.
The NSW Government has defined ‘secure yield’ as the maximum supply rate that can be
maintained by the supply system without exceeding any one of the following three
acceptability criteria (NSW Government, Water Supply and Sewerage Management
Guidelines, 1991):
Reliability – The proportion of the supply that is unrestricted. Over any extended
period, restrictions should not be in place for more than 5% of the time. In
Eurobodalla for example this would mean that based on 100 years of stream flow
the total duration of water restrictions would be less than 60 months.
Robustness – The average frequency of restriction should be less than once every
10 years. More precisely there would be less than a 1-in-10 chance of having to
impose restrictions in any one year.
Security - The storage will not be drawn down to below a critical level that would
prevent Council providing even a basic supply or require alternative supply
measures. The IWCM strategy guards against this scenario by ensuring that the
system can supply 80% of unrestricted demand from the time restrictions are
imposed. This is based on the conservative assumption that the full drought of
record could recommence at this time.
The above three acceptability criteria are commonly referred to as the 5/10/20 rule. Implicit
in this rule is the trade-off between risk, community costs and social expectations. This rule
was developed and adopted by the NSW Government in the mid-1980s and has since been
used in the water supply planning for nearly all country towns in NSW. Although the IWCM
strategy guards against storage levels being drawn down to a critical level by ensuring that
the system can supply 80% of unrestricted demand from the time restrictions are imposed,
this margin will reduce over time with aggressive demand management.
The calculation of secure yield requires modelling of the water supply source and system,
which is related to the following:
Weather pattern in the water supply catchment and the urban areas
The environmental needs of the rivers including access sharing rules with other
water users
The quantity of water that can be stored