Economic Aspects of Decision Making in Reliability Assessment of Water Distribution Networks - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 230

8.10 CONCLUSIONS

The research developed and discussed in this chapter has compared the implications of
increased investment and/or operational costs for the reliability of two network
configurations with different supplying scheme and with different topographic and design
scenarios. The economical aspects have been taken into consideration by applying the Present
Worth method that includes actual costs of operation for particular energy tariff and the costs
of annual maintenance calculated in simplified way. With this, an attempt has been made to
analyse the effect on the reliability if the same amount of money is invested to enlarge the
network and/or to improve its operation. The network layouts have been manipulated through
five different topographical patterns, three altitude ranges and four economic scenarios. Each
of the 120 scenarios was further tested for additional 20 design scenarios comprising gradual
increase of the pipe capacity combined with the reduction of pumping capacity, while
targeting the similar minimum pressure. The network reliability has been assessed by the two
resilience indices introduced in Chapter 5, namely the NBI and I n .

The analyses of the results done on total 2520 network scenarios lead to the conclusions that
can be summarised in the following bullets:
1. Reliability analysed on the level of whole network, or significant part of it, will be much
more efficiently maintained by investing into additional pipe capacity rather than by
installing additional pumping capacity. The solutions that favour additional pumping on
account of reduced pipe diameters hide calamities with potentially serious implications
for service levels.
2. In the economic scenario of high O&M costs and unfavourable loans leading to
substantial annuities, the least cost design scenario is likely to be more reliable than the
one with increased costs for O&M.
3. The design scenarios with balancing tank appear to be more reliable and less costly than
those with direct pumping, provided that the tank is properly designed and operated in
combination with pumping station.
4. The reliability of networks laid in topographies which allow substantial use of gravity for
water conveyance, appears to be more dependent on network capacity i.e. pipe volume.

In the process of analyses using the NDT, the following observations have been made:
1. The tool allows quick execution of large number of design scenarios but has limitations in
targeting the minimum pressure for different combinations of network conveying capacity
and pumping capacity. This is (currently) not done by using GA optimisation, except for
the initial scenario, because it would very likely be a time consuming process for any
larger (sample of) networks.
2. The alternative method that is using diameter multiplier and pump speed multiplier is a
very basic surrogate of optimiser that is pretty fast and reasonably effective. Nevertheless,
the selection of increments asks for trial and error procedure to avoid results outside
realistic operational range. Even with this, the fine-tuning of each and every design
scenario that could result in the preferred (fixed) minimum pressure is virtually
impossible if fixed multipliers are used across the entire range.
Finally, a few conclusions about the reliability measures used in the analyses:
1. The NBI-values have appeared to be much more responsive to the various terrain
configurations that is the case with the values of I n . Again, the conclusions made in