Economic Aspects of Decision Making in Reliability Assessment of Water Distribution Networks - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 212

2. Slope up . In this setup, the consumers are located higher than the source(s). The supply
scheme would be the same as for flat terrain except that higher-lift pumps are needed.
3. Slope down . In this setup, the consumers are located lower than the source(s), which gives
possibility of supply by gravity. If the available elevation difference is not sufficient,
additional pumping may be necessary. Nevertheless, those pumps will be of lower lift
compared to the situations under 1 and 2.
4. Hill . When the source and the consumers are separated by an area of high altitude, the hill
can be used to position a reservoir that should be supplied by constant flow from the
pump, and supply the variable demand by gravity.
5. Valley . If the distribution area is located in a valley, a possibility exists to locate the
reservoir on the opposite side of the source. The benefit is twofold: (1) the pumps can
supply average flow constantly throughout 24 hours while part of the area on the side of
the reservoir are supplied by gravity, and (2) as a consequence of such supply scheme
more balanced selection of pipe diameters makes future extensions of the network easier.

Each situation in reality can be covered by one of the above setups or, in more complex
networks, by their combination.

8.5 HYDRAULIC RELIABILITY AND ITS COSTS

The above five cases have different implications on the cost of operation and network
reliability, which can be summarised in the following bullets (assuming single source of
supply):
- Pumps in any of the first three terrain configurations will be designed on the maximum
consumption hour of maximum consumption day. This capacity can be reduced if the
reservoir is introduced within the service area; the more volume has been introduced to
balance the demand, the smaller the pumps can be.
- Pipe diameters will be gradually reduced, from the source towards the end of the system
in case of single source supply in the terrain configurations 1, 2 and 3.
- In case of gravity supply applied in the terrain configuration 3, larger pipe diameters will
be needed to utilise the topography as much as possible, on account of reduced head-
losses. The more of pumping capacity has been introduced, in combination with gravity,
the smaller the pipes can be.
- The pumps in the terrain configurations 4 and 5 will be designed on the average
consumption hour of maximum consumption day. This capacity can be increased if the
volume of the balancing reservoir has been reduced.
- Pipes connecting pumps and reservoirs in the terrain configurations 4 and 5 will be more
balanced in diameter than in case of the terrain configurations 1, 2 and 3.
- The reliability of supply with balancing tank should be higher in general, compared to the
direct pumping supply scheme.

Nevertheless, the cost of storage comes on top of the cost of pipes once the investment costs
are to be considered with the operational costs. More reservoir/pipe volume means:
- increased investment costs,
- reduced operational costs,
- more buffer in the network,
- higher maintenance costs (for storage).