Decision Support Tool for Design and Reliability Assessment of Water Distribution Networks - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 238

There is a clear hydraulic dependency between the demand and pressures in any water
distribution system. Delivering a satisfactory level of service will mean that the maximum
demand expected in the system under regular supply conditions will be satisfied maintaining
the pressures above accepted threshold. Further increase of the demand for any reason, for
instance a fire fighting, will cause a corresponding drop of the pressures. The service will
also be violated in case of a failure where parts of the system become supplied though
alternative routes, causing increased energy losses. As a consequence, the drop of pressure
below the threshold will start to affect the demands causing their reduction. As the pressure in
many systems in the developing countries is below the above-listed limits, for various
reasons, its impact on the demand is even more profound than in case of well-operated
systems.

As for the hydraulic gradients, the design criteria depend on adopted minimum and maximum
pressures, the distance over which the water needs to be transported, local topographic
circumstances and the size of the network, including possible future extensions. Trifunović
(2006) suggests the following values as a rule of thumb:
-

5-10 m/km, for small diameter pipes,

- 2-5 m/km, for mid-range diameter pipes,

- 1-2 m/km, for large transportation pipes.

Hydraulic gradients tell something about the network conveyance i.e. the balance between the
energy input and energy loss and as such, the balance between the investment and operational
costs. They eventually reflect whether the minimum pressure in the network has been created
through increased pumping or enlarged pipes, which also has the implications on network
resilience against the whole economy of water distribution, as discussed in Chapter 8.

Finally, the velocity range is also to be assessed in the network design process. Too low
velocities have potential implications for water quality (sediment accumulation, low chlorine
residuals, increased corrosion), while too high velocities are mostly corresponding to high
hydraulic gradients, indicating exceptional head-losses. The recommended design values are:
- ± 1m/s, in distribution systems,

- ± 1.5 m/s, in transportation pipes,

- 1-2 m/s, in pumping stations.

These values are however not easily achievable where particular (high) reliability is deemed
necessary, be it in the form of fire demand, provision of capacity in case of repair and
maintenance, or even as a consequence of overwhelming demand forecasts.

Figure 9.1 shows the pipe flushing diagram in one location of the city of Rotterdam in The
Netherlands, which is a result of sediment removal accumulated by very low velocities
(Source: Water Supply Company 'Europort', currently 'Evides', Rotterdam). The diagram
shows normal operating pressure of approximately 2.7 bar (27 mwc) while the velocity is
barely 0.1 m/s. By opening and closing the hydrants, a sufficient velocity is generated (close
to 1.5 m/s) to remove the sediment from the pipe. During those moments, the pressure drops
down to one bar. The pipe flushing will be conducted in normal working hours, which does
not affect the consumers in surrounding areas significantly because of sufficient extra
capacity in the rest of the system. Evidently overdesigned network in this case originates
from two facts: (1) high fire demand requirement and (2) specific demand prognosis done a
couple of decades ago that did not take into account the water demand management
programme successfully implemented in the last two decades. Consequently, the reliability