Decision Support Tool for Design and Reliability Assessment of Water Distribution Networks - Pattern Recognition for Reliability Assessment of Water Distribution Networks - page 236

9.1 INTRODUCTION

Design of water distribution networks aims at solutions that will guarantee acceptable service
levels. The common understanding of the word 'acceptable' will depend on the availability of
water at the source, needs and habits of consumers and their ability and willingness to pay for
expected level of service. Once all boundary conditions have been properly assessed in the
planning phase, the design would initially be approached from hydraulic perspective, the
main objective being sufficient water quantity delivered at guaranteed minimum pressure.
Until 15-20 years ago, the issues of water quality in distribution networks and network
hydraulic reliability were less considered due to absence of powerful methods and tools for
such analyses. As far the network reliability, the story would be kept short by including
comfortable safety factors in design solutions. Those were driven mostly by designers'
engineering experience combined with reasonable economic logic applied during assessment
of several scenarios. That however, done with little worries about potentially detrimental
impact of large pipe diameters on the water quality.

IT-revolution has significantly changed such a practice, whilst the awareness of consumers
about their service levels has grown in parallel, practically all over the world. Consequently,
water companies are increasingly pressurised to improve their efficiency at affordable costs.
Next to the consumers, the second main driver of this transformation has been the need to
protect ever limited drinking water sources in some countries and prevent their uncontrolled
depletion. The result of this new paradigm, possible due to better computer tools available,
has been a more stringent design process with significantly sharpened economic and
reliability considerations. This assumes design parameters taking into considerations reduced
pressures (and leakages), reduced fire demands, the effects of water demand management
programmes, quick isolation of the network areas affected by calamity or regular
maintenance, etc. The quality of the related analyses has improved mainly due to application
of pressure-driven demand (PDD) algorithm, which eventually enabled more insight into
irregular supply scenarios caused by various calamities, pipe bursts, electricity failures, water
rationing, etc. Consequently, the safety factors applied in the design could have received
more or less justification once the accompanying costs have been compared with the network
performance in stress situations, while assessing many more alternatives.

9.2 DESIGN PARAMETERS AND RELIABILITY CONSIDERATIONS

The aim of any network design is to provide conveyance of sufficient water quantities and
then to preserve the water quality obtained at the source i.e. after the treatment process has
been completed. From the perspective of public health, this is somewhat twisted logic driven
by assumption that sufficient pressure safeguards the quality of water preventing intrusion of
pollutants. Furthermore, the problem of overdesigned pipes leading eventually to water
stagnation can be mitigated by additional disinfection and pipe cleaning activities, which is
mostly true but costs money. More threatening water quality problems actually arrive from
inadequate construction and operation of the network, such as poor laying and jointing of
pipes, poor selection of materials and appurtenances, intermittent operation or poor protection
of pipes against internal and external corrosion, etc. This all is difficult to predict and/or
prevent in the hydraulic design phase.