BURNING ISSUES IN WATER INFRASTRUCTURE SYSTEMS
by: Balvant Rajani, Principal Research Officer
National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6
Three issues faced by most North American water utilities are: (1) Ageing water supply system (past), (2) increasing failure events (present), and (3) deciding what to do about it (future) with limited financial resources. These issues are discussed on how the past practices in the design and operations of water supply systems have led to the current situation of increased failure frequency and consequences. This is followed by a discussion how alternative strategies can help utilities take actions that will reduce risk.
How did utilities get to the situation they find themselves in? Current inventory of pipes in most water supply systems consists of different pipe materials such as cast iron (pit and spun), ductile iron, asbestos cement, PVC, prestressed concrete cylinder pipe (PCCP) and steel. A large proportion of the mains are cast and ductile iron pipes, a mere reflection of the availability of these materials when significant urban growth occurred. Past pipe design considerations that did not always consider issues such as cold temperatures, soil shrinkage and swelling as well as external and internal corrosion, etc are discussed in the context of how these could significantly contribute to the ageing (deterioration) of pipes in pertinent local conditions.
What are the manifestations of deteriorating pipes? A pipe usually breaks structurally at a specific location while hydraulic and water quality failures usually are manifestations of deficiencies on a wider scale. Thus a distribution system is deemed to have failed when it is unable (momentary or extended) to meet performance criteria, e.g., any pressure drop below a specified minimum, any unscheduled service disruption, any event of water safety breach, water aesthetic complaints, etc. Challenges to define failures and quantify failure frequencies and how these can influence management strategies for water supply systems are discussed.
What should or can be done? Allocation of limitation financial resources to alleviate problems caused by the deterioration of buried infrastructure requires careful examination of past performance, current condition and appropriateness of currently available technologies. However most utilities lack the necessary data required in most decision-making tools available to assist planners and engineer to make rational decisions regarding their renewal. Possible modelling approaches where data are scarce and cause-effect knowledge is imprecise are outlined.
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