Balancing Supply and Demand in the UK

The supply/demand balance of water distribution in the UK is under stress. From the demand side, there are four clear upward trends at present:

• demographic forecasts show an increasing UK population
• an equally important demographic factor is the continuing reduction in average household size, which leads to greater per capita consumption (single occupation can double the per capita consumption compared to households of 3 and more)
• summers are expected to be hotter and dryer in the future, with implications for higher demand
• national leakage figures, which reached a low point in the year 2000 at just above 3000 megalitres per day nationally, have been on an upward trend since then.

To meet this increasing requirement, the supply side needs very careful management. Increasing the supply of water depends on the good management of both existing and new resources, optimising the use of both to meet the demand pattern. In this paper we propose that one contribution to achieving this objective can be the concept of “conjunctive use”.

Conjunctive use is essentially using different water sources according to the conditions and maintaining flexibility between these sources. In wet conditions there is surface water available for use in watercourses and lakes, whilst in dry conditions such water is not available and ground water sources such as bore-holes, springs and adits are the likely source. Water companies can move flexibly between the two at different times of year or under different weather events.

The principle of flexible usage of sources in this way has always been available to water engineers; but there are three principal reasons why this option is often resisted. First, there are potential problems of water distribution to be solved - if the different supply sources have different entry points to the network, there must be confidence that water can be distributed at adequate pressure throughout the network from all entry points. Second, there are issues of whether customers would accept the water quality changes that would result as supply changes between differently sourced waters. Third, there is the likelihood that if the point of entry to the network is switched, the direction of flow will reverse in some pipes and sediment may be disturbed.

In any exploration of conjunctive use, we advocate a high level feasibility and cost benefit analysis before looking at the detail. The hydraulics issues can be addressed using an advanced network modelling tool such as InfoWorks WS, which can also address the issues of sedimentation. Results from the hydraulic and water quality simulations obtained in the network modelling can provide the base data for an advanced analysis using specialist software such as WRc’s Water Quality Prediction Tool. Such analysis can give high level answers to questions of the feasibility, capital and operating costs of alternative scenarios, and optimisation. Only after the outline viability has been established should the detailed level of analysis be undertaken.

If the high level analysis indicates that conjunctive use is feasible, detailed analysis will be required to prove the case. This should cover the same ground as the higher level analysis - hydraulic modelling, water quality analysis, capital and revenue costing of the options, and a final cost-benefit analysis - but should go to such detail that the case is proven.

The last of these, customer acceptability, is often the final judgement that has to be made regarding the merit of conjunctive use of water sources. The hydraulic modelling may show that it is entirely practicable, the cost/ benefit analysis may show it is financially sensible, the water quality parameters may show that it is within all statutory limits and engineering limits on water quality. But the final unknown is customer acceptance. Perhaps consumers as well as engineers need educating in the benefits of conjunctive use of water sources.
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Dene Marshallsay, Head of Demand Management at WRc plc. First presented at Wallingford Software’s 2003 International User Conference.