External Drivers

In practice, there will be many external influences on the various aspects of the supply demand balance. Figure 9.6 illustrates this.

Figure 9.6 shows that it will be necessary to look at apparent loss management strategies as well as real loss strategies. Although apparent loss management strategies do not in themselves reduce water production they will generally increase the recorded

Figure 9.6 Supply/demand drivers. (Source: Dave Pearson/Stuart Trow.)

water used making this more transparent. This in turn may then make demand management strategies more cost effective. A well-developed strategy for apparent losses will also reduce wasted expenditure looking for real losses which do not actually exist.

However, it is strictly the supply/demand balance itself that drives the final solution. In this case, demand is the sum of real losses and consumption. The evaluation has to be carried out at water resource zone level, that is, where all the customers have the same level of security of supply taking into account all possible internal and external drivers.16

External drivers on water abstracted may include

• Environmental concern over low flows

• Environmental damage from over abstraction

• Environmental drivers, for example, European directives such as the Water Framework, Birds and Habitats directives, or equivalent

• Carbon footprint of water production

External drivers on water use may include

• Regulatory water efficiency targets17

• Sustainable water use targets18

External drivers on the supply/demand balance may include

• Security of supply requirements

• Risk of supply restrictions in drought conditions

• Impact on social and economic progress

• Risk of additional environmental damage in drought conditions

External drivers on leakage performance may include

• Regulatory minimum comparative performance

• Social and economic cost of disruption

• Possible political target

• Carbon footprint of repairs

• Carbon footprint of detection activity

The least-cost solution to meeting the supply-demand balance can be found using a standard optimisation method, for example, genetic algorithm or unconstrained mixed integer optimiser, using a formulation such as

Minimise the total cost of operating the system including

• Pressure management

• Proactive leakage detection

• Reactive leakage detection

• Rehabilitation

• Water production

• Demand management options

• Apparent loss strategies

• Resource development

• Abstraction mitigation

Subject to achieving (at least)

• Security of supply target

• Leakage target

• Water use target

• Apparent loss target

• Carbon footprint target

• All environmental constraints (low flows, habitats, etc.)

9.6 History and Experience 9.6.1 England and Wales

England and Wales (E&W) have a well-developed supply system with over 99% of properties connected to public water supply networks. Continuous supply is available 24 hours a day with less than 0.02% of premises receiving low pressure (usually taken to be less than 15 m) at any time during the year.19 Only 34% of properties are metered,20 the rest pay for water based on the value of the house. However the network is of mixed age with some parts of the network well over 100 years old. There is a small number of operating companies (less than 25 covering over 20 million properties), which were privatised in 1989, and there is a strong environmental and economic regulatory regime. Figures on leakage are reported to the regulators each year and audited by independent assessors. Every 5 years the companies have to develop business plans for the following 20 years, which include a full engineering assessment of their assets and a financial model of forecast income and expenditure. This is used to establish the price limits for the next 5 years. Part of the engineering submission involves the assessment of the economic level of leakage and whether this is constrained by headroom or not. Following the severe drought in 1995-96, leakage levels have been reduced by over a third and leakage targets are set by the regulator each year based on companies' assessment of their ELL. Most companies are operating at or close to their assessed ELL. Several companies are operating at a level that is constrained by headroom.

The assessment of ELL within England and Wales has a long history. Although there were many papers on ELL, the first national study and report on the topic was published in 1980.21 This set down a methodology for the assessment of ELL, and it identified the benefits of pressure control and sectorization in managing leakage. This led to the implementation of sectors (DMAs) in most companies in England and Wales. The findings of this report were updated by a major national research programme that reported in 1994.9 This and subsequent reports have led to greater understanding of the relationship between pressure and leakage and other activities which allow the construction of models to forecast the effect of changes in operating regime on leakage. There is a very high level of monitoring, and hence data availability, within England and Wales, for example, 15 minutes flow and pressure data on each sector. Most companies now have fully calibrated all mains hydraulic models of their networks. As a result of the drought in 1995-96 a number of companies initiated major leakage management programmes based on economic assessment outlined in this paper. One of these involved the construction and implementation of over 2000 pressure management schemes within a 3-year period. As a result of this, a company supplying over 3.2 million properties reduced their average night time pressure from over 50 m to less than 40 m.3 All companies implemented a free or heavily subsidised programme for the repair or replacement of customer supply pipes in order to speed the repair of leaks that previously required the serving of statutory notices.

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