Page 126 - Water Loss Control

108 Cha pte r Ni ne

Uneconomic
Additional cost/repair Economic

0 10 20 30 40 50 60 70
Repair time, days

FIGURE 9.3 Economic repair time. (Source: Stuart Trow.)

curve is a static situation, then there are less leaks running at any one time at lower
levels of leakage. Thus, moving from one point to a lower point will mean that addi-
tional leaks are brought in for repair before the situation reaches equilibrium again.
Transitional costs should generally be fairly low and they can be added, with appropri-
ate discounting as they are a one-off investment, into the calculation of the economic
level of leakage to obtain a slightly revised economic level of leakage.

9.3.4 Leak Repair Activity
A similar methodology to that for ALC can be applied to developing the economic level
of speed of repair. Very short repair times can be achieved but at the cost of possible
overtime for weekend and evening working for the repair teams. This may or may not
be economic. There will be a relationship between cost and repair time as in Fig. 9.3.
Leakage level will be related to the average repair time, and so a similar curve to the
ALC curve can be produced. The benefit from reducing repair times can be estimated
using a component loss model. The economic repair time can therefore be determined
in the same way as described above for ALC. At this point the marginal additional cost
of repair will equal the marginal cost of water production.

9.4 Long-Run ELL
Some leakage control activities will involve an investment decision, and hence a pay-
back longer than the short-run period. This will typically apply to options such as pres-
sure management and mains rehabilitation. In these cases, it will be economic to make
an investment on pressure management or rehabilitation to reduce leakage if the cost of
water saved over the investment period would pay for the cost of carrying out the
works. Once the investment has been made, there will be a new (lower) economic level
of leakage, which has to be recalculated using the method above.
9.4.1 Pressure Management
Leakage will reduce as a result of pressure reduction due to two factors, namely,

• Both background and leak flow rates will reduce, as leakage flow is directly
related to pressure by a factor called the N1 relationship. 11