The HIDDEN LEAKS OF DISTRIBUTION NETWORKS IN MOUNTAIN AQUEDUCTS: A NECESSARY EVIL?

1) INTRODUCTION

In the mountain regions, one of the main features in the drinking water distribution network, working with elevated exercise pressure is that of accusing remarkable hidden leaks. Rare are not the cases where the water scattered in the ground overcomes the 50% of the total one introduced in the network.
This is a widespread phenomenon which, besides involving the above-mentioned scattering of that precious good which is water, causes a serious economic damage of the management too.
The note proves, on one side, that the presence of leaks in such remarkable quantities paradoxically presents a positive connotation; on the other side, that the elimination of the leaks is almost impossible unless the drinking water supply network is radically modified.


2) DESCRIPTION OF THE PROBLEM
A regular working of the drinking water distribution network requires a piezometric as much parallel as possible to the ground and with values of relative pressure included between a minimum of 20 and a maximum of 50 metres of water column as regards the soil.
Though such result is easily attainable in the lowlands, this is no more the case with notable gradients of hill or mountain side built-up areas. The rules to follow in such a case are shown in a detailed article with the title: "The Drinking Water Distribution Network in Areas with Remarkable Topographical Gradients". This article shows the importance of dividing the network in a variety of different "sub-networks", each of them with an equal altimetry and a height to remain within defined limits. Furthermore the pressures of these "sub-networks" have to be constantly checked.
These ideal conditions of employment, although granting in themselves an adequate piezometric in constant parallelism to the ground, are rarely adopted being the management boards of aqueducts rather orientated towards unifying the networks, despite the altimetry of the territory to supply. The result is an extremely simplified water service, either in the construction of the plants or in their management, which however shows the above-mentioned drawbacks. In particular, an absolutely inadequate pipe pressure subjected to frequent variations. As soon as the water supply shrinks, the pipe pressure increases until nearing the static level; at that precise moment the pipe pressure values are so high to cause different drawbacks as we will see later on.
Let's take, for example, a single network supplying a built-up area situated on a 300 metres' gradient slope. Pressure is kept within normal values when expenditures are elevated, whereas when they are close to zero, pressure achieves in the lowest parts of the territory, values of about 300 metres on the ground, which are therefore absolutely unacceptable.
If we divide the distribution network into five "sub-networks" with an average height of 50 metres each, pressure would be kept parallel to the ground, both in periods of higher expenditure and in periods of lower one. If this is true for this last type of distribution network, whose pressures may be constantly kept under control, though it follows a step-by-step tendency, it certainly may not be adopted with unified distribution networks.
In this last case, in fact, a huge number of hidden leaks leads pressure to constantly correct itself. This causes an excessive increase in water flowrate scattered in the ground. So doing, at the end, the operation to zero consumption and therefore the static level of piezometric in the distribution network are absolutely unattainable aims.
During the effective working, the amount of leaks constantly and automatically changes. It acquires minum values during the moments of the day with a greatest need, to increase hugely during the night or other moments of the day with a slightest need. This causes such relevant leaks in the water distribution network to draw water pressure within acceptable values.
We may therefore conclude that:
1. In unified management of distribution network in mountain areas, hidden leaks, though constantly subjected to controls, are extremely varying according to the period. They are in fact almost unrelevant during the day time, with the greatest users' request of water supply. Hidden leaks become, on the contrary, considerable during the night or the moments of the day with the least users' request of water supply.
2. Hidden leaks in the distribution network mentioned in the previous point, though causing a serious damage, that is the scattering of precious water, are nevertheless important to adjust pressure which, without these leaks, would be absolutely unacceptable.
3. The elimination of the hidden leaks in such a distribution network is not possible. Paradoxically, the real management of aqueducts should simply repairs the biggest pipe leakages, while the frequent smallest ones, which very much depend on pressure variations, automatically adjust the distribution network pressure, thus contributing to bring it back within normal values
4. The real working of such a unified distribution network in mountain areas is therefore paradoxical since it leads us to consider that hidden leaks are useful. What above written, cannot but confirm the general need to divide the unified distribution network into homogenous "sub-networks", with controlled pressure, as it is shown in the article "The Drinking Water Distribution Network in Areas with Remarkable Topographical Gradients".

3) CONCLUSIONS:
The critical analysis of unified drinking water distribution networks in hill or mountain side built-up areas with notable gradients, draws to the conclusion that the presence of important and frequent hidden leaks, whose working is not yet clear, has a double contradictory aspect.
They, from one side, represent a great disadvantage due to the huge scattering of water in the soil, but from the other side they represent an advantage adjusting pressure which, otherwise would achieve unbearable values. Another important conclusion is that the total elimination of hidden leaks from distribution networks is an almost unattainable aim, as far as the unified type of distribution network exists.
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