Water Supply

Choice of Sciprce.

Recent developments in the methods of water purification have made it possible to consider many waters as potentially suitable for public supply purposes which but ten or twenty years ago would have been entirely ruled out. It fol lows that the choice between utilising a relatively pure water from a distant elevated source or a water of a much lower standard from a nearer source which calls for pumping and greater work ing cost in filtration and other treatment, is now largely a question of cost of water. This means the cost per i,000 gallons delivered at the main service reservoir after taking into account interest and sinking fund charges and annual working expenses—cost of labour, chemicals, fuel, repairs and maintenance.

The initial cost of water from a gravitation scheme is usually high because the impounding reservoir and all or part of the aqueduct must be constructed at the outset of sufficient size to meet the ultimate requirements, although the initial demand may be but 20 or 25% of the ultimate. Another factor, which applies particularly to British work, is that it has been the practice of Parliament to insert a Clause in the Act authorising the construc tion of impounding reservoirs whereby one-tenth to one-third of the yield of a catchment area is to be released in a more or less regular flow from the reservoir as "compensation water," and this of course may place a serious additional burden on the under takers. In the United States the riparian and other interests are most frequently compensated by monetary payments.

It is therefore evident that for the economical development of impounding schemes, a relatively large initial demand is desir able and this, in a measure, explains the modern tendency to the formation of joint Water Boards with the consequent reduction in the cost of water to each constituent authority.

In a pumping scheme, on the other hand, the initial capital expenditure is, as a rule, much smaller, and the working expenses are larger but roughly proportionate to the quantity of water pumped. The capacity of such schemes can also be augmented from time to time as required to keep a few years ahead of the demand at relatively small cost, whereas when the demand is approaching the yield of an impounding reservoir it is necessary to construct another similar reservoir and duplicate increasingly long lengths of the aqueduct, so that it is usually advantageous for a single authority to embark on a pumping scheme where there is any choice.

Impounding Reservoirs.

The considerations which deter mine the capacity of the impounding reservoir are dealt with under RESERVOIRS, and the type of dam by which the reservoir may be created under DAMS. It may, however, be emphasized that frequently as much as 3o or 40% of the cost of the dam may be expended on subsurface works for preventing undue leak age through pervious beds under the structure or round its flanks, and a masonry dam may have to be carried to considerable depths to ensure that the structure rests upon material strong enough to withstand the loading which is the resultant of the water pressure and the weight of the dam itself.

An essential preliminary to the selection of the site for a dam is a thorough geological investigation of the area within the limits assigned to the dam, and a general examination of the valley above the dam ; such an investigation will not only prevent waste of money in locating a dam at an unsuitable site, but will also enable a much closer estimate of the cost of the work than would otherwise be possible. In this connection the use of boreholes only as a means of exploration is insufficient, and may lead to erroneous conclusions : they should be used to supplement the results obtained from trial shafts and to clear up doubtful points.

The cementation process by means of which cement is injected under pressure through drilled holes over the site of a dam, has of recent years been used to strengthen rock which otherwise might have had insufficient strength, and also to seal up fissures and joints through which water might escape. This process therefore makes it possible to use sites or structures which might otherwise have been considered unsuitable, and in more favour able sites, to reduce the extent of the subsurface work.

Gravitation Aqueduct.

In fixing the size of the various portions of the aqueduct an allowance over the average daily sup ply which the aqueduct is intended ultimately to carry, apart from any allowance required to meet loss of capacity due to encrusta tion, must be made to cover the filling up of the terminal service reservoir of ter depletion consequent upon a burst in the aqueduct, and also to cover seasonal variations. The precise allowance de pends on the length of the aqueduct, its liability to interference, accessibility and the economic possibilities of terminal storage; in general i o per cent. would be sufficient : this might be reduced if a relatively large storage-5 or 6 days—could be economically obtained at the end of a long and costly aqueduct, and increased if the aqueduct were short or only 2 or 3 days' storage could be obtained. The aqueduct is chiefly of cut and cover or tunnel with relatively short connecting lengths of pipe line or lines where the general route can more or less contour the hillsides, or entirely or chiefly of pipe line with short intervening lengths of tunnel where the route crosses hills.