[CHAPTER X.]
THE STORAGE AND DELIVERY OF WATER.
The methods of storing and delivering water will vary with its source. In rural districts, deep wells and springs are the best sources of supply; but in large towns they are found to be insufficient for the wants of a rapidly-increasing population; and they can only be multiplied in a given district within certain limits, as every well drains a large surrounding area. The supply from surface wells in gravel or sand beds or in chalk districts is liable to fail in seasons of drought; but deep wells in oolite or chalk formations, and in the new red sandstone, generally yield a constant and abundant supply.
When the water is supplied from upland surfaces, springs, or small streams, a collecting reservoir is required. This is generally a natural valley below the level of the source of supply, but of sufficient elevation above the place supplied to allow the water to be distributed by gravity, without any pumping apparatus. The reservoir should be large enough to hold five or six months’ supply, and its embankment should be perfectly water-tight, and of great strength.
When water is collected from upland surfaces, it is important to know the amount of rainfall to be reckoned on. If we know the area of the surface which drains towards the reservoir, and the average rainfall, the total rainfall is easily calculated. This will, however, differ greatly from the available rainfall, owing to the losses from penetration into the ground, evaporation, and other causes. The amount lost will vary, according to the season, from one-half to seven-eighths of the total rainfall; and according to the soil (page [68]). The proportion of percolation in the chief water-bearing strata surrounding London varies from 48 to 60 per cent. (Prestwich). It is less when the ground is steep and the rainfall rapid, and usually less in winter than in summer.
Water collected near its actual place of fall, and from uncultivated districts, is always purer than that collected further from its source, and from cultivated land.
From the collecting or impounding reservoir, water is carried by the aqueduct or conduit either directly into the service-pipes, or when the pressure is too great, into a second service-reservoir, resembling the impounding reservoir in general structure, and capable of holding a few days’ supply.
This must be high ground, above the level of the highest houses to which water has to be supplied, as water cannot rise above its own level. When this cannot be arranged, the water is pumped into tanks at a higher level, and distributed from them.
The greatest hourly demand for water being double the average hourly demand, the water-mains supplying a town must have double the discharging power that would be required, supposing the demand was uniform. The first requisite of a supply of water is that it should be abundant, and sufficient in amount for any extra strain on its capacities. Water ought to be laid on to every house, and to at least two floors of the house. Anything preventing free access to water, militates against cleanliness.
Cast-iron is the most serviceable material used in the construction of the main water-pipes; it is coated with pitch, or Dr. Angus Smith’s varnish, or with magnetic oxide of iron (Barff). The service-pipes to each house are generally made of lead, and the ease with which this material can be bent and curved, and carried to the different floors of a house, makes its use very convenient. Lead pipes, furthermore, can be easily obliterated in case of bursting, and so any waste of water and flooding of the house minimised. Some kinds of water, unfortunately, act on and dissolve lead; this is especially true of soft waters and those containing organic matter. Shallow wells, being very liable to organic pollution, ought never to have the supply-pipe of their pumps made of lead. With hard waters, lead pipes may generally be used safely. When the quality of the water renders lead pipes objectionable, the use of iron, tin, zinc, tinned copper, earthenware, gutta-percha, and other materials, has been suggested. Of these, cast and wrought-iron pipes are the most serviceable, or pipes composed of an inner lining of block-tin and an outer of lead, a layer of asbestos intervening to prevent galvanic action between the metals. According to Rawlinson, “supply-pipes of wrought-iron are cheaper, stronger, and more easily fitted than service-pipes of lead;” but it is urged against them by Perry, that with soft water they become choked by rust in a few years. If galvanized they are more durable. Cast-iron pipes are rusted less easily than wrought-iron.