1902 Encyclopedia > Water Supply

Water Supply




WATER-SUPPLY. An ample supply of pure water is of the utmost importance for the healthiness of towns. When the population of a district is scattered it is possible to supply individual wants by means of streams, springs, or shallow wells ; but when a number of people are con-gregated within the limited area of a town the natural supply of water in this area is liable to be insufficient, and is also in danger of being contaminated by sewage and house refuse. Accordingly, works for the collection, storage, purification, and distribution of water are indis-pensable necessities in towns, for the preservation of health and the promotion of cleanliness. The remains of aqueducts near Rome and at other places (see AQUEDUCT) show that important works for providing water were undertaken many centuries ago; and water for irrigation and other purposes has for ages been stored, in the rainy season, in tanks by the natives of India. Artificial pro-visions for the supply of water were, however, entirely neglected in Europe during the Middle Ages, and the colossal works of earlier times were allowed to fall into decay; and most of the present systems of water-supply are of comparatively modern date.

ORIGIN OF SUPPLY.

Rainfall.—All supplies of fresh water come primarily from the clouds, though portions may eventually be drawn from the bowels of the earth. Water when distilled is obtained in its purest form ; and the heat of the sun is continually drawing up large quantities of moisture from sea and land, forming clouds which return it as rain to the earth. Some of the rain is quickly evaporated from the surface of the earth, and returned to the clouds ; some sinks into the ground to feed springs and underground stores of water; and some passes into streams and rivers, whence it flows into the sea, from which the greater portion of the rain is derived. The available water-supply of any district, accordingly, principally depends on the rainfall of the locality and the extent of the gathering ground. The rainfall varies greatly in different places, and at different periods of the year (see METEOROLOGY) ; and, in England alone, the average annual rainfall at the Stye, the wettest part of the lake district in Cumberland, is about eight times the rainfall at Hunstanton, the driest locality in Norfolk. The rainfall, moreover, varies from year to year; and the driest years must be taken into consideration in estimating the available water-supply.

Evaporation.—The proportion of the rainfall which is actually serviceable for water-supply depends greatly upon the season of the year in which the rain falls ; for evaporation is very active in the hot season, whereas in the cold season its influence is slight. Accordingly, for hydrological purposes, the year may be divided into two seasons in the latitudes of the British Isles,—the warm season, extending from the beginning of May to the end of October, and the cold season, embracing the other half of the year. Comparatively little effect is produced by variations in the amount of rain in the warm season, except in extreme cases, owing to the large proportion drawn off by evaporation, whereas the rainfall during the cold season is of the utmost importance for replenish-ing the sources of supply which have been drained during the summer. The period of the year, therefore, in which the rain falls is of more consequence than the total amount in the year; and a drought is much more likely to result from a dry winter than from a dry summer. Any deficiency in the supply is generally felt towards the close of the warm season, when the reserves of water, furnished by the rains of the preceding cold season, have been reduced to their lowest level by the demands of the summer.

Other circumstances also modify the influence of evapor-ation. Eain descending in a heavy continuous fall, by sinking into the ground or escaping into the water-courses, is less exposed to diminution by evaporation than several separate showers of rain, equivalent in total volume, but spread over a longer period. Forests and vegetation shelter the ground from the influence of evaporation ; and thereby, in spite of abstracting some of the moisture for their own requirements, they augment the proportion of available rainfall. The nature and slope of the surface stratum, moreover, notably affect the loss from evaporation. Rain falling on an impermeable stratum is almost wholly evaporated in hot weather, when the surface is flat; but it flows off from a steep slope before evaporation can produce its full effect. Rain readily sinks into a porous stratum; and when a depth of three or four feet below the surface is reached it is to a great extent withdrawn from the influence of evaporation.

Percolation.—The percolation of rain through porous strata is the origin of springs and subterranean reservoirs of water, from whence so many supplies are derived. Sand, gravel, chalk, and sandstone are very absorbent strata; whilst the oolites and other limestones are permeable to a smaller extent. The excess of rainfall over evaporation sinks into the ground till it reaches the level of saturation of the stratum by previous rainfalls, and adds to the underground supply. The water thus intro-duced is prevented from sinking lower down into the earth by encountering an underlying impermeable stratum; and it is hindered in flowing away to the lowest point of the outcrop of the permeable stratum by friction, which causes the surface of saturation to slope towards its outlet, the inclination varying with the head of water and the resistance offered to its flow. The amount of percolation depends upon the rainfall, the porosity of the stratum, and the extent of its exposed surface ; and it varies inversely as the evaporation, being greatest in winter and during heavy long-continued rainfalls, and least in the summer and with short showers of rain.

Watercourses.—On impermeable strata, the y whole of the rain not removed by evaporation finds its way into the watercourses. The streams, however, draining these strata have a very variable discharge, as they are rapidly swollen after a heavy fall of rain, and soon subside (see RIVER ENGINEERING), whilst in fine summer weather they are liable to be dried up. Accordingly, torrential streams, in their natural condition, are not suitable for water-supply, as they tend to fail when they are most wanted, and owing to their rapid flow they carry along a large quantity of matter in suspension. The flow of streams draining permeable strata is more regular, both on account of the smaller fall generally of the river-bed, and also owing to the rainfall being delayed by percolation in its passage to the river. Some of the rain sinking into a permeable stratum tends to find an outlet outside the valley in which it falls, but most of it reappears again in the form of springs which feed the river gradually at a lower part of its course ; and the loss which may occur is more than compensated for by the greater regularity of flow. In olden times, towns, villages, and monasteries were established on the banks of these streams, owing to the ready, ample, and never-failing water-supply thus ensured.

SOURCES OF SUPPLY.

Tanks.—The simplest method of procuring pure water is to collect the rain as it falls from the clouds; and this method is a necessity where, as in tropical countries, there is an excessive rainfall during one period of the year, followed by a long drought, unless the rain sinks into a permeable stratum whence it can subsequently be drawn. These open tanks, however, excavated in the ground, have to be numerous, and often large in extent, to collect sufficient rain to supply the wrants of the surrounding population for several months; and the water in them is subject to loss from evaporation during the dry season.

The collection of rain-water is also advantageous in the rural districts of temperate regions, especially for large institutions and isolated mansions and farms, where, by a simple arrangement of gutters and pipes, a large pro-portion of the rain falling on the sloping roofs can be stored in underground tanks. In large towns, the rainwater is liable to contamination by smoke, dust, and other impurities, and is only serviceable for gardens and water-closets, or where softness is of more consequence than purity.

Springs.—A very valuable source of water-supply is provided by springs. These springs appear at the lowest point S of the outcrop of a permeable stratum, where it rests upon an impermeable stratum (fig. 1); and they constitute the outflow of the rain which has percolated c

Fig. 1.

through that stratum. A spring depends for its supply upon the extent of the underground reservoir furnished by the permeable stratum ; and its discharge is regulated by its level in relation to the line of saturation of the stratum and the resistance offered to its flow. The gathering ground of a spring consists of the portions AB, DS of the permeable stratum drained by it which are actually exposed at the surface, provided the surface slope is not very steep, and also of any impermeable surfaoe strata BCD sloping from a higher level towards the permeable outcrop. The position of the spring is deter-mined by the dip AS of the underlying impermeable stratum and the line of least resistance to the under-ground flow. When the permeable stratum covers the surface, and is of small extent, as when it forms a thin cap to a hill, an outflow only occurs after a fall of rain. Where a permeable stratum, with a limited gathering ground, has a sufficient depression at some point to cause the line of saturation to sink occasionally below the level of the outcrop, the outflowing spring is intermittent; and the time of the appearance of such springs (or bournes, as they are termed) can be accurately predicted by observing the rise of the water in the neighbouring wells sunk into these permeable strata. A spring is generally clear, and free from organic impurities, as particles in suspension are removed by the natural filtration, and organic matters are oxidized and eliminated in the passage of the water through the ground. The water, however, collects any soluble gases and salts which exist in the strata through which it flows ; and most springs contain some inorganic compounds in solution, depending upon the nature of the strata and the distance traversed. Occasionally springs are so strongly impregnated with certain substances as to receive specific names, such as sulphuretted, chalybeate, and saline springs; but in such cases they are of more value for medicinal purposes than as sources of water-supply. The abundant springs derived from the chalk, though containing considerable quantities of carbonate of lime, are quite suitable for domestic purposes. Springs from large underground supplies possess the advantage of a fairly constant temperature, as their sources are pro-tected from atmospheric changes ; but underground waters are subject to the rise of temperature, experienced in descending below the surface of the earth, of 1° F., on the average, for each 52 feet of depth.

Small springs frequently supply little hamlets ; and a shallow tank, formed at the spring, into which the water trickles, serves as a reservoir, which is gradually filled, and from which water is readily drawn. Large springs may afford adequate supplies for towns ; but before relying upon such a source it is essential to gauge their discharge at the close of the autumn, in a dry year, so as to ascertain their sufficiency under unfavourable conditions. When springs have been selected, it only remains for the engineer to design suitable conduits for conveying the water to the place to be supplied. The fine chalk-water springs of Amwell and Chadwell, in Hertfordshire, have been thus utilized by the New River Company for supplying London, by means of a conduit 40 miles long, completed in 1613 ; and the springs issuing from the Malvern Hills furnish Malvern with a plentiful supply of the purest water.

Springs were naturally much prized in ancient times, when the simplest means of procuring water had to be resorted to ; and ignorance of their real origin led to their being the subject of mythological legends. They were used for the supply of public fountains by the Greeks and Romans (see FOUNTAIN), and provided water for some of the Roman aqueducts.

Streams and Rivers.—In olden times, the only other obvious sources of water-supply, besides rain and springs, were the watercourses which carried off the surplus rain-fall. Streams and rivers afford the most ample supply, but they become turbid in flood-time ; and when they have a rapid fall, and drain an impermeable basin, they fail in times of drought. These objections have, however, been overcome by settling-tanks, filter-beds, and storage-reservoirs,—so that now the principal supplies are drawn from these sources. The increase, indeed, of population, and especially the introduction of the system of discharg-ing the sewage of towns into the adjacent streams, have polluted many rivers. Fortunately, in process of time, some of the organic impurities are removed by aquatic animals and plants, and some become oxidized and thus rendered innocuous (see WATER),—so that, after a sufficient length of unpolluted flow, the river again becomes suit-able for supply. The enactments against river-pollution have attacked this evil at its source ; but some rivers, passing through large manufacturing centres, are hope-lessly contaminated with refuse products. The best sources of water are found in streams draining uncultivated mountainous districts, where a plentiful rainfall on steep impervious strata affords a very pure though somewhat intermittent supply. The freedom from habitations, the rapid flow of rain off the surface, the absence of organic impurities and of soluble salts, prevent any chance of contamination beyond occasional discoloration by peat.

Distance, however, from hilly regions and a considerable population render it frequently necessary to resort to larger rivers, which, passing towns and villages in their course, become more or less contaminated, and, being fed by springs from permeable strata, contain the salts dissolved by those springs. London, for instance, draws its principal supplies from the Thames and the Lea; and, though microscopical investigations and the reports of the water-examiners are not always reassuring about the qualities of the water thus supplied and the recuperative powers of nature, waters containing some organic matters and certain kinds of micro-organisms do not appear to be injurious, as is evidenced by the general absence of specific diseases in the inhabitants supplied from these sources.

Wells.—There are two distinct kinds of wells, namely, shallow wells, sunk into a superficial permeable stratum ; and deep wells, sunk through an impermeable stratum into an underlying permeable stratum. Both kinds of wells tap the underground waters which are the sources of springs, and furnish artificial outlets for waters which, would either find a natural outlet in springs at the outcrop, or which, owing to a depression of the strata, may not possess a natural outlet at a low enough level ever to drain the lower part of the underground reservoir.

Shallow wells, sunk in the ordinary manner, have long been used for collecting moderate supplies of water, where a permeable stratum, such as the Bagshot sands, or the gravel covering parts of the London basin, overlies a watertight stratum such as the London Clay, especially where a slight depression in the impervious stratum towards the centre, or a considerable expanse of the surface stratum, prevents a ready outflow from the per-meable beds. Many parts of London were supplied for a long time in this manner; for the rain percolating the bed of gravel flowed into the wells sunk in it, from whence the water could be drawn up. Indeed, as pointed out by Professor Prestwich, the growth of London was restricted, till the regular establishment of waterworks, to those districts possessing a gravel subsoil, in which water could be readily procured, as no such facilities existed where the clay rose to the surface. The sites also of many of the older towns and villages were doubtless determined by similar considerations. Shallow wells are still very useful in supplying scattered populations, but they are exposed to the worst forms of contamination when the houses are near together. Any surface impurities are washed in with the rain ; frequently cesspools are given an outlet into the permeable stratum from which the water-supply is derived ; and pumping in the well, to increase the supply, creates a flow from the contaminated areas to the well. Such utter neglect of sanitary pre-cautions has led to serious outbreaks of illness; and the sparkling water from some of the old wells in the City of London has proved very deleterious, from the infiltration into them of the decaying matter from graveyards and elsewhere. Shallow wells, in fact, must be resorted to with great caution, and only when an absence of habitations, or a thorough inspection of the district drained by the well, affords assurance of freedom from organic pollution.

Deep wells, passing generally through impervious beds into a permeable water-bearing stratum to a depth at which an adequate supply of water is obtained, are mostly free from organic impurities, partly owing to the protection of the superincumbent impervious stratum, and partly to the filtration any impurities must undergo before reaching the well. The wells are usually formed by sinking a shaft lined with brick for the upper portion, and then carrying down a boring below to the requisite depth. The level of the water in the well depends upon the water-level in the stratum; and generally the water has to be raised by pumping to the surface. Occasionally, owing to a de-pression of the strata, the top of the well is below the level of the lowest part of the outcrop of the permeable stratum into which the well is sunk, and the water rises in the well directly this stratum is reached, and flows over if the hydrostatic pressure is sufficient to overcome the friction. These latter wells, known as Artesian wells, have been already described (see ARTESIAN WELLS) ; and the methods of boring other deep wells are precisely similar. The most favourable strata for deep wells in England are the Chalk, Oolites, New Red Sandstone, and Lower Greensand. The yield of these wells depends upon the extent of the portion of the underground reservoir which they can drain; and the reservoir depends for its supply, as in the case of springs, on the extent of the stratum exposed at the surface, the drainage it may receive from adjoining impermeable strata, and the amount of rainfall over these areas. As these points can only be roughly estimated, it is impossible to judge of the yield beforehand ; and much depends on the fissures the well may happen to pierce, as the main flow in many rocks takes place along their fissures. It is disadvantageous to sink a well where the superincumbent impervious stratum is very thick, not merely because of the depth that has to be sunk before reaching water, but also on account of the slow rate of the underground flow at a long distance from the outcrop, and owing to the com-pression of the porous stratum by the mass above it. For instance, the deep well on Southampton Common, sunk through 465 feet of impervious beds, has only yielded a small supply of water, though carried 852 feet into the chalk. A well may also prove a failure owing to a fault or an impervious barrier interrupting the underground flow, if it is sunk on the side of the fault or barrier away from which the dip of the stratum inclines. Thus a well sunk at A (fig. 2) will receive the water flowing along water, on account of the interruption of the flow. As there is a limit to the underground waters, only a limited number of wells can be advantageously sunk within a certain area; and a multiplication of wells, such as has occurred in the London basin, permanently lowers the water-level of the underground reservoir, and involves an increased lift in pumping to maintain the supply. Wells drawing their supplies from the same sources as springs reduce the yield of the springs issuing from the strata which they pierce; and when these springs feed rivers the volume of these rivers is thereby somewhat diminished. Wells, however, sunk into strata draining to the sea-coast merely intercept water which otherwise would be absolutely lost. A useful well for small sup-plies is a tube well, which consists of a series of strong wrought-iron pipes, between 1 and 2 inches in diameter, the bottom length being terminated in a point, and perforated with little holes for a short distance up. The point is driven into the ground by a falling weight, as in pile-driving; and, as the tube descends, fresh lengths of pipe are screwed on the top. When the perforated pipe reaches a water-bearing stratum, the water enters through the holes and is raised by a pump (compare descriptions and diagrams of tube-wells under PETROLEUM, vol. xviii. pp. 716-718).

EXTENSION OF SUPPLY.

A supply obtained from wells may be increased by reaching the water flowing through undrained fissures or lying in untouched cavities, either by sinking fresh wells, or by driving headings from the bottom of existing wells in various directions, both of which courses were adopted for extending the Brighton water-supply. Continued pumping sometimes improves the supply when the stratum is well saturated and the drain is not sufficient to lower the water-level permanently. This result is due to the steepening of the gradient of flow towards the well by the depressiou of the water-level in the well, which increases the velocity of flow, whereby the channels of access are cleared out and enlarged, so that the water flows more readily and quickly into the well than at the commence-ment.

The supply from springs and streams can only be increased by storing up the excess of supply in the wet season, to make up for the deficiency in the dry season. This can be accomplished by means of storage reservoirs, which sometimes are found suitably provided by nature in the form of lakes, or may be constructed in mountain valleys by means of dams.

RESERVOIRS.

Lakes as Reservoirs.—A lake is a natural reservoir of water, caused by the influx of a stream into a depression of an impermeable stratum, which is barred to a certain height by a ridge across its outlet, over which the water has to rise before it can flow away (see LAKE). The water of lakes is generally of exceptional purity, owing to its being usually supplied by the drainage from the impervious uncultivated ground of uninhabited mountainous districts, and its general freedom from pollu-tion, and on account of the lake serving as a deep subsiding reservoir for any matters in suspension contained in the inflowing streams, of which the Lake of Geneva in relation to the turbid upper Rhone is a notable instance. Glasgow is supplied with excellent water from Loch Katrine (see AQUEDUCT, vol. ii. p. 224); it was at one time proposed to supply Edinburgh from St Mary's Loch, and London from Bala Lake; and works for the conveyance of water from Thirlmere, in Cumberland, to Manchester are in progress. To increase the storage capacity of a lake intended to serve as a reservoir, and avoid injury to vested interests, the ordinary water-level of the lake has to be raised by heightening the barrier at its outlet. By this means the lake is not unduly lowered by the drain upon it during the dry season, and compensation water is pro-vided to supply the water-rights along the stream below. The extent to which the water-level of the lake has to be raised depends upon the area of the lake, the influx into it, and the supply drawn off; thus Loch Katrine, with an area of 3000 acres, needed only a rise of 4 feet in order that, with a maximum lowering of 7 feet, it might provide a storage of 5687 million gallons for a supply of fifty million gallons per day; whereas the water-level of Thirlmere, with an existing area of only 350 acres, requires raising 50 feet to furnish a storage of 8100 million gallons for a similar daily supply. The amount of water that can be collected depends on the catchment area, and the rainfall of the driest years, less the loss from evaporation ; and the supply which can be relied upon from any definite gathering ground is given by the formula Q = 62.15 A (|-R-E), where Q is the daily supply in gallons, A is the catchment area in acres, R is the average annual rainfall and E the loss from evaporation, both in inches. The storage capacity must be regulated by the number of consecutive days, in time of drought, that the supply might have to be drawn from the reservoir without its receiving any accession of water. This period has been variously estimated at from 70 to 300 days according to the locality; for, owing to the smaller fluctua-tions in the rainfall and in the periods of drought in very rainy districts, and the less amount of evaporation, a much smaller storage suffices for very wet districts than for very dry ones.

Storage Reservoirs.—Where no natural lake is available for a reservoir, an artificial lake may be formed by con-structing a dam across a narrow gorge of a mountainous valley, thereby impounding, in the winter, the stream draining the valley, and storing up a supply for the following summer. To prevent an escape of the water thus impounded, the reservoir must be formed on an impervious stratum, and all cracks and fissures closed, or the bed and banks must be rendered watertight by a surface layer of impervious material. Occasionally the configuration of the ground and the amount of storage re-quired render it neces-sary to form a series of impounding reservoirs at different levels along a valley, thereby in-creasing the number, but keeping down the height of the dams. For instance, in supplying Manchester from the Long-dendale valley, six large reservoirs, having a total area of 497 acres, and a capacity of 4160 million gallons, were formed in steps by dams from 70 to 100 feet high (see AQUEDUCT, vol. ii. p. 224).

RESERVOIR DAMS.

The capacity of a reservoir depends upon the form and levels of the valley in which it is situated, and the height of the dam retaining it. As, however, the extent of the water surface is considerably increased by raising the water-level, an additional height of dam adds largely to the capacity of a reservoir. Thus Thirlmere, with an existing maximum depth of 108 feet, will have its area increased from 350 to 800 acres by raising the barrier at its outlet 50 feet. Accordingly, dams of considerable height are sometimes erected: as, for instance,the Entwistle embankment of the Bolton waterworks, retaining a reservoir 120 feet deep, the Villar dam, for the supply of Madrid, founded 158J- feet below the water-level of its reservoir (fig. 7), and the Furens dam, near St Etienne, with a height of 164 feet at the water-level (fig. 5) ; whilst the Gileppe dam, near Verviers, was made 147| feet high (fig. 6) in preference to four clams of 95 feet.

A reservoir dam is constructed either with earthwork in an embankment sloped on each side, and with a water-tight puddle or concrete trench along the centre, or of masonry. Earthwork embankments have, till within the last three or four years, been exclusively adopted in Great Britain ; whilst masonry dams have been long ago con-structed in Spain, and more recently introduced into France.

Earthen Embankments.—In moist climates, and for moderate heights, embankments of earth are adopted with advantage for reservoir dams, more especially when ample materials can be readily obtained, either by excavations in the reservoir, thus enlarging its capacity, or elsewhere near at hand, and where a rock foundation is not easily attainable. All loose material must be removed from the site of the dam ; and the puddle trench in the centre must be carried down to a solid impervious bed (fig. 3). The embankment must be brought up in thin layers carefully

FIG. 3.—Nethertrees Reservoir Embankment, Paisley.

punned or rolled, the most retentive materials being placed near the middle, and the looser materials towards the outside. The inner slope, facing the reservoir, is usually made 3 to 1, and pitched on the surface to protect it from the wash of the waves. The outer slope is formed to the angle of stability of the material employed, generally 2 to 1; and occasionally berms are introduced, diminish-ing the liability to slips (fig. 4). The best puddled clay is used for the central trench; but the remainder of the embankment should not be composed exclusively of clay,

FIG. 4.—Gladhouse Reservoir Embankment, Edinburgh.

as stiff clay under the influence of the weather, especially on the exposed outer slope, tends to slip. An earthen dam possesses ample stability if it is perfectly solid ; but it may fail from the infiltration of water through it, owing to faulty construction, or from settlement, leading to its overtopping by the water in the reservoir.

Masonry Dams.—In hot dry countries, an earthen embankment is liable to crack and become somewhat dis-integrated ; and high embankments, owing to their flat side slopes, require a very large amount of material. Accordingly, in Spain, masonry dams have been adopted; and they are preferable to earthen dams when the height exceeds about 80 feet, and where a rock foundation can be secured. The Spanish masonry dam of Puentes (Lorca), 164 feet high, was indeed built upon piles; but it was eventually undermined, and settled; and the outburst of the water from the reservoir on its failure in 1802 caused the loss of 608 lives. Besides a solid rock founda-tion, the conditions of stability of a masonry dam are that the maximum pressure shall not exceed the limit that the masonry can sustain without injury, and that the lines of resultant pressures, with the reservoir empty and full, shall not anywhere pass outside the middle third of the section of the dam, so as to prevent the possibility of tension at the faces. With the reservoir empty, the pres-sures on the dam are merely those due to its weight; and the line of resultant pressures is the locus of the points of intersection of the verticals from the centres of gravities of the several portions of the dam above a series of horizontal lines, with the base lines of those portions (figs. 5 and 8). With the reservoir full, the water exerts a horizontal thrust against the inner face of the dam, equal to the weight of a column of water having the depth of the water resting against the dam, and acting at the centre of pressure, which is at two-thirds of the depth down from the water-level. The line of resultant pressures, in this case, is the locus of the points obtained by the intersection of the resultant lines of the pressures of the masonry and the water on the portions of dam, at different distances from the top, with the horizontal base lines of these portions (figs. 5 and 8). The direction and amount of the resultant are readily obtained graphically by the parallelogram of forces, the point of application being at the intersection of the vertical from the centre of gravity with the horizontal line one-third up from the base; for, by drawing the horizontal and vertical lines proportionate in length to the water-pressure and weight of the dam respectively, the diagonal represents the resultant of these forces both in magnitude and direction. When the inner face of the dam is battered, the weight of water resting on this face must be added to the weight of the dam when the reservoir is full. The resultant pressures neces-sarily increase with the depth; and the maximum pres-sure is at or near the base of the inner face when the reservoir is empty, and of the outer face when the reservoir is full. The top has only to be made wide enough to resist the shock of the waves and floating ice in the reservoir; but the base, having to bear the weight of the dam together with the water-pressure, requires widening out adequately for the safe limit of pressure on the masonry not to be exceeded; and, as the water-pressure with the full reservoir deflects the resultant towards the outer face, this face is given a considerable batter (fig. 5). All dams have to be raised high enough not to be overtopped by the highest waves in a storm, depending on the size and exposure of the reservoir.

Sections of three of the largest masonry dams erected within the last twenty-five years, namely, Furens, Gileppe, and Villar (figs. 5, 6, and 7), as well as the Vyrnwy rubble concrete dam, now in course of construction (fig. 8), drawn to the same scale, illustrate the forms adopted for these dams. The Furens and Villar dams follow closely the theoretical requirements; whilst the Gileppe and Vyrnwy dams, with their excess of thickness, impose an unnecessary weight on the base, and absorb extra material, without any adequate compensating advantage. Provision, however, was made in the section of the Gileppe dam to admit of its being raised at some future time, and for a roadway along the top, which in some measure accounts for its excess of width; and the Gileppe and Vyrnwy dams are the first examples of such structures in Belgium and Great Britain.

The Furens dam (fig. 5), constructed in 1859-66, across the Gouffre d'Enfer, for forming a reservoir with a capacity of million cubic feet, has a maximum height of 183 feet, and a length along the top of 337 feet; and the maximum pressure on the

Scale to Figs. 5, 6, 7, and 8.
_ _ . , . . 0
i I I i i i i

masonry is 641 tons per square foot on the inner face, a few feet above the base, with the reservoir empty; whereas, with the reservoir full, the maximum pressure on the outer face is under 6 tons. No allowance was made for the arched form of the dam, in plan, towards the re-servoir, which reduces the pressure, due to the head of water, at the lower part where the valley is very contracted, and would have admit-ted of the omis-sion of the pro-jecting outer portion for the bottom 60 feet. A very low limit of pres-sure, in addi-' tion to the ex-cess of strength just referred to, was adopted, owing to the unprecedented height and type of the Furens dam. By raising, however, the limit to 6-6 tons per square foot, it was possible to reduce the section of the Ban dam, 138 feet in height at the water-line, making it 110J feet wide at the base, as compared with 116J feet for the Furens dam at the same depth; whilst with the same limit applied to a dam of the full height of the Furens dam, the reduction in width

100 FEET

at the base would be from 161 feet to 154 feet. Any further raising of the limit of pressure, which might be safely effected, would be of little advantage for dams down to a depth of about 100 feet, as the reduction in width is restricted by the condition of the middle third ; but beyond that depth the width is regulated by the pressure.

The Gileppe dam (fig. 6), built in 1869-75, having a maximum height of 154 feet, and a length along the top of 771 feet, retains a reservoir with a capacity of 423j million cubic feet.
The average pressure on the base is 8 '2 tons per square foot; so that, even allowing for the specific gravity of the masonry being about one-seventh greater at Gileppe than at Furens, the maximum pressure on the Gileppe dam is considerably greater than on the Furens dam, in spite of its greater base and the smaller head of water (compare figs. 5 and 6), owing to the excess of material employed in its construction.

The Villar dam (fig. 7), built across the river Lozoya in 1870-78, convex towards the reservoir as at Furens and Gileppe, has a maximum height of 168f feet, and a length along the top of 546 feet; and it forms a reservoir having a capacity of 70| million cubic feet. It differs
W/////Mk the Furens dam; but the form given
to the batter of the outer face is cal-culated to render the pressures more uniform towards the base.

The Vyrnwy dam (fig. 8), now (1888) in course of construction, across the Vyrnwy in Montgo-meryshire, for form-ing a reservoir, 1100 acres in area, to supply 40 million gallons per day to Liverpool, is to sustain a maximumhead of water of only 70\ feet; but, as it has to be carried down about 60

FIG. 7.-Villar Reservoir Dam, near Madrid.

feet ^ gur_
face, in the centre of the valley, to reach solid rock, its maximum height is about 140 feet, and its length along the top is 1350 feet. The maximum pressure on the inner face with the reservoir empty has been estimated at 8'7 tons per square foot, and with the reservoir full at 6 "7 tons on the outer face. These pressures are considerably in ex-cess of the maxima pressures on the Furens dam, though the head of water to be supported is much less, and the width at the base is greater in propor-tion to the depth. The pressures at the base in the Vyrnwy and Gileppe dams show that a super-abundant mass of material in a masonry dam, whilst involving a larger outlay, imposes a greater pres-sure upon the masonry,
whereas the stability is

FIG. 8.-Vyrnwy Reservoir Dam. ample in the Furens type .

and the oozing of water through the dam should be provided against by the quality of the materials and workmanship, rather than by an extra thickness of masonry.

Accessory Works.—Sometimes a small dam isplaced across the upper end of a reservoir, so as to form a small settling reservoir, in which the inflowing stream can deposit any sediment before passing into the main reservoir.

A waste weir is provided at a suitable place in the dam with its sill slightly lower than the highest proposed water-level in the reservoir, so that the surplus water, when this level is reached, may flow over into the lye-wash. The length of the weir should be made sufficient for the discharge over it to pass off the inflow during a flood, so as to ensure the dam against being overtopped by a rise of water in the reservoir, which would be fatal to an earthen dam. To provide for a large discharge without a great length of weir, the sill of the weir may be placed somewhat lower, and planks placed temporarily across it to retain the water at its highest level on the approach of the summer.

The water is drawn off from the reservoir, as required for supply, through an outlet culvert passing from a low level in the reservoir into a conduit in the valley below. This culvert was formerly frequently placed through the lowest part of the dam, being readily formed during the construction of the dam, and giving command of all the water in the reservoir. Accidents, however, have often been traced to the unequal settlement of the earthen embankment near the culvert, or to infiltration of water into the embankment, either by escaping from the culvert fractured by settlement, or by finding a passage along the outside line of the culvert or pipes. Thus the bursting of the Dale Dike embankment, 95 feet high, near Sheffield, in 1864, on the occasion of the first filling of the reservoir behind it, having a capacity of 114 million cubic feet, which entailed the loss of 238 lives, was attributed to the unequal settlement and consequent cracking of the puddle wall over the trench in the rock in which the outlet pipes were laid, aggravated in this instance by the defectiveness of the material in the main bank, the rough manner in which the bank was raised, and the rapid filling of the reservoir. The percolation of the water under pressure along the line of outlet pipes was the cause of the gradual failure of the embankment, 41 feet high, across the Lynde Brook, Worcester, Mass., which burst in 1876 and set free a reservoir with a capacity of 110 million cubic feet.

No possible considerations can justify the burying of outlet pipes at the base of a high embankment, with the valves regulating the discharge at the outer extremity, whereby the water-pressure always acts along the whole length of the pipes, and their inspection is impracticable. In some cases a valve-tower is erected in the centre of the embankment, by which means the water can be shut off from a portion of the pipes. If, however, the outlet pipes are carried under the embankment, they should be laid in the solid ground, and should be commanded along their entire length by a valve-tower placed at the inner toe of the embankment (fig. 4). Nevertheless, it is far safer to carry the outlet pipes in a tunnel constructed through the side of the valley, beyond one end of the embankment. In the case of masonry dam=, the outlet is generally con-structed in the solid rock aistinct from the dam ; but at Villar the outlet culvert has been carried through the centre of the dam (fig. 7).

CONVEYANCE OF SUPPLY.

A reservoir in a mountain valley is at a sufficient elevation for the water to flow by gravitation to the locality to be supplied; and it is only necessary to form a conduit by canals, tunnels, aqueducts, and pipes, of adequate size in relation to the gradient, to convey the daily supply required (see HYDROMECHANICS). In olden times hills were contoured, and valleys were crossed by colossal aqueducts, at great expense, to obtain a regular inclination, which was reduced by the circuitous route that had to be taken (see AQUEDUCT). NOW, however, hills are pierced by tunnels; and, by the employment of cast-iron, deep wide valleys can be crossed by inverted siphons following the depressions of the land,—so that a much straighter course is attainable, affording better gradients, and therefore enabling smaller conduits to be adopted, which is of great importance when long distances have to be traversed. Thus the waters of Thirlmere, after being discharged through a tunnel formed under the Kirkstone Pass at the south end of the lake, instead of at the natural outlet to the north, will be conveyed to Manchester by a conduit 96 miles long, with a total fall of 178 feet. Portions of the route are in tunnels, 7 feet square, the longest tunnel being a little over 3 miles long; and there are several inverted siphons, to be formed of five cast-iron pipes, each 3J feet in diameter, the longest siphon being miles, and that under the river Lune having to bear a water-pressure of 416 feet. The water from the Vyrnwy reservoir will be conveyed to Liverpool in a conduit 67 miles long, of which 4 miles will be in tunnel, and will furnish a supply of 40 million gallons per day. A large supply of water from the river Verdon for the district round Aix, serving for irrigation and manufactures, as well as for domestic purposes, is conveyed across the valley of St Paul in an inverted siphon formed of two wrought-iron tubes, each 5f feet in diameter.

The water obtained from rivers in low districts, and from wells, has to be raised by pumping to the height necessary to obtain a proper pressure for supply ; and the pumps have to be in duplicate, to prevent a failure in the supply in the event of a break-down. Thus the water-supply of London has to be raised by pumping to fill the service reservoirs.

PURIFICATION.

The water obtained for supply is frequently not sufficiently pure to be at once distributed for domestic purposes. The impurities to which water is liable are of three kinds, namely, particles of matter in suspension, inorganic substances in solution, and organic matters in solution or of extreme minuteness. Suspended matters are readily removed by subsidence if the particles are heavy, and by filtration if the particles are flocculent or light. Some inorganic compounds are readily removed, whilst others cannot be dealt with in a practical manner. The removal of organic impurities is of the most importance, and the most difficult, which has led to the great care exercised in selecting unpolluted supply.

Settling-Reservoirs.—Allusion has already been made to subsiding reservoirs formed at the head of large storage reservoirs; the object of these, however, is rather to pre-vent the accumulation of silt and sand in the principal reservoir than for the purpose of purification, but the principle is the same. Supplies from large reservoirs are generally free from matters in suspension, except some-times during heavy floods, owing to the subsidence in the reservoir itself; but river supplies have often to be led into settling-reservoirs, where the water, whilst at rest, can deposit its heavier particles before passing on to the filter-beds for the removal of the lighter portions.

Filter-Beds.—Over the bottom of brick tanks layers of clean material are spread, decreasing in coarseness from small rubble to sharp sand, with a total average thickness of about 4 feet. The actual filtration is effected by the upper layer of sand; and the lower layers allow the passage of the water unaccompanied by the sand. The efficiency of the filtration depends upon the slowness of the passage of the water, which should not exceed a How

of from to 3 gallons per square foot of area per hour. The filter must periodically be cleaned by scraping off the top surface of the sand, which becomes choked with the matter removed from the water ; and after a time a fresh layer of sand has to b« provided. Filtration, though in itself a purely mechanical operation, has been found to reduce the organic impurities in the water, which must be due either to their oxidation from exposure in thin layers to the air in the process of filtering or to the actual removal of very minute organisms floating in the water, or probably to both causes combined.

Filters of spongy iron mixed with gravel were set up at the Antwerp waterworks in 1881, and arrested a quantity of matter which had passed through an upper layer of sand, and proved very efficient agents for purification. Their large cost, however, their becoming rapidly clogged and caked on the top, necessitating the removal of the upper layer of sand to break up the hard top iron crust, and the rather frequent renewal of the iron required, as compared with sand, were a bar to the extension of the system, which had become inadequate for its work. Eventually, it was found that the purification could be effected more economically and rapidly, and quite as effectually, by scattering cast-iron borings in the water as it flowed through horizontal revolving cylinders, furnished with projecting plates fastened at intervals round the inside, which, in revolving, scooped up the iron at the bottom, which had fallen through the water, and dis-charged it on reaching the top. The iron is first converted into ferrous oxide, FeO, which dissolves, at least partly, as bicarbonate, and then by further oxidation into ferric oxide, Fe203, which readily precipitates, and is easily removed by filtration through sand, a small portion only of the oxide remaining in solution. Iron appears to purify water from organic matter, partly by its fatal influence on the growth and even life of micro-organisms, and partly by dragging down the floating organisms in the process of precipitation.

Softening Water:—Many waters drawn from springs, wells, and rivers fed by springs contain inorganic salts in solution, and, though innocuous and pleasant for drinking, are not good for general domestic and manufacturing purposes, owing to their curdling soap and encrusting kettles, boilers, and pipes. This quality, known as hardness, is mainly due to salts of lime, which are found in large quantities in waters drawn from the chalk. Most of the lime in solution is in the form of bicarbonate, Ca0.2C02, having been produced from the very slightly soluble carbonate of lime, CaO.C02, of which chalk and other limestones are mainly composed, by combination with free carbonic acid, C02. It is only therefore neces-sary to remove half of the carbonic acid from the bicar-bonate to reconvert it into the insoluble carbonate. This can be done either by boiling, which drives off the excess of carbonic acid, depositing the carbonate of lime which forms the troublesome incrustation in pipes and boilers where chalk water is used, or by adding caustic lime, CaO, to the water, which, combining with the excess of carbonic acid, reduces the bicarbonate, forming carbonate of lime, which is precipitated. This reaction is indicated by the formula CaO + Ca0.2C02 = 2CaO.C02, and is the basis of Dr Clark's process for softening water. To indicate the degree of hardness of various waters, Dr Clark's scale of 1 degree of hardness for each grain of chalk in 1 gallon of water is employed. Some waters have 22°, or even more degrees of hardness; and all waters are termed hard which contain more than 5°; but by the softening process waters of 22° of hardness can be reduced to about 5°. The remaining or permanent hardness consists of sulphate of lime and other soluble salts. The conversion of the bicarbonate in the process of softening is rapid ; but the difficulty of dealing with the fine precipitate, which requires time to settle, has hindered the general adoption of the process, though it has been applied successfully at various works deriving their supply from chalk wells. The precipitation of the carbonate of lime in the softening process has been observed to remove to a great extent the micro-organisms in the water, confirming the view expressed above, that precipitation sweeps down with it the minute germs, as in this case the chemical action could not influence them. The waters obtained from mountainous districts are very soft, and therefore very valuable for manufacturing districts; but they have more action upon lead, and are more liable to absorb organic impurities than water highly charged with inorganic salts.

STORAGE.

Quantity of Daily Supply.—The water-supply required is estimated in gallons per head of population, with additions in manufacturing districts for trade purposes. The consumption varies greatly in different towns, ranging from about 12 to 50 gallons per head per day; and it depends more upon the fittings and other sources of waste than upon the habits of the population, though the consumption per head is greater in the wealthier quarters. An ample supply, for domestic and general requirements, is from 20 to 25 gallons per head daily. The actual rate of consumption varies with the time of day, and also with the period of the year, being greatest between 7 and 10 A.M., and in June, July, and August, and least from 9 P.M. to 5 A.M., and in January, February, and March.

Where the quality of the supply drawn from different sources varies, as in London (the water from the deep wells in the chalk being far superior to that derived from the Lea), and where filtration has to be largely adopted, it is unfortunate that the best supply cannot be devoted to drinking and cooking, and the inferior in quality and unfiltered waters used for cleaning, for gardens, stables, water-closets, flushing sewers, watering the streets, and extinguishing fires. Besides, however, the cost of a duplicate system of mains and pipes, with the addition, in London, of difficulties between independent companies, the carelessness of persons in drawing from the two supplies has been considered a bar to this separation. It is possible, however, that, when the population becomes still more dense, and pure water more difficult to obtain, these objec-tions may be overruled, and the purest supply devoted to special uses.

Service-Reservoir.—To provide a sufficient reserve for sudden demands, such as for a fire, and to ensure an adequate supply to every house, a service-reservoir has to be constructed, into which the water from the source of supply is led. The reservoir consists generally of a brick or concrete tank, rendered inside with cement, sunk in the ground, and roofed over with brick arches resting on the side walls and intermediate pillars, over which a covering of earth is spread. By this means light and heat are excluded, which, together with a depth of at least 15 feet of water, prevents the growth of minute aquatic plants, of which the germs are found in some well waters, particularly from the New Red Sandstone, and maintains the water at a tolerably even temperature. The reservoir should have a capacity of not less than twenty-four hours' supply, and should be at a sufficient elevation to command the whole of the district it serves, and if pos-sible afford a good pressure on the fire-hydrants. Where a town stands at very different levels, separate reservoirs at different elevations for supplying the high-level and low-level districts are advisable, to equalize the pressure.

DISTRIBUTION.

The water is led from the service-reservoir through cast-iron mains to the branch mains, from which the service pipes convey it to the several houses. Lead is generally preferred for house connexions, owing to the facility with wnich it can be adapted to structural requirements. The only objection to it is that it is attacked by some very soft waters, oxide of lead being formed, which is partially soluble and very injurious to health. With free carbonic acid, however, the pipe becomes coated with carbonate of lead, which is insoluble in water and protects the pipe from further action. Peat also in water protects the lead pipes, by depositing a surface film. In the case of hard waters, the lead soon becomes coated with sulphate of lime. Accordingly, it is only under exceptional conditions that the employment of lead for pipes is deleterious; but it should be prohibited for the lining of cisterns where the water may be stored for long periods.

Domestic niters are very valuable for local well and spring supplies, and afford an additional safeguard against accidental impurities in public supplies (see FILTER).

Intermittent and Constant Supply.—Formerly the com-mon form of supply was on the intermittent system. On this plan, each house is provided with a cistern, into which water from the main is admitted for a short period, once or twice a day, by means of a valve on each service main, which is opened and closed by the turncock for each separate district. When the cistern is filled, the inlet pipe is closed by the rising of the floating ball shutting the ball tap. The supply is, accordingly, limited to the contents of the cistern, except during the short period the water is turned on ; and the cistern is proportioned to the accommodation of the house. The water in these open cisterns is liable to contamination from impurities of various kinds settling in them and not being cleaned out; and it is often exposed to heat, a smoky atmosphere, and dust, and sometimes to sewage gas. Moreover, in the event of a fire, the turncock has to be summoned before a supply of water can be obtained. Accordingly, the adoption of the constant system has been urged, and in many places carried out. The advantages of drawing a fresh supply always direct from the main, and of having an ample supply constantly at hand to meet any emergency are unquestionable; but the constant supply of water necessitates the strengthening and very careful inspection of the pipes, joints, and fittings, to prevent fracture and avoid leakage under a continual and increased pressure, and is liable to lead to a careless waste of water if unchecked by a meter. Before substituting a constant for an intermittent supply, it is essential to overhaul thoroughly the pipes and joints, and to substitute screw-down taps, which close gradually, for the leaky suddenly-closing plug-taps, which throw a sudden pressure on the pipes. Waste in water-closets can be stopped by the insertion of a waste preventer, which only allows a definite quantity of water to pass each time the plug is raised (see SEWERAGE). The detection of accidental waste from leakages has been much facilitated by the introduction of a waste-water meter, which records graphically, on a revolving cylinder, the amount of water which is passing the place where the meter is fixed. By fixing the meter on one of the district mains at night, when most of the recorded flow is running to waste, and shutting off successively the service pipes through which water is heard to be flowing, the change in the diagram of flow at each closing of a service pipe localizes the position and extent of each source of waste, showing at what places leaks must be occurring, and which are the worst, needing attention first. This method of inspection was first adopted at Liverpool in 1873; and, besides effecting a considerable economy, it enabled the constant service to be restored, which the previous waste had rendered impracticable.

Water-Meters.—There are two classes of water-meters, —the positive and the inferential. The positive meter, such as Kennedy's and Frost's piston meters, measures the actual quantity of water passed through it, as recorded by the strokes of a piston working in the cylinder, which is successively filled from the top and bottom, and affords a measure of the water introduced ; whilst the inferential meter, such as that of Siemens, measures only the revolutions of a turbine actuated by the flow of the passing water, of which the quantity is deduced from the velocity. The positive meter is more accurate, and measures very small flows; whereas the turbine meter may sometimes not be turned by very small flows which are gradually increased. Measurement by meter would seem naturally to follow the adoption of the constant service for domestic supply, as well as for manufactories. Its general adoption has, however, been hindered by the fear that a charge by quantity, instead of by rental, might press unduly upon the poorer classes, and induce them to stint themselves of a proper supply, and also the difficulty of obtaining a very cheap and at the same time a perfectly trustworthy meter of adequate durability. To avoid the possibility of checking a sufficient use of water in the poorer tenements, it has been proposed to allow a definite supply at the ordinary rate, and only to charge by meter for any excess over this amount. (L. F. V.-H.)


Footnotes

Theory and Practice of Hydro-Mechanics, Inst. C. E., p. 44.








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