In all descriptions of reservoirs it is important that precautions should be taken to prevent the accumulation of alluvial matters of a nature to fill them up ; and, in the case of water-works reservoirs, this is essentially necessary because the waters standing for a long time over them must contract impurities from those matters. The nature of the surface of the drainage area will regulate the nature of the precautions to be taken ; for, if the soil should be one easily moved by tho surface rills, it will be necessary to form depositing pits on the courses of the feeding streams ; on hard limestone, slate, or plutonic formations, it may often only be necessary to prevent largo boulders from being carried into the reservoir ; and on peaty uplands, it will usually be found that the surface of the country will itself suffice to prevent the transport of much alluvial matter. The water obtained from peaty lands, it is to be observed, is not fit for the purposes of a municipal supply ; and hitherto no satisfactory mode of removing the qualities communicated to the water by the peat has been discovered. For canal purposes peaty water is as good as any other, inasmuch as its colour and taste are of no importance for purposes of navigation ; but either for irrigation, or for town supplies, such waters are inadmissible.
In selecting the position for a reservoir the most important con ditions to be considered are, firstly, the nature of the bed ; and secondly, the position of the retaining wall. The bed must not only, as said before, be of a nature not to communicate impurities to the water, but it must be for all practical purposes, impermeable. The value of a reservoir, in fact; depends on its power of storing, that is to say, of its holding water ; but when the strata of which its bed is composed are so much fissured, or are of so permeable a nature as to allow the water to escape, it becomes necessary both to increase the original volume of the water stored, and to guard against the danger of the water's finding its way under the seat of the dam. The con struction of the latter is, however, the most important part of the works required for the formation of a reservoir, for the depth of water against its face frequently is not less than 40, or even 70 feet, so that the pressure calculated to turn the reservoir over upon its outer edge, or to thrust it forward horizontally, is enormous; and the difficulties of construction increase precisely in proportion to the height and the length of the dam. Its position must therefore be selected in some portion of the gorge to be converted into a reservoir where the sides approach one another so as to contract the opening of the valley; the foot of the dam must be carried down far enough below the surface of the ground or of any subjacent permeable strata, to prevent the lateral passage of the water; its thickness must be made superior to that required to resist any possible dynamical effort it may be exposed to; the materials of which it is composed must he able to resist the passage of water ; it must be made high enough to prevent waves, by whatever cause produced, from washing over its crown ; and finally the overflows, bye washes, waste weirs and accessory structures (such as the sluices, draw off pipes, &c.) must be made perfectly water tight, unless in the parts
especially designed for the passage of the water. To secure the condition of the impermeability of the seat of a dam it is often necessary to carry the foundations as far below the surface as the dam itself rises, and instances of foundations fifty feet deep are by no means rare. Telford even recommended that when the strata on which a reservoir had to be formed were of a porous nature, the whole of the valley should be paddled ; so great an importance did he attach to the necessity for opposing this dangerous action.
The'sklams of reservoirs are either formed of earthwork, or of masonry, or of a mixed system of earthwork and masonry. Local circumstances must guide the engineer in his selection of the precise nature of the materials he will employ; hut the result of careful observations on the movements of large works of this description has led to the general recognition of the following practical rules for tho thicknesses to be given. When masonry dams are used, it is found to be necessary to make the thickness of the wall, x, equal to the following proportions of the height of the dam above the surface of the bottom of the reservoir near its foot : at the top . x = /i, x 0.30 in the middle x = h x 0•50 at the bottom x = h x 0.70 When earthwork is used the dam should be made from 15 to 16 feet wide at the top, with slopes of at least 11 to 1 towards the water, and of 21 to 1 on the dry side; the water side must be protected against the abrasion of the waves by stone pitching, and below the line of agitation its face must be carefully puddled. A wide puddle trench, or a thick wall of hydraulic masonry, should be carried up in the heart of the bank, and every precaution must be taken to resist the passage of the water through the various layers of tho body of the work. As a general rule the materials of the bank should be deposited in layers of from 0 to 9 inches in thickness, and well and carefully rammed ; care being taken to prevent anything like a perfect horizon tality of bedding, by making occasional steps in the layers of earth. Similar precautions must be taken in the formation of masonry dams, and the Layers, or horizontal beds must be stepped. It is also to be observed that there is a tendency in all long straight dams, of earth, or of masonry, exposed to the constant action of a great weight of water to assume in time the form of a catenary curve ; and in order to resist this tendency it is customary to make earthwork dams segments of large circles in plan, with their convex side towards the water ; and in masonry dams, it is usual to form strong counterforts ou the down side, even when the same segmental horizontal outline, and the before quoted thicknesses have been adopted. The usual rule is, it may here be added, to make the top of the dam about 5 feet above the maximum water level, in order to resist the action of the waves in the reservoir ; a precaution which was proved, by the accident to the dam of the Croton reservoir, to be by no means exaggerated.