Allusion has been made to provision for scouring out sand and deposit, especially in the dams of Algeria and of Spain. The amount of sand, etc., brought down by the floods is something enormous, and the question of the best means of getting rid of it has occupied much attention. In the old Moorish reservoirs the flushing gallery, piercing the lower part of the dam, was closed by iron doors on the down stream face and blocked with timber at the upper end. When required to be flushed out, laborers passed through the gallery and broke down the timber barrier, the silt forming a wall sufficiently thick to resist the pressure of the water for the time being, and allow of the retreat of the Forlorn Hope - if the latter had luck - before giving way.
One method adopted in Algeria, which has the advantage of permitting the sediment to be utilized together with the irrigation, this sediment being very fertilizing, is to pump air down through hose extending to the bottom of the reservoir, the pumps being actuated by steam power or turbine, and the sediment thus stirred up and run off with the water through the irrigation pipes. As an example of one of the early types of masonry dams in France, reference may be made to Fig. 13, on which is shown an elevation and cross section of the Lampy dam, forming a large reservoir for feeding the Languedoc canal.
I will now refer to some of the most notable masonry dams in existence, commencing with France, where perhaps the finest is that known as the Furens, in connection with the St. Etienne Water Works, constructed between the years 1859-66, and designed by the engineers Graiff and Grandchamps. It is curved in plan, struck with a radius of 828 ft. from a center on the down stream side, and founded upon compact granite, the footings being carried down to a depth of 3 ft. 3 in. below the surface of the rock. It is of rubble masonry, in hydraulic mortar, carried up in courses of 5 ft. in depth.
The height is 170 ft. on the up stream side and 184 ft. high on the lower side, with a breadth of 9 ft. 8 in. at the crest and 110 ft. at the base, and the cross section is so designed that the pressure is nearly constant in all parts, and nowhere exceeds 93 lb. to the square inch - 13,392 lb. to the square foot. The contents is equal to 52,000 cubic yards of masonry, and the cost of erection was £36,080. The capacity of the reservoir is equal to 352,000,000 gallons.
The reservoir discharges into two tunnels (see Fig. 11), driven one above the other through a hill into an adjacent valley. The lower tunnel contains three cast iron pipes, with a masonry stopping of 36 ft. long. Two of these pipes are 16 in. diameter, with regulating valves, and discharge into a well, from whence the water can be directed for the town supply or into the river. The third pipe, of 8½ in. diameter, is always open, and serves to remove any deposit in the reservoir, and to furnish a constant supply for the use of manufacturers.
The author drew attention to the difference in the section of the Furens dam, Fig. 11, as compared with that of Alicante, and of Puentes, which is similar to the latter. These two last illustrate the ancient Moorish type, and the former that of the present day. The Gileppe dam at Verviers, in Belgium, Fig. 14, although quite recently erected, viz., between the years 1869 and 1875, differs very much from the Furens type, in so far as it is of very much larger sectional area in proportion to its height, but this is accounted for by the desire of the engineer, M. Bodson, to overcome the opposition to its construction, and meet the objections and combat the fears of those whose interests - and those serious ones, no doubt - would be affected in the event of its rupture, the body of water stored being 2,701,687,000 gallons, or about eight times as much as the capacity of the Furens reservoir.
In addition to this, there was another reason, which was quite sufficient in itself to account for the extra substantiality of the dam. This reservoir is for supplying water to the cloth factories of Verviers, on the Belgian-German frontier. It is curved in plan to a radius of 1,640 ft., with a length of 771 ft., and the additional strength of the structure due to so flat a curve is probably slight.
It is built of rubble masonry, with ashlar facework, laid in hydraulic mortar. The total amount of masonry is 325,000 cubic yards. There are two weirs, at a level of 6 ft. below the crest, each 82 ft. wide. The total height, including the foundations, which are carried down from 3 ft. to 5 ft. into the rock, is 154 ft., and the breadth of the crest, which carries a road, is 49 ft. 3 in., and at the base 216 ft. The outlet pipes are carried through tunnels, which are driven on the curve into the hill side a considerable distance clear of each end of the dam.
Another very important structure is the Villar dam, Fig. 15, in connection with the water supply of Madrid, and situated on the river Lozoya. The storage capacity of this reservoir is very considerable, viz., 4,400,000,000, or nearly thirteen times as great as that of Furens. The height of the dam is 162 ft., with a breadth of 14 ft. 9 in. at the crest. It is built on the curve to a radius of 440 ft., and the length of the dam measured along the crest is 546 ft., of which 197 ft. is by-wash, thus describing nearly one-fifth of a circle, and consequently well designed to resist pressure. The dam is built of rubble masonry in hydraulic mortar, and cost £80,556.
The Stony Creek lower reservoir dam of the Geelong water supply, Fig. 16, colony of Victoria, is interesting as being constructed of concrete, in the proportion of 1 to 8½. Its erection occupied eighteen months, and cost about £18,000. It is curved in plan to a radius of 300 ft., and the greatest depth or head of water is 52 ft. 4 in. The width at the crest is only 2 ft. 8 in., although surmounted by a heavy coping of bluestone 3 ft. 3 in. broad and 1 ft. 9 in. deep. There being no facility for making a by-wash at the side, the center of the dam is dished to form a weir 30 ft. long. There are both outlet and scour pipes, and valves of 2 ft. diameter, and the capacity of the reservoir is 143,145,834 gallons.