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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.
The slope on the inner face is 3 to 1, and on the outer 2½ to 1. The by-wash is 6 ft. below the crest, which is about the average difference. The storage capacity of the reservoir is 2,400,000,000 gallons, or sufficient for 200 days' supply to the city. The puddle wall is 6 ft. wide at the top and 18 ft. at ground level, the bottom of the puddle trench about 40 ft. below the surface of the ground.
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 sandbank now showed as an isolated patch about two hundred yards wide and perhaps half a mile long, with what looked like a by-wash channel of about one hundred yards wide flowing between it and the mainland, the latter being a sandy beach backed by sand dunes clothed with a rank creeper-like vegetation, and a few stunted tree tops showing behind them.
These remarks equally apply to earthwork dams, as regards sufficient provision of by-wash, careful execution of work, and security of foundation, but their area of cross section, supposing them to be water-tight, on account of the flatness of their slopes and consequent breadth of base, is, of course, far in excess of that merely required for stability; but in these latter, the method adopted for the water supply discharge is of the very greatest importance, and will be again referred to.
The length will be 1,300 ft. and the height 170 ft. above the river bed, or 277 ft. above the foundation. The water by-wash is 7 ft. below the crest, and the dam is 26 ft. broad at the crest and 216 ft. at the base. The capacity of the reservoir will be 32,000,000,000 gallons, or nearly a hundred times as great as that of Furens. The geological formation at the site is sienitic gneiss.
The area of the reservoir is about fifteen square miles, the dam about 102 ft. high, with a breadth at the crest of 76 ft., and of the section shown in the diagram. The by-wash is cut in the solid rock altogether clear of the dam; the outlet culverts, however, are carried under the bank.
For getting rid of the large deposits of sand to which all reservoirs in that country are liable, two scouring outlets were provided of the same description as those in the old Moorish dams. The profile was calculated from Delocre's formula, and was correct in this respect, supposing the by-wash to have been sufficient.
At the trench crossing there would be a bed of puddle 50 ft. in thickness beneath the pipe, in the gullet a bed of 1 ft. in thickness. So much as regards the laying of the pipes. The embankment had scarcely been completed when, on March 11, 1864, a storm of rain came on and nearly filled it up to the by-wash, when the bank began slowly to subside.
Some years previous to the failure of the Dale Dyke reservoir there occurred, in 1852, a failure of a similar character though, as far as the author is aware, unattended by such disastrous results at the Bilberry reservoir at Holmfirth, near Huddersfield, which had never been filled previous to the day of its failure, and arose from the dam having sunk, and being allowed to remain at a level actually below that of the by-wash; so that when the storm occurred, the dam was topped and destroyed.
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