Gimber.
AQUEDUCT BRIDGE AT CLENDINNING VALLEY.
The situation was chosen as one affording the proper elevation: but its formation was such as to present difficulties in the way of making the Reservoir perfectly water-tight; the surface, in tracing it from 79th to 86th street, was quite undulating, a portion of it in the southern division of the Reservoir falling below the proposed bottom, and that portion of the surface which was earth, forming only a covering to the rock, which over the whole island, presents a singularly broken and uneven formation. In almost every instance of excavation, the rock was found above the proposed bottom of the Reservoir, and the difficulty of preventing leakage along the surface of this rock may easily be conceived; but considering that measures are taken to prevent such an occurrence, another difficulty is still presented in the formation of the rock: the veins and fissures which are frequent in this gneiss formation would possibly afford courses for the water to escape; the rock being unsound in many instances, would render such an occurrence still more liable. A Reservoir has however, been constructed here which proved, when it was filled with water, that sufficient precaution was used to prevent leakage, and that the difficulties which presented themselves before the commencement of the work were no longer to be feared.
The embankments forming the Reservoir are made of good assorted earth, and a portion of the bank is puddled, or made compact and impervious by wetting the earth and using a spade to force it into a compact state. They are about 20 feet wide on the top, and increase in thickness towards the base by a slope on both sides: the outside face of the Reservoir bank has a slope of 1 foot horizontal to 3 feet vertical: the inside has a slope of 1½ foot horizontal to 1 foot vertical. The outside face is protected by a stone wall 4 feet thick having the face laid in mortar: the inside face is protected by a slope wall of stone laid without mortar, 1¼ foot thick. The top of the bank is 4 feet above top water line, and the inside slope wall terminates at 2 feet above top water line, leaving the remainder of the face to be covered with grass, so as to present a belt of green above the water on the bank entirely around the Reservoir.
A neat fence bounds the outside and the inside of the top bank, forming a walk of a mile in length around the entire Reservoir.
The greatest depth of water in the northern division is 20 feet: it was originally intended to excavate so as to give the water a depth of 20 feet over the whole, but a quantity of rock was left, as the capacity was thought to be sufficient without taking it out.
The southern division has 30 feet of water where the bottom was filled in with embankment, and 25 where excavation was made. A portion of rock was left in this division for the same reason as that in the northern division; the greater part of it being in the south-west corner, where it rises above top water line.
The capacity of the Reservoir when both divisions are full, is 150,000,000 Imperial gallons.
The surface of water in the northern division covers 18.13 acres, and in the southern division, 12.75 acres; making in both nearly 31 acres.
[Plate XXIII]. is a plan of the Receiving Reservoir.