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8

to a height of about 46ft., and it attains its maximum discharge when the basiii is full (48ft. Bin.), and the flow reaches a total of 8,640 gallons per twenty-four hours. This undoubtedly proceeds from the reservoir, and I believe percolates under the pitching and side walls of the overflow, as I had the joints of the stones forming the floor well grouted with cement and sand before the water covered them. I excavated for the outlet of the escape on exterior face of embankment, and found the water issuing in a constant stream from a joint in the main reef. Another contributor to the main outflow was tapped close to the shoot in the east spur, and it furnished an average supply of 1,680 gallons per diem with a full reservoir, which decreased to 127 gallons after a fall of 13ft. in the water-level. I am satisfied that part of this flow proceeds from the reservoir, though in wet weather it is considerably augmented by surface-wata*1. At the main east branch the principal feeder supplies about one-half of the total outflow, and the water emerges from the main reef in the east spur; and I have no doubt that the run is all derived from springs and local surface-drainage after the storage-water in the reservoir has fallei* to certain levels. This supply, with a reservoir depth of 48ft. 4in., reached a total of 291-9 gallons per hour, decreasing to 85'7 gallons, with a depth of 36ft. Having detailed the various operations I have had recourse to and their results, I will now state the conclusions I have come to after having looked into the matter from every point of view—(1.) That there is no leakage from the reservoir through the puddle-wall of the dam. (2.) That there is an escape or leakage of water from the reservoir round the puddle-wall on the east side, percolating through the rocky spur, which is of an extremely broken and shattered nature, and finding its way to the lowest level along the original creek-bed to the main outflow at the face of the dry wall. (3.) That the leakage occurs in the upper portion of the reservoir at the east side from the level of the overflow to a point not lower than about one-third of the depth. (4.) That if there is any soakage through the floor of the reservoir it is of an infinitesimal quantity, and may be'entifely disregarded. The question now arises whether any risk is incurred in allowing a leakage of the kind indicated to exist without taking the necessary remedial measures. In considering this question, it must be remembered that the only point that still admits of any doubt is the run of water at the shaft, which decreased to 38 gallons per hour when the reservoir had still 36ft. of water impounded, and of this quantity only a proportion was due to leakage, and it is not as if the water was oozing from the embankment, but it is now running almost 35ft. below the toe of the bank alongside the solid reef. The actual source of this water can very easily be proved by extending the drive I have commenced about 30ft. further, following up the run; but in the meantime I am of opinion that no immediate danger exists, but think it would be prudent for the Council to undertake at some convenient time certain extensions or additions to the puddle-wall on the eastern side of the dam as may be found advisable on stripping the surface. The reservoir-dam is designed in the usual manner, with inner and outer slopes of 3 to 1 and 2 to 1 respectively ; and if the work has been carried out faithfully, which I have no reason to doubt, I am satisfied it is sufficiently secure for the required purposes, especially as it is greatly strengthened by the rocky spur that abuts the outer face of bank. The pipes (supply and scour) are at present carried through the artificial embankment; but I think this is a dangerous method, as, in the event of a burst in the pipes within the dam occurring during the night-time, considerable damage might be incurred before the accident was discovered. The pipes have been tested lately, and do not show any sign of leakage. It may not be out of place in this report to point out generally for the Council's information some improvements that might be effected at the works of the northern reservoir that would materially conduce to the safer and more efficient working of the water-supply. In the first place, an additional bye-wash should be constructed on the western side of the reservoir, as the present one, in time of flood, is insufficient to carry away the storm-water without some portion being turned into the reservoir; in fact, on the occasion of the late flood on the 18th August the upper basin overflowed its embankment, running over into the main reservoir. It must also be borne in mind that the chance of a waterspout bursting on the watershed that supplies Boss's Creek is by no means a remote one, and an enormous body of water and debris may be precipitated into the reservoir without much warning ; and every facility should be available for meeting such an emergency. The phenomenon referred to actually occurred, in several places some twelve months since within a few miles of the Flagstaff Range. Another much-needed addition is a large scour-pipe, the present one, 9in. in diameter, being of little practical use; and, in the event of any necessity arising for quickly emptying the reservoir or lowering the water-level, the present appliances would be incapable of making any appreciable difference in the height of the water under a considerable period of time, the 12in. main and the 9in. scour being the only available outlets from the reservoir. In addition to the element of safety gained by carrying out this supplementary work, the power of being able to change the water in the reservoir expeditiously from time to time as opportunities occur, is also a useful and valuable advantage. The tables appended show the actual discharge from the west, middle, and east flows separately and combined from the 28th July to the 26th September, together with the depth of reservoir; and. also rainfall extending over the same period, which Mr. Skey was good enough to supply me with, and, as my gaugings commenced at 8.30 a.m., and Mr. Skey's observations are taken at 9 a.m., within half a mile of the reservoir, they are especially valuable and applicable in the present instance. The plan herewith shows the points at which the various excavations were made, and will explain itself. I have to express my thanks to the Corporation officials, who have cheerfully rendered me every assistance during my investigations; and. I must apologize to the Council for the length of time I have occupied ; but it was utterly impossible to arrive at a satisfactory conclusion earlier on account of the broken weather experienced during the last few weeks. I remain, &c, The Town Clerk, City of Dunedin. R. Hay, M.lnst.C.E.