H.—29

1889. NEW ZEALAND.

ROSS CREEK RESERVOIR, DUNEDIN (REPORT ON), BY THE DISTRICT ENGINEER.

Laid on the Table by the Hon. E. Mitchelson, with the leave of the House.

The Distbict Engineer, Dunedin, to the Engineeb-in-Chief. Bib,— Public Works Office, Dunedin, 30th July, 1889. Agreeably to instructions contained in your telegram of the 10th instant, informing me that the Minister wishes me to make a preliminary investigation into the question of the safety of the Boss Creek Reservoir, and to report as to whether I advise a thorough examination, which will probably necessitate the emptying of the reservoir, and whether I think the emptying of it will show where the leak is, I have the honour to report that I have inspected the reservoir, made all necessary surveys, and obtained all the information possible as to its construction. The work was commenced in May, 1865, in accordance with the designs of Mr. Ealph Donkin, Engineer to the Dunedin Waterworks Company, and carried on under him till June, 1866, when he left the service of the company, and was succeeded by Mr. James M. Balfour, the then Colonial Marine Engineer, »vho superintended the works to their completion in November, 1867, Mr. John McGregor, C.E., acting as his assistant, and Mr. James Crawford, as inspector of works, under both Mr. Donkin and Mr. Balfour. The contractor for the work was Mr. David Proudfoot. The reservoir when full contains 50,000,000 gallons. The water was turned into the main on the 9th December, 1867, on which date the works were opened. Ultimately they were purchased from the Company by the Dunedin City Corporation. To explain more fully my remarks I attach the following drawings : No. 1, Plan showing the position of the reservoir in relation to the city; No. 2, Plan and cross-sections showing embankment, cross-sections through it, puddle-trench, &c. ; No. 3, Plan showing how pipes (supply and scour) are laid through embankment; No. 4, General plan of reservoir, upper basin, outflow of leaks, &c. On drawing No. 1 are figured in blue the levels —taken from the railway datum, namely, 47ft. below high-water mark of Dunedin Harbour—from Pelichet Bay Eailway-station, up the Leith Valley, through Woodhaugh and the reservoir creek to the top of embankment of the reservoir. From Pelichet Bay to the junction of Duke Street with King Street, near the Water-of-Leith Bridge, there is a rise of 74'70ft., with a further rise of 44'57ft. to Woodhaugh Bridge. From Woodhaugh Bridge to junction of reservoir creek with Water of Leith, 33 56ft. From this junction to outflow of leak below reservoir, 12(j-26ft., being up a narrow rocky gorge, averaging about one chain and a half in width from about the level of road to waterworks; and from the outflow of leak to the top of embankment, 95-47ft.; or a total height above rail-level at Pelichet Bay of 374-56ft. Drawing No. 2 shows the position of the embankment, toe of outer and inner slope of same, by-wash, storm-water channel, well-tower, bed of creek under embankment, longitudinal section of embankment, cross-sections through same, surface of ground under embankment, depth of puddletrench when finished, enlarged cross-section showing batter of inner and outer slopes of embankment, width and position of puddle-wall, and development of east and west walls of by-wash. Drawing No. 3 is the most reliable one I can obtain showing how the pipes (supply and scour) are laid under the embankment —namely, on masonry pillars 4ft. square, founded on the solid rock, through the bed of the creek, and masonry blocks on the hard ground. The pipes, I am informed, were specially cast for this work, and were 6ft. in length between the joints when laid, the joints coming between the pillars. Puddle was well rammed between and all round the masonry pillars, blocks and pipes, and the trench filled in for 2ft. above with best puddle procurable. The supply-pipe is 12in. in diameter, and the scour 9in. Mr. Balfour contemplated putting in a culvert to carry the pipes through, and, I am informed, had actually drawn out the plans for one, but afterwards altered his mind, and put in the pipes as above described, probably on the plea of economy. It would undoubtedly have been better had he adhered to his former opinion, and at the same time put in a much larger scour-pipe. It must be borne in mind that these works were constructed by a Company, and in all probability the expense was kept as low as possible. The puddle-trench shown on drawing No. 2 was taken out in 1865 and the puddle-wall proceeded with along with the embankment, which was formed in lft. layers. A roller was not used to consolidate the bank, in th^ usual way, as it proceeded, but it was carted over in all directions instead. The trench was cut 4ft. into the solid rock, where the embankment crossed the creek, and for some distance beyond ; and I am informed by the Inspector that rock was met with in the bottom of the trench generally throughout between cross-sections Nos. 4 and 11, the material between 11 and 13 being of a clayey nature mixed with haematite, and that between 4 and 2 rock of a more or less shattered nature. I—H.—29.