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THE ROSS GREEK RESERVOIR., Issue 7986, 15 August 1889
THE ROSS GREEK RESERVOIR.
MR USSHER’S RECOMMENDATION.
[From Our Parliamentary Reporter.]
WELLINGTON, August 15,
A lengthy report on the Boas Creek Reservoir was received by the Engineer-in-chief from District Engineer Uaaher this afternoon. Mr Uaaher writes that on July IG lie received instructions from the Minister of Works to make a preliminary investigation into the safety of the Ross Creek Reservoir, and to report as to whether he advised a thorough examination, which would probably necessitate the emptying of the reservoir, and whether be thought the emptying of it would show where the leak was. He reports that ho has inspected and made surveys, etc., and attaches drawings to bis report—(l) a plan showing the position of the reservoir in relation to the City; (2) a plan and cross sections showing the embankment, cross sections through it, puddle trench, etc.; (3) a plan showing how the pipes (supply and scour) are laid through the embankment ; (4) a general plan of the reservoir, the upper basin, the outflow of the leaks, etc, Drawing No. 1 shows the levels taken from the railway station. From Pelichet Bay to near the Water of Leith Bridge there is a rise of 74.70 ft, and a farther rise of 44.57 ft to Woodhaugh Bridge ; from Woodhaugh Bridge 33.56 ft to the junction of Reservoir Creek with the Water of Leith ; from this junction 126.26 ft to the outflow of the leak below the reservoir, being up a narrow, rocky gorgeaveraging about 11.2 ft again; and the width from the outflow of the leak 95.47 ft to the top of embankment; making a total height of 374.76 ft above the rail level at Pelichet Bay. Drawing No. 2 shows the position of the embankment, the toe of the outer and inner slope of the same, the by-wash, storm water channel, well tower, bed of the creek under the embankment, a longitudinal section of the embankment, cross sections through the same, etc. Drawing No. 3 is the most reliable that Mr Ussher can obtain to show how the pipes (supply and scour) are laid under the embankment viz., 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 were 6ft in length between the joints, the joints coming between the pillars. The supply pipe is 12in in diameter, and the scour pipe 9in. Mr Balfour contemplated putting a culvert to carry the pipes through, but altered his mind. “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 (remarks Mr Ussher) 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 one foot layers. A roller was not used to consolidate the bank in the usual way as it proceeded. When the trench was cut a spring of running water was met witli between cross sections 2 and 3, etc. On the 26th November, 1867, the reservoir was filled up and nearly running over the by-wash, and on the 26th the inspector observed water running over the surface of the slopes of the embankment between cross sections 2 and 3. By reference to the cross sections of the embankment it will be observed that Us outer toe is well set into the rising ground; in fact, in one or two places to rocky spurs. Drawing No. 4is a general plan showing the reservoir and its surroundings, References are then made to Professor Black’s and Mr Hay’s reports. _ The dry stone wall is shown, also the cut in the face of the shaft drive and to the pipe referred to in Mr Hay’s report. Mr Ussher says:—“l have shown in this drawing a surface drain from the upper basin to the foot of the slope on the west side of the reservoir, and discharging itself into the reservoir creek below the bridge. This drain also intercepts the water from a small gully near the watchman’s house, and no doubt in time of wet weather carries a considerable amount of water, which must more or less percolate into the old spoil bank and made ground (shown on the plan) and find its way to the outflow of the leak. Much material has been deposited in and round the top of the outer slope of the embankment for the excavation taken from the upper basin about 1875, this materially increasing the area of the outer slope, into which a portion of the rainfall penetrates, and must find its way to the low ground at the outflow. When on the ground I noticed a depression in the pitching of the inner slope of the embankment. The caretaker says it is many years ago since he first observed it, and he is not aware that it has increased, The 4-iuch tile drain shown in the drawing, and known as the ‘concreted pipe,’ discharges water when the reservoir is within 2ft 2in of being full, and is about the spot referred to by the inspector in his statement of finding water running over the surface of the outer slope of the embankment between cross sections 2 and 3 on the 26th November, 1867, which clearly shows that this has been a weak point in the embankment ever since its construction.”
Mr Usaher goes on to say:—“ On the first occasion (28th July) when 1 measured the flow of the supposed leak there was a depth of 18ft lin in the reservoir, and the total discharge from the three flows was 139.7 gal per hour. And the last measurement taken by Dr Black before the water was turned into the reservoir (after having been emptied with the exception of a small pool) was 222.1 gal per hour. This pool Mr Gillies, the caretaker, states is about 170 ft long, averaging 40ft wide towards the lower end. When the reservoir was emptied during Mr Black’s investigation a trench was cut from the pool to the scour pipe, and when water ceased to run into the scour pipe the depth in the lower portion of the pool was 10ft, with the length and width as above stated. This pool, being the lowest ground, is doubtless the bed of the old creek; in fact its position points to this. The water in the bottom of the pool stands 32ft Sin above the outflow of the supposed leak, and 10ft below the mouth of the scour pipe. The first gauging of the alleged leak by tbs City Surveyor was taken on the morning of April 8,1875) the discharge for the twenty-four hours being 8,020ga1, with a depth of water in the reservoir of 25ft Sin. Gaugings I have taken from July 22 to July 25 averaged 4,210 gal for twenty-four hours, with an average depth in the reservoir of 31ft 7in, no rain having fallen from the 18th inst., when there was 0.4 of an inch, the weather since being fine days and frosty nights.” With reference to the safety of the reservoir, Mr Ussher says;—“By inspection of the tables showing the gaugings of the flow and rainfall, it will be observed that so far back as April, 1875, there was a large quantity of water issuing from the outflow of the supposed leak, and that up to a date extending over a period of fourteen years the quantity has been a variable one, governed more or less by the state of the weather, and has not been at all a steadily increasing one. The site for the reservoir is a remarkably good one, the embankment being near the foot of a valley at the commencement of a narrow rocky gorge, and the outer slope of the bank in several places abutting against rooky spurs. From all 1 oan learn, the seat of the embankment is good ground, and it has been carried out with the utmost care. It is unsatisfactory, however.
to know that such a large quantity of water is met with at the 1 outflow ’ ; and in order to trace the source from which it comes, I would suggest that the upper b isin and the reservoir be emptied during a dry season, leaving only the pool, and that a staging be erected over this pool and the water bo pumped from it and passed through the scour pipe, which could be quickly done by using a small engine, .'leasumnents of the flow should be taken previously to mid during the pumping operations. Tins would conclusively prove whether there is any connection between the reservoir and the outflow. If such a connection exist.-, and assuming that the old bed of the creek int.'faceted by the puddle nail is thoroughly Sealed by the puddle being taken 4ft into the solid rock, and the pipe track cut through solid rock also in the bed of the creek and thorouguly puddled, I would feel disposed to seek for the leak by sinking a shaft through theembankmenttoadepth exceeding that of the bottom of the pool, driving under the by-wash; and, if the How were found here, I would cut a trench right up to the surface and under the by-wash to a short distance beyond the storm-water channel into the solid ground, filling it up with the best clay pndctle procurable, thus forming a continuation of the puddle wall. At the same time, if the bulk of the water were not found, after having sunk the I would recommend extendi." the drive put in by Mr Hay towards the puddle wall, which doubtless would prove whether or not the water is coming through the embankment. During heavy floods the upper basin has become filled to such an extent that the water overflowed its embankment into the reservoir, which caused the storm-water channel to be so strained that it was unable to carry off the flood waters. This, to my mind, is a decided element of danger, and should not be allowed to exist. Provision should at once be made to carry off this surplus water by either constructing a new storm-water channel on the western side of the reservoir or enlarging the existing one. I would favor the enlarging and strengthening of the existing one, selecting from the excavation the best of the clay, to bo tipped into the pool to act as a puddle, and generally seal over the pool, so that all the water in the reservoir would tiud its way to the scour pipe, and thus allow the reservoir to be emptied and cleaned out at any I am of opinion that if these recommendations are given effect to the reservoir would be perfectly safe."
THE ROSS GREEK RESERVOIR., Issue 7986, 15 August 1889
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