CITY WATER SUPPLY

1 ♦ MR COTJSTON'S BEPORT,The following is the test of the report by Mr B. Couston, one of the two engineers appointed by the City .Council to j report on the Dunedin Water Supply. It may be mentioned that in places we have inserted headings not appearing in the original;— * —Present System.— The City of Dunedin is supplied with water fronT two sources—the WaitatiLeith and the Silverstream. The watei from Fergusson's Creek, Burns Creek, Jefferson's Creek, and Williams Creek in the Waitati catchment area is collected in earthenware pipes laid round the contour <tf the hill, and conveyed over the LeithWaitati saddle in a cast iron pipe to the head of the Leith. The Waitati watei . then flows in conjunction with the Leith Stream to Sullivan's Dam. Sullivan's Dam is a reservoir capable of storing 50 million gallons, and is connected direct with the service reservoirs at Maori Hill, Koslyu, and Mornington. At times of drought the draw off from Sullivan's Dam is greater than the supply, and consequently no water is by-passed into the Leith, but provision is made foe the water to flow past the dam altogether if so desired. Following down the Leith, the first feeder of any size is Morrison's Creek, and its waters aro intercepted at a high level, and connected by a branch pipe with the main from Sullivan's Dam; and the watei - gathering between this intake and the Leith is again intercepted and conveyed in a second pipe to Ross Creek reservoir. Jn times of drought all the water is taken, and provision Is made also and freely made use of for intercepting the whole of the water in the Leith at this point, and con veying it along with Morrison's Creel; water to Ross Creek reservoir. In addition to these supplies Ross Creel reservoir is fed by the two branches o; Ross Creek. The reservoir has a capacity of 50 million gallons, and supplies l large portion ofthe lowex area 6 of tbf City. The Maori Tfi.ll reservoir is, properlj spe;iking, a service reservoir, but has i storage capacity of two and three-quartei million gallons. It serves the high leveli of the (Sty, Jforth-east Valley, and Wesl Harbor, and is used in addition to in crease the pressure in the town mains as occasion requires. The reservoirs at Ros lyn and Mornington are also service reser vojrs, and their combined capacity is s little over one million gallons. The other source of supply is the Silver stream, and its waters are conveyed in ai open water race a distance of 19 mile; along the eastern side of the Silverstrean Valley and the western and southen slopes of Abbot's and Kaikorai Hills t< the southern reservoir. In times o drought all the water in the streams in tercepted by the race, including the Coa Creeks, M'Rae's Creek, M'Kenzie's Gully Cameron Creek, and Flagstaff Creek, 'ii utilised to augment the supply; the watei inGow's and Abbot's Creek is taken to ai extent restricted only by the oppositioi of riparian right owners, and the wate; in M'Quilkan's Creek is taken when con sidered sufficiently pure. In the_ Silverstream Valley the wate: race is situate on the eastern side of thi stream, and does not therefore intercep any feeders that flow "into the stream fron the west. The principal feeder at this altitude from the west is Powder Creek which joins the main stream about a mil< and a-half below the water race intake Immediately below the, junction an elec trically-operated pumping plant has beer installed, and in times of drought thi combined waters of Powder Creek and tin water that gathers in the Silverstream be low the race intake are intercepted anc - pumped into the race to augment th< supply _to the southern reservoir. ThL Teservoir has a storage capacity of 2? million gallons, and from it a pipe is laic through the old railway tunnel to Caver sham and is connected up with the City re ticulation. The southern reservoir serve; also by means of a branch pipe tht borough of Green Island. the watershed of the streams that art utilised for supply put-poses is corporatior freehold, in the Leith area all the lane within the watershed of Sullivan's Dan and Morrison's Creek is corporation free hold, but some of the land that drains tr the Leith between Sullivan's Dam anc Morrison's Creek is privately owned. Ii the Ross Creek watershed the land is prin cipally corporation property on the mail branch, but largely private property or the V/akari branch. In the Silverstrean watershed all the drainage area of Powdei Creek ;> the Silverstream, Coal. anc M'Rae's Creeks .is corporation property and the drainage areas of M'Kenzie'; Creek, Cameron's Creek, M'Q.uilkan's Creek, and Flagstaff Creek are partly corporatior freehold, partly corporation leasehold, anc partly private property. The other race feeders all flow through private property. —Previous Reports.— 1 have been supplied by the City En gineer with a number of printed reports and have read these carefully through, anc the one matter, that is most strongly irn pressed on _ the mind is that the Silver stream drainage areas failed in times o drought to supply anything like the qnan tity of water to the reservoir that was anticipated. In the Leith-Waitati area I have care fully measured the drought flow, and an satisfied that with the exception of Fer gusson's _ Creek all the water that wai flowing in the 'creeks was being deiiverec to the northern reservoirs. In the Silver stream area I also measured the inflow o the tributary feeders to the race and tht quantity of water that was being sup plied to the southern reservoir, and whilst owing to the absence of facilities for tak ing absolute measurements of flow at fre quent intervals along the race, I am un able to say with exactness what water i: lost in different sections, I know for certain that a considerable amount of wate: is taken into the race that does not fine its way to the reservoir. In. the earlier reports on the Silver stream watershed it was estimated tha with a drainage area of 10,000 acres thi Silverstream, Coal Creek, Whare Fla Creeks,M'Quilkan's Creek, Flagstaff Creek Cow's Creek, and Abbot's Creek wonh give a minimum discharge of four milJioT gallons per day. That is an allowance fo minimum drought flow of 400 gallons pe acre per day. It is true that part of thi drainage area may not properly be utilisec by the corporation, but at times of drough it is used with very small exception, an< yet the quantity delivered at the sonthen reservoir for nine days in March, 19U was less than half a million gallons : day, and, even after the installation of thi pumping auxiliary supply a fc Powde' Creek, which added over 2,000 acres t< the drainage area for 13 days in April - 1920, the records show a daily flow to thi reservoir of less than one million gallons. More recent estimates of supply for thi combined Ross Creek and Waitati-Leitl schemes were based on a minimum drough flow of 200 gallons per acre per day, and as the area that is reasonably free fron pollution is 3,000 acres, this should yieli a- drought supply of 600,000 gallons pe day. As a matter of fact, the Waitati Leith supply above Dam alon has never.fallen below that quantity, anc as, in addition to the water from thi 3,000 acres, the Leith itself is picked U] in times of drought at Morrison's Creek I take the recorded measurement of 790,CQ gallons as truly representing the minimun quantity that can be depended npon fron the combined northern watershed, and i is safe to assume that this quantity wa always be delivered. —The Drought Yield I am satisfied from my own observa tions that, although individual creeks wer. at the end of the drought actually dry and a number of those with small catch ment area or -non-retentive nature o gathering ground yielded at times les than one-half of, the 200 gallons per acr per day, yei over the combined water sheds it is at all events true to say tha an average quantity of 200 gallons pe acre per day • can be depended upon -our inz.to* drought period.

J f In tha SDveisfcroara watershed I have established the dependable area of gathering ground reasonably free from pollution at 8.000 acres, and, with a minimum flow of 200 gallons per aero per day, this should yield 1,600,000 gallons, or, with 10 per cent, loss, supply 1,440,000 gallons per day to the Southern Reservoir, and, allowing only 600,000 gallons from the northern " gathering grounds, give a total daily eupo ply of 2,040,000 gallons. t The storage capacity of the combined e reservoirs is stated at 106,574,300 gallons; but probably 90 to 100 million gallons is e the capacity utilised- The present daily consumption is five million gallons unrestricted and two and a-half million gallons , restricted. The longest period of drought f 1 has been 120 days. If, then, the limited quantity which x should bo available of 2,040,000 gallons •> were delivered at the reservoirs during the n drought, and consumption was at once rea stricted to, say, two and three-quarter r million gallons per day when the supply '• fell to that figure, there would be a daily 6 shortage of 710,000 gallons, which the x reervoirs, if full at the commencement of b drought (assuming their capacity at 100 n million gallons), would be able to supply I- for the 120 days. But with unrestricted hj consumption, the daily shortage would be i- 2,960,000 gallons, which the reservoirs could only supply for 33 days. a I mention here—because it is obvious -, that, if the water which runs to waste d during the wet season of the year could r be impounded at reasonable cost, there is x no necessity to look elsewhere to augment the supply—that it is usual to allow t that 6in of the annual rainfall, or 136,125 d gallons per acre, is available for storage. L, And, as the reasonable cost must necese sarily be compared with that of a supply , x from an outside source, I give the figures e for the Lee Stream proposal. The estimated supply is five million gallons per ._ day, the estimated cost £128.738, • the J' annual cost £6,836 —that is, about one g penny per thousand gallons. e The questions, therefore, to which at- ' tention has to bo given are:—. j. First,_ why the Silverstream supply is not maintained to the assumed drought , capacity. . Secondly, whether any creeks other than those lifted might bo utilised to augment - the supply. Thirdly, whether the storage capacity of le present reservoirs might be enlarged, or w-hether there exist in the watershed suit- ■ 3 able sites for the construction of new a reservoirs, sr ] 6 —Leakage at Silverstream.— 3 5 The answer to the first question is either j. that the assumed drought flow is not de- ! Spendable, or that there is undue loss by s _ soakage and leakage from the water race". r _ I cannot speak authoritatively as to the a condition of the race prior to the drought, but I do know that, when I first inr_ spected it after receiving instructions, n there was every indication that many leaks a , had recently been staunched, and from £ measurements taken of the inflow and outflow I know that even then the loss due to leakage was large. , There are some sections of the race where it is impossible to give an assurer ance that the race will not develop a leak ~ between inspections; there are other seey tions where it is reasonable to conclude | s that with proper attention the race will - r carry the quantity for which it was de■n signed without undue loss. n It is only possible, by a proper svs--r tern of gauging the creeks at their inflow *- and a series of gauges along the race itself, to locate all leakage, and this would -r be a necessary preliminary before decidle hi£ -whether certain sections of the race n should be concrete-lined or piped. rn As far back as 1877 Mr Blair foresaw :s that this might ultimately become necesi, sary when he said: "It "is not proposed .e to line or pitch the Silverstream race at b. present, but this can be done when the c- demand increases and the finances of the ;n City permit of the additional expenditure." lB r _ —Leith-Waitati.— la The second question, whether any creeks B : other than those lifted might be "utilised 0 to augment the supply, has reference to ! . e both the Leith-Waitati and Silverstream P watersheds. So far as Waitati is concerned, it is apparent now that, for d drought supply, the present pipe line has L '- already been extended beyond its line of 3- utility, as Fergusson's Creek at the in;s take on the day of mv visit measured te only seven and three-quarter gallons per minute, and the two small creeks that n are crossed between Fergusson and Burns •e Creeks measured respectively 12 gallons n and five and a-quarter gallons per minute d —that is, a total of 25 gallons per minute, n or httle more than 36.C00 gallons per j- ctay—and while I do not suggest that o this supply, however small, should have d been neglected, I give the exact figures to n illustrate what was obtainable. i- it is quite evident, from following up n ', the courses of these creeks when excepn tionally_ low, that there is an elevation n for each creek beyond which it is inad,r visablc to go, as the Waitati has to flow d by gravitation over the Leith Saddle, the ,- t elevation at which each supply can be • s tapped is fixed, and in my opinion (without - ; recourse to tunnelling through the saddle, n which would be an expensive undertaking) d there is no other feeder of the Waitali , e worth consideration. As an example of how quickly the water gathers from the'hillsides in this locality, I instance Fergusson's Creek, which, at x _ the intake, measured only seven and three--3 quarter gallons per minute, while at the Leith-Waitati road, half a mile away, the s flow was 60 gallons per minute. Burns Creek gathers 30 gallons per minute be/tween the intake and the road—a distance or 40 chains, and Williams Creek gathers 10 gallons per minute in a distance of to 15 chains. ■, c . }?■ tlle Leit ' n watershed, after passing -- Sullivan's Dam, some small' creeks join ™ the Leith before Morrison's Creek is reached, and these together contribute the "-S water that was previously referred to as a diverted in an emergency to the Morrison i- Creek supply. There is a drought flow 3t here of 230 gallons per minute, or 331,200 ie gallons per day, and it is apparently too > valuable to disregard at times when the reS servoirs are low ; but I do not think the i- water sufficiently pure to utilise without e- filter beds or acquiring the freehold of the i- drainage area. Two creeks further down is are Nichol's Creek and Mount Cargill r- Creek, both good streams at their inter;r section with the Leith ; but as it would id be necessary to extend pipe lines up both creeks to the level of Morrison's Creek r- intake, to enable the supplies to be taken it conjointly with that supply, and necessary ie also to acquire the freehold, whilst indiifc eating the source from which temporary k, relief might be obtained, I do not recomld j mend their permanent taking, unless , n I supply from an outside source is ruled jr out. particularly as I recognise that the 3 r limit has been reached to which the Leith i e can bo deprived of its waters lower down. *[ (To be continued.) id ■■