DRAINAGE SYSTEM.

New Zealand Herald, Volume LI, Issue 15549, 5 March 1914, Page 9

DRAINAGE SYSTEM.

OPENING THIS MONTH.

TOTAL COST, £450,000.

23 MILES OF CONDUITS.

PROGRESS OF FOUR YEARS

Wednesday, March 25, will be the occasion of the official opening, at Orakei, of the great drainage system of Auckland city and suburbs. The sewage works constitute the largest single undertaking ever earned out in New Zealand, and represent a total expenditure of close upon half a million sterling. The legislation empowering the Auckland and Suburban Drainage Board to raise moneys for the purposes of the work fixes the total amount available by the raising of loans at £500,000, and the estimated total cost of the whole work is set down at £450,000. Up to date the amount actually spent if about £370,000?' The actual work was commenced on October 25, igo9, four years and four months ago. On that date the first sod was turned, in the presence of a large gathering at Orakei; and the long, and often difficult, construction has gone on steadily since, under the guidance of the Drainage Board's engineer* Mr. W. E. Bush. The Drainage Board and the contractors have been remarkably fortunate in the class of material which has had to be worked through, and have generally met with comparatively few serious difficulties. The greater part of the work has been done by contract, though the board has earned out a considerable amount by day labour, with very satisfactory result*. Labour troubles, however, ' prevented the extensive adoption of the day-labour svsteni.

Ihe total length of the main intercepting sewer, which runs from Orakei to near Cox s Creek, is 81 miles, and the branches by means of which the system is brought into touch with th^spreading of the city and suburbs aggregate another 15 miles. The main sewer serves, of course, an area along a great, part of its length and of limited width; but the branches, meandering through thickly populated areas, and designed to be linked up with all parts by the reticulations which are the concern of the various local bodies, will ultimately reach effectively the whole population of the drainage area. At the present moment, if all the reticulation were completed, the sewers, as far as they are made, would servo a total population of 89,000 persons. The reticulation, however, is still in progress, and is not likely to be completed for a long time. The local bodies have, in fact, to deal with difficult problems in bringing their people into communication with the sewers. Nor does the total of 89,000 represent tho ultimate capacity of the system, which is designed for a population within the drainage area of about three times as many, calculating upon an allowance of 35 gallons per head per day. The Beach of the System. There are 13 branch sewers, of which two are secondary branches from one of the principal branches. Four of the branches, again, are short ones, serving part of the area which might otherwise have been dealt with by the main sewer, but introduced to avoid a deviation of the great conduit, so saving expensive construction and enabling more fall to be obtained.

Ihe respective branches serve the following areas:—No. 1 (4646yd5), Remuera, eastward of Victoria Avenue, and part of One-tree Hill; No. 2 (1062 yds), the westerr, part of Remuera: No. 3 (2835 yds), Newmarket, Epsom, Parnell, and Mount Eden; No. 3a (491 yds), and 3b (596 yds), in Parnell, are two of the small branches referred to as saving a deviation of the main sewer; No. 4a (956 yds), and No. 4b (359 yds). in the city, are the other two small branches with a similar purpose; No. 5 (3596yd5), skirts Ponsonby on the seaward side as far as Heine Bay; No. 6 (1309 yds), runs- from the upper end of the main sewer along the shore of Cox's Creek towards the extremity of No. 5 branch, the two thus almost enclosing the Ponsonby area; No. 7 (3767yd5), is the well-known Archhill Gully sewer; No. 7a (1340 yds), and No. 7b (482 yds) are branches from the Archhill Gully branch; No. 8 (4900 yds), the longest branch, runs up the valley of the Meola Creek, and drains the Mount Albert and Mount Eden districts. Undermining the City. People who are unacquainted with the nature of the drainage scheme would probably be astonished at the dimensions of the work. For a considerable distance from the Orakei end, the sewer, which in the main portions is all "egg-shaped" in Section, measures Bft 6in in height, and sft- Bin in width. Various sections have lessening dimensions, till at the upper end the main sewer measures 3ft 6in in height and 2ft 4in in breadth. The branches are mostly circular in form, and range from Bin in diameter up to 3ft. By far the greater part of the main sewer is thus large enough to be walked through. Nearly the whole of the work has been done in "tunnels. These have been of various dimensions up to 10ft in height, and within them the contractors have had to build the sewers of concrete, in bulk or in blocks, and many ingenious and interesting devices have been used in the work. The extent of the tunnelling that has gone on, a great part of it under the city itself, has probably never been realised by the citizens, though at times numbers of them have felt aggrieved at the succession of muffled reports which, in the dead of night, have disturbed their rest, and assured them that the hidden workers were toiling while others slept. Of the main sewer, the only portions which were not built in tunnels are the long stretch across Hobson Bay, the sweepingly curved portion on the Orakei Beach, and a trifling piece in the city. About one-third of the total length of the branches was laid in open trenches.

Tunnelling at Depth. Much of the tunnelling has been done at a considerable depth below the surface, as may be realised when it is understood that the line of work passes from the Domain gate in Stanley Street into the city by a "route which takes it right under Albert Park. The deepest portion, however, is in Ponsonby Road, at the corner of Summer Street, where the sewer lies at a depth of 180 ft below the road surface. To enable the work to be carried on, a number of shafts were sunk, as in ordinary deep mining practice, and the deepest of these is in Nelson Street, where the sewer is 85ft below the roadway. The shafts will be used as means of access to the sewer, and altogether there will be over 200 manholes, of which 34 will be in the completed main sewer. ' The main sewer, between the upper end and the receiving tanks at Orakei, has a total fall of 23ft. Enormous Storage Tanks. One of the most important and interesting pieces of work in the whole system is the great storage tank at Orakei. This is a huge triple reservoir of concrete, heavilv reinforced. It is 1300 ft. or nearly a quarter of a mile long, and 106 ft wide, and is divided longitudinally by two concrete walls reaching from the floor to the ceiling. The three tanks are about 10ft high, and they have an aggregate capacity of 7j} million gallons. At the landward end of the tanks, in addition to a distributing device to carry the sewage from the sewer into whichever chamber is in use at the time, is a machine which will remove silt and other heavy solid matter from the entering flow. At the seaward end the chambers communicate by sluices with the outfall, by means of which the contents are led into the sea and discharged at a distance of 1213 ft, and a depth of 30ft below ordinary high water. The making of the outfall was a work which afforded those responsible for it much gratification. As may be imagined, there might have been great difficulties but the work went ahead with the

minimum of trouble. Over that area of the sea, bottom bared at low tide, the work was done much in the ordinary way, the 4ft twin-pipes being laid in a concrete foundation. Beyond that point, however, the pipes are laid in cast-iron cradles let into the rock and secured with concrete. The problem of this interesting submarine construction was admirably overcome. The outlet of the pipes is nearly a quarter of a mile seaward of the tanks; and its position is indicated by a beacon 875 ft north-east of the end of "the Orakei Jetty. Beauty Combined With Utility. The long and roomy top of the receiving tanks has been specially designed to form part of the beautiful waterfront esplanade, which it is intended to make in the not far-distant future. Its width, of over 100 ft, is ample for any such purpose; and the structure is specially strong so as to carry any amount of traffic. One part of the width has been so reinforced that it can carry tram traffic, and the concrete top has been provided with channels for the reception of the rails. The curved portion of the big conduit lying on the Orakei beach ill also play a part in the same scheme, being intended to be used as a retaining wall for the future reclamation which will be necessary for the formation of the road. The section of the sewer crossing Hobson Bay stands upon piles, and there has been apportioned a sum of money, now beinor partially used, inasmuch as the interest is available for expenditure, for the purpose of building between the piles a close masonry wall, so that this, too, will become a retainer. The area behind the wall so formed is supposedly destined to become an important and beautiful recreation ground. The Working of the - System. The drainage system is not intended to deal with any great quantity of storm water; nor is it the practice for any sewer to be designed of such a size that it is regularly full to above a level which, to the inexpert, would seem rather low, until it is remembered that a sewer must be made, not to deal with average quantities, but with ordinary maximum flows. There is therefore left in the sewer at ordinary times a sufficiency of room for the passage of a considerable quantity of rain water; but when that quantity is exceeded it overflows by special means into other channels which carry it by direct routes to the sea. It is well known that the whole of the sewage is to be led into the great tanks at Orakei; and these have often been called loosely septic tanks. As a matter of fact, they are nothing of the kind. A septic tank is a vessel in which sewage remains til], by the* operation of bacteria, it is broken down into a harmless fluid, which is very often, used for the irrigation of a farm specially provided for its absorption. But the Orakei tanks are to be merely containers, which will be emptied at high tide. The Harbour Board is naturally the authority most concerned with the protection of the harbour waters from pollution, and its engineer, some time ago, carried out a series of tests by moans of floating buoys, to determine the period during which the sewage could be discharged without threatening the cleanness of the enclosed water of the Waitemata. These tests established that from the outfall provided in the drainage scheme the sewage could safely be discharged at a period lasting from 2g hours before high water until four hours after high water, during; which period the tidal flow from the harbour carries all floating and suspended matter out into the outer channels with little prosnect of any notable quantity of it returning. The tanks therefore have a capacity ample for purposes of storage between tides, and arc fitted with sluices enabling the contents to be released at the proper times. At the present time only one of the three divisions of the tanks need be used at a time; but as the amount of sewage increases it will be feasible to use two tanks per tide, or all three, if it should become necessary. The Financial Aspect. The enormous cost of such a work, involving the borrowing of nearly half a million pounds, is met by a charge levied upon the different local bodies governing the drainage area. The method of apportioning the cost is that the Government valuations of the areas are adopted as a statement of the value of each local body's area, and the total amount of the interest is divided up among the bodies in proportion to the respective valuations, and is paid by the local bodies half-yearly.