Dunedin Drainage and Sewerage Works.

Progress, Volume I, Issue 7, 1 May 1906, Page 165

Dunedin Drainage and Sewerage Works.

By J. T. Noble Anderson.

The most comprehensive scheme for drainage in the colony is nearmg completion in Dunedin. The city presented exceptionally difficult problems to the sanitary engineer, being situated on what seems to be the crater of an extinct volcano. The hills which flank the Otago harbour on all sides rise with steep grades, in some parts streets being found where the grades have a rise of one foot m every three-and-a-half feet. Among these hills there are of course many valleys which do not all flow together. Some six or seven of these which contain the city proper and the suburbs of Maori Hill and Northeast Valley, run down into the harbour, while the important valleys of Caversham and Mornington discharge into what was at one time a morass or swamp, but which has been gradually reclaimed, and arterial drains run through it into the harbour. Up to now all the sewers in these valleys discharged into the harbour, and the first cause of the present scheme being adopted was the difficulty which arose between the Harbour Board and the local authorities. At an early date a scheme had been suggested to divert all these sewers by one main intercepting sewer which would run along the harbour foreshore and would convey the sewage to the ocean, and on leaving the foreshore of the harbour such a sewer would have to cross a mile of the old morass to reach the sand hills which fringe the ocean beaches. The result of trial bores showed that such a scheme would be almost impracticable, owing to the last two miles of this main sewer being located in ground where the mud was very fluid and ending m seams of running sand. The matter then remained in abeyance for twenty-five years, and m the meantime more than three dozen engineers devised schemes, no two of which were alike. On his appointment the author revived the original scheme, but, as the result of ocean-float trials and other observations, found that it would not be practicable to discharge sewage on the beaches and that the sewer outfall would have to be located either on some islands situated over two miles along the coast to the east, or else at the foot of some very

high cliffs, which are over 300 feet plumb, and nearly three miles to the east of the originally projected site. The scheme which has been carried out brings both stormwater and sewage to the ocean, and for economy the large double sewer, which carries combined sewage and stormwater, has only been carried to the first rocky point, where it can with safety to the structure be discharged into the ocean. This is at a point known as Lawyer's Head, situated one mile east of the originally proposed outlet m the sands. Here at present the sewage is being discharged, but the tunnel for the smaller sewer has been driven some 1,400 feet further, and eventually the sewage will flow to one of the more remote places mentioned above. Two-and-a-half miles of the mam intercepting sewer along the harbour frontage has also been completed, and for the past eleven months the drainage from this has been pumped to the ocean. For the latter half of this time sewage from w.c.'s has been also passed through this sewer. At the present moment there are over 3,500 w.c.'s connecting with sewers, over a third of which number are connected with this new intercepting sewer, while it is hoped in about a month's time to turn almost all the others into it. The pumping plant is particularly interesting, -since it comprises the first installation of Diesel engines erected south of the line. Tt consists of three Gwynne " Invincible " centrifugal pumps, each with a normal discharge of I2| million gallons per diem, and capable of being driven, in case of an emergency, each up to 18 million gallons per diem, giving an ultimate capacity equal to the combined capacity of the two main sewers which supply it, or about 55 million gallons per diem. These two main sewers are respectively 5 feet 6 inches diameter and 3 feet diameter. The Diesel engines are peculiarly applicable for such work, because they do not require stokers to be continually standing by to keep up steam for an emergency, as would be needed in case of a steam plant. They can be got under full power within two minutes from starting, and are very clean and cheap to run. The estimated cost of running a complete steam pumping plant was over per annum, but it does not seem likely that the cost of the present plant will reach half this figure. There are several other very interesting features of the scheme, such as the Dortmund tank erected on the stormwater overflows into the harbour. The necessity for these has not, however, as yet been appreciated by the community, and the two tanks which have been erected seem likely for some time to be left unused. They were used for r5 months with excellent results, but when they became full of detritus the cost of cleaning them was postponed. A second feature of interest is that the main sewers have all been constructed of ferro-concrete, on the Monier system, while the rising mains have been constructed of jarrah and ironbark timber in accordance with the most modern views. The pumping station, which requires no unsightly chimney stack, is a striking architectural feature of the suburb of St. Kilda, being built in the classic style with a lofty and well proportioned peristyle of Doric columns, and occupies an open space of nearly four acres adjourning the Culling Park. This space has been laid out with avenues and shrubs, and will eventually be a handsome park. The total cost of loan works to date, including a portion of the interest charges, has been about /i 80,000. A considerable sum has also been spent in replacing obsolete and worn out sewers The present extent of the stormwater drains and sewers in the district of a permanent character is 60 miles, and the population which they will serve in a year or so is over <;0,000, or nearly QO per cent, of the population of the district under the Board's control. Estimates have been made for sewering the whole area of the district, or nearly 20 square miles, with a system of sewers 170 miles in length, and when this is accomplished the estimated expenditure will reach £400,000. While a great deal of such detail as ventilation and the connecting up of several of the branch sewers yet remains to be done, all the works presenting any real engineering difficulty have been completed.