DRAINAGE OUTFALL SCHEME.

New Zealand Times, Volume XXXIV, Issue 5549, 10 January 1879, Page 3

DRAINAGE OUTFALL SCHEME.

The City Engineer has preapared the following:— Wellington, January Bth, 1878.

Sir, —I have the honor to enclose a plan, showing proposed new outfall for sewage into Ohiro Bay, also three sections taken on different lines of route, with a view of ascertaining the best and most economical plan for reaching the point in Ohiro Bay thought to be the most suitable for the purpose intended. Ohiro Bay is a small inlet entirely surrounded with steep and rocky cliffs, with a line of reef running out for a considerable distance into the straits, and terminating in about six fathoms of water. It is, in my opinion, admirably adapted for receiving the sewage of Wellington, as it cannot possibly become a nuisance to anyone, the land surrounding it being totally unfitted for residences.

No. 1 Route commences at a point on the Adelaide-read about six chains from Sussexsquare, and runs in a direct line under town acres, Town Belt, and the high land lately purchased from Mr. Hunter ; this route is the shortest possible between the reef I have fixed on as the point of outfall and the termination of the main intercepting sewer to be hereafter alluded to. The total length of this route would be 3 miles 11 chains, 2 miles 74 chains of which would be in tunnel. The sewer would commence at 56ft. above high water mark, und ruu out to the outfall with a grade of 4'4sft. per mile, discharging its contents two feet above highwater mark. It would be necessary to work this tunnel with seven shafts, and at each end; the shafts are necessarily very deep, and would be expensive to work. On this account I determined to try another route, although longer.

No. 2 Route commences at the same point as No. 1, and follows the line of the Adelaideroad to town belt sections adjoining Mr. Hunter’s late property, and thence in a directline to outfall.

The length of this would be 3 miles 17 chains, with three miles in tunnel. The grade would be 4'36 per mile. The same number of shafts would he required to work this tunnel, but the total depth would be 812 ft., as against 1119 ft. in No. 1 route, so that the decrease in construction and in lift would make the tunnel cost something like £2OOO less. Another advantage would be that it would not go under any private property, except the Island Bay estate.

I was stilkdissatisfied with the difficulties to be overcome, so had another line surveyed. No. 3 Route.—This also commences at the same point on the AdeJaide-road, follows the same route to the Town Belt as No. 2; it here slightly deviates from the line of street in order to follow the low ground in the Island Bay estate, and proceeds to within 26 chains of the beach at Island Bay ; thence it proceeds through the hills to the same discharging point as the N®. 2. The total length is 3 miles 31 chains, on which there will be two tunnels 1 mile 56 chains and 47 chains respectively ; 17 chains along the Adelaide-read, and 71 chains on the Island Bay property, will be in open cutting. The grado would be 4Toft. per mile, aud the total length of shafting necessary to work it 509 ft. There is still an alternative route in connection with No. 3, that is to go round the beach instead of the last 47 chains of tunnel; it would increase the length of sewer by 20 chains, but the cost would be much the same. The tunnel route would be the most direct, and a better grade obtainable. A great advantage in the adoption of No. 3 route, is, that it will be available for the sewage of the Island Bay estate when cut up into building allotments, and the work would cost considerably less than either of the other routes, and the time occupied in construction fully one-third less. I will now proceed to show how Mr. Clark’s scheme will be affected by the change of outfall. It will be seen that the whole of the gradients can be much improved ; the rising main will be reduced in length from 71£ chains to 56 chains ; the height to be pumped will be reduced from 37ft. to 30'25ft., and an additional area of approximately 20 acres will bq intercepted, consequently less sewage will require pumping. The compensation and purchase of laud for utilisation of the sewage will be nil, and there is no probability of at any time its becoming a nuisance ; the direction of the flow will take it well out into the straits, and even if it should be brought back by the tide, it would be deposited upon a beach where it would |be far away from any habitation.

The following are the details of alterations in Mr. Clark’s scheme, rendered necessary by the alteration of the outfall. According to his scheme the intercepting sewer would commence at 96ft. above datum, I propose to alter this to 91tt., so as to increase the depth of cover over the sewer, and intercept a larger area. The first gradient with a 9in. pipe I alter from 1-500 to 1-350, for 31 chains; this will bring the termination of the Ist grade to S4'lß above datum, giving a velocity of 147 ft. per minute. From this point I continue with the same pipe, viz,, a 12in., and laid to the same distance as proposed by Mr. Clarke. I alter the gadient to 1-450, which gives a velocity of 138 per minute ; the termination of this sewer will thus be above datum 72'01. Thence carrying on with a 15in. pipe for a distance of 15 chains, as in Mr. Clarke’s scheme, and altering the gradient to 1-350, it gives a velocity of 140 ft. per minute, and brings the sewer down to 69T3 above datum. Thence, using the same sized pipe, viz,, 18in., laid to the same distance of 28 chains, I alter the gradiant to 1-600, giving a velocity of 147 ft, per minute, and leaving termination of this sewer at a point 66 05 above datum. From this point I continue with the 21in. pipes fora distance of 56 chains, or 15 chains less than proposed by Mr. Clarke, to the proposed termination of the intercepting sewer, about 6 chains from Sussex-square ou the Adelaide-road, with a grade of I’6Bo, and velocity of 160 ft. per minute, terminating at a height of 59'89 above datum, thus giving a cover of 3 36 above top of pipe. Low-level Sewer.—l propose commencing with this sewer at the same point Mr. Clarke did, but at a reduced level of 45 instead of 48ft. above datum, so as to have additional cover. Using the 12in. pipe with an altered grade of I'OOO for a distance of 76 chains, and velocity of 130 per minute, it brings its termination to a height of 34'97 above datum. From this point continuing the 15in. pipe as proposed by Mr. Clarke for the same distance with an altered grade of I‘6oo, and increased Velocity of 134 per minute, to a height at termination of 32 - 56 above datum. Thence, as done by Mr. Clarke, I proceed with an 18iu. pipe for a distance of 26 chains, at a grade of 1-650, and velocity of 141 feet per minute, which brings the sewer height above datum to 29'92. Thence continuing the 21in. pipe for a distance of 12 chains, at a grade of 1-650, and velocity of 153 feet per minute, giving sewer height above datum at termination of 2870. From this point I propose to carry on the same description of sewer as given by Mr. Clarke, viz., 2ft. 4in. x 3ft. 6in. brick oval, with a grade of 1-1100, and velocity of 144 ft. per minute, reducing height of sewer at termination to 25'64, or at a depth of 14'36 below high-water mark. From this depth it has to he pumped to a height of 17*89 above high water-mark, thus giving a drop from the end of the rising main to tunnel sewer of l'B9ft. I estimate difference of cost, as compared with Mr. Clarke’s scheme, and taking his prices as basis (and I may add they are extremely liberal ones), after making deductions of £2OOO for carrier channels, and the saving in the intercepting sewer and rising main, at £8950 more. As a set-off against this sum has to be placed Mr. Crawford’s and other persons' claims for compensation in the event of the original scheme being carried out. The proposed outfall at Ohiro Bay will not necessitate the acquiring of any land for purposes of sewage deposit. Neither do I think that any person or persons will be injured through the works being carried out in the manner now proposed. —I am, &0., J. D. Baihd, City Surveyor. The Town Clerk, Wellington.