WELLINGTON DRAINAGE.

New Zealand Times, Volume XXXII, Issue 5185, 3 November 1877, Page 2 (Supplement)

WELLINGTON DRAINAGE.

The following is Mr. Napier Bell's (consulting engineer) report to the Mayor on the scheme of drainage works for this city : Sir, —In accepting the position of consulting engineer to the drainage works of your city, and in compliance with the instructions of your honorable City Council, I have investigated the plans and estimates of your engineer ■for a scheme of sewerage for the City of Wellington, and beg to submit the following : In preparing the schedule on which I have to report, your engineer mentions that the only conditions imposed ou him by his instructions were :—That drainage should be provided foi a population of 50,000, at a cost not to exceed £Bo,ooo t and that the sewage should be dis- ' charged into the bay ; my report, therefore, ■ should be confined to the consideration as to whether the scheme has fulfilled the conditions imposed, together with my opinion of the method employed. Your engineer having decided how he should deal with the sewage, viz., by pumping, seems to have had three proposals under his consideration for the position of the outfall, Ist. ■ To pump the sewage into the sandhills at the rear of Evans Bay ; 2nd. To pump it and discharge it into the sea at Lyell Bay ; 3rd 1 To pump it and discharge it into the sea at Island Bay. In his report he discusses the first project on the supposition that Mr. Crawford will convey the sewage [at his own expense to irrigate the sand hills, the sewage to be delivered to him at only 8 feet above high water. It appears, however, that objections having been raised by Mr. Crawford to the first, and by the Government to the second project, your engineer has adopted the third, and this I presume is the project which my report is to consider. The principal feature of this project is its main intercepting sewer, which begins at Lambton-qnay opposite Charlotte-street; here the tap is 1 foot 9 inches below the surface, and the sewer descends by various inclinations tinder liambton-quay, Custom-house-street, . Manners-street, and Conrtenay-place to Kentterrace ; along Kent-terrace to Sussex-square and up Adelaide-road to Drummond-street ; thence by tunnel under the Adelaide-road to the pumping station at the Town-belt; here the tunnel is 11 feet below low water and 57 feet ' below the surface. The engines to be erected here are intended to pump the sewage from the bottom of a well and lift it 24 feet higher, when it will flow by a sewer to the sea at Island Bay. In this manner the whole of the sewage, the stated amount of rainfall, and all flashing and subsoil water of the tunnel is intended to be pumped. From the pumps to Taranaki-street, a length of nearly three • miles, ; there can be no overflow on ' account of the depth of the sewer below high water. The pumps can therefore get no relief in case of accident, or of heavy storms of rain, but must take all that comes to them from that part of the town south of Majoribankstreet and Dixon-street, being about two-thirds of the whole area, the pumps being placed at the bottom of a shaft 57 feet deep, and the sewer terminating in a " dead end" with a lift of 21 feet. As long as the rainfall exceeded the pumping power, and in case of accident to the machinery or of the pumps getting chocked, the sewer would fill up to a depth of 13 to 14 feet above the bottom at the pumps, causing delay and inconvenience, as the sewage of a great part of the sown would stagnate in the tunnel and sewer as far as the overflow at Taranaki-street, a distance of nearly 3 miles. Another inconvenience attending this arrangement would be that supposing the engines to work 12 hours daily, during the night the sewage would collect in the sewers and the sludge would settle down on the bottom ; if the pumps failed to remove it during the daily pumping it might have to be removed by hand up to the shafts, as I would expect that flushing the sewer would not be "very effectual seeing it terminated at a closed end. It is usual when sewage has to pass through pumps to screen it firat in order to remove every thing which might choke the pumps or lodge in the valves or buckets, which obstructions are very difficult to be kept out of the sewers. (I have seen trousers and hats come down to the pumps.) In this plan the screening would need to be done at the bottom of the shaft, 57 feet deep, which would be extremely awkward, to say the least of it. The recommendation in the report of your engineer, that the ends of all drains should be guarded by gratings, would be effectual only in proportion as it was carefully attended to. In considering the question of pumping it is important to decide whether it 13 to be carried on day and night, or only during the day; the former can of course be done if necessary ; but. I think it should be avoided if possible. By thi3 arrangement of the sewers a great amount of night pumping would be unavoidable whenever there was more than one-tenth of an inch rainfall, as the capacity of the sewers up to the nearest available overflow is about 217,000 cubic feet ; but my estimate of the night flow of sewage for 50,000 people at 5 cubic feet per added to O'l inch of rainfall on 1000 acres would collect in the sewer during the night to the amount of 244,000 cubic feet, bo that whenever there was one-tenth of an inch rainfall the sewers would overflow into the harbor, unless pumping was continued during the night. This might not be considered a very great nuisance ; but as the sewers were designed to carry 0 - 3 inches rain and all the sewage, if they overflowed, with- 0"1 inch rain they would not be serving their intended purpose. The fault could be remedied by enlarging a part of the tunnel, which, although it would add considerably to the cost, would be preferable to primping all night whenever the weather was rainy. "Your engineer has estimated for the sewage of 50,000 people at 5 cubic feet each, together with 0-3 inch rainfall over 1,339,000 cubic feet to be raised 24 feet in, say, 12 houra (pumping during the day only). This amount of work which the engines would have to do in rainy weather would require them to be of 83 net horse power; but it is necessary to add a considerabls per centage to this for friction of the engines, loss of effect in the pumps, and other aource3 of loss of power, so that to keep the sewers from overflowing with three-tenths of an inch rain, and do the work in 12 hours, an engine of considerably more than 83 horse power would be required. No doubt 0 - 3 inch rain admitted to the sewers is an excessive amount where partial separation of rain from sewage is intended to be carried out, but that amount is given in the report, and I presume the sizes of the sewers were designed to accommodate it. It will be evident that for safety's sake it would be necessary to provide two engines to the pumping' station, so that if one went wrong or wanted clearing, or the pump got choked, the other could do the work; but if both pumps should get choked, which is not impossible in sewage pumping, there would be no means of emptying the sewer, and, as mentioned above, it would fill up to its-overflow level, about 14 feet at the pumps. It will, therefore, be evident that to the usual inconvenience and risks attending the pumping of sewage, there is in this project the additional risks and inconvenience arising from the peculiar position of the sewer and pumps. As to the efficiency of the scheme when in good working order, I will call attention to ■ the following considerations : Where nil the sewage of a district is intended to be lifted by pumping, it is of the greatest importance that an much separation of rainfall from sewage should be effected as is possible; but this separation, apart from the question of pumping, is an expensive and inconvenient method of dealing with the sewago of a town, as it gives rise to the necessity of having two systems of sewers in the streets—one to convey the rainfall to the nearest creeks or the harbor, and auother to convey the sewage. In the case of Wellington, where the ground is extremely uneven, and the mountain gullies and creeks numerous, a great many of the streets might with little expense be made to discharge" the rain water falling on them to the nearest watercourse : but there are streets where much injury would be caused to the street and annoyance to the people unless the rainfall we e conveyed away under ground in" pipes, 0-. culverts, which otherwise would flood the street and overflow through private property; BflcL in, such cases, the expense of a double system of newer* would be felt. If

the sewage is not to be pumped, it is of little consequence what rainfall from the streets is admitted to the sewer?, and economy is often better served by allowing it to be admitted. The plea usually urged that a separation of rainfall from sewage allows the use of smaller sewers is of less effect in Wellington than in most towns, because the streets are so steep that small sewers will convey large quantities of water, and the excuse that it would enable a smaller main outfall sewer to be use 1 is in this case of no consequence, as the outfall sewer designed on the plan 3 is capable of discharging 1440 cubic feet, while the sewage and rainfall estimated to enter the snwers only amounts to 930 cubic feet per minute, so that there is room for a very large increase. From these considerations it would appear that this scheme offers a maximum of risk and expense for a minimum of convenience and efficiency.

If economy in the size of the sewer has not been attained in this scheme, there ought to be some other advantages which would be gained by pumping all the sewage of the town, and I am at a loss to think what they can be, seeing that four-fifths of Wellington is built on hills. I do not think sufficient reasons have been shown to justify this method of dealing with the difficulties of the place. I notice that your engineer in his report has suggested that the pumping might be done at the top end of the outfall sewer, and that a part of the sewerage might then flow away by gravity, but there is nothing in the plans or sections to Bhow how this was intended to bo done, and I take it that either on his own decision, or on that of the committee to which he refera the subject, it was not intended; but on the contrary that the whole of the sewage and rainfall to the amount of three-tenths of an inch should be allowed to descend by gravity 11 feet below low water, and be lifted 24 feet by pump, so as to enable it to flow away to the sea. In my opinion the waste of money which would be incurred by such a method of dealing with the sewage could only be justified on the ground that it was impossible to convey any by gravitation or to separate that part which might flow by gravity from that which must be pumped. This is a consideration of the first importance, and if any such impossibility exists it has not been pointed out in the report of your engineer with sufficient clearness to be convincing. It must also be borne in mind, as affecting the .question of the economy of pumping the sewage after it has passed through the tunnel, that the latter is situated in the centre of a deep valley with high mountains on each side. In such a position I should expect to find the rocks shivered and disintegrated aud charged with water at a great pressure, the tunnel being at an average of 8 feet below low water ; consequently, however carefully the sewers were built, I should expect the influx of underground water into it to be enormous. In many English towns it amounts to one-half, and even two-thirds of the quantity of the sewage, and it is evidently important that this water should not have to be pumped with the sewage. I think I have advanced sufficient reasons against the practicability of this scheme, and taking these into consideration, together with its great cost, and the large annual expense necessary to keep it in working order, in my opinion the City Council would not be justified in adopting it, unless it could be shown that no other was practicable. The sewage of the city at present is insignificant, and even when the population amounted to 50,000 would not be anything very great compared with many European towns similarly situated but with far greater populations. There are more ways than one in which the sewage of a town may become a nuisance to it, and by this complicated method of treating its sewage I think your city would soon find itself burdened with a very serious nuisance.

Before proceeding to consider the rest of this scheme I shall make a few observations on the alternative outfall which was proposed, which was to pump the sewage to irrigate the sand hill 3at the head of Evans Bay. Theoretically thi best method of disposing of town sewage is to irrigate land with it, but every town should be yuided as much by the practical considerations affecting its own case as by abstract conclusions, and I think your town would be undertaking unnecessary precautions against the nuisance arising from its sewage by adopting this method of disposing of it, both because of its insignificant amount, and because the places at which it may be di charged are not likely to become a source of annoyance on that account. If the sewage had to be pumped, at any rate then it would have been as well to make some use of it, if this could be done without seriou3 increase of cost. By irrigating the sand at the head of Evans Bay the sewrfge would be filtered and purified, and in time would fertilise the land. I estimate that it would cost about £IO,OOO less to the sewage to the sand hills than to Island Bay, but the annual working expense would be about £750 more, which might be recovered in rents after some years when the sewage had fertilised the land.

But I should not advise your City Council to pump any more of the sewage than was absolutely necessary, and I see no practical method of getting the sewage to the sandhills by gravity, also the question of utilising a few hundred acres of sand is of small importance in a new country. The irrigation of sand with sewage is very generally advocated in England, but it is usually in cases of inland towns where the sewage must otherwise be discharged into fresh water rivers to the annoyance of other towns lower down. In the case of Wellington it is possible to make too much of the supposed annoyance which is going to arise from the discharge of the sewage, and to overdo the remedy altogether. It must be remembered that the evils which Wellington is supposed to suffer from its drainage do not arise from the discharge of the sewage in the neighborhood of the city, but rather from its Itaky drains, which allowing the sewage to be absorbed by the soil, leave the solid filter to rot under the streets. There is always considerable difficulty in devising a scheme which will be capable of accommodating a large town in the future and yet be suitable for a small one at present; and in Wellington this difficulty would be experienced at the very outset, especially from the number of streets yet unformed and the scattered position of the houses. Thus a great many of the sewers would have to be omitted for years to come, and the others would run half empty, because they had been designed to carry the sewage of five or six times the number of people at present existing. Such incongruity between the design of a scheme and the purpose it serves in the meantime may be unavoidable ; but it is a good reason why the design should be as simple as possible and capable of development as the necessity arises. Next to the question of the outfall sewer, the most important is the arrangement and size 3of the street drains. The arrangement wherever possible should be such that the sewers can be flushed from one to the other with the least expenditure of flushing water, and of labor. The plans are not sufficiently finished to allow me to judge how far this object can be attained, although I have noticed one or two places where it has been neglected, evidently in order to keep the sewers at a shallow depth below ground.

As the town water is limited, a number of small reservoirs should be constructed on the upper parts of the various creeks running through the town to collect water for flushing the sewers, but as neither the creeks nor their levels are shown on the plans, there is no means of judging whether the levels of the sewers in the streets are such as to allow of them being flushed in this way.

I have been unable to find out the considerations which guided your engineer in proportioning the size of the pipes to be laid down in the streets, as neither the inclinations at which they arc laid, nor the areas which they are intended to drain, appear to mo to correspond to the sizes given on the plana ; thus, in the Tinakori-road, there is a 12-inch diameter pipe, the flattest inclination of the line of pipes being one in 108, and the area which it is Intended to drain 42 acres ; but a 12-?cuh pipe at 1 in 108 with three-tenths inch rain, and the sewage of fifty people per acre, contributing 5 cubic feet per day each porson, is capable of draining 200 acres. In Hawk-stone-street there is a 15-inch pipe at an inclination of 1 in 50 draining at most-50 acres. This pipe would suffice to drain 500 aeresand would

discharge as much as the intercepting sewer on Lambton-quay, to which it and other sewers are tributaries. In Hill-street, there is a 12-inch pipe laid at 1 in 26, draining 15 acres; such a pipe at such an inclination would suffice to drain 480 acres. At Ingestre-street an 18-inch pipe, inclination 1 in 4S, draining about 100 acres, and capable of draining 800 acres; lower end of Wellington-terrace, 15-inch pipe at 1 in 25, draining 25 acres, capable of draining 890 acres ; Constable-street, a 15-inch pipe, at 1 in 260, drains 76 acres, capable of draining 230 acres ; at Vivian-street a 21-inch sewer is increased to a 24-inch diameter, receiving an additional drainage area of only 25 acres. At the same time the inclination is increased from 1 in 76 to 1 in 38 ; but a 21-inch sewer at 1 in 38 would discharge two and a-lialf times as much as the same at 1 in 176, and the 25 acres additional drainage would be accommodated by the 21-inch sewer if it were laid at a very slightly steeper inclination, but the 24-inch sewer at an inclination of 1 in 38 could discharge lg time as much as the outfall sewer at Pirie-street, to which it and all the sewers of the town are tributaries. From these cases which I have investigated, I come to the con-

clusion that all the pipes, except the smallest, are too large for their purpose. Not only would such disproportionately large pipes incur a needless expense, but they would otherwise be very unsuitable from the waste of flushing water they would entail, which is a consideration not to be disregarded, as the capacity of the tjwn waterworks is not unlimited, and the creeks run nearly dry in summer. It must be admitted that in Wellington, owing to the inequalities of the streets, it would be impossible to lay down pipes of sizes exactly to correspond with the areas which they are to drain, but whilo making every allowance, I still think your engineer has made an oversight in giving such large pipes to the streets.

I have endeavored to check the estimate of cost of the whole scheme, although the plans are not sufficiently finished to be able to arrive at very exact results, and I attach hereto the details of my estimate, from which it will be seen that I differ very considerably from your engineer as to the cost of the bchemes. The principal difference consists of the estimated tunnel, To estimate this is a matter of great uncertainty, as the nature of the rock and ths quantity of water to be encountered in driving the heading can only be guessed at ; but considering that all the excavated rock and all the wat-er has to be hoisted up shafts from 60 to 140 feet deep, and all the materials have to bo lowered down the same, I think there can be no doubt that the cost will be very great. Almost 70 chaius of the outfall sewer is so deep underground as to be virtually in tunnel, and the above observations will apply to it also. I have kept the estimate for the tunnel and outfall sewer, properly so called, distinct from the estimate for the street pipes and sewers for the purpose of being able to see the cost of the outfall above. I have not put any sum to the estimate for the cost of land, as I have no means of knowing what it will amount to. I have also omitted the cost of management, as this will depend on the staff employed by the City Council and the rapidity with which the waterworks were carried on. It ought to vary between 3 and 5 per cent, of the total cost.

In conclusion I hope I have made sufficiently clear to your Worship and your honorable City Council the points in this project to which attention should be directed, and that the many objections which I have found necessary to make to the details and in the scheme as a whole, will be judged as being fully supported by the reasons given. I am glad to have this opportunity to express my thanks for the attention and kind assistance which I received from your engineer during my stay in Wellington.—t have, &c. C. Napier Bell, M. Inst. C.E. Christchurch, 29th Oct. 1877.