DUNEDIN DRAINAGE SCHEME.

Evening Star, Issue 12067, 12 December 1903, Page 11

DUNEDIN DRAINAGE SCHEME.

AN AMERICAN OPINION. Mr Arthur S. Turtle, M. Am. Soc. 0.K.. and at present engineer iu charge of a mo-t important branch of the New York Metropolitan Board of Works, ha* forwarded to a friend in Dunedin a brief resume of the above, and which we have pleasure in publishing for our reader-/ benefit : I have read the preliminary engineering reports ou the drab-age of Dunedin ivkii much interest. Ido not know that I can give a very intelligent or valuable opinion upon the various plans recommended, for the reason that the local conditions are not described in the printed reports: that ike names of the districts frequently referred to are not shown by the map ; and that the drainage map gives no contours, the only elevations which appear being those on ihe profiles, which are very incomplete, and rather indistinct. I also' feel somewhat uncertain as to your rain records, the diagram submitted not being fully explained. These criticisms, and others of a similar nature which might be made, are no reflection upon your engineers, for I assume that, the missing elements are either too well known there to require making them a, matter of record or that they are covered by drawings not published. I make this explanation so you wili understand that my suggestions may be conditioned upon false premises and incorrect assumptions. From a brief consideration of the reports I should offer the following hastily-ma dt notes:—lt is unfortunate that your drainage engineer and the consulting engineers have hot been able to come togtthcr .md reconcile their differences. Both seem to have good points in respect to the projects they offer, and while I feel that both, plans may be adversely criticised. I am inclined in the main to favor your drainage engineer. As between the separate and combined systems, the choice should be purely a matter of first cost and expense of operation ; sometimes it is desirable to treat cmsection by one system, and an adjoining one by the other. For such a comparison it is not fair to omit the cost of the surface drains required if ibe separate system be adopted, unless existing surface channels or drains can be utilised. The mains are pro, bably below the ground water-line in at least a considerable part of their length, but I do not note that, the capacity of the system pioposed in either report provides for handling seepage into them of ground water, as would undoubtedly occur if my assumption be correct. The flow-line of the sewer should be above high-water line at all points lwck of the ocean frontage: the profiles lead me to believe that under the plans of the eonstilting engineers the salt water will back into the sewer at high tide, thus checking the. velocity, and causing the solids to be deposited. The profile submitted by tile drainage engineer is not carried to the outfall, but a similar criticism would apply to the connection proposed by him into "the pump well. The grades for all sewers should be such as to give a velocity of flow of not less than 20in to 2ft per second, and not more than 6ft per second ; if less than the former requit-einent' there will be a deposit in the_ sewer. The grades to be used for the smali sizes are not given in either ieport, but those named by Engineer Cuthbeit (page 35 of report) are much Hatter thin permitted by good practice, and are not to be commended. The proposed inverted syphon to Bird Island is objectionable; stoppage is liable to take place here, and I do not note that provision has been made for removing the same. Such syphons should be avoided if possible. The outfall discharge at or near tide level is undesirable, as there will surely be a nuisance at that point; it would be better to discharge the sewage below tide level, and. if possible, the outfall should be carried in a submerged pipe out to a good depth of water — say 100 ft. The map indicates that proper currents may be found for the final removal of sewage in the vicinity of Bird Island, and not at Lawyer Head. If, as in the drainage engineer's report, the storm flow is to be discharged at Lawyer Head, and the diy flow at Bird Island, a considerable saving' could be effected by using only a single pipe line from the Mus*. selburgh section to the " dividing well," where an overflow could be built. If catch basins are located at the inlets to storm sewers, I see no necessity for the usie of detritus tanks. The necessity of septic tanks, as provided in the plans' of the consulting engineers, is by no means clear. Septic tanks are of some value in connection with and as preliminary to a system of filtration, their sole function" be:nj,' now generally conceded to be the removal" of a small portion of the solids—say about 25 per cent. The plan, however, calls for final disposal into the ocean ; if purification works are required at all they should also include filtration. The septic tank alone would not prevent the pollution of Tomahawk Beach, as is stated by the consultins: engineers If electrically - operated centrifugal "'pumps are to be used, the motor should be placed well above the sewage and dampness, and it should be belt-connected with the pump —not as shown bv the drawing submitted by the consulting engineers. If I understand the drawing of tV intercepting weir as proposed bv the drain-ace en<rineer, I do not believe that it will give satisfactory results.

From such information as I have, I should be inclined to suggest a plan somewhat as follows:—Build a main intercepting sewer at the lowest, possible elevation, to secure a gravity system, and to take all of the bouse drainage, and the storm water from higher levels (combined system); this sewer should be carried to the nearest deep water of the ocean, where proper currents can be found. By use of an overflow near the water's edge, storm water could be discharged near the shore, thus permitting of use of a small submerged pipe to carry the dry Slow out to deep water. The size of the main would be sufficient to permit of a ver\' slight grade, and it would be flushed with every "storm,; the flat grnd* would permit of locating it on very low land. Build all sewers, directly tributary to this main on the combined system. For

the territory at lower elevation use the separate system, dividing the area up into house drainage districts as large as possible, and providing a central well and pumping station for each, where the sewage would be pumped into the main intercepting sewer. The pumps would be of the centrifugal type, and electrically operated, if power can be purchased at reasonable rates. , Storm drainage for the low district should be carried to the nearest water, either by surface channels or by pipes, the street refuse being collected in the catch basins, or in detritus tanks if basins are not used. The main intercepting sewer could be provided with overflows into the harbor to relieve it from exceptionally severe storms, thus permitting use of a moderate size of main. Plan thi system to utilise all of the present sewers, where it is possible to do so without injurious effect on the design. If storm water is to be removed, build the sewera large enough to care for the same, otherwise the municipality will be liable for damage caused by floods. A few months ago we authorised an expenditure of nearly 200,000d0l to reinforce the outlet sewer from one district, where the saving from suits now frequently and successfully brought will many times justify the expenditure. I suspect that the drainage engineer's plan does not provide for the immediate removal of the storm water, except for ordinary rains. I should recommend that pumping and purification works be dispensed with as far as possible ; the first cost may possibly be more for a system such as I have outlined, but even if such is the case (and I believe it doubtful) the expens? of maintenance will be very materially less, and admits of far more accurate determination. My experience has always shown that the more simple the system the better the satisfaction, and that liability to neglect or incompetent management is greatly diminished, a factor bv no means insignificant in municipal works I cannot believe that the surface drainage into Otago Harbor from the small area at lower elevation than the main intercepting sewer could cause complaint, nor could overflow from the main during periods of exceptionally severe storms, the first wash in the latter case currying with it the street refuse, being carried out to sea in the main drain. B-. disusing of the sewage in deep water, and permitting the storm flow to discharge at the beach, and where the outfall dips into the ocean, tine re should be sufficient dilution of the refuse, either sewage or storm water, for effectually rtisposinc of it for all time to come, [f your appropriation is limited, it should, nevertheless, be possible to design and adopt a proper system, building as much of it now as the treasury permits, and extending as more money becomes available ~nd necessity demands. I trust that this review may be of some service to you. and rearet that mv information concerning the local conditions is not such as to warrant me. in making more positive statements.