THE DRAINAGE QUESTION.

Auckland Star, Volume XL, Issue 147, 22 June 1909, Page 5

THE DRAINAGE QUESTION.

** "■*- ' 6 ARE THE SUBURBS FAIRLY 1 TREATED? 1 Mr. M. Raymond 'Creagh, a civil engi- f neer, and one of the delegates from the t One-tree Hill Roaa Board, at the con- ] I ference held shine time back in regard to 1 1 the Auckland drainage scheme and itsjl proposals anent Remuera and Gne-tree Hill, strongly supports the case made 11 out by the Kemuera Road Board's engi- i j neer, and says the position is a most 1 1 '. serious one as far as all the local bodies [, are concerned. the conference," | j Mr. Creagh states, "I roughly calculated j what I considered was about the size , 'of pipe required for iseWerage only. My •. ! data consisted Of ultimate density of : ! population as ten, allowance of fluid 35 : >! gallons per head per diem. This was ; » taken frbra'Mi. Midgely Taylor's report. -1 The grade I did not know for certain, t! but naturally, to be.oh the safe side, took '■ 11 ft as fairly' fiat. This gave the size 'of I ripipe at the commencement" of 'the sewer j a | on, the.boundary- of- Remuera- and-flne- j 11 tree Hill for sewerage only as a 9in 11 pipe. To carry six times this flow re- i i j quired a 21in pipe. Finding this was | I what Mr. Wilson had specified, I felt j j satisfied I was fairly correct. I naturt ally felt disconcerted when told that | t the drainage engineer considered that a Sin pipe Would carry feix tithes the bare i sewerage flow. On being supplied with the data that this calculation was based j on, I found that it Was absolutely cori rect. The reason of these apparent dis- j . I crepancies in the calculations of different *| professional men is.the point. I wish to " | emphasise, and, with all respect, would ? like to urge those other suburban bodies b that are at present inclined to resent | £ the precautions taken by Remuera to j : carefully digest and apply to their own j 8 j cases. The difference was caused entirely by the difference in data: whereas II ] I used 35 fralions per head-j>er diem,! Pi-he |i-he drainage engineer only used 25. i d ! Six times 35 is 210t six times 25 is i i 150 gallons per Head. This,- -however, i | vrag ir.'EJaiised by the estimated poptl- I i lation hciag raised to 20 -per acre, as j against 10. The.other important factor; was the grade. This the drainage engi- j neer stated was 1 sft 70. Not being sure, I naturally allowed a very much easier grade. This, I hope, explains they . difference in calculations, and I wish now to try and assist in driving home j_ the import of it. In the city tho allowance Is taken, I believe, at 60 gallons per head, with a probable future 1 density of 50 people per acre. This ! , gives 3000 gallons per day per acre. I Compare this with the. allowance in. the ; ' j disputed calculations—Le,., 25 > gallons per _ j day and 20 people per acre, giving 500 \ gallons per acre. This shows a- large "I difference, hut as it is the actual amount ■of sewerage arising from these different "populations, it has to be carried, and, therefore, as far as this sewerage flow is concerned, the sewers will differ. When, however, the modified allowance for storm water is considered, the difference is of vast importance. Six times 3000 is ft.OOO gallons per acre; six times 500 is 3000 gall6ns per acre. In other words, six times the amount of rain is to be allowed to fall on a city acre, compared with that to be provided for on a suburban acre. The reply likely to be given is that all the rain water gets to the city sewers, while in the suburbs a great portion soaks into the sround and never reaches the sewers. This may be true for some parts, ?av, round Mount Eden; but on clay or other semi-impervious land it is certain that more than one-sixth of the rainfall will reach the sewer. It is difficult to deal with this subject, and at the same time refrain from technical expressions which might be confusing to the lay mind. T think it is very startling to find the unlimited power placed in the hands of the drainage engineer, wno is but human, and the contemplated total -departure from the fundamental points laid down in previous reports "