SEW

419

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That

•hey aVe to show that their size is unnecessarily large,hoj ® re _ larger than required for the ordinary drainage can-tha’ , e lrn agine, be a matter of question, after the result ofkiln-. 0Ve observations and experiments have been made

° Ve observations andj i

0tl this point, given by persons practically acquainted

5Ho w ooservations ana experiments

n 5 nor can we attempt to disprove the unvarying evi

,j — *U A LA A j g I I oil 1/ J O A OV/llO 1 Ctv tlwill k j civv^ LA LAI LA IV/U

^ci(j ' e ® u ^j®ct ; the table of gaugings by Mr. Roe seems to18 -Jq 6 ^'' s luost'on, for during the extraordinary storm, int°t l ''’ e find that by far the majority of the examples werel ° n ^ lat occasion, nor, indeed, nearly so. Out of 16"‘es^only two were filled on this occasion, one of which®Cp es tQa 'n line of 13-feet capacity, d raining an area of 110an< * the other a 15-inch drain, relieving an area of 0.84, Coring the experiment in the Earl-street sewer,lla Ppened in a wet season of the year, a 15" pipe was, rri01 'e than sufficient for the drainage of 44 acres offi roun fi> with only one exception, which occurred30 0 lleav y storm, when the discharge amounted to aboveper minute. In the experiments in Upper>H 'v' str eet, during a continuance of 96 hour’s rain, the 12-on e ^ e Was never observed to be more than half full; butlr * e fiea ° Ccas ' or b during a violent storm, the water rose 18a , a fi°ve the pipe ; this state of things, however, lasted

»t f|

a sh,

‘eet

0r t time. The area drained by the Upper George-

was also 44 acres' v h f . t i 1 ’ Putting aside for the present the question as tot l le pipes used in the above experiments were of’ ^'nlt Ca P a city, Mr. Roe’s experiments, which give, we! ' Ve may say, the very greatest amount of dischargeti)..-^ 11 ever take place, are decisive thus far—that by far^ °1' existing sewers are very much larger than can

tlfv °i it . ltne necessary; and with respect to the exceptionalXi^uins a question, whether under an improved systemJailer ., a S e l fiey would ever have been put to such a test, buts"’Ur ( ) °ld system, when, occasionally, a sewer got smaller, ait • lta out let, the levels were ill-arranged and defective,0 ps t f not to be wondered at, that sometimes one sewer had^re a j 0rtn more than its fair share of work, whilst others° s t inactive; if the sewage had been more equally dis-'f^i ,tl0 °ne sewer would have been so much overcharged.

A t 0 S.tfien the maximum quantity ever discharged, as thei'I th a ^ Ul ^ e us >n fixing the proper dimensions of sewers, wef* 1Ue S ti ^' e °lfi system is too capacious; but beyond this iti° f Su ch° na ^ e whether it is necessary or advisable to provide/•porta ex traordinary flow, and this question becomes of more, Ce > if it can be proved that extra capacity is detri-of o efficient discharge of sewage matters. In thisp 'Si'abl ' 6 case ’ ( ' ie rea l matter at issue is, whether it bep f ai'y e to be subject to a constant inconvenience, or a tem-all e lla Problematical one. Some say we must providep te iticli ,l ' ei '® enc ‘ es > others would rather run the risk. Wetj^thgj, towards the latter opinion. But there remains,‘Sument in favour of large sewers, and that is, that

atir) “'Unit tt ...o- —> —— —— —» ——

, 5 Per' a- P assa g e °f a man > for the purpose of inspectionl ein ,, 0c *ical cleansing, which the advocates of the old'hat u deem ,_°’. :<■

t0 ' Je necessary ; if, however, it can be proved5 ex; t . ;l powers will keep themselves free from deposit byten ‘

t| Ve f°city and scour of the water, this objection will

tin %sri d - • ■^ L S a ’n, this objection will not bold good against'Otigij utl °n of very small pipes for those sewers which,* ot c„ mUch l‘ u 'ger than requisite for the ordinary flow,Ip’t Cai| nVeil >ently be entered for this purpose ; for if theC* d Wiq 68 ^ leat discomfort to the labourer, it is certain! a r p(l at tt°t be performed efficiently, if, indeed, it be per-ig] lt al > we should imagine that sewers less than 4 feet'deli ’• ttIe not inspected much more frequently than awould be.

We have now to consider whether drainage may be moreefficiently discharged by a small than a large sewer. Thesmall quantity of water ordinarily passing through the largeexisting sewers, is allowed, on all hands, to be very sluggish inits movement; and that a deposit of solid matter is always goingon, and that to a considerable extent, is evident from thefact, that large quantities of solid matter are obliged to beperiodically removed by flushing or hand-labour, and inseveral cases when old drains have been opened, they havebeen found to be almost choked up by such accumulations.It is certain, therefore, that large sewers do not of themselvesefficiently remove the refuse matters. This defect wasobserved some years since by Mr. Roe, and the cause beingdetermined upon, it was attempted to remedy it by reducingthe width of the invert of the sewer, and thus narrow anddeepen the channel for the water, thereby producing less fric-tional area, a greater depth of water, and consequently agreater velocity and improved scour. This practice wasfound to succeed, and was afterwards adopted in other locali-ties. It may be noted in passing, that where much deposithas taken place, it is observed that the stream forms foritself a sort of gut or narrow channel, thus giving visible evi-dence of the requirements of nature in this particular.

As the question relating to the velocity and scour of abody of water in various-sized channels was of considerableimportance in determining upon a system of drainage, a seriesof experiments upon the flow of water through various-sizedpipes was made, under the direction of the MetropolitanCommission; and amongst the results given, we find thata 3-inch glazed stone-ware pipe, 50 feet in length, with aninclination of 1 in 120, and being fully charged at the head,will discharge 100 gallons of sewage water in 3 minutes; andthat a 4-inch pipe under precisely similar circumstances, willdischarge 200 gallons in the same time; and further, thatsuch a flow is sufficient to remove any, and even more thanordinary and usual semi-fluid deposit, such as is usuallyfound in house-drains. A mixture of sand with water,in proportion of from to tj the volume of water, was alsoremoved. Also that the hydraulic mean depth of a 3, 4, 6,and 9-inch pipe when half full, is respectively .749, 1.00,1.5 and 2.18 inches, and that the fractional line, under thesame circumstances, would be 4.71, 6.28, 9.42, and 14.13inches respectively. Further, that 1 gallon of water througha 3-inch pipe, moved lib. ; through a 4-inch pipe, 31b. ;and through a 6-inch pipe, ^lb. ; and that 3 gallons of waterwill carry off 11b. of solid faces through a 6-inch pipe witha fall of 1 in 10 ; but to make these results of use, we oughtto be acquainted both with the fall and velocity. Anotherstatement is this : if 81 seconds suffice for the dischargeof 50 cubic feet of clean water, 84 will suffice when ^ ofsolid matter is added, and 91 seconds when the solid matteramounts to fa

Such results are generally in favour of the small pipes, butit is to be regretted that the experiments were not carriedon on a more extensive scale, for they afford little informa-tion respecting the larger kinds of pipes. It will be generallyallowed, however, that a concentrated body of water willmove with greater velocity than the same amount wherespread over a large surface, the resistance offered by frictionwill be less effective in retarding its progress, and the greaterdepth afforded by narrowing the channel will tend to improvethe scour. The scour will depend both upon the depth andvelocity of the stream, and will therefore in both ways bemore efficient in a contracted channel. On these two pro-perties, depend the efficiency of the drain ; for if the velocitybe greater, the discharge will also be greater, and the powerto keep the sewer free from deposit will increase in likv