184.8.] 



THE CIVIL ENGINEER AND ARCHITECrs JOURNTAL. 



76 



use, and a more regular and abundant supply of water, the sewers 

 would, by this system of arranging them and economising tlie 

 currents, keep themselves thoroughly clean. 



All head sewers, horn want of backwater, have a tendency to 

 choke up, and their ventilation is also very bad, consequently there 

 ^ould be as few of them as possible. 



The general surface of the metropolis, on the north side of the 

 Thames, is most admirably situated for being efficiently drained, 

 as the ground continues to rise with an easy acclivity from the river 

 to tlie hills some miles to the northward. The surface is divided 

 into several natural areas, each of %vhich has its main outfall sewer 

 nmning through the lowest level of the valley, and discharging 

 into the Thames, and into these main or valley sewers the whole 

 of tlie sewers on the sides of the declivities discharge themselves. 

 This mode of drainage is a very objectionable one, and should 

 never be resorted to if it be possible to avoid it. The declivities 

 of ail natural areas are generally in two directions, namely, trans- 

 versely towards the valley line, and longitudinally towards the 

 outfall. Now, if attention be paid to the levels, and the sewers 

 on the sides of the declivities be judiciously arranged, a perpetual 

 circulation of water may be kept flowing throughout the whole of 

 them from the sewer on the summit at the head of the natural 

 area to the outfall in the river ; that is to say, a system of col- 

 lateral or concentric sewers should rise one al)Ove another from 

 the valley line to the ridge or water-shed line of the district ; each 

 collateral sewer skirting tlie entire area, and discharging itself 

 into the river by a separate outlet, or in the manner previously re- 

 ferred to. It will be seen that, when the sewers running trans- 

 versely are connected at their upper and lower ends on the same 

 levels with those running longitudinally, a facility is afforded for 

 the drainage to circulate from the highest sewer to the one imme- 

 diately below, from this to the one next lowest, and so on through- 

 out. 



Mr. Phillips proposes fourteen graduated forms of branch se- 

 condary and principal main lines of sewers of the egg-shape for 

 the di-ainage of a district in which the sewers and the water supply 

 are under one and the same authority. Fig. 13, Plate V., shows the 

 form of one of the sewers together with the radii of the several 

 curves. 



Much caution is required in the building of sewers in a clayey 

 soil ; otherwise, from the treacherous character of this ground — 

 its liability to expand and slip, — the sewers may be forced in. 

 The tliickness of a sewer should be proportioned to the nature of 

 the ground and the pressure it has to bear ; but its stability is very 

 mudi dependent on the goodness of the workmanship. A half- 

 hi-ick sewer, under ordinary circumstances, will, if executed well 

 and soundly, the joints made thin, and the sewer worked true to 

 the curve, be quite strong enough, and would be found to answer 

 eva-y required purpose. The thicknesses depend upon the ma- 

 terial and strata. The equilibration may be altogether destroyed 

 by a want of uniformity in the working of the curve. The greatest 

 pressure of the ground acts laterally from the sides downwards. 

 Much of this pressure may be prevented by leaving in the trench 

 from the surface downwards short lengths of earth, say of 10 to 

 'iO feet, and about 50 to 80 feet apart, to be tunnelled through for 

 the sewer to pass. These benchings, as they are termed, will keep 

 the sides of the trench from sinking and slipping, and so from 

 jM-essing against the sides of tlie sewer. 



The smootJier the surface the less will be the friction, and con- 

 sequently the greater will be the velocity and discharge ; and the 

 friction in a glazed pipe must be considerably less than in a brick 

 drain, as commonly built. I am not prepared to say that the 

 friction would be diminished so much as one-third ; I think not so 

 much. The smoothest glass pipes throw off traus^'erse motions 

 which greatly impede the flow. There is a difference in the flow 

 •f pure clean water and of sewage water ; the latter moves more 

 sluggishly. This is caused by its being thicker and more viscid. 



from having matter chemically combined and mechanically sus- 

 pended in it. 



As the velocity increases, so does the transverse section of tlie 

 area occupied by the stream decrease. This is a natural law ob- 

 servable in all moving streams, for we see that in a mo\ing mass 

 of water the discharge is the same, whatever form and size tlte 

 channel may assume, the velocity being greater where tlie channel 

 is narrow and deep, and less where it is wide, flat, and irrcguliir; 

 but the exact ratio of decrease of area, from decrease of friction 

 aud increase of flow, can only be determined by actual experiment 

 and by taking into account all the attendant circumstan«es which 

 influence and govern the motion of the stream. 



Have you at all considered the capacities of sewers necessary for 

 draining different areas of ground? — Yes, I have given the subject 

 much attention. If the consideration of the sizes of sewers was 

 confined solely to the carrving off the water supplied by the several 

 water companies, then I apprehend that pipes somewhat larger in 

 size than the siip])ly-pipes themsehes would suffice ; but provision 

 has to be made for receiving and conveying away the waters of 

 heavy rains. In London continuous heavy falls of rain are not of 

 long duration, lasting seldom more than from one to four hours. 

 About one-fifth of the quantity that falls is absorbed partly by the 

 dryness of the surface of the roofs, the paving, and the ground 

 and partly by the porosity of the ground itself. A farther propor- 

 tion is also prevented from ffowiiig to the drains and sewers at all 

 by hollows in the surface, and again reascends into the atmosphere 

 as vapour. There is also a small quantity that enters into the 

 composition of animal and vegetable bodies. Then there is the 

 resistance the flow experiences from the friction of the entire sur- 

 face, being accelerated or detained in proportion as the surface is 

 more or less inclined. To provide for the discharge of a fall of rain 

 of two inches in depth has been considered by Mr. Hawksley, C.E., 

 the extreme datum upon which to proportion the capacities of 

 town sewers generally. Now I believe that, practically, the sizes 

 in his table, although they may appear theoretically correct, are 

 (excepting for the smallest sizes) too large for sewers in London. 

 It is extremely violent rains alone that produce a depth of two 

 inches per hour, and such rains occur only once in four or five 

 years, if so much. I am of opinion that it is unnecessary to pro- 

 portion the sizes of the sewers to meet an extraordinary occurrence 

 that may probably happen only once in so many years. My reason 

 for not fearing any serious damage from an excess of rain at remote 

 intervals being provided for in surface channels, excepting, pei»- 

 haps, in situations peculiarly liable to inundation (for instance, at 

 the foot of a long or steep declivity, or where the waters may, 

 from any cause, be suddenly congregated at one focus) is, that I 

 have obser^•ed, that in towns entirely destitute of undei-gi-ound 

 drains, no such inconvenience is felt as would justify the formation 

 of enormously large sewers, or the e.xpenditure ot large sums of 

 money to provide against it. In August 1846, a most extraordinary 

 fall of rain occurred in London. The storm lasted nearly two 

 hours, and from the best information I have been able to obtain, 

 the depth of rain amounted to about four inches. Much damage 

 resulted therefrom, by the water in the principal main lines situate 

 in the valleys flowing up the drains and branch sewers, and inun- 

 dating the rooms and cellars below its level by the influence of its 

 pressure. The inundation of lands and the damaging of property 

 in the valleys could not happen if there were parallel catch-water 

 lines of sewers on the sides of the declivities to convey the drainage 

 into the river by separate outlets. The average fall of rain in 

 London is about 22 inches in a year, or about 2i inches in depth 

 per thousand hours. Now after observing and calculating the 

 depths of different falls of rain in London, it appears to me that if 

 the sewers were of sufficient capacity to receive and discharge, as 

 fast as it falls, a quantity of water equal to the produce of a full of 

 rain of one inch in depth per hour, they would be found large enough, 

 and that more particularly if they were built on the intercepting 

 or catch-water principle, and so as to communicate with each other, 

 and all be filled with running water at the same time. The steps 

 to be taken to proportion the capacities of sewers to receive and 

 convey away the waters of heavy rains should, I think, be as fol- 

 low, although I fully admit our present knowledge of the subject 

 to be very elementary : — 



To ascertain the number of superficial yards or acres to be drained 

 by each sewer separately ; progressing in a uniform gradation frofl-i 

 the entire natural area to be drained by the largest outfall sewer, 

 to the small tract of land to be drained by the least sewer on the 

 summit. Taking the hourly fall of rain, therefore, upon one acre 

 at one inch in depth, we must provide for the discharge of a quan- 

 tity of water (•'-?^#i') =: 3630 cubic feet per hour, or one cubic foot 

 nearly per acre per second. Then taking into account the loss 



