512 



NATURE 



[September 24, 1903 



of the dam for the whole or a portion of its length, thus 

 obviating the necessity for and the cost of an independent 

 bye wash. 



Whilst masonry dams have the advantage over earthen 

 dams of not being liable to be breached by a waterspout, I 

 am not aware of any case in which an earthen dam has 

 been destroyed in that manner, and so far as I am able 

 to form an opinion the accidents due to other causes have 

 been as frequent in the case of masonry dams as in that 

 of earthen dams. The destruction of masonry dams has 

 in some instances been the result of too great reliance 

 having been placed on theoretical calculations, without 

 sufficient allowance having been made for the many defects 

 in material and workmanship which might occur iii a work 

 of that kind. It was the opinion of the late Mr. Thomas 

 Hawksley that in some cases the destruction of masonry 

 dams had been occasioned by the neglect of the effects of 

 uplift due to the pressure exerted by water finding its way 

 beneath the bottom of the dam, a possible condition which 

 he was very careful to take into account when designing 

 the masonry dam of the Vyrnwy reservoir of the Liverpool 

 Waterworks. 



Examples of large masonry dams in the United Kingdom 

 may be seen in that constructed by Mr. G. H. Hill at 

 Thirlmere Lake, from which the city of Manchester is 

 partly supplied with water. Also at the Vyrnwy reservoir 

 of the Liverpool Corporation Waterworks, designed by and 

 partially carried out under the direction of the late Mr. 

 Thomas Hawksley, after whose retirement it was completed 

 by Mr. G. F. Deacon, who presided over Section G on the 

 occasion of the visit of the British Association to Toronto 

 in 1897 ; and again at the reservoirs near Rhayader, in 

 Wales, now approaching completion, from the designs and 

 under the direction of Mr. James Mansergh, F.R.S., Past 

 President Inst.C.E., for the supply of water to the city of 

 Birmingham. 



From the impounding reservoir the water has to be con- 

 veyed to the point of distribution by an aqueduct. This 

 aqueduct, which is sometimes of great length, rnay consist 

 either wholly of metal pipes, usually of cast iron' or partly 

 of a conduit constructed of masonry, brickwork or concrete 

 following the contour of the ground, with occasional tunnels 

 where high ground has to be passed through, and metal 

 (inverted syphon) pipes where valleys ha*-e to be crossed. 

 These conduits may be either open or covered, the latter 

 method being generally adopted, when they become what 

 is technically known as " cut and cover " conduits. In the 

 case of a continuous pipe-line of considerable length it is 

 divided into sections by means of break-pressure tanks inter- 

 posed at suitable elevations, each tank being say loo feet 

 or thereabouts below the preceding tank, by which means 

 the pipes are relieved from the excessive pressure to which 

 they would be subjected if the head due to the elevation of 

 the impounding reservoir was carried forward to the service 

 reservoir, from which the water is distributed to the con- 

 sumer. Steel pipes are frequently used abroad vi'here the 

 cost of carriage is great, but they have not yet been much 

 employed in this country, sufficient experience not having 

 yet been gained in reference to the deterioration of steel 

 pipes due to the action of the water from within and of the 

 ground in which they are laid from without. 



The lines of pipe are provided at intervals with suitable 

 stopcocks, sluice-valves, and air-valves, and also in some 

 cases with self-acting valves which close automatically in 

 the event of the velocity of the water in the pipe becoming 

 abnormally increased owing to the bursting of a pipe 

 beyond. 



I have already stated that most waters obtained from 

 gathering grounds are much improved bv filtration. The 

 process of filtration may be carried on where the water 

 leaves the impounding reservoir or at any convenient point on 

 the line of conduit thence to the place of distribution, pro- 

 vided the filter-beds are situate at such an elevation as to 

 place them on the line of hydraulic gradient. Various con- 

 siderations will influence the determination of their position, 

 but it is desirable that the water should not be subjected 

 to long exposure to light after filtration. Filtration by the 

 slow passage of the water through a bed of sand from' two 

 to three feet in thickness, supported by small gravel or 

 other suitable material, is the method usually adopted in 

 Europe, though what is known as mechanical 'filtration has 



NO. 1769, VOL. 68] 



been used to a considerable extent in the United States, 

 and may under certain conditions be usefully employed. 

 However I do not think it is likely to take the place to any 

 considerable extent in this country of the efficient system 

 of sand-filtration introduced so long ago as the year 1828 

 by the late Mr. James Simpson, Past President of the 

 Institution of Civil Engineers. The rate of filtration, to 

 be thoroughly effective, must depend on the condition of 

 the water to be filtered, but a rate of from 450 to 550 

 gallons per square yard of surface of sand per day (i.e. 

 twenty-four hours) is usually found to be efficient. Filter- 

 beds are generally open to the sky, but occasionally, when 

 situate at considerable elevations, they are covered by roofs 

 to prevent interruption by the formation of ice in times of 

 severe frost. In certain exceptional cases in which the 

 water is difficult to treat it is twice filtered with excellent 

 results. The water after filtration should be discharged 

 into a pure-water tank or service reservoir of sufficient 

 capacity to enable the process of filtration to proceed at a 

 uniform rate by night as well as by day, without regard to 

 irregularities in the rate of demand in the district of supply. 



The particles in suspension in the water, which are inter- 

 cepted by the process of filtration, gradually form a film 

 over the surface of the sand, and thus improve the filtra- 

 tion ; but this film at last becomes so thick as unduly to 

 reduce the rate at which the water passes through the 

 sand. The filter-bed is then laid off and, the water having 

 been withdrawn, the surface of the sand is scraped off to a 

 depth of about a quarter of an inch ; the sand thus removed 

 is washed in suitable machines to free it from the matter 

 intercepted during the process of filtration, and is after- 

 wards replaced in the filter-bed either immediately or after 

 several similar scrapings have taken place, care being taken 

 that the thickness of the sand left in the bed shall not at 

 any time be reduced below that required to ensure efficient 

 fiUration. From time to time the sand is removed to a 

 depth of several inches and washed, and occasionally it is 

 taken out and washed to its full depth. From the fore- 

 going description it will be understood that the filtration 

 of water, although a simple process, is one which necessi- 

 tates constant watchfulness on the part of those responsible 

 for the management of those waterworks undertakings in 

 which the water undergoes filtration. 



As near to- the termination of the aqueduct conveying the 

 water from the impounding reservoir to the point of distri- 

 bution as the levels of the ground will permit, a service 

 reservoir should be constructed for the purpose of equalising 

 the flow of water along the aqueduct, and for maintaining 

 the supply to the district during any temporary interruption 

 on the line of aqueduct due to a burst pipe or otherwise. 

 The service reservoir should contain not less than one day's 

 supplv, two or three days, and, in exceptional cases, even 

 more being sometimes desirable. Service reservoirs should 

 by preference be covered so as to exclude light, and thus 

 prevent the growth of vegetation which would otherwise 

 take place. The covering, when consisting of brick arches, 

 has also the advantage of keeping the water cool in summer, 

 and preventing the temperature from becoming too much 

 reduced in winter. The rate of draught on the service 

 reservoir is continually varying throughout the day and 

 night according to the hourly requirements of the popula- 

 tion which it serves. This variation is very considerable, 

 amounting during certain hours of the day to at least twice 

 the average rate of consumption during ' the twenty-four 

 hours. It will therefore be apparent that were it not for 

 the equalising efi^ect of the service reservoir the aqueduct 

 must have a capacity at least double that which is needful 

 where a service reservoir is available. At Southport, foi 

 example, although the water is distributed from a service 

 reservoir, that reservoir is situate at a distance of about 

 seven miles from the town, because, owing to the great 

 extent of comparatively flat land in the neighbourhood of 

 Scuthport, it was impossible to obtain a suitable elevation 

 nearer to the town than Gorse Hill, on the summit of which 

 the reservoir stands. Consequently the main pipes thence 

 to the town have to be of sufficient capacity to convey the 

 water at a rate corresponding with the demand at the time 

 of maximum consumption, or, in other words, of about 

 twice the capacity which would have been needed if the 

 service reservoir could have been placed close to the town, 

 when these pipes would, for the greater part of their length, 



