TRANSACTIONS OF SECTION G. 7 Go 



about four-fifths of the average taken over a long period — say, forty or fifty years. 

 From the mean rainfall of the three driest consecutive years a deduction must be 

 made for loss by evaporation, which is usually between twelve and sixteen inches. 

 The result is known as the available rainfall, and represents the quantitv of water 

 which can be drawn continuously from an impounding reservoir without fear of 

 failure in the driest years. But the whole of this water can rarely be abstracted 

 from a stream without injuriously afl'ecting mill-owners or other riparian owners 

 on the stream below the reservoir ; therefore they have to be compensated for the 

 injury they sustain. This is sometimes done by payments in monev, but where 

 the mills on the stream are numerous it is generally more economical to make 

 compensation in water delivered into the stream immediately below tlie reservoir 

 because the same water compensates each mill in succession as it flows down the 

 stream , 



It has now become an accepted principle that one-third of the available rainfall 

 flowing down a stream in a regulated quantity day by day throughout the year is 

 of greater benefit to the mill-owners (Avith few exceptions) than the whole of the 

 rainfall allowed to flow in the irregular manner in which it is provided bv nature. 

 This compensation water is discharged from the reservoir into the stream either 

 during certain hours on working days or by a uniform flow throughout the 

 twenty-four hours of every day ; a method nov/ frequently demanded by County 

 Councils on so-called sanitary grounds, but which is in my opinion not infrequently 

 detrimental to the interests of mill-owners without a correspondino- advantao'e to 

 the public. 



Where compensation in water is given there remains for distribution in the 

 district to be supplied a quantity equal to only two- thirds of the available rainfall. 



Assume for the sake of illustration a case in which the gross annual rainfall is 

 40 inches. Then we have : — 



Inches 

 Gross annual rainfall ......... 40 



Deduct to an-ive at the mean annual rainfall of the three driest 

 consecutive years— say one-fifth of forty 8 



Mean annual rainfall of three driest consecutive years ... 32 

 Deilnct for evaporation, say 14 



Available for supply if no compensation water be given . , 18 

 Or if compensation water be given deduct one-third ... 6 



Leaving available for supply 12 



Having now ascertained the amount of the rainfall available for the supply of 

 the district, it remains to be seen whether or not the area of the gathering ground 

 above the reservoir is sufficient to give the required quantity of water. If it is 

 not, the area may in some cases be extended by means of catch-waters in the form 

 of open conduits cut along the sides of the valley below the embankment of the 

 reservoir, and at such an elevation as will enable them to discharo-e the waters 

 they collect into the reservoir above its top water line. 



Almost all waters derived from gathering grounds are much improved by 

 filtration before use for potable purposes. In some cities and towns in this country 

 more especially in Lancashire and Yorkshire, the benefit derived from filtration 

 has not been sufficiently appreciated, and the water is still delivered into the 

 houses iinfiltered; but I am of opinion that the time will come when nearly every 

 town of importance supplied with water derived from gathering grounds will 

 adopt filtration, for it not only removes matters in suspension but it also diminishes 

 the discoloration due to peat which is to be found in most moorland waters. 



Reservoir dams in Great Britain consist either of earthen embankments or 

 masonry walls. Of the former, examples of considerable size may be seen at the 

 reservoirs of the Manchester Waterworks, designed by Mr. J. F. Bateman F.R.S. 

 Past President Inst.C.E., who was President of Section G of the British Association 

 at the Manchester Meeting in 1861 ; and at the Eivington reservoirs of the Liver- 

 pool Waterworks, designed by my father, the late Mr. Thoma? Hawksley, F.R.S.^ 



