September 24, 1903] 



NATURE 



5" 



entail the construction of reservoirs of enormous capacity, 

 at a cost incommensurate with the object to be attained ; 

 it is therefore customary to make them of such size as to 

 enable the supply to be maintained without risk of failure 

 throughout the three driest consecutive years, the mean 

 annual rainfall of which years generally amounts to 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 quantity of water which can be drawn con- 

 tinuously 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 

 affecting 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 money, but where the mills on the stream are 

 numerous it is generally more economical to make com- 

 pensation in water delivered into the stream immediately 

 below the 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 regu- 

 lated quantity day by day throughout the year is of greater 

 benefit to the mill-owners (with a few exceptions) than the 

 whole of the rainfall allowed to flow in the irregular manner 

 in which it is provided by 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 

 now frequently demanded by County Councils on so-called 

 sanitary grounds, but which is in my opinion not infre- 

 quently detrimental to the interests of mill-owners without 

 a corresponding advantage 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 

 40 



Gross annual rainfall 



Deduct to arrive at the mean annual rainfall of 

 the three driest consecutive years — say one- 

 fifth of fortv 8 



Mean annual rainfall of three driest consecutive 



years 32 



Deduct 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 avail- 

 able 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 

 discharge 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 unfiltered ; 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 

 • ppat which is to be found in most moorland waters. 



I^pservoir dams in Great Britain consist either of earthen 



.l)ankments or masonry wails. Of the former, examples 

 NO. 1769, VOL. 68] 



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 186 1 ; and at the Rivington reservoirs of the 

 Liverpool Waterworks, designed by my father, the late Mr. 

 Thomas Hawksley, F.R.S., Past President Inst.C.E., who 

 was President of this Section at the Meeting at Nottingham 

 in 1866. 



Earthen embankments are formed of the most suitable 

 materials to be obtained by excavation in their neighbour- 

 hood ; the water is retained by a wall of watertight clay 

 puddle forming the core of the' embankment, extending for 

 its whole length and continued at each end into the natural 

 ground forming the hillsides. This puddle core has to be 

 carried down into the ground until watertight strata be 

 met with, occasionally necessitating a puddle trench having 

 a depth of 80 feet or more below the bottom of the valley 

 and 200 feet or more in depth in the hillsides. W'here the 

 strata forming the sides of the valley are not watertight, it 

 is necessary to continue the puddle core along the sides of 

 the reservoir by means of wing trenches. The determin- 

 ation of the depth and extent of the puddle trench in order 

 to secure the watertightness of the reservoir is one of the 

 most difficult and anxious duties of the engineer on whonv 

 rests the responsibility of its construction. In forming his- 

 judgment he has to rely entirely on his experience for guid- 

 ance, this being one of those matters which cannot be learnt 

 at an engineering school or even in an engineer's office. 

 How much depends on the exercise of a wise and trained, 

 judgment may be understood when it is realised that art 

 error in this respect may result in very costly works having 

 subsequently to be undertaken to stop an escape of water 

 which might in the first instance have been prevented by a 

 comparatively small outlay. 



Provision has to be made for the passage of flood-waters 

 during the construction of the embankment. This is 

 ordinarily effected by the construction at about the level of 

 the stream of a tunnel of sufficient diameter to convey 

 with onh' a slight head the volume of water produced by 

 t'le greatest flood which experience has taught us to antici- 

 pate. This tunnel is sometimes formed beneath the em- 

 bankment, but preferably, where the circumstances are 

 favourable, it is carried through the natural ground near 

 to one end of tfie embankment. A shaft is built in con- 

 nection with the tunnel, in which, after the embankment 

 has reached its full height, are placed the outlet valves of 

 the reservoir. 



It is of the utmost importance that ample provision should 

 be made for carrying off the flood and other surplus waters 

 coming from the gathering ground when the reservoir is 

 full, for if this be not done serious consequences may ensue, 

 including the washing away of the embankment with re- 

 sulting destruction of property and even of life. The 

 surplus waters sometimes fall down a shaft erected within 

 the reservoir, and make their escape by means of the tunnef 

 previously mentioned, but more frequently they flow over a 

 masonry weir and reach the stream below the embankment 

 by means of a bye wash formed in the hillside. In my 

 opinion the latter' method is in most cases to be preferred, 

 as being free from the risk of blockage by ice to which 

 th-; shaft and tunnel are liable. Engineers are occasionally 

 reproached with extravagance in the magnitude of the 

 provision made for the escape of flood waters, but it must 

 always be borne in mind that a maximum flood has to be 

 provided for, such a flood as may occur only once jn twenty 

 o"- thirty years, but which must find a means of escape 

 when it does occur, without danger to life or property. 



Masonry dams are not so frequent in this country as 

 eaithen dams, partly by reason of their greater cost and 

 partly because the geological conditions are generally not 

 favourable to their formation, for not only do they require 

 a supply of suitable stone near to hand for their construc- 

 tion, but they also need an incompressible foundation, such 

 as rock or very strong shale. .Any irregularity in the com- 

 pression of the foundation occasioned by the weight of the 

 dam would be liable to fracture the masonry of which it 

 was built. 



In the case of masonry dams a tunnel for the passage of 

 flood waters during construction is formed at a suitable 

 level in the masonry of the dam, and after completion of 

 the work they are generally allowed to pass over the top 



