ON THE BRADFORD WATERWORKS. 455 



are completed, is ten millions of gallons per day, a quantity equal to 36 gallons 

 per head for the population of the district of supply. 



• The sources of supply of these works would, however, if fully developed, 

 yield more water than the quantity named ; 700 acres of gathering-ground, 

 on which there is a rainfall of 44 inches per annum, will yield one million 

 gallons of water per day if a reservoir is made to contain 180 days' supply. 

 With a rainfall of 36 inches per annum, the drainage-area would require to 

 be about 900 acres to give the same quantity of water per day. The quantity 

 of water to be impounded, 180 days' supply, 180,000,000 gallons, is equal 

 to 11-4 inches in depth on 700 acres, and to 8-805 inches in depth on 900 

 acres, about one fourth of the total rainfall in each case. These quantities 

 may vary, however, to some extent with the character of the gathering- 

 ground ; sometimes it happens that there are large springs within the drainage- 

 area, whilst in other cases the ground may be so absorbent that part of the 

 water may pass down to springs below the level of the works. 



The supply is also dependent upon the distribution of rain throughout the 

 year ; if the rain falls in heavy floods with a long period of drought, so much 

 of the fall cannot be utilized as during years when the rain is more equally 

 distributed. 



In determining the value of any given area of gathering-ground after the 

 average rainfall is ascertained, one fourth is to be taken off to arrive at the 

 quantity for dry and exceptional years, one third of the remaining quantity 

 is then to be deducted for loss by evaporation, absorption, discoloured and 

 turbid water, and unmanageable floods. These quantities show that only one 

 half of the total average rainfall can be collected and used. These quantities 

 and particulars, however, apply only to gravitation works in districts similar 

 to those in which the Bradford works are situated. 



The Bradford reservoirs are formed in the manner usually adopted for large 

 works — i. e, by embankments made across the valleys, such sites being almost 

 the only practicable ones where reservoirs could be made of sufficient size 

 for the large quantities of water to be collected. 



The mode of construction adopted for such reservoirs is to make the em- 

 bankments of earthwork, the earth being excavated from the site of the 

 reservoir itself. In the middle of the embankment a vertical core or wall of 

 puddle is made, to render it impervious. This puddle-core must be continued 

 to such a depth that the water cannot pass uuder it ; and it must also be 

 contiaued so far into the sides of the hills which form the valley, that the 

 water cannot pass round the ends. 



The strata underlying the site of the reservoir are not always regular ; in 

 some cases the bottoms of the valleys have been raised by drift many feet in 

 thickness. It is necessary to find some stratum or some number of strata 

 which together wUl make an impervious bottom, and which underlie nearly 

 the whole of the site, and to continue the puddle-work of the embankment 

 (by means of open-cut trenches) into them, so as to form a complete basin or 

 inclosure within which the water is to be contained. 



It is necessary in some cases to continue the puddle-trenches from the ends 

 of the embankment up the sides of the valley to some point where the dip of 

 the measures brings the impervious stratum to the height required for the 

 surface of the water when the reservoir is full : advantage is also to be taken 

 of faults and dislocations in the natural strata ; in this district these faults 

 are nearly always impervious, and they are sometimes of great service in 

 reservoir works. 



In making the deep trenches for the puddle-work, it frequently happens 



