1898.] on Bringing Water to Birmingham from Wales. 683 



first suggested by the veteran waterworks engineer and hydrologist, 

 the late Mr. Hawksley. The greatest rainfall was in 1872, viz, 93-86 

 inches ; and tlie least was in 1892, viz. 43 '44 inches. 



Diagram No. 3 shows the rainfall. The upper one gives the 

 yearly fall at Nant-gwillt from 1871 to 1896, with the meau of that 

 term, and also the mean of the lowest tliree consecutive dry years. 

 The lower one shows the monthly fall at the same place for the same 

 period. It is very usual to take 14 inches as the amount of evapora- 

 tion, but, in order to be on the safe side and to allow amply for loss 

 by overflow, 19 inches were deducted from the 55, leaving 36 as 

 collectable by means of the reservoirs intended to be constructed. 

 Taking the mean of three consecutive dry years, the rainfall in one 

 year upon the watershed area would be equivalent to 252,495,491 tons 

 of water, of which 63,950,823 would be lost by absorption or evapora- 

 iton, and 2^,154,608 tons by overflow, leaving 165^390,060 tons as 

 collectable in the reservoirs. In a year of maximum ram like 1872, 

 the total quantity falling upon the watershed would be 431,116,756 

 tons, and the volume overflowing from the reservoirs into the river 

 would be correspondingly increased. Further observations since the 

 Bill was in Parliament have satisfied me that we may calculate on 

 obtaining from the works 75 million gallons a day for supply, in 

 addition to the 27 millions for compensation. 



Considered geologically the whole of the watershed area consists 

 of rocks of Lower Silurian age, principally inferior slates, but in parts 

 of very hard grits and conglomerates. It is the presence of thick 

 bands of the latter stretching across the Elan, at a place called Caban 

 Coch, and resisting degradation, which has determined the position of 

 the contraction in the sides of the valley and rendered it so eminently 

 suitable for the location of a barrier dam. At this spot the bed of 

 the river is 700 feet above Ordnance datum, the bottom of the valley 

 being about 200 feet wide, and at 120 feet higher, only 600 feet. Im- 

 mediately above this contraction the valley widens out into a broad 

 "/a^," and 1540 yards higher up, the river C'laerwen joins the Elan 

 on its right bank. 



These conditions pointed unmistakably to the Caban as the site 

 of the lowest dam, and consequently determined the area of gathering 

 ground to be utilised. 



The height of the wall to be built was after milch consideration 

 fixed at 122 feet above the bed of the river, and the contents of the 

 reservoir behind it will be nearly 8000 million gallons. As compared 

 with the height of this wall above the river, the Vyrnwy (Liverpool 

 works) is 85 feet, and the Thirlmere (Manchester works) 50 feet. 

 The river Elan has in the part aflfected by this dam a rise of 30 feet 

 in a mile, so that the 122-feet barrier backs the water up that valley 

 4 miles, and up the Claerwen, which is somewhat steeper, about 

 2J miles. The length of drought which it was deemed advisable to 

 guard against was fixed at 180 days, and consequently the total 

 storage to be provided was nearly 18,000 million gallons, or 10,000 



