684 Mr. James 3Iansergh [March II 



millions more than the Cahan Coch reservoir would contain. For the 

 purpose of selecting the positions of other reservoirs than the Caban 

 higher up, the two valleys were levelled and closely contoured to 

 above the highest possible site on each, and by this means the exact 

 positions of five others were determined, giving the greatest impounding 

 capacity icith the least amount of structural wcrJc. 



On Diagram No. 4 are given longitudinal sections of the two 

 valleys, showing that on the Elan, above the Caban, there is to be 

 a dam at Pen-gareg, and another at Craig-yr-allt-goch, and on the 

 Claerwen at Dol-y-mynacb, Cil-oerwynt and Pant-y-beddau. In the 

 order named, tlieir respective heights are 128 feet, 120, 101, 108, and 

 98 ; and tlie reservoir capacities 1330, 2000, 1680, 3150, and 1940 

 million gallons respectively. 



A unique feature in the scheme is the provision of what has been 

 called a suhmergcd dam, to be built across the Caban C<5ch reservoir 

 at a point nearly a mile and a half above the main wall, and called 

 Caregddu, its precise function being to hold the water up behind it 

 high enough to charge the aqueduct conveying the water to Birming- 

 ham^ It is described as submerged, because until the water has 

 been lowered 40 feet it will be drowned and out of sight. The neces- 

 sity for this device comes about in the following way, viz. at the 

 Birmingbam end of the aqueduct the water is to be delivered into 

 a large service reservoir at Frankley, about 6 miles from the centre 

 of the city, whose top water will be 603 feet above O.D. From 

 the commencement of the aqueduct in the side of the Caban reser- 

 voir to Frankley is a distance of nearly 74 miles, and in this 

 length the fall required to convey the water is in round figui*es 

 170 feet, so that the invert of the aqueduct at its inlet will be 770 feet 

 above O.D. or 70 feet higher than the bed of the river at Caban 

 Coch. Now the water must of necessity never fall below this inlet, 

 or the aqueduct could not be charged, and therefore the submerged 

 dam is to have its crest at 782 O.D. being high enough to fill the 

 aqueduct; the cross-section of which will be described later on. 



Diagram No. 5 explains this more fully. A is the main Caban 

 wall, built at a spot where the bed of the river is, as before stated, 

 700 feet above O.D., its crest being 822 ; B is the submerged wall, 

 with a crest level of 782 ; c is the entrance to the aqueduct, with its 

 invert level at 770. 



When the reservoir is full, the layer of water between 822 and 

 782, 40 feet thick and having a surface area of 600 acres, contains 

 4585 million gallons. Now suppose very little water were coming 

 down the rivers in a time of great drought, 27 million gallons have still 

 to be sent out for compensation every day at a, and dealing with the 

 first instalment, another 27 millions have to go down the aqueduct 

 to Birmingham ; then this combined draught of 54 millions would 

 draw down the water from 822 to 782 in about 80 days. The quantity 

 of water below 782 between Caban Coch at a and Carregddu at b 

 is 2565 million gallons, and would therefore sufSce to pay the com- 



