19<^ THE WATER SUPPLY OF KINGWILLIAMSTOWN. 



the (lam. and greatly facilitated the work of building, as it 

 enabled a tramway to be constructed throughout the whole length 

 of the work for the conveyance of concrete and " plums," or 

 displacers, which could be tipped from the railway direct into 

 the work. As the height of the dam rose, so the tramway was 

 lifted higher and higher up the railway metals. 



Last year it was discovered that considerable erosion had 

 taken place on the downstream surface of the rocks. The 

 holes formed by this erosion have been filled in with concrete 

 and the cushion wall raised about 3 feet, so as to increase the 

 depth of the water in the cushion. The manner in which this 

 erosion took place was rather interesting. The first year the 

 overflow from the dam formed a hollow all round, about 15 inches 

 deep ; during the next five years it increased to two feet ; during 

 the following year it went from 2 feet to 6 feet. The explanation 

 was that there was a hard layer of shale about 2 feet thick, 

 underlain by a much softer layer 4 feet thick, so that the moment 

 the hard layer was worn through, the softer layer eroded very 

 rapidly; it also eroded in a horizontal direction to a width of 

 4 or 5 feet underneath the hard layer, necessitating the work 

 above referred to. 



The reservoir is stocked annually by the Council with trcut, 

 which seem to thrive well, and some good fish have been taken, 

 both rainbow^ and brown varieties. 



Pipt- Line. 



The pipe line has a total length of 13 miles. The first 

 63/2 miles consist of lO-inch diameter cast-iron pipes with turned 

 and bored joints, laid in 1881 in connection with the old scheme ; 

 the remaining 6^. miles, from Izeli to the dam, are of 15-inch 

 diameter inserted joint steel pipes three-sixteenths of an inch 

 thick. 



The new pipe line crosses the Buffalo River five times, and, 

 owing to the i)recipitous banks and great depth of water, in 

 two instances bridges are used to carry over the pipes. Both 

 are of the " cantilever suspension type," the one at Izeli having 

 a clear span of 150 feet, and the one in Haynes' Farm,- near the 

 dam, two spans of 45 feet each. In both cases the pipes are 

 carried by iron supporters underneath the bridge decking, and 

 provided with corrugated expansion joints. 



The steel pipes have given very little trouble ; only two small 

 sections have shown any signs of leakage. One section w;as 

 near the dam, the length of pipe afifected being about 50 yards. 

 This was in made-up ground with a considerable amount of 

 soakage passing through it. The author covered these pipes 

 with concrete, reinforced by means of No. 11 black iron wire; 

 since then there has been no further trouble. The other section 

 which gave trouble was at the bottom of a narrow kloof about 

 four miles below the dam, the length of pipe line affected here 



