USE, CONSTEUCTION, AND COST OF SERVICE RESERVOIRS. 151 



that a Service Reservoir should be covered in, but when it is high 

 and sufficiently remote from the liability of contamination from 

 dust off roads &c, covering in may be safely omitted. 



When selecting the site for a Service Reservoir the engineer 

 seldom has a wide selection of available sites to choose from, the 

 necessary height and configuration of the surrounding country 

 invariably points to but one or two possible sites. For convenience 

 and economy of construction the site selected should be as nearly 

 level as possible, although frequently, the choice must fall on a 

 hill-side in which case one half the Reservoir may be in deep 

 cutting, while the other half requires a retaining wall • such a site 

 naturally points to the desirability of adopting a long and narrow 

 rectangular Reservoir, but as a general rule the circular form will 

 be found the most economical to adopt, unless when the land 

 available is square and limited, under which circumstances to make 

 the most of the land, the square form is in no wise objectionable. 



Generally it will be found most advantageous to construct 

 the Reservoir about two-thirds in excavation and one-third out, 

 but no hard and fast rule can be laid down on this point. Much 

 will depend upon the nature of the ground and whether "head" is 

 an all important factor or not ; in many cases " head " is of the 

 utmost importance owing to adjacent houses being built on nearly 

 the same level. It was this fact which led the Department to adopt 

 the Steel Reservoirs hereinafter described, so that the water might 

 be all stored above the surface. Spoil from the excavation should 

 be well rammed into a substantial bank surrounding the Reservoir 

 to back up the walls where above ground. 



Of the thirty-nine Reservoirs hereinafter described, thirty-five 

 have been constructed under the direction of the Harbours and 

 Rivers Branch of the Public Works Department ; six are rect- 

 angular, three square, and thirty are round. Twenty are built 

 with brick walls, seven with concrete walls, and ten are constructed 

 of iron or steel, one has dry pitched slopes, and one is cut in solid 

 rock. 



