244 PARKS 



cultural tile (either round or octagonal) and vitrified clay pipe (called 

 vitrified sewer pipe or salt glazed or Akron pipe). There are two grades 

 of vitrified sewer pipe known as first quality and second quality. Where 

 the source of supply for such pipe is close at hand, it is sometimes equally 

 as satisfactory to use second quality vitrified sewer pipe at a less initial cost 

 for the main lines into which the agricultural tile lateral drains empty. The 

 agricultural tile of unglazed burned clay, being quite porous, is most com- 

 monly used in the various types of subdrainage. Some engineers claim 

 that the octagonal tile will hold its alignment much better than the round 

 tile, otherwise there is no real reason for the use of one type in preference 

 to the use of another type. Some experienced men say that since most of 

 the underground water enters the pipe through the joints in any case, and 

 since there is no extra cost except that of the pipe in installing vitrified 

 pipe with open joints, this is preferable to agricultural tile because more 

 permanent. 



Computing sizes of pipes required for drainage. In the work of deter- 

 mining the required size of pipe for drainage in order to meet the require- 

 ments of any specific drainage problem, one must first consider the problem 

 of surface water removal and second the problem of ground water removal. 



(a) Surface water. The area to be drained should be studied to deter- 

 mine its smaller individual areas, each of which collects its own quantity 

 of surface water and each of which should be served by one or more inlets 

 or catch basins. With this information in hand, compute from rainfall and 

 run-off tables the size of the outlet pipe required to remove the water as 

 rapidly as it will collect at any given low point, allowing in the computa- 

 tions for the time necessary for the surface water from the periphery of 

 the drained area to reach the inlet. The second step is to join these units 

 into groups by combining the quantity of surface water collected in pipe 

 lines coming from two or more catch basins and adopting a proper increase 

 in pipe diameter. From these simple groups more complicated groups may 

 be combined until finally the entire surface water is emptied into one or 

 more main outlet lines. 



Since the greater the grade on which the pipe is laid, the faster the flow 

 and the smaller the pipe which is required, for these and other considera- 

 tions it is more economical at times to depart from the apparent natural 

 outfall location and to incur additional expense for excavation on some 

 other part of the system in order to procure a steeper grade on the drainage 

 lines. The time allowed for the removal of storm water is an important 

 element in computing pipe capacities. Many situations arise where tem- 

 porary backing up is allowable, and consequent economy of cost is effected 

 through the use of smaller pipe. The calculations for the size of pipe should 



