USE OF DIAGRAMS 191 



By the diagram on Plate 4, assuming that a 3-foot pipe 

 will be needed, we find the velocity to be 6 feet per second, 

 so that it will take 254 seconds, or a little more than 4 minutes, 

 for the water to get to the creek a total time from the 

 farthest point of 8 minutes. Adding 3 minutes for the time 

 necessary after the storm starts for the rain to reach the 

 gutters and catch-basins/the duration of the storm which will 

 give a maximum discharge is n minutes. 



Now consulting Kuichling's diagram, Fig. 6, we find the 

 maximum rate of a storm lasting n minutes to be 3.18 inches 

 per hour. 



On larger areas it will be necessary to determine the time 

 of concentration separately for the different parts of the same 

 area in order that the rate of rainfall may decrease as the 

 combined parts give larger and larger total areas. In the 

 present case, however, the entire area is so small and the time 

 of concentration to the bottom of the hill is so short that no 

 greater accuracy would be secured by computing the rate of 

 rainfall separately for the smaller district. For large areas, 

 however, the times of concentration must be completed in 

 order. 



We assume, then, that there is a rain falling at the rate 

 of 3.2 inches per hour which is to be cared for, and note that 

 by the topography no water from the upper side of Eddy 

 Street or above will enter this drain; and that as the velocity 

 is high and the buildings are residences, so that a temporary 

 filling of the gutters will not be an annoyance, the pipe need 

 not begin until the water has reached the corner of Stewart 

 Avenue and State Street. At that point there is a contrib- 

 uting area, scaling it from the map, of 1750X580 = 1,015,000 

 square feet, or 23.3 acres. An inch an hour is practically the 

 same as a cubic foot per acre per second, so that the discharge 

 from these 23.3 acres will be 23.3X3.2 = 74.5 cubic feet per 

 second, provided it all flows off. The area has a population 

 of about 25 per acre, and, by the table given on p. 74, 25.3 

 per cent of the rainfall will flow off. 74.5X25.3 equals 18.8 



