626 PROCEEDINGS OF SECTION H. 



a =Area of waterway to be provided, in sq, feet, 

 c =Co-efficient for nature of catchment. 

 V =Velocity of flow in outlet waterway in feet per 

 second. 



Then Q= ^ ^ = c r Ai 

 A* 



, Q c r Al 



To avoid the necessity for selecting values for c it was sought 

 to modify the formula so as to embrace large as well as small catch- 

 ments while maintaining the value of c constant. Curves were 

 accordingly plotted for values of Q when c = l,0'8, 0-6, and 0*4, 

 and a mean curve was determined, which at every point gave full 

 values, but not excessive values, for all areas generally met with 

 in practice. The curve corresponded to the formula Q=r57 

 cr As, giving results equivalent (in original formula) to c = rO up 

 to 1,000 acres, c=0-6 up to 100,000 acres, c=0-4 up to 10,000,000 

 acres. 



The value of r57 c when C=l gives volumes of flow equivalent 

 to full maximum intensity of rainfall up to 10 acres, volumes 50 

 per cent, above the possible for one acre, and above 10 acres the 

 results are generally about 25 per cent, above the author's records 

 and are believed to be full to that extent. Instead of r it is con- 



sidered preferable to use ( f,)" where R represents the average 

 annual rainfall in inches. 



The formula thus modified becomes Q = l-ll C Ri As- But 

 as it has been established that not more than 80 per cent, of the 

 rainfall flows off even paved surfaces during the time the storm 

 continues, the volume of flow becomes Q = -888 C Rs As- The 

 author, however, adopts for preference Q= c R^ Ai which gives 

 results at least 10 per cent, higher than have proved sufficient, so 

 far as has come within his knowledge. In this formula C is a co- 

 efficient, never less than unity, varying with the nature of rainfall 

 in the locality. Its value has not been determined for tropical 

 (monsoonal) conditions ; but for temperate zone unit value is 

 beheved to be ample. 



Adopting then Q—c Rh At 



When C is given a value equal to unity 



Q=R^ At Q = l-414 r At 



a = m At a = 1.414 r At 



~V" V 



D= 113 Ri Ah D = 1-342 ri A^ 



Vi Vi 



D=diameter of pipe or culvert in feet. 



But with the customary value of C=0-9. 



