EXCESSIVE RAINS 51 



necessary for water to flow from the farthest laterals. This 

 time with its corresponding intensity will give the greatest 

 probable discharge at the outfall. A secondary maximum 

 may occur as follows: If a part of the contributing territory 

 should be steep and near the outfall, it may be that the higher 

 rain-intensity corresponding to the shorter time for that section 

 will give more storm-water at the outfall than the less intensity 

 over the whole section. It can be worked out by trial in a 

 few sections and the rain conditions for the real maximum 

 determined. The time required for the passage of the water 

 from the farthest point to the outfall is a matter of trial and 

 judgment. From five to eight minutes is allowed for the rain 

 to pass along the ground from where it fell to the nearest inlet 

 to the sewer or to the gutter if inlets are not located at each 

 block corner. Two feet per second may be taken as a min- 

 imum rate of flow in the sewer, and 15 feet per second as the 

 maximum, but between the two the rate of flow will depend 

 on the surface grade and on the size of the pipe. Therefore 

 the size and grade of the imaginary sewer must be assumed 

 for a preliminary trial. From the surface grade and intensity 

 thus established the sizes can be roughly worked out, and if 

 very different from those assumed at first, the new intensity 

 must be found and the sizes redetermined. It must be remem- 

 bered that not all of the rainfall is carried off by the sewers, 

 and that only a certain proportion is to be considered, a subject 

 taken up in the next chapter. 



It has been pointed out by some writers * that, since no rain 

 can be assumed to flow uniformly for any length of time, but 

 that rather it increases in intensity to some maximum and 

 then decreases, it is an error to require the theory of the duration 

 chosen to depend on the assumption that the entire area is affected 

 equally by a storm of any certain intensity. It is argued that 

 if a certain time is required for a drop of water to reach the 

 outlet from the most distant point, then those distant points 



* Engineering News, Vol. XL VIII, p. 387, Jour. Assn. Eng. Soc., Vol. XX, 

 p. 204. 



