PROPORTION REACHING THE SEWERS 69 



With the effect of the size of the district, i.e., the time of con- 

 centration, upon the rate of rainfall in mind, a proper value 

 for the latter can readily be selected, but the proper percentage 

 of this to be used in determining the size of the sewer is far 

 more uncertain. The methods developed in the next chapter 

 will be of service for such a purpose. 



PROBLEMS 



21. Show that for a rainfall of 2\ inches per hour on an impervious 

 area, the run-off is approximately i\ cubic feet per second. /^jUx euA** 



22. From any city plan available, scale not less than 200 feet to the 

 inch, determine the percentage of some district, of about 100 acres, that 

 is devoted to street surface. 



23. In a residential district of a city, inspect three selected blocks, 

 pacing distances between street centre lines and estimating as closely 

 as possible the areas of roofs, determine the percentage of roof area and 

 street pavement area to total areas of the blocks. 



24. Assume an area of 100 acres to be of three different shapes, wide 

 and shallow, square, narrow and deep. Assume further that the path of 

 the sewer may be either around the edge of the area or along a diagonal. 

 Show how such changes would affect the time of concentration if the 

 velocity remained the same. 



25. Assume a square area of sides equal \ mile and assume that 

 this area has varying slopes from o to 10 per cent (take i per cent, 

 2 per cent and 5 per cent as intermediate values). Assuming a rainfall 

 to follow Fig. 6 (lower curve) with 35 per cent of the area impervious 

 and the rest not contributing. Show the effect of the slope on the time 

 of concentration and on the sewage flow. Establish a relation between 

 Q and S. 



