PROPORTION REACHING THE SEWERS 



61 



automatically measuring the height of waves in sewers.* The 

 heights thus obtained, plotted on the same sheet and to the 

 same scale as the depths of rainfall, give at a glance the relation 

 between the two. The article in question is largely devoted to a 

 description of the apparatus, but the relation between five 

 storms and the resulting sewage-flow is given. The largest 

 percentage found is as follows: 



Fig. 40 shows the daily record of sewage-flow as recorded by 

 the apparatus. 



It is interesting in this connection, although the percentages 

 have no bearing on the present question, to compare the results 

 of the gaging of the Sudbury River watershed, as given in 

 the Geol. Report of N. J., Vol. II, p. 6, with like data of 

 many other streams (see Table VII). 



The tables following illustrate a relation between the rain- 

 fall and the discharge of a watershed of 78 square miles, very 

 similar to that at first thought to exist between the same 

 quantities in the case of sewers, and show that while the annual 

 average holds not far from 50 per cent, the monthly relation is 

 much more variable. In the case of sewers, in order to reach 

 the true relation between the rain and the discharge, the unit 

 time must be reduced from the month not only to the day and 

 hour, but to the five-minute or minute interval. 



Baumeister in considering this subject says: "In England 

 from o to 70 per cent of the rainfall reaches the drains, averaging 

 about 50 per cent. In different districts of London from 

 53 to 94 per cent has been registered. It required from three 

 to four times the duration of the storm to carry off the water, 

 and the maximum flow per second in the sewers rose as high as 



* At Omaha, Neb. 



