HYDROLOGY OF NEW YORK 



457 



Q p = the mean discharge in cubic feet per second for any given 



Qhi + Qh 2 



period, as for instance, Q = and 



2 



Qh 2 + Qh 3 



Q Pl = , etc 



2 



S = inflow from catchment area, taken in the present case at 



30,000 cubic feet per second; and 

 t = the time in seconds in which the water will rise to any given 

 value of h above crest. 

 Whence we have the formula, 



C 



t*= (41) 



S— Q p 



b} r which table Xo. 80 has been computed. 

 On referring to table No. 80 we learn : 



1) That, with water surface in reservoir at level of crest of 

 overflow weir and a constant inflow of 30,000 cubic feet per second, 

 it will be about 6.5 hours before the outflow will reach 15,000 cubic 

 feet per second. 



2) That under the same conditions it will be about 24 hours 

 before the outflow will reach approximately 30,000 cubic feet per 

 second. 



3) Inasmuch as the original assumption was that the inflow 

 should only be at the rate of 30,000 cubic feet per second for 24 

 hours and then gradually decrease, we may therefore say that the 

 flow at rate of about 30,000 cubic feet per second would only be 

 for say two or three hours, instead of at least 24, as it would have 

 been without the assistance of the surface storage of the reservoir. 



4) The total inflow in 22.5 hours would be 2,431,782,000 cubic 

 feet, of which 34 per cent of the whole would be stored during 

 that time temporarily in the reservoir. 



Other deductions can be made, but the foregoing are enough to 

 show the value of such a reservoir as a moderator of floods even 

 when entirely filled at the beginning of the maximum flow. 



In the same way if we assume the reservoir full and an inflow 

 at the rate of 40,000 cubic feet per second, we learn on making 

 the numerical computation that about 19 hours ivould elapse 



