456 



NEW YORK STATE MUSEUM 



worked out for Genesee river, the following is herewith included 

 to illustrate the subject : 



The most unfavorable case that can be assumed is that of the 

 occurrence of an extreme runoff when the reservoir is full to the 

 flow line. Even under such circumstances the reservoir will still 

 act as a gTeat mitigator of an extreme flood-flow, as may be seen 

 by inspecting table No. 80, which has been prepared specially to 

 illustrate the point in question. The following discussion will 

 indicate the principle embodied in this table. 



The efficiency of a storage reservoir as a flood moderator will 

 depend upon the storage capacity in relation to the quantity of 

 water flowing in from the catchment area. This capacity in- 

 cludes all storage space, whether above or below the crest of 

 the overflow weir, which may 'be available at any time of heavy 

 storm. Water is stored in the space above the crest only tem- 

 porarily, but this space may still play an important part in 

 reducing the maximum discharge below the reservoir, by extend- 

 ing the time within which the total surplus has to be passed down. 



Inasmuch as extreme flood-flows are of short duration, we may 

 neglect the effect of evaporation, absorption and leakage, whence 

 it becomes evident that the discharge by the overflow weir or 

 sluices will be equal to the quantity received, less the quantity 

 retained, whether temporarily or otherwise. 



We will assume that the water stands at the level of the crest at 

 the instant when the inflow becomes equal to 30,000 cubic feet per 

 second, and that the inflow remains constant at that figure for 24 

 hours, after which it gradually decreases. We desire to deter- 

 mine the length of time which will elapse before the outflow 

 reaches 30,000 cubic feet per second, and the approximate time 

 it will remain at about that figure. With the following notation : 



h=>any given hight above the crest in linear feet and h 1? li 2 , 



h 3 , etc., successive equal hights. 

 C = storage capacity corresponding to h, and C 2 , C 3 , etc., 



successive capacities. 

 Q = discharging capacity of the overflow weir in cubic feet per 

 second, as determined by the formula Q = 3.33 V h 3 x L 

 for the given values of h, h lr h 2 , h 8 , etc. 



