SPILLWAYS FOR RESERVOIRS AND CANALS. 39 



In cases where ordinary overflow spillways are operating, trie 

 water released and allowed to continue down the watercourse would 

 increase and decrease with the head on the overflow, and would at 

 no time during the discharge be greater than the volume of water 

 contributed to the reservoir above the structure. The watercourse 

 under such conditions would be performing its natural functions. 



In the case of an ordinary siphon spillway, from the instant the 

 siphon comes into action the volume in the channel below the struc- 

 ture becomes the full spillway capacity and the burden of conveying 

 this suddenly imposed surplus may be compared to that of a simi- 

 larly released flood ; this for the reason that the spillway capacity is 

 usually assumed to care for both normal and flood flow into the 

 reservoir. 



A battery of siphons may be regulated so as to bring each unit into 

 action separately or in pairs by placing their crests and air intakes 

 at different elevations. The regulating parts are usually placed 

 near the high- water line of the reservoir, where flow into it results in 

 a slow-rising water surface. 



Varying the elevation of the priming parts of the different units 

 need not utilize a range of more than from 1 to 2 feet in height, 

 while it would regulate the outflow to conform more closely to the 

 conditions of an ordinary overflow spillway and the volume released 

 to the watercourse below the dam would vary with the reservoir in- 

 flow. 



The lower unit should be fixed to operate at the level where it is 

 desired to maintain the reservoir surface, and the remaining units 

 should have their crests and air intakes set to operate at slightly 

 higher elevations — still maintaining a safe freeboard above the high- 

 est air vent. 



The writer is of the opinion that this practice has been followed on 

 several of the larger installations, but for the purpose of regulating 

 the pond level and not to govern the discharge to the stream below. 



There are conditions where the siphon spillway is not adapted to 

 the site or to the requirements which it is intended to serve, and its 

 failure to perform under the improper conditions has led to condem- 

 nation of the structure as a type. For instance, one case cited in a 

 criticism to the writer, refers to the tendency of a siphon to check 

 the velocity of the stream near Its intake and encourage the deposit 

 of silt in front of the structure, where it was used as a regulator at 

 the end of a canal. The design of this siphon did not take into 

 consideration its utilization as a scouring device, but was intended 

 only to skim the surface water and prevent the overflow of the banks 

 of the canal. Some provisions should have been made to temporarily 

 take care of the silt problem, since it was intended, as an ultimate 

 development, that a power plant would be located near the site of 



