SPILLWAYS FOR RESERVOIRS AND CANALS. 39 
In cases where ordinary overflow spillways are operating, tne 
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 fult 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 
snoulcl 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 
