4 BULLETIN 831, U. S. DEPARTMENT OF AGRICULTURE. 
discharge any excess over the safe capacity of the canal in countries 
where heavy rainfall occurs periodically or where melting snow may 
affect surface rim-off irom the higher lands tlraiiiiiig into the' cafial 
during the operating season. It iiiay be desirable to collect pari ©f 
all Of such drainage and carry it as additional storage Escape 
structures should also be provided with means for taking care of any 
surplus water in the canal produced by regulation of the flow in one 
part of the canal system without having provided for changes in all 
other parts of it. Their location will be desirable above points of 
questionable strength, above stretcher of canal located on steep side- 
hill where slides are apt to occur, just above the intake of any impor= 
tant structure where there is any danger of erosion around its intake, 
or above any structure whose direct loss Would not bfe material com- 
pared to the resulting damage to valuable property either connected 
with or foreign to the canal system. Capacity should be computed 
from the possibilities of combined flows resulting from conditions apt 
to develop above the structure, 
The overflow spillway, WasteWay, and sluiceway are similar in the 
common characteristic of requiring the addition of a wasteway 
channel to divert the waste water to a point away from the vicinity 
of the canal, ... . 
There are two general types of spillways, overflow spillways, 
and siphon spillways. The distinguishing features of the two types 
are that the capacity of the overflow spillway depends upon the 
length of the crest and the height of the water above the crest, and is 
increased in no way by the distance through which the water falls 
below the crest; while the capacity of the siphon spillway depends 
upon the area of the cross-section at the smallest part of the siphon 
and the difference in elevation of the water surface at the intake and 
outlet ends. In other words, the siphon utilizes the fall from the 
water surface in the reservoir or canal to the discharge end of the 
siphon to increase capacity, while the overflow spillway makes no use 
of most of this fall. The two types of spillways are discussed in 
detail, as follows : • — «— ~ •* -^-~*w 
OVERFLOW SPILLWAYS. 
Overflow spillways are of three general t} T pes, the "ogee," the 
" steps," and the simple inclined type. Some dams combine the first 
two of these types by utilizing the top part as an ogee and having 
the lower portion stepped to break the velocity of the falling water. 
Flow over a spillway is produced by the velocity resulting from 
the head measured from its crest to the surface of the water in the 
pond above. It depends entirely upon the stored head to increase 
volume per unit length, and, regardless of the height of the crest 
above the pool into which the water is spilled, no part of the fall 
below the crest level is effective either directly or indirectly. 4 In 
