SPILLWAYS FOR RESERVOIRS AND CANALS. 3 
used to discharge the waters from a canal when it becomes filled be- 
yond its normal capacity. When thus used the structure is essentially 
automatic in action and serves as a safety valve to prevent the canal 
from being overloaded and consequently overflowing its banks with 
disastrous results. 
The escape can be either a spillway, a wasteway, or a sluice gate, the 
last named differing from the spillway of the overflow type in that it 
can be used for partially or completely emptying a canal. It is 
seldom automatic in action and requires some means of rapid opera- 
tion, since it is usually required to operate immediately in case of a 
break in the canal banks below it. Only local conditions can suggest 
the advisability of locating one of these structures on a canal system 
or determine whether or not it is necessary, but the protection it 
affords the canal and its appurtenant structures usually warrants the 
cost. The number and distance apart at which wasteways should be 
located are dependent upon the importance of the canal, the topog- 
raphy of the ground above and below the canal, and the character of 
the service the structure itself is to perform. 
A spillway acts automatically and operates to prevent a rise in the 
canal level beyond a safe freeboard. This rise may be the result of 
an excess of water coming from the heaclgates, surface run-off from 
lands higher than the canal, an excess flow produced by the closure of 
lateral gates above the structure, or by an obstruction falling into the 
canal, or the closing of an outlet or checkgate below the escape. A 
spillway must be designed to take care of the most adverse conditions 
resulting from these causes and in its design there must be considered 
the maximum quantity of water the structure will be required to 
discharge and the maximum rise above normal water surface which 
the canal will stand. These factors are often assumed and are rarely 
absolutely reliable. 
The escape or spillway may include a checkgate as a part of the 
structure, in which case it must not only be capable of undertaking 
the duties of an overflow spillway, but those of an escape as well 
where the full capacity of the canal must be discharged. Ordinarily, 
however, the principal function of an overflow spillway is to dis- 
charge the surplus water above the desired normal canal capacity, 
whereas an escape is intended to spill the entire flow of the canal if 
necessary. An escape, on the other hand, embodies both types, but is 
intended for the protection of the system below it and to divert the 
entire flow of the canal to some natural drainage channel in case of a 
break or other emergency. It also may be used as a scouring or 
sluice gate, ridding the canal of deposited silt. The functions of the 
three may be obtained by building the structure in combination with 
a check immediately attached or a short distance below the spillway, 
escape, or sluiceway, as the case may be. Provision must be made to 
