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 headgates, 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 



