4 BULLETIN 831, U. S. DEPABTMENS Of AGBlCitrt/EtJSfi, 



discharge any excess over the safe capacity of the canal in countries 

 where heavy rainfall occurs periodically, or where melting snow may 

 affect surface run-off from the higher lands draining into the canal 

 during the operating seaion. It may be desirable to collect part or 

 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 stretches 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 an}? danger of erosion around its intake, 

 or above any structure whose direct loss would not be 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 capaeit} T , while the overflow spillway makes no use 



of most of this fall. The two types of spillways are discussed in 



detail, as follows : 



OVERFLOW SPILLWAYS. 



Overflow spillways are of three general types, 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. In 



