GATE STRUCTURES EOR IRRIGATION CANALS. 13 



est factor in favor of the safety of such a structure lies in the design 

 and construction of the upper wings and cut-off walls. If water 

 tops, goes around, or under these there is little chance of saving the 

 gate unless it is paved on top, so that unusual floods may pass over 

 it harmlessly. 



Headgate out in the stream channel. — Where the bed of a stream is 

 very wide and the low-water discharge very small as compared with 

 the floods which have determined the location of the main stream 

 banks, it may be necessary to place the river gate out in the stream 

 bed. It is then protected from above and below by more or less exten- 

 sive cribbing or rock riprap. A canal bed, well protected on the river 

 side by riprap, is then built up from the gate to the point where the 

 canal line intersects the main bank of the stream. 



This kind of construction also is resorted to in some cases where 

 the river bank is more or less precipitous; it is not practicable to 

 install a high diversion weir, and the desired canal grade intersects 

 the river above the line of flow in time of low water. In this combina- 

 tion of conditions, which is quite common throughout the West, it is 

 the usual practice to continue the grade of the ditch or canal on up the 

 river bed, usually just under the bank of the stream, until the grade of 

 the ditch approximates that of the stream, at which point the gate 

 may be installed. 



Headgate below upper end of the canal. — The condition spoken 

 of in the last paragraph usually is met by constructing the ditch up 

 the river bed as just described, but installing the gate at the point 

 where the artificially-built bed of the ditch intersects the main bank 

 of the stream, at which point a waste gate forms an " L " with the 

 river gate and the surplus water is turned back into the stream. 



The conditions of anchorage and foundation may not be favorable 

 at the point where the line of the canal intersects the main stream 

 bank and then the headgate is installed at some distance down the 

 canal with open channel between it and the river water. 



In connection with such a river gate it is most advisable to install 

 waste gates of such capacity that the water in the channel leading 

 to them will have sufficient velocity to keep the silt moving and not 

 choke up the intake channel. 



Headgate on bank of secondary channel. — Some of the most satis- 

 factory structures in the West are built at the banks of secondary 

 channels to main streams. In most of such cases the water enters 

 from the head of the channel and is regulated roughly by logs and 

 brush or bowlders at the point where the secondary channel leaves 

 the main stream. In other cases the water is made to back up from 

 the lower end of the secondary channel and in this way much of 

 the silt and sand is confined to the main channel of the stream. The 

 structure shown in Plate V. figure 2, is from a secondary channel 



