46 BULLETIN 115, U. S. DEPARTMENT OF AGRICULTURE. 



fice 3 inches high and 4 feet wide the pressure head required will be 

 about 6 inches. If this discharge be increased to 5.5 second-feet, then 

 the pressure head on the same opening will be about 14 inches. If 

 the same volume of water is passing over a crest — giving approxi- 

 mate weir conditions — of the same width (4 feet), then the head on 

 the weir will increase only from 5 to about 6^ inches. Therefore 

 this given fluctuation raised the level of the water in front of a 

 given submerged orifice 8 inches, while the same amount of fluctua- 

 tion raised the level on a crest but 1| inches. 



The great factor against securing all of the control of the water 

 above a check by means of crest discharge is that the stilled water 

 tends to cause the deposit of all sedimentary matter in the water, 

 and if the check is not " pulled " regularly this deposit will become 

 the cause of trouble and expense. On the other hand, water dis- 

 charging through an undershot gate issues from the structure in a 

 very turbulent condition and tends to cause damage by erosion of the 

 banks below the gate. In the opinion of the writer the form of 

 shutter shown in figure 8 (p. 37) gives the best general solution for 

 a cheap check shutter. The loose flashboards allow for crest regula- 

 tion, and the fact that the shutter may be lifted as a unit by the 

 stem attached to the lowest board of the series allows the opening to 

 be " pulled " and the silt scoured out. This operation is not tedious 

 or so liable to be shirked as is the case if ordinary flashboards are 

 used. 



The elevation of the crests of various checks on a system is a 

 matter of great importance. It is usual to design the extreme top 

 of the structure at least 6 inches above the maximum height to which 

 water is to be checked. Some companies mark a line on the structure 

 as the limit of safe operation, because, as a rule, the levees above the 

 check are made to conform to the height of the check or, as a matter 

 of safety, 6 inches above the check. 



- In an uneven flat country the cost of developing in detail contours 

 of small interval is so great and so much time is necessary to accom- 

 plish this that it is a very common sight in such irrigated regions 

 to see a great number of checks which have been built up a foot 

 tor two in order to check up the water to reach high knolls which 

 were not noticed during the original construction. On the other 

 hand, it is quite common to see checks on which the high-water mark 

 shows that there was more material put into the structure than was 

 necessary. In other words, a check gate is one of the structures of 

 which a careful study must be made in order to determine whether 

 it would not be better to install wood in the initial construction 

 and replace with concrete when the wood decays. By this time 

 the ditch tenders handling this structure will know exactly what 



