46 BULLETIX 115, U. S. DEPAETMEXT 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-J 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 1J 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 S (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 
