GATE STRUCTURES FOE IRRIGATION CANALS. 
33 
If the first condition holds, then it is easier to maintain a constant 
delivery by an undershot gate, as the discharge through such a gate 
varies as the square root of the head on the opening, while the dis- 
charge over a crest varies as the cube of the square root of the head. 
Thus, the best combination of check and delivery for the purpose of 
delivering a constant head is to check the water up to pass over a 
crest, the wider the better, and to pass the delivered water under a 
gate. If the second condition holds, then it is desirable to deliver 
the water in the same way as the water passes the check; that is, if 
the check has an undershot discharge, then the water should be deliv- 
5c obey. 
Fig. 7. — Small lateral headgate or delivery gate, Imperial Valley, California. 
ered under the gate, but if the check is a, crest device, the water deliv- 
ered should then be passed the same way and the ratio of the crest 
lengths should be in the same proportion as the desired ratio of 
delivered water to volume allowed to pass on down the canal over 
the check, the crests being set in the side walls so that the contraction 
is practically suppressed. 
SMALL LATERAL HEAD GATE OR DELIVERY GATE. 
The water companies in the Imperial Valley, CaL, use a box 
on the general plans of the one shown in figure 7. This is a modifica- 
