32 BULLETIN 115 ; U. S. DEPARTMENT OF AGRICULTURE. 



The open type of gate replaces the levee for the width of the gate, 

 and it must be j)repared, therefore, to withstand all of the condi- 

 tions of variation in water level in the canal that are required 

 of the levee. The front, wing, and side walls need not be any higher 

 than the crest of the levee adjoining the structure. On the other 

 hand, if they 'are lower, then the high-water line in the canal is 

 lowered accordingly. It may be desired to have the lateral gate act 

 as a spillway for surplus water to pass from the canal into the 

 lateral and on into some natural drainage. Such a condition as this 

 may be met by so designing the front of the lateral gate structure 

 that water may top it and be collected in the culvert below without 

 damaging the anchorage within the wing walls. 



The same thing may be said of the tightness of the shutters in a 

 lateral gate as of a check gate. If the shutters act as regulators only 

 and there is always more or less water being delivered into the lat- 

 eral, then there is no necessity for the gate to be water-tight, but if 

 the lateral is of such small size or the conditions of delivery are such 

 that water is turned out for intervals, then it is desirable that the 

 structure be made water-tight. This is true particularly when the 

 Avater carries silt in suspension. 



Experience has shown that it is very desirable to design a struc- 

 ture leading from a canal so that the general shape of the canal 

 bank is changed as little as possible. This is comparatively easy to 

 do when a lateral gate is designed to be placed in the bank after the 

 canal has been operated for some years and the bank has assumed 

 the general form which will remain and which may be called one of 

 the individualities of the canal at that particular point. This change 

 in the shape of the levees or banks is very marked in most systems. 

 When first constructed the loose earth usually takes a slope of 1£ or 

 2 to 1, but after some years grass and weeds appear on the levees 

 and the bank near the water line stands nearly vertical, overhung 

 with earth and grass in many cases, while the slope near the bottom 

 becomes flatter than it was in the initial construction. When this 

 final position of the canal banks can be determined, then the face of a 

 delivery structure may be made to conform to and be flush with the 

 steeper slope, and if the shutters are set well to the front of the gate 

 structure there will be very little break in the canal bank and the cost 

 and trouble of maintenance due to the deposits caused by eddies and 

 quiet water in nooks in the canal bank will be reduced to the 

 minimum. 



The system of delivery through a lateral gate may be such that a 

 constant quantity is desired regardless of fluctuations in the surface 

 of the water in the canal, or it may be such that it is desired that any 

 fluctuation in the water of the canal be shared in proportion by the 

 lateral. 



