IRRIGATION IN INDIA AND 

 AMERICA. 



BY. E. H. PARGITER, OF THE IRRIGATION BRANCH, PUBLIC WORKS 

 DEPARTMENT, PAN JAB. INDIA. 



(Continued from last month.) 



During the flood season the river water is heavily laden with silt 

 or sediment, mostly sand. This, entering a canal with the water, is 

 soon deposited on the bed in the first few miles, as the velocity in the 

 canal is very much less than that in the river. Wherever there has 

 been a heavy erosion of its bank by the river, upstream of a canal 

 head, as much as six feet of sand may be deposited on the bed of the 

 canal channel at the head, in one season of three months, decreasing 

 perhaps to one foot some five or six miles down. But where the head 

 is in a good position, with no erosion near, and especially when it is 

 in a long creek or small side channel of the river, there may be not 

 more than two feet of silt at the head: and nothing at all three miles 

 down. As the river falls after the end of the rainy season, a canal 

 whose bed is heavily silted will of course run dry much sooner than 

 one with little silt in it. The level of the canal bed is usually fixed at 

 the lowest cold weather level of the river water surface at its head, so 

 that the silt clearance goes down to the spring level there; it is not 

 easy to dig deeper, for the annual silt clearances are heavy enough 

 usually with the bed at this level. The bed is graded at a slope or 

 grade of from one in 10,000 to one in 2,000, according to the size of the 

 canal, and the natural grade of the country traversed by it. As soon 

 as the river begins to rise, water can flow down the canal, but in 

 practice it is not usual to open a canal with less than two feet depth 

 of water, as a mere dribble is of no use, and deposits its silt very soon 

 These canals are opened in March, April or May as required, and flow 

 until September, October or November. Occasionally it happens that 

 a canal flows throughout the cold weather months, it being free from 

 silt, and the bed level having been scoured out deeper than usual; but 

 this does not often occur. For these large inundation canals, a bed 

 grading of one in 5,000 is given wherever the natural slope of the 

 country will allow of it; but in some places a flatter grading has to be 

 adopted in order to bring the level of the water surface in the canal 

 more speedily above the level of the ground, and so allow of land be- 

 ing irrigated near the head of the canal. 



During the cold weather months, when these canals are dry, they 

 are cleared of all silt deposits, banks are strengthened, bridges or 

 other works repaired, and new ones constructed where necessary. 



