18 BULLETIN 304, U. S. DEPARTMENT OF AGRICULTURE. 



is necessary to keep the plant operating; at other times the low- 

 water flow of the ditch will be sufficient to warrant the plant operating 

 only a small portion of the time. This feature will be discussed in 

 more detail under the heading, ''Necessary capacity of pumping 

 machinery." 



The greatest amount of sediment is likely to be washed into the 

 main ditches during a heavy storm when the ditches are bank full. 

 It should be noticed that at such a time there is an important differ- 

 ence between the condition of these ditches and that existing in main 

 ditches of gravity drainage systems. In the latter, when more than 

 normally full, the velocity is a maximum, and hence there is the least 

 possible tendency toward the deposition of sediment in the ditches. 

 But in a pumping district, on the contrary, when the main ditches 

 are unusually full, the velocity will be low, and hence sediment may 

 be easily deposited. As a rule, the velocity in these ditches will 

 always be so low that they are not self-cleaning. On this account, as 

 well as on account of the drawing down of the level in the ditches by 

 the pumps, such ditches should be of unusually generous dimensions. 

 The engineer will wisely give particular attention to the designing of 

 the ditches to insure ample depth and breadth to provide the requisite 

 capacity. In general, it is probable that the main ditches should be 

 dug from 8 to 10 feet deep. Figure 1 of Plate V is a view of the main 

 ditch, Coal Creek Levee and Drainage District, Illinois, which was 

 constructed in 1909 with a dipper dredge having side spuds. The 

 view was taken before the ditch was completed at the lower end. 

 The water level as shown is about 4 feet below the ground surface 

 and about 4 feet above the normal level after the ditch was com- 

 pleted. Figure 2 of Plate V is a view looking up Branch A of east 

 lateral, from near the outlet of tile drain No. 7. An inspection of 

 these ditches at the end of 1914 showed them to be in very good con- 

 dition and remarkably free from deposits of silt. This very desirable 

 condition of the ditches is largely due to the complete diversion of 

 silt-bearing waters from the hills. Other districts already in opera- 

 tion would do well to investigate the advisability of treating their 

 hill drainage in a similar manner. 



Experience shows that to put bottom land into perfect agricul- 

 tural condition considerable tile drainage is necessary. This has 

 been demonstrated on all the districts along the Illinois and Mis- 

 sissippi Rivers, and landowners who have already been put to con- 

 siderable expense for the main drainage of the district can not afford 

 to have their land in any but the most fertile condition; it is, there- 

 fore, money exceptionally well invested to go to the additional 

 expense involved in the necessary tiling. Tiling in such lands should 

 be done with the same careful attention to the slope and to the 

 nature of the soil and subsoil as is necessary in getting the desired 



