COST OF DRAINAGE 



189 



Provisions should be made to keep stock from tramping 

 on the outlet and destroying it. In drains that are dry 

 part of the time, screens to keep out rodents and other 

 troublesome animals should be placed over the outlet. 



Manholes at intervals in the system assist in locating 

 trouble. These manholes may be constructed in such a 



FIG. 30. SILT Box WITH LID. THE SILT THAT 

 SETTLES IN THE Box CAN BE SPADED OUT. 



way that they serve as silt basins and thus eliminate 

 from the system silt that might clog the tile. These silt 

 basins are particularly necessary if the fall of the drain 

 has to be reduced. A good type of combination silt trap 

 and manhole is shown in Fig. 30. 



Cost of Drainage. The cost of installing a drainage 

 system varies so much with conditions that definite figures 

 cannot be given. Hart (5) estimates the drainage of 

 irrigated land to vary from $15 to $30 with $20 as an 



