r I" 



ENGINEERING FOR LAND DRAINAGE. 



moisture and texture in all conditions of weather. 

 Where the soil is alike in the tenacity with which it 

 holds moisture the system should be uniform, and 

 every part of the ground brought under the influence 



Fig. 5. — ^Grouping System. 



of drains at regular intervals. But when the soil varies, 

 or the surface is diversified by ponds, sloughs, and 

 draws, thorough drainage means lines with reference to 

 the different conditions. The gridiron system consists 

 of equidistant parallel lines with mains and sub-mains 

 for collecting and conducting the water to some point 

 of exit. It is economy to have the laterals enter the 

 mains at right angles, but for completeness and effi- 

 ciency they should so enter that the currents of the two 

 streams may coalesce and increase rather than retard 

 the flow of the main. This system is illustrated in Fig. 6. 

 Double-main System — This is applicable in broad, 

 flat sloughs where it is desirable to use two lines of 



