76 AGRICULTURAL ENGINEERING 



more desirable to have an increase than a decrease in the 

 grade. Where the grade is reduced there is a reduction 

 in the velocity of flow at that point, which permits the silt 

 in the water to settle in the tile. 



Joining Laterals to Mains. When laterals or submains 

 are joined to another drain, it is advisable to have a slight 

 fall, or drop, as it is called, into the main at the end of the 

 drain. The amount of drop should be proportioned to the 

 size of the tile into which the drain discharges. Thus for 

 the 6-inch main the drop from the lateral should be 0.2 foot; 

 for an 8-inch, 0.3 foot; for a 10-inch, 0.4 foot; and for a 

 12-inch, .5 or % foot. To compute the elevation of the start- 

 ing point for each drain when a drop is to be provided, the 

 amount of the drop should be added to the grade elevation 

 of the main at the junction. 



Construction Figures. It is customary for the engineers 

 having the work in charge to indicate upon the guide stakes 

 the cut at the various stations. For convenience of those 

 digging ditches, the engineer often changes the decimal of the 

 foot to inches. It is also customary to furnish to the tile 

 ditcher a tabulated list of the cuts at the various stations. 

 Sometimes this is furnished and the marks on the guide 

 stakes are omitted. 



The Final Map. After the drainage system has been 

 located and all the field observations made, all data should 

 be reduced to a permanent map. This map should show the 

 /ocation of each drain, its length, head, outlet or junction 

 with another line; the number and size of tile required; 

 location of all surface inlets, silt basins, etc. It is also well 

 to record the grade of the drain from point to point and the 

 surface elevations and cuts at representative places. No 

 reputable engineer would think of undertaking the design of 



