1 66 ENGINEERING FOR LAND DRAINAGE. 



will not give way and slide by. reason of the superin- 

 cumbent weight. A distance of 7 feet from the outer 

 edge of the ditch to the inner edge of the waste bank 

 w^ill be sufficient, yet more than this will make the 

 after working down of the waste bank more convenient. 



Surveys for Open Ditches. 



After having done the necessary preliminary topo- 

 graphical work for the purpose of determining in a gen- 

 eral way where the ditch or ditches should be located, 

 the lines should be surveyed, the grades established, 

 and the quantities of earth to be excavated computed. 

 The first work is to run the center line of the ditch, 

 which is done in the manner described for running a 

 line for a tile drain. Stakes should be set, levels 

 taken, and the line located for mapping purposes as 

 heretofore described. 



Locating the Grade. — The most satisfactory method 

 of determining upon a suitable grade for a long line is 

 to first make a profile upon profile paper, which will 

 show to the eye the relation of different points along 

 the line with respect to their elevations. Draw trial 

 grade lines upon this profile in pencil until one is found 

 which will give a desired fall and depth to the several 

 sections of the ditch. Then run this in on the notes 

 and compute the cuts as directed for tile drains. If 

 the line is short, say a mile long, the grade may easily 

 be fixed in the manner described for tile drains. In 

 considering the matter of grade and depth, it should 

 always be found whether or not all of the land that it 

 is desired to drain can be well drained through the 



