1/6 ENGINEERING FOR LAND DRAINAGE. 



every survey stake near the line of work will be de- 

 stroyed during construction. The top width of the 

 ditch should be laid out and defined by the stakes. 

 The first work of the contractor should be to mark the 

 outside lines of the ditch by a well-defined furrow, and 

 he should also make a note of the depth of the pro- 

 posed ditch at such points as he may be able to 

 identify. The ditch should be inspected and tested for 

 grade as the construction proceeds. To do this, begin 

 at the zero or beginning point of the ditch, and by 

 measurement reproduce the station points, take levels 

 at these points on the bottom of the ditch, and compare 

 the elevations obtained with those required. It will 

 be remembered that bench-marks have been established 

 at convenient intervals along the line for the purpose 

 of reference during construction, and no ditch should be 

 considered finished which has not been tested in this 

 way. 



Discharge of Side Ditches into Main, 



Much trouble is experienced in alluvial soils or those 

 which wash easily at points where lateral surface water 

 enters the main ditch by reason of the earth which is 

 washed Into the main and is frequently found deposited 

 in bars at or near the junction. This is particularly 

 observed at points where a large main ditch crosses a 

 public highway and the water from the shallow road 

 ditches is discharged into the deeper main by an over- 

 fall which rapidly cuts away the earth in time of 

 freshet. Much of this difficulty may be avoided by cut- 

 ting the lateral ditches down to such a grade that the 

 point of discharge will be near the bottom of the main 



