PREVENTION OF EROSION BY TERRACING. 



33 



inexperienced or careless operator, the results obtained are far from 

 satisfactory. Surveys of seven level-terraced fields where a level of 

 the above type was used showed an average variation in level along 

 the terraces for each field ranging from 0.4 to 2.6 feet. The engi- 

 neer's Y level is by far the most satisfactory instrument for this 

 work and the results obtained warrant the small expense of employ- 

 ing a competent engineer or surveyor who has such an instrument. 

 A number of surveys of 

 terraced fields laid off with 

 an engineer's level in the 

 hands of experienced level- 

 men showed remarkably lit- 

 tle variation from the level 

 or uniform grade line. 



Laying off level-terrace 

 lines is a simple leveling 

 proposition which consists 

 merely of following con- 

 tours of the field with a 

 chosen vertical interval be- 

 tween them. The terrace 

 nearest the top of the field 

 should be laid off first. The 

 level instrument should be 

 set in a position near the 

 middle of the terrace line so 

 as to command a view of the 

 whole length of the terrace, 

 and sufficiently high so that 

 the bottom of the rod, when 

 set at the highest point in 

 the field, is slightly below the level line of sight of the instrument. 

 If, for instance, the reading observed on the rod at the highest point 

 be 0.5 foot and the vertical distance between terraces is to be 3 feet, 

 the rod is placed at a point directly down the slope where the rod 

 reading is 3.5 feet. To establish the line of the terrace, points of 

 equal elevation should then be located to both ends of the terrace at 

 intervals of 25 to 60 feet, the closer spacing being used for land of 

 irregular topography. Invariably a point should be established 

 where the terrace line crosses a draw, gully, or depression. The 

 points established may be marked permanently by stakes to be used 

 subsequently as guides in the construction of the terrace. A very 

 common method is to lay out and construct the terrace at the same 

 time, or at least to plow one furrow to establish definitely the line 

 of the terrace. In this method the rodman is followed by a man with 



Contours 

 Terraces 



Fig. 14. — System of level-ridge terraces on a field 

 having regular slopes. 



