18 BULLETIN 512, U. S. DEPAETMENT OF AGEICULTUEE. 



ridge terraces can be obtained from the curves in figure 4 by adding 

 one-half the vertical distance to the width of the bench for a 

 horizontal-bench terrace. 



'Were it not for the fact that the terraces would need to be placed 

 very close together on steep slopes, thus necessitating a greater num- 

 ber of terraces, it would be well to reduce the height of the terrace as 

 the slope of the land increases. This would obviate the difficulty 

 encountered in the construction of large terrace embankments on steep 

 slopes. 



The equation from which the curves in figure 7 were constructed is 

 based upon the assumption that the ends of the terraces are closed. 

 In the field investigations many terraces with closed ends were found. 

 Some followed contours completely around a knoll or hilltop, form- 

 ing a closed circuit with no outlet. (See fig. 6.) But most of the 

 level terraces examined had outlets at either one or both ends. In 

 the foregoing discussion the terrace was taken as 1 J feet high ; with- 

 closed ends it would overflow for a rainfall in excess of 8 inches in 

 48 hours. However, if one or both ends of a terrace be left open 

 a liberal factor of safety against overflowing is provided. To pro- 

 vide a factor of safety for terraces with closed ends it is recommended 

 that they be made about 1| feet high. 



General discussion. — The success or failure of a broad-base level- 

 ridge terrace depends largely upon whether or not it is laid out on an 

 absolute level. Since the surface of the water stored above the terrace 

 is level, it is imperative that all points along the top of the terrace be 

 above this water level. If one point is low, the water flows over and 

 soon washes away a section of the terrace. All of the water above the 

 terrace then flows toward this crevasse and contributes to the further 

 destruction of the terrace and often erodes a deep gully down through 

 the field. Hence, in laying out a level terrace, the top should be main- 

 tained at the same elevation throughout its length. 



It is desirable also that the base of the terrace follow the contour of 

 the ground as closely as practicable. This often necessitates the use 

 of very sharp curves and abrupt bends, but it eliminates the exist- 

 ence of any low places or pockets above the terrace which collect and 

 hold water on impervious soil. These sharp bends occur usually at 

 crossings of draws and depressions. Most farmers object to them on 

 account of inconvenience in cultivation and prefer to give the ter- 

 race a gradual bend by crossing such places at a lower elevation. 

 Then it is necessary to build the base of the terrace on lower ground 

 and still maintain the top at the same elevation as that of the rest of 

 the terrace, which requires that the terrace be built higher and wider 

 at the base. (See fig. 8.) One landowner who was experienced along 

 this line advised that a terrace crossing a gully or depression be built 

 one-third higher than the required height of the terrace, to provide 



