PREVENTION OF EEOSION BY TEREACING. 



17 



The values in the above table are based upon a field study of the 

 effect of soil and slope upon the run-off. A knowledge of the soil, 

 slope and design of several terraced fields which were known to have 

 withstood heavy rainfall successfully for a number of years furnished 

 data from which was estimated the percentage of rainfall that runs 

 off. These figures were used as a basis for interpolating the inter- 

 mediate values. 



The figures given in the above table are to be used in conjunction 

 with the curves in figure 7 to determine the proper vertical spacing 

 of broad-base level-ridge terraces. For example, if it is proposed 

 to terrace a field having an average fall of about 5 feet per hundred, 

 a pervious subsoil, and a deeply plowed clay-loam topsoil containing 

 considerable humus, then the percentage of water flowing off as taken 

 from the table would be 55. This is found by following the space 

 marked " clay lo^m " to the right until' the column headed 5, " slope 

 of land in feet per hundred," under " pervious subsoil," is reached. 

 The value 65 is found at this point. Now the note below the table 

 specifies that 10 should be deducted from the table values for soil 

 deeply plowed and containing much humus. Hence, the value to be 

 used is 65 minus 10, or 55 per cent. To determine the proper vertical 

 spacing from the curves (fig. 7), extend a horizontal line from the 

 ordinate 55 per cent until it intersects the curve marked 5 feet per 

 hundred, and from the point of intersection project a vertical line 

 to the horizontal axis. Such a line intersects the horizontal axis 

 at about 2.6 feet, which is the required vertical distance between 

 terraces. 



Where the average slope of a field is less than 5 per cent, use the 

 vertical spacing as obtained from the 5 per cent curve. For inter- 

 mediate slopes for which no curve is given, the vertical spacing can 

 be obtained by interpolating between the curves plotted. Where the 

 rate of fall of a field varies down a slope, the vertical spacing may be 

 varied between the terraces to suit the slopes. However, a very small 

 portion of a field often has an excessively steep slope as compared 

 with that of the rest of the field. In such cases the vertical spacing 

 should be chosen to suit the lesser and more general slope of the field. 

 This will place the terraces on the steep slope very close together, 

 but it undoubtedly is the most satisfactory solution of the problem. 



It can be seen from the curves in figure 7 that the vertical spacing 

 between terraces decreases as the slope decreases, which precludes the 

 possibility of an excessive slope distance between terraces. This rela- 

 tion minimizes erosion between terraces by reducing both the volume 

 of water and the distance traveled by the run-off water from one ter- 

 race, to another. The horizontal distance between broad-base level- 

 71775°— Bull. 512—17 3 



