PEEVENTIOK OF EEOSIOF BY TERRACING. 5 



clearing it was found impossible to control or check the erosion. 

 Such lands should be reverted to timber; otherwise the ravages of 

 erosion will reduce it soon to a state of barrenness. It is known 

 that erosion is least active in forested areas, because of the penetra- 

 tion and binding power of the roots and the accumulation of a thick 

 layer of leaves and organic matter on the soil surface. The soil 

 possesses great coherence and power of resistance to the erosive action 

 of the water and the layer of humus protects the surface and also 

 absorbs considerable water. 



UNDERDRAINING. 



It can be seen readily that by the underdrainage of land to carry 

 off the excess water from the soil space is created for the reception 

 of more water from the surface. The water falling upon the sur- 

 face sinks into the soil, percolates through it, and is conducted away 

 by the underdrains to an open drainage channel without running 

 over the surface and causing destructive erosion. Entrapped air, 

 which often prevents the entrance and free movement of water in 

 the soil, finds a means of escape through subdrainage channels. The 

 physical condition of the soil is altered by underdrainage through 

 the aeration and flocculation of the soil particles. A perceptible 

 expansion and a slight upheaval of the soil take place, resulting in 

 an increase in the size of the individual pore spaces. Hence, the 

 rainfall percolates more easily and quickly into the soil and a 

 diminution in the run-off follows. This system of draining is 

 accomplished best by the use of tile drains. 



USE OF HILLSIDE DITCHES. 



Hillside ditches, as the name implies, are ditches constructed on 

 hillsides to intercept run-off water and carry it at a low velocity to 

 the nearest open drainage channel. Wherever this method of pre- 

 venting erosion is employed there is likely to be a constant, percep- 

 tible draining off of the finer particles of soil, and a continual en- 

 largement of the ditch takes place, the extent depending upon the 

 amount of fall given to the ditch. (See PL III, fig. 1.) It is inad- 

 visable, therefore, to resort to this method except when it is neces- 

 sary to intercept surface water from adjoining higher land on which 

 methods of preventing erosion are not employed. Sometimes hill- 

 side ditches are constructed to serve as outlets for systems of graded 

 terraces where natural drainage outlets are not available. 



TERRACING. 



The greatest benefits from the foregoing methods of prevention 

 come when they are applied in connection with a system of terraces. 



