PREVENTIOlSr OF EEOSION BY TERRACING. 



29 



Sometimes hillside ditches are constructed as outlets for terraces. 

 Such ditches should have a fall two or three times that of the terraces 

 and should be located so as to cross them and discharge into the 

 nearest available drainage channel. Often wooded strips of land 

 are left in fields to afford a place for the discharge of the water with 

 a minimum amount of erosion. 



General discussion. — Many of the failures of graded terraces may 

 be attributed to irregularities in grade. Breaks occur often with 

 abrupt reductions in the grade. This causes a piling up of the water 

 and a consequent overtopping of the terrace by reason of the inability 

 of a full channel to carry the same amount of water on a light grade 

 as on a heavy one. With a variable-graded terrace there is less like- 

 lihood of overtopping because the grade is increased at short inter- 

 vals along the terrace. 



Again, breaks in graded terraces are very frequent where gullies 

 and depressions are crossed and at abrupt bends. Such breaks are 

 due to sudden changes in the direction of flow or to a change in 

 grade, and often to both. The usual practice of crossing depressions 

 at a low elevation to avoid abrupt bends, as explained under " The 

 broad-base level-ridge terrace," results in an increase of grade to the 

 middle of the depression and a decrease beyond the middle. In order 

 to avoid a break due to this diminution in grade it becomes necessary 

 to maintain the top of the terrace at a uniform grade. This necessi- 

 tates the building of a high and broad embankment across the depres- 

 sion similar to the one described for level terraces. Wherever it can 

 be done without increasing the grade to such an extent as to cause 

 serious erosion, it is advisable to make the grade greater for that- 

 portion of the terrace leading away from the middle of the depres- 

 sion thaii for the portion leading to the middle. 



The graded terrace is adapted particularly for use on impervious and 

 worn-out soils, and on shallow open soils with an impermeable sub- 

 soil foundation — in general, soils that are incapable of absorbing 

 much water. Since the object of terracing is to prevent erosion, and 

 as this is accomplished best by securing the least movement of the 

 surface water, it can be seen readily that, within limits, the efficiency 

 of a graded terrace varies inversely with the amount of fall given to 

 it. The greater the fall the greater the velocity and, hence, the 

 greater the erosive power of the moving water. 



The embankment of a graded terrace, being subjected to the erosive 

 action of the water on its upper side, is often washed considerably, 

 particularly at bends. Plate VI, fig. 1, shows a graded terrace 

 embankment cut away practically half by erosion and the heavier 

 parts of the soil deposited along the upper side of the terrace. The 

 deposit of soil in the terrace channel reduces both the grade and the 

 cross-sectional area of the channel and renders the terrace extremely 



