Page 326 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

7.20 THE WORK

FIGURE 7.13 Check dam.

Belt loaders, of both standard and special terracing models, give excellent results.
If the channel or cut depth is greater than that of the topsoil, a barren strip will be left which
will yield poor crops. This damage may be reduced by overcutting the channel so as to leave it
below grade, and blading some topsoil from above to cover it.
Maintenance. Terraces will serve their purpose best if plowing and cultivating are done along
contour lines—that is, parallel with the terrace lines.
A terrace can be enlarged by plowing so that dead furrows are in the channels and lands on the
ridges, as in Fig. 7.14.
Any accidental blocking of a channel or damage to a ridge should be repaired immediately, as
it might cause the terrace to fail in a heavy rain, with possible destruction of the terraces below it.

GULLIES

Characteristics. So far as this discussion is concerned, a gully is a drainage channel that has
become so deepened or enlarged that its banks are unstable and tend to extend destructively into
surrounding land (Fig. 7.15).
Control of gullies is largely an agricultural problem, but may also be required to protect high-
ways or structures.
Gullies are a sign of the beginning of a new cycle of erosion which tends to dissect smooth
slopes or high levels of ground into tablelands separated by steep-walled channels or canyons.
Unless controlled, it will eventually narrow such tables into peaks. Geologically, they are small
examples of the type of stream erosion which carves rising land into mountains.
The new erosion cycle may be started by land rising so that steepening channels add to the
velocity of flowing water; by lowering the outlet of a stream with the same result, or by reducing
the resistance to erosion of the land.
Gullies are caused most frequently by the destruction of the vegetation which protects the land
surface, although they may also be started by lowering of outlets, due to highway or stream channel
work, or land slips.
The majority of gullies contain intermittent streams which flow only during or immediately after
rains, or in wet seasons. Permanent streams are less often affected as their beds are unsuitable for
agriculture. These are most apt to be raised and choked by silt deposits resulting from bad farming.

Growth. When a slope is covered with vegetation, whether sod, bushes, or forest, rainwater
tends to move downward as a flowing sheet, and is only gradually gathered in definite drainage-
ways. Its eroding action on the ground is slight, as it is held from contact with the dirt and its
velocity is lowered by stems and roots that form a protective mat.