Page 325 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

LANDSCAPING AND AGRICULTURAL GRADING 7.19

FIGURE 7.12 Terrace line correction.

If the line is moved uphill to cut off a bend on a ridge, the channel will have to be dug more
deeply to allow flow of water through it. However, such a ridge may be used as a divide or drainage
head, in which case little water will be present.
Stakes are ordinarily used only for location guides, but may be marked with grades where the
terrace is to be higher or the channel deeper than standard.
It is desirable that the top of the terrace system be also the top of the drainage area. The top
terrace should serve the same width of ground as the terraces below it. If a larger area must be served
because of flow from higher fields, the channel capacity must be increased proportionately, or some
other type of intercepting drain used.

Grades. Ridge terraces usually have a level grade. Channel terraces may be level for the section
most distant from the outlet, and slope increasingly to about a ⁄2 percent grade at the outlet. Drainage
1
is normally from ridges toward hollows. Short terraces require less maintenance than long ones.
Outlets. The discharge from a terrace should be into a waterway that is capable of carrying the water
directly down the slope, without eroding. Shallow depressions carrying a permanent sod are often sat-
isfactory. These should have enough drop from side to center to ensure gathering of all water discharged
from the channels, but should not concentrate enough flow at the center to cause erosive velocities.
The strength of the sod, as affected by fertilizing and grazing, and the condition of the soil will
determine the maximum safe gradient for a meadow outlet. This is ordinarily 6 percent or less.
Sod should extend several feet above flow lines on the sides of the waterway. Steeper or nar-
rower channels may be protected by ungrazed and uncut growth of grass, weeds, bushes, or trees.
For extreme conditions, a channel protected with check dams (Fig. 7.13), pavement, or other
artificial structures may be required. Steep banks require sod or artificial flumes for the terrace
discharges.
Permission of owners of land below the farm must be obtained if the terrace system alters the
path or concentration of water on their properties.
Outlets must be completed and protected before terraces are built.

Construction. Terrace construction is primarily a matter of sidecasting. The work is commonly
done with graders of either the powered or towed types. Bulldozers working at right angles to the
terrace are also effective, particularly in the channel type.