Page 323 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

LANDSCAPING AND AGRICULTURAL GRADING 7.17

FIGURE 7.10 Terrace types.

Terrace Types. Three principal types of terrace are used. Each is constructed along level or con-
tour lines. The ridge terrace, Fig. 7.10(A), is a ridge built of soil obtained from both sides. The
channel terrace, (B), is a ridge constructed of dirt from the upper side only, and the channel
formed by this excavation is an essential part of the structure. The bench terrace, (C), has a stair
structure with steep risers separating relatively flat cultivated areas.
Ridge and channel terraces are usually built with sufficiently gentle slopes to allow farm
machinery to work along or across them. Best results are obtained if farming operations are done
parallel with their centerlines.
Ridge Terraces. The ridge or absorptive-type terrace is used primarily to conserve water in
regions of deficient rainfall. Each ridge serves as a dam for a pond, which is deepest in the exca-
vated area immediately above it. Water may also be impounded in the trough formed below this
ridge by borrow of material.
A larger area and quantity of water can be held on slight gradients than on steep ones, by any
one size of ridge. Not only is more water retained per yard of dirt used in the ridge, but also its
distribution over the land is more uniform.
Too great a depth of water may drown out crops immediately above the ridge.
It is ordinarily not economical to construct terraces for water conservation alone on slopes
over 3 percent, and structures for reducing soil erosion are more often of the channel or inter-
mediate types.
Overflow channels may be provided to carry off rain in excess of that for which the system
was designed. These should be protected like channel terrace spillways.

Channel Terraces. Channel or drainage-type terraces are essentially shallow diversion ditches
which catch water flowing down a hill and lead it off to drainageways that have been protected
against erosion.
The channel depends on the ridge of excavated material for much of its capacity. Its grade is
flat, or nearly so, so that only extremely fine soil particles can be carried by the water it discharges.