Page 110 - Soil Degradation, Conservation and Remediation

3.4 Control of Water Erosion 97

ORIGINAL LAND
PROFILE

CUT FILL

6,00 a 12,00 m
A. WIDE-BASE TERRACE

3,00 a 6,00 m
B. MEDIUM-BASE TERRACE

3,00 m
C. NARROW-BASE TERRACE

Fig 3.18 Types of terraces on size of the terrace base (FAO 2000 )

(b) Ridge or Mangum terrace: a long, low ridge of earth with gently sloping

sides and a shallow channel along the upper side. These terraces are con-
structed by excavating the soil from both sides of the embankment (Morgan
1986). They are recommended for slopes less than 10 %, for low rainfall
regions, and for permeable soils (FAO 2000). Ridge terraces control erosion
by diverting surface runoff across the slope instead of permitting it to fl ow
uninterrupted down the slope.
3. Size of the Terrace Base (Fig. 3.18 )

(a) Narrow-base terraces: where soil movement is limited to about 3 m

(b) Medium-base terraces: where soil movement is 3–6 m

(c) Wide-base or broad-based terraces: where soil is moved more than 6 m, but

normally less than 12 m
4. Terrace Shape
According to their shapes, terraces could be of the following two types (Bertolini
et al. 1989).

(a) Normal terrace: Normal terrace consists of a ridge or bank and a channel,
which may be constructed on a gradient or level. This type of terrace is
normally used in areas where the slope is less than 20 % (FAO 2000). These
terraces mostly include broad-based terraces.

(b) Bench terraces: These terraces form a series of level or nearly level strips of
earth and a steep or vertical downhill face, constructed on or nearly on the
contour (Fig. 3.19 ). Bench terraces are constructed by cutting and fi lling and
are used in land with slopes in excess of 20 %. The bench terrace is perhaps
one of the oldest forms of terraces. All other types of terraces have been
derived from this terrace type.