Page 109 - Soil Degradation, Conservation and Remediation

96 3 Soil Erosion by Water

3.4.13 Terracing

Terracing refers to the building of a mechanical structure, a channel and a bank or
an earthen ridge or a stonewall on the land to reduce steepness of slope and divide
the slope into short gently sloping sections (Morgan 1986 ). Terraces are created to

encourage infiltration, to intercept surface runoff, or divert toward a predeter-
mined and protected safe outlet at a controlled velocity to avoid soil erosion
(USDA Soil Conservation Service 1980 ; FAO 2000 ). The critical runoff velocity,
at which soil particles that have been detached from soil aggregates begin to be
−1
−1
transported over the surface, is 5 m s in sandy soils and 8 m s in clay soils
(Rufi no 1989 ; FAO 2000 ). Terracing reduces runoff velocity below this threshold
values. It is one of the oldest means of saving soil and water. Moreover, it is the
most widely used soil conservation practice throughout the world (Hanway and
Lafl en 1974 ; Mountjoy and Gliessman 1988 ; Sutikto and Chikamori 1993 ;
Skinner and Porter 1995 ; Drechsler and Settele 2001 ; Bokhtiar et al. 2001 ; Kasai
et al. 2001 ).

3.4.13.1 Types of Terraces

Terraces can be naturally formed upslope contour hedgerows (Poudel et al. 1999 ),
vegetative fi lter strips (Stark et al. 1999 ), and grass barriers (Aase and Pikul 1995 ;

Walle and Sims 1999 ). Man-made terraces can be classified on different criteria.

They can be classified according to (1) their main function, (2) the construction
process, (3) the size of the terrace base, and 4) the shape.
1. Main Function of the Terrace

(a) Retention terraces, also called absorption or level terraces (Morgan 1986):

these are designed to accumulate and retain runoff in the terrace channel so
that it will eventually infiltrate and the sediment accumulates. These ter-

races are recommended for low rainfall areas, permeable soils, and for
land of less than 8 % slope. They are normally broad-based terraces (FAO
2000).

(b) Graded or diversion terraces: these are sloping terraces, designed to inter-
cept or divert runoff into protected waterways. These terraces are recom-
mended for high rainfall regions, for slightly or moderately permeable soils,
and for slopes of between 8 and 20 % (FAO 2000).
2. Construction Process

(a) Channel or Nichols terrace: these terraces are constructed by excavating soil
from the upper side only to form a channel and depositing it downhill to
form a bank (Morgan 1986). They are recommended for slopes up to 20 %.
They are used in high rainfall regions and in soils of low or medium perme-
ability (FAO 2000).