Soil and Water Conservation Works - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 93

Fig. 5.3 Contour dykes

soil, so that the soil that comes with runoff will add up to form a horizontal terrace. To
provide reverse slope, the height of the wedge at the dike-end should be made abruptly
rising so that the net slope of the soil surface is opposite to the slope of the terrain.
Section of the wall as well as of the soil filled should be measured and it should
carefully be ensured that it complies with the specified section. Wall section is
checked simply by measuring its width and height. Volume of the soil filled will
vary from place to place, but the average quantity can be estimated for a given
contour, assuming uniform ground slope upward of it.

5.6 Bench Terraces and Gradoni

Bench terraces are structures of considerable cost and complexity. These are con-
structed along contour on hill slopes that are steeper than 15 %. Excavation is started
at the centre line of the section and soil is heaped down below, toe level upwards,
forming the built-up part of the terrace as work continues. In the end, half of the
terrace surface is built-up area and the other half is an excavated surface (Fig. 5.4 ).
It is sometimes desirable to provide reverse slope (2 % or less) on the terrace for
more effective storage of water. Uniform excavation all along the contour must be
ensured so that no longitudinal slope exists. A bund may be constructed at the outer
edge of the terrace on which seeds of suitable hedge species are sown so that the
terrace becomes long lasting and stable.
Terraces that are narrow—up to 1 m wide—and run along the contour lines, usually
at a spacing of 7-10 m, are called gradoni. A gradone may have a slight reverse slope
or it may have a small bund at its outer edge on which a contour vegetative hedge
(CVH) should be raised by sowing seeds.

5.7 Contour Bunds and Contour Furrows

Contour bunds are constructed by excavating soil from nearby borrow pits and form-
ing this soil into a proper section and compacting the soil using manual methods. A
faster method is to use a tractor-drawn disc plough to loosen the soil and then form
the bund with relative ease and speed (Fig. 5.4 ).