Soil and Water Conservation Works - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 90

sections marked with a double line indicating the width of the trench are to be dug.
Excavated soil should be carefully heaped on the lower edge of trench and formed
into a bund of side slopes that are natural to the soil. Edge of the bund should be at
least 5 cm clear of the edge of the trench to avoid possibility of the soil sliding back
into the trench. Even though the trench is dug as a continuous trench, yet after every
10 m of run, a stretch of 0.45 m should be left undug so as to form a barrier (baffle)
to the flow of water within the trench.
The section of a trench should be uniform and in accordance with the specifica-
tions all along. In slopes higher than 5 %, the two sides of the trench section will
have significantly different depths. In such cases, it is the lower side that should be
measured as a check, as this is the depth that will determine the amount of water
stored in the trench. It is more convenient, expedient, and accurate to get a frame
of wood or some other light material made—measuring the designated size of the
section—and have it run through the section to ensure that it is of the right size.
In many cases, it is desirable to curve up the ends of each continuous trench so as
to prevent water from spilling off at the ends. Trench-flooding may result in breach
of the bund from somewhere in between if the ends are curved up far too high. It
should therefore be practised only in the soils that are more friable and light, where
the ends of the trench section are more likely to be eroded as a result of getting
overtopped. Alternatively, each continuous stretch of the trench can be provided
with a surplussing arrangement in the form of a weir on its bund made of dry stone
masonry.
Lower edge of the bund can be pitched with a line of stones or boulders laid out in
a row or two, to prevent sliding of soil down the slope as a result of disturbance from
rain-drops or wind. It is more required in steep slopes—more than 15 %—and friable
and light soils than in heavy soils. In case no stones are easily available, local mulch,
scrub, brushwood, or any other material may be used for the purpose of stabilising
the soil bund at its lower edge.

5.3 Digging of V-Ditch

A contour V-ditch (Fig. 5.2 ) is dug on a contour line and in continuous form, leaving
out baffles of about 0.45 m after every stretch of 20 m. Untreatable sections, if any,
should be left out. The upper line marked in alignment—marking is in double line
indicating the width of the ditch—should be visible even after excavation of the ditch.
A good technique of arriving at the correct section is to first excavate a thin trench
of required depth at the lower end, and then to scrape out the soil on the upper side
by progressively excavating until the desired slope and width are obtained. Workers
should be trained for this operation as it is more likely that they will not understand
the section correctly. Often they will leave the sloping part of the cross-section not
flat but bulging out and consequently less water will be stored. The edge of the bund
should be kept at a distance of 5 cm from the edge of the ditch. Natural slopes of the
soil should be maintained on the bund.