Soil and Water Conservation Works - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 88

the levels may be read, and all the readings for a given contour should be checked for
being equal. An error of 2 cm may be fixed as the tolerable limit. In case no dumpy
level is available, the hydrostatic level (HSL) may be used for checking. For this,
random points may be selected on a contour line and readings of the levels may be
compared. Correct spacing of the lines is also to be checked at this stage. If two lines
are too close, one of these may be erased; and if too far apart, an intervening line
may be marked. If lines are not properly smoothened out, this should be checked
and corrected too.
After having marked the contour lines and checked their accuracy, it will be
necessary to mark the actual alignment of the structures along these lines. Alignment
of linear structures such as contour trench, V-ditch, contour bund, contour dike, and
contour furrows is marked with whitewash in two prominently visible parallel lines
indicating the width of the structure at its base. Where the contour line already drawn
forms part of such marking, only one extra parallel line needs to be drawn.
The alignment of planting pits is provided in between contour lines. The size of
the pits is marked as a box of white line. Spacing can be regulated using a staking rod
or a similar device. Untreatable areas such as barren rocks should be left out. While
indicating alignment it should be ensured that no pit is aligned near an existing plant
(Fig. 5.1 ).
Loose-stone check dams are wedge-shaped structures built across streams to re-
duce the velocity of water and to trap its silt load. These should be aligned keeping in
view their basic objective. A series of check dams should be planned in a top-down
fashion, such that the complete slope of the streambed is converted into terraced
steps. Alignment therefore requires use of a levelling instrument such as the dumpy
level. Alternatively, the HSL may be used. The basal area of the proposed check
dams should be marked with whitewash on the ground.
Impervious check dams or stop dams, should be so aligned that the preceding
slope on the upstream side is gentle. Such locations can be found by reconnoitring
the area or using contour maps if such maps are available. The number of stop dams
in a plantation will depend upon the number of water bodies to be created. If a
large number of economical sites are available—a stop dam site is economical if the
slope in the streambed is less than 1 %, and width of streambed upstream is much
more compared to the width of the streambed at the site of the structure—maximum
number of such structures should be planned, since each such structure stores water,
and thereby recharges the subsoil moisture. Location of each of the proposed check
dams should be marked on ground, rechecked using levels, and confirmed as an
economical location.
Sometimes, check dams and stop dams may be located on the fence of the planta-
tion. This special case arises because the stone wall fence cannot cross a streambed.
The vertical face of the check dam or the stop dam will then look like a fence, from
outside, and prevent entry of animals. However, this will be possible only when the
slope of streambed and other factors are favourable to enable locating the structure
at the place.