Soil and Water Conservation Works - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 100

Ordinarily, if the slope of the stream-bed is less than 1 % (which it should be for
economical choice of a stop dam), no such anchoring will be required.
Foundation and superstructure should be constructed using the largest available
pieces of stone. The upstream surface of the superstructure should be finished with
cement-sand pointing even if the masonry has been done in lime-sand mortar. The
top surface should be finished with cement concrete coping. Weep holes should be
kept on the downstream side, nearly at the base of the structure. Use of perforated
metal pipes is a convenient method of providing weep holes.
As stop dams are overflow type structures, extensive pitching should be done on
the downstream side. Pitching length should not be less than three times the height
of the dam. At least two courses of dry stone masonry should be laid to ensure safe
passage of water rushing down. The banks of the resulting water body should be
lined or pitched with masonry and brushwood layers to protect them from erosion.
Maintenance of the stop dams is important. Behaviour of the structures should
be watched during the first downpour, and if any damage is seen likely, immediate
steps should be taken to prevent it. If pitching is inadequate or the stones of pitching
are dislodged by the force of water, the same should be repaired immediately. In
due course, the dam will get filled with soil and silt, and will act as a retaining
wall. At this time, vegetative barriers should be established in the accumulated soil.
Working from the outer edges inwards, trees, shrubs, and grasses can be used to
create a permanent barrier to flow of water. In case of certain streams with very
large untreated catchment, this stage may not come since water will flow at ever
faster speeds and will make it impossible to raise vegetative barriers. Ordinarily,
such streams should not be treated in isolation, but with an integrated treatment of
the entire catchment.

5.11 Earthen Embankments

Earthen embankments are preferred where the streambed is too wide to be eco-
nomically dammed with a masonry structure. Embankment costs only a fraction of
masonry and is employment intensive. The only requirement for a good embankment
is availability of soil with good binding property, such as heavy loam or clay. Where
the stream is constituted by local runoff from a small catchment, only limited dis-
charge is expected and the stream can be dammed with a purely earthen embankment.
Where the stream has a larger catchment, however, arrangement must be made for
safe passage of excess water, otherwise the embankment will get overtopped and con-
sequently washed out. Outlet for excess water can be provided in form of a spillway
suitably located in the embankment's length. Alternatively, a saddle outlet may be
provided on a side valley, if such a formation is available. The latter, however, is pos-
sible only in rare cases of site selection where detailed contour maps are used to locate
the structure with a view to availing natural topographical features for the purpose.
Construction of embankment involves a few simple steps. After design of the
embankment has been finalised (Figs. 5.8 and 5.9 ), its base width is marked on