SMALL DAMS PETITS BARRAGES

For very small reservoirs (fetch of only a few hundred meters and good face orientation),
it may be tempting to provide no upstream protection, or only a grass cover, as can be seen in
Figure 5.13. It is always possible to take action after the face has deteriorated.
Fig.5.13 – Upstream slope protection with grass at the Chibarro dam, 12 m high, in Brazil
(fetch = 300 m).
During the rip-rap construction it is important to avoid the use of rock materials that can
disintegrate along the time, as a consequence of the several wetting and drying cycles along
the time, and the fast weathering of the rock.
5.10.3 Downstream Slope Protection With Grass
The downstream slope of an earthfill dam must be protected from the effects of rainfall
runoff. Grass is practically always planted to this end on the downstream slope of small dams.
For fill dams over 12 meters high, it is recommended to install an intermediate berm halfway
up the downstream slope. For others dams over 15 meters high, this recommendation
practically becomes a requirement. This berm offers two advantages:
� It limits the effects of runoff along the slope;
� It gives access to piezometers halfway up the slope, as well as to spread topsoil, to
plant grass, and later to maintain the slope.
Planting grass on the downstream slope is made easier through use of synthetic or
natural geotextile, fertilizer and a straw substratum are incorporated, or some kind of
Honeycomb geosynthetic mats can also be placed on the dam body. These techniques are
advisable in a climate where periods of severe drought and intense storms make planting
more difficult. In any case, all varieties of shrubs must be prohibited.
The root system of the grass cover holds the surface soil in place and protects very
successfully the slope from any kind of erosion. Most of the dams which resisted and don’t
failed were that with a grass cover. But it is very important the grass cover be maintained by
irrigation and cutting. Grass cover is useful also during the periodic visual inspections, when
fresh green spots appear, they are an indication of seepage or high phreatic surface. Of
course an inappropriate vegetative growth, will screen deficiencies as cracks, sinkholes and
animal burrows.
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