SMALL DAMS PETITS BARRAGES

� Hand-placed riprap failed in 30 percent of the cases it was used; failures were due to
the usual method of single-course construction.
� Concrete pavement failed in 36 percent of the cases it was used; failures were
generally due to inherent deficiencies with this type of construction.
This survey substantiated the premise that dumped riprap is by far the most preferable
type of upstream slope protection. Figure 5.12 presents the riprap at the upstream slope of
Paraitinga dam, in Brazil, which is a very well dam built for water supply using a good gneiss
as rock material.
Fig. 5.12 – Upstream gneiss rip-rap at the Paraitinga dam, in Brazil.
The superiority of dumped rock riprap for upstream slope protection and its low cost of
maintenance compared with other types of slope protection have been demonstrated so
convincingly that it has been considered economical to transport rock considerable distances
for major dams.
At a small dam, the surface area of the reservoir is often very small when the reservoir is
almost empty. Furthermore, the period in which the reservoir is at a low level generally lasts
only a few weeks (e.g. at dams built for irrigation purposes at the end of the summer). In such
cases, it may be possible to provide no protection for the lower part of the upstream slope. In
this case, a berm should be installed at the base of the protected top part. That berm will serve
as a support for the protective layer and will extend horizontally outward for at least one meter
past that layer. The berm elevation should be at least 2 h (h = wave height) below normal
reservoir water level.
Of course, partial protection of the upstream slope can be envisaged using the same
technique for dams with little variation in water level, such as lakes for recreational purposes,
diversion dams, etc. In this case, protection by plant cover may be considered if wave height is
less than 0.50 meter.
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