SMALL DAMS PETITS BARRAGES

It is important to emphasize that most old farm dams have been built without a good evaluation
of the maximum discharge, what explains the large number of small dams’ failure during severe
floods, in some areas of several countries. Therefore, when developing an EAP it is very
important to initially check the real capacity of the dam outlet or spillway, in order to evaluate
the probability of the dam to endure severe storms.
8.3.2 Piping and Internal Erosion
Several terms and classification systems have been used to describe failures and accidents
caused by water flowing through or under an embankment. Piping is usually defined when
internal seepage occurs through a soil causing the progressive removal of soil particles by
percolating water, leading to development of internal channels.
An embankment dam can fail or experience serious distress if water flows without adequate
controls through the embankment itself or through the foundation soil and bedrock on which it
rests. Small dams are more vulnerable to internal erosion, related to large dams, as a
consequence of the inexistence of internal filters and transitions, or filters poorly constructed.
Internal erosion in a dam can breaches the embankment when it creates a tunnel through the
embankment that is large enough to empty the reservoir suddenly, resulting in the uncontrolled
release of the reservoir.
The analyses of the piping mechanism have to concentrate on the type of soil, the hydraulic
gradient and the confining stresses, at the dam base and along the interfaces between
embankment and the concrete structures.
8.3.3 Earthquake
Earthquakes are the second natural event that have to be considered in the application of an
EAP, through the analysis of the location of the small dam over a seismological map. With such
analysis it is possible to predict the intensity of the maximum probable earthquake, and its
frequency along the time.
Considering that most small dams are of the embankment type, it is recommended to check the
possibility of liquefaction of the dam foundation or of the dam embankment itself, which occurs
more frequently with well graduated sands and silts. In second place it is important to check the
actual level of the dam crest and make a prediction of its settlement after a strong earthquake,
to avoid dam overtopping in case of severe storms.
8.3.4 Failure of Upstream Dams
It is very usual to have several dams built along a determined river, as can be seen on Fig. 1, in
which the farm used to stock water for cattle, for irrigation purposes, for human supply etc. In
such cases it is very important not only to consider the safety of a determined dam alone, but its
safety as a consequence of the failure of another dam upstream. This scenario would usually
take place during severe storms, that can cause dam overtopping if its reservoir is already at its
maximum.
A probable solution to improve the safety of the downstream dam is to protect the downstream
slope with gabion, RCC, articulated concrete blocks, as already presented in Chapter 5 –
Rehabilitation Practices for Existing Small Dams.
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