SMALL DAMS PETITS BARRAGES

5.2 DESIGN FLOODS
Inadequate flood handling capacity not meeting design standards is normally caused by
underestimating the design flood, not acknowledging the effect of the upstream approach
channel on the hydraulic gradient or not ensuring that the dam was constructed with its total
freeboard over the entire crest length of the dam. The hydraulic control must be correctly
defined for the design flow by determining the hydraulic flow and energy lines based on the
correct sectional information.
In rural areas, in which usually is very seldom inhabitants downstream, it is applicable the
design floods indicated in Table 5.1, like in South Africa.
Table 5.1 – Suggested Minimum Design Floods
Potential Hazard Classification -
PHC
PHC = 1 PHC = 2
Recommended design flood 1:20 year 1:100 year
Safety evaluation check flood
The following are important regarding Table 5.1:
1:50 year
1:200 year
No overtopping to occur for this flood and freeboard remaining for protection against
wave run-up
Overflowing is acceptable, but very limited in duration and depth, and risk of
catastrophic failure must be very low.
Floods to be reduced when upstream control is applicable.
Flood attenuation in the reservoir above full supply level studies will determine the
discharge flood.
The owner may adopt a lower risk as indicated and in any case when the incremental
cost for upgrading to a higher standard is low.
In more populated areas and countries like France, for instance, the draft Guidelines for the
design of dams spillways contains the following recommendations for small embankment
dams.
Table 5.2 – Suggested Minimum Design Floods in France (April, 2011)
Potential Hazard Classification –
PHC
PHC = 1 PHC = 2
Recommended Design Flood 1:300 year 1:1000 year
Safety Evaluation Check Flood
1:1000 year
1:10000 year
It is possible to see that the safety considerations could differ from one country to another,
depending to the attitude of the society in regards to acceptable risk and on the level of
economic development. But is always important to increase the requirements related to the
dam safety, in order to reduce the risk and avoid any possible dam overtopping during severe
storms. Most of the small dams are embankment dams, and this type of structure not accepts
overtopping without the risk of failure.
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