SMALL DAMS PETITS BARRAGES

2
U f Fe
S
1400 D
Where Uf is the design wind velocity over water, and D = average water depth along the
fetch.
The freeboard requirement for wind-generated waves is the sum of the wave run-up (R)
and wind setup (S). Freeboard is generally based on maximum probable wind conditions
when the reference elevation is the normal operating level. When estimating the freeboard to
be used with the probable maximum reservoir level, a lesser wind condition is used because it
is improbable that maximum wind conditions will occur simultaneously with the maximum flood
level (USACE, 1997).
5.4.4 First Approximations for Freeboard Requirements
As a first estimate, the U.S. Bureau of Reclamation (USBR, 1992) uses the empirical
guidelines summarized on Table 5.3 for preliminary studies of small dams. The values shown
on Table 5.3 were based on wind velocities of 160 km/hour (100 miles/hour) for estimating
normal freeboard and 80 km/hour (50 miles/hour) for minimum freeboard.
Table 5.4 – Freeboard Requirements for Preliminary Studies of Small Dams for rock faced
slope (USBR, 1987, 1992)
Longest fetch
Normal Freeboard Freeboard – MFL(*)
(km)
(m)
(m)
< 1.6 1.2 0.9
1.6 1.5 1.2
4.0 1.8 1.5
8.0 2.4 1.8
16.0 3.0 2.1
(*) MFL = Maximum flood level.
A large number of dams have failed due to overtopping and consequently greater
attention must be paid to this feature. Freeboard should not be less than 1,0 m, even for small
dams, as pointed out by Lewis (2002).[4]
In the French Guidelines for Small Dams, 2006, it is recommended that the minimum
freeboard for fill (providing a safety margin from maximum water level, settlement and
upstream cracking of the crest) depending on the parameter H V
2
, in which the minimum
values are presented in Table 5.5. Of course, if calculations using more detailed formula give a
higher value for freeboard, that higher value should be used. In such a case a less rigid wave
wall (e.g. gabions) can provide protection between minimum freeboard (Table 4) and
calculated freeboard.
Table 5.5 – Minimum freeboard for fill dams according to parameter H V
2
H V
2
5 30 100 200
Rmin (m) 0,40 0,60 0,80 0,70
According to practice in Czech Republic, and considering that the breakwater at some small
dams with large reservoir and long run-up of water waves can be considered.
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