SMALL DAMS PETITS BARRAGES

y multiplying the 1 hour chart values by factor of 0.96. Adjustments are then made for overwater
wind speeds by multiplying the map values by the velocity ratios listed on Table 5.3.
Table 5.3 – Wind Velocity Relationship – Water to Land (USBR, 1992)
Effective Fetch (Fe) Wind Velocity Ratio
(km)
(over water/over land)
0,8 1.08
1,6 1.13
3,2 1.21
4,8 1.26
6,4 1.28
≥ 8 1.30
The relationship between the wind velocity (Uf) over water and wind duration is found
using charts based on the effective fetch (Fe).
Compute the Design Wave
Design wave terms are defined on Figure 5.3. Significant wave height (Hs) and wave
period (T) are first computed from the design wind and effective fetch (steps 1 and 2), using
design charts or equations. The deep water wave length (L) is then computed as follows:
2
L 5. 12T
Wave run-up (R) is defined as the vertical height above still-water level (SWL) to which
water from an incident wave will run up the face of the dam. Wave runup is calculated as
follows:
H
R
s
0.
5
H s 0.
4 cot
L
Correction factors are applied for run-up when the wave propagation direction is not
normal to the upstream face of the dam. For embankment dams with smooth upstream faces,
the computed run-up is increased by a factor of 1.5.
Different design charts are used to evaluate run-up on rockfill dams or on earthen dams
with a rock U/S protection.
S
SWL
D
Fig. 5.3 – Definition of Terms for Design Wave Parameters
Wind setup (S) is computed as follows:
L
Ѳ
R
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