SMALL DAMS

Chapter IV
PROTECTION OF FACES AND CREST
Placing a layer of gravel on the crest will in particular avoid the formation of ruts due
to traffic and desiccation of the last layers of compacted clay materials.
The dimensions of the protection for the upstream slope (including the filter blanket)
should take into account the effect of waves and the type of protection selected.
ICOLD Bulletin 91, published in June 1993, specifically deals with protection of
upstream slopes for fill dams and the reader is invited to consult it.
That bulletin gives rules for dimensioning rip-rap protection (median block mass,
grading, thickness of the layer, thickness and grading of the filter blanket, material
quality). It also describes in detail specifications concerning protections made of:
!" soil-cement;
!" cast-on-site concrete slabs;
!" pre-cast concrete blocks;
!" bituminous concrete.
Wave action depends essentially on reservoir size and geographical location (wind
rose). The choice of type of protection and its dimensions are therefore independent
of dam height. From this standpoint, small dams cannot be considered as special
cases unless the reservoir surface area is small.
Wave height h is calculated as indicated above and in table 4 hereafter.
U: wind velocity (m/s)
D: depth of water (m)
F: length of fetch (m)
g: acceleration due to gravity (m/s 2 )
h = 0.26 th 0.578 (
g D
) 3/4
U 2
0.01
U2
th
(
g.F
) 1/2
U 2
th 0.578 (
g D
) 3/4 g
U 2
75
D
U 20 25 30 35
F
300 600 1000 2000 3000 300 600 1000 2000 3000 300 600 1000 2000 3000 300 600 1000 2000 3000
5 0.28 0.39 0.50 0.67 0.78 0.35 0.49 0.61 0.81 0.94 0.42 0.58 0.73 0.96 1.10 0.49 0.67 0.84 1.09 1.24
10 0.29 0.40 0.51 0.71 0.86 0.36 0.50 0.64 0.88 1.06 0.43 0.60 0.76 1.05 1.25 0.50 0.70 0.89 1.21 1.44
15 0.29 0.40 0.52 0.73 0.88 0.36 0.50 0.65 0.90 1.09 0.43 0.60 0.77 1.08 1.30 0.50 0.70 0.90 1.25 1.50
20 0.29 0.40 0.52 0.73 0.89 0.36 0.51 0.65 0.91 1.11 0.43 0.61 0.78 1.09 1.32 0.50 0.71 0.91 1.27 1.53
25 0.29 041 0.52 0.73 0.89 0.36 0.51 0.65 0.92 1.11 0.43 0.61 0.78 1.10 1.33 0.50 0.71 0.91 1.28 1.55
Table 4 - Wave height h expressed in metres
Depending on wave height h, table 5 gives the recommended dimensions for classic
rip-rap: thickness e of the rockfill layer (measured perpendicularly to the face) and
diameter d 50
such that 50% of the blocks by weight have a diameter equal to or
greater than d 50
. The dimensions of the largest blocks are limited to e. The smallest
elements should not have a diameter less than 0.10 metre.