SMALL DAMS PETITS BARRAGES

The supporting layer for the rip-rap is intended to protect the fill from the hydrodynamic
effects of waves and from erosion. The U.S. Corps of Engineers recommends the following
rockfill diameter (D50) and thickness of rip-par layer presented at the following table [3].
Table 5.15 – Rip-Rap layer recommended by U.S. Corps of Engineers.
Wave height
Average rockfill diameter – Layer thickness
(m)
D50 (m)
(m)
0 – 0.60 0.25 0.30
0.60 – 1.20 0.30 0.46
1.20 – 1.80 0.38 0.61
1.80 – 2.40 0.46 0.76
2.40 – 3.00 0.53 0.91
The same corporation recommends the following filter layer or bedding thickness
between the rip-rap and the earthfill.
Table 5.16 – Transition thickness layer recommend by U.S. Corps of Engineers
Maximum wave height (m) Bedding thickness layer (m)
0 – 1.20 0.15
1.20 – 2.40 0.25
2.40 – 3.00 0.30
For wave height from 0.30 to 1.55 m it is also possible to use the following table, present
by the French Guidelines [5].
Table 5.17 – Dimensions of upstream rip-rap (French Guidelines on Small Dams)
Wave height
Thickness - e
Block diameter – D50
(m)
(m)
(m)
0.30 0.30 0.20
0.55 0.40 0.25
0.80 0.50 0.30
1.05 0.60 0.40
1.30 0.70 0.45
1.55 0.80 0.50
The supporting layer may be replaced by a puncture-resistant geotextile in cases in
which the fill material is fairly resistant to erosion.
5.10.2 Selecting the Type of Upstream Protection
Experience has shown that in most cases, dumped riprap furnishes the best upstream
slope protection at the lowest ultimate cost. Approximately 100 dams, located in various
sections of the United States with a wide variety of climate conditions and wave severity, were
examined by the Corps of Engineers. The results of this survey were used as a basis for
establishing the most practical and economical means for slope protection. The dams were
from 5 to 50 years old and were constructed by various agencies. This survey found that:
� Dumped riprap failed in only 5 percent of the cases it was used; and failures were due
to improper size of stones.
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