Environmental Engineering Reference
In-Depth Information
Table 13.3.
Rip-rap size vs wave height.
Rip-rap size - metres
3H:1V slope
2H:1V slope
Wave height (m)
D
50
D
100
D
50
D
100
0.5
0.19
0.27
0.21
0.30
1.0
0.37
0.55
0.42
0.63
1.5
0.55
0.82
0.63
0.95
2.0
0.73
1.10
0.84
1.26
2.5
0.92
1.38
1.05
1.58
Table 13.4.
Damage to rip-rap in percent as a function of
design wave height (US Corps of Engineers,
1984a).
H/H
DO
Damage (%)
1.00
0-5
1.08
5-10
1.19
10-15
1.27
15-20
1.37
20-30
1.47
30-40
1.56
40-50
H
actual wave height; H
DO
design wave height.
For rock with a specific gravity of 2.6 the estimated sizes of rockfill are as shown in
Table 13.3. The wave height shown is the 10 percentile height. Values are given for
upstream slopes of 3H:1V and 2H:1V.
Note that specifying rip-rap grading by linear dimension can sometimes lead to argu-
ments about the acceptability of the rip-rap. This problem can be overcome by specifying
the grading by mass of particles.
13.2.1.3
Selection of design wind speed and acceptable damage
On any project the decision has to be made on an acceptable degree of damage. This will
influence selection of the design wind velocity and recurrence period. A useful guide can
be obtained by considering alternative design wind velocity recurrence intervals, e.g. 1 in
10 year, 1 in 20 year, 1 in 50 year, 1 in 100 year, and assessing the effects of using a high
recurrence design velocity using the damage estimates from Table 13.4. The figures in
Table 13.4 are for quarried rock rip-rap.
The methods outlined in Thompson and Shuttler (1976), a CIRIA publication from the
UK, are also valuable tools not only for designing the rip-rap but also for defining the
expected damage.
What damage is regarded as acceptable will depend on the importance of the structure
and the ease of access for repairs.
For existing dams, the authors' experience is that even rip-rap which is significantly smaller
than a normal design has not suffered extensive damage after 30 years of operation. If the
owner is willing to accept on-going maintenance, it is usually economic not to provide
new rip-rap to modern design standards. Nevertheless it is important to recognize that
inspection and maintenance are essential. If local failures are left without appropriate
