Environmental Engineering Reference
In-Depth Information
(e.g. Thomson dam in Victoria where granite was used for the rip rap zone as distinct
from sandstone/siltstone used tor the rest of the rockfill). Only in dams where severe wave
action is anticipated and Zone 2B rockfill is too small or not sufficiently durable would a
separate layer be placed.
Where the reservoir is operated so that the water level is maintained at a high level, i.e.
there is a minimum operating level well above the base of the dam, it may be possible to pro-
vide lesser or no rip-rap protection on the lower part of the dam. Because wave action affects
about two times the wave height below the water level, rip-rap should be provided to mini-
mum operating level less two times the design wave height. A small berm should be provided
at this level to support the rip-rap layer and prevent undermining of the rip-rap by erosion
when the reservoir first fills as shown in
Figure 13.5(c)
.
On some smaller dams, particularly those on mine sites where earth and rock moving
equipment and waste rock are readily available, rip-rap may be designed in the knowledge
that damage will occur due to larger waves or to breakdown of non durable rock, but that
the damage can be readily repaired.
13.2.1.2
Sizing and layer thickness
The sizing of rock needed for rip-rap, the layer thickness required, and filter layer require-
ments are determined from the size of waves expected on the reservoir and the nature of
the earthfill or rockfill under the rip-rap. The procedure suggested for sizing of rip-rap is
that given in the US Corps of Engineers Shore Protection Manual (US Corps Engineers,
1984a). The steps involved are:
- Determine the design wave height H. This is taken on the average of the top 10% of
the wave, which is 1.27 times the significant wave height determined as outlined in
Section 13.1.3.
- Calculate the weight of the graded rock in the rip-rap from:
H
K S 1) ot
3
r
W
(13.5)
50
3
RR
r
where W
50
weight in kilonewtons of the 50 percent size in the rip-rap;
r
unit
weight of the rip-rap rock substance in kN/m
3
; H
design wave height in metres;
S
r
specific gravity of the rip-rap rock relative to the water in the dam (S
r
r
/
w
);
angle of upstream slope of the dam measured from the horizontal in degrees;
K
RR
stability coefficient
2.5 for angular quarried rock and non breaking waves.
The maximum weight of graded rip-rap (W
100
) is 4W
50
and the minimum 0.125W
50
.
This is equivalent to the maximum size being 1.5 times the D
50
size, and minimum size 0.5
times D
50
. In practice if smaller minimum size rock is included it may be washed out under
wave action. These values allow for less than 5% damage under the design wave. US
Corps of Engineers (1984a) give factors which allow for greater damage.
For single size rip-rap, K
RR
is replaced by K
D
with K
D
2.4 for smooth rounded rock
and 4.0 for rough angular rock. These factors assume a rip-rap layer thickness allowing
two layers of rock.
The equivalent sieve size of the rip-rap is approximately 1.15(W/
r
)
0.33
. Where a thin
rip-rap layer is being adopted, the average layer thickness can be calculated from
0.33
r K(W/
)
(13.6)
r
where r
average layer thickness in metres; n
number of sub layers of rip-rap weight W
in kN in the layer; K
1.02 for smooth rounded rock and1.0 for angular quarried rock.
