cessfully been used along the Canadian and the
an event is believed to occur rarely, the repair to
Alaskan offshore regions. The rubble ice may not
ice-damaged riprap was done in a conventional
remain stable in water depths greater than 10 m
manner, rather than adopting a different, perhaps
(33 ft), and an underwater berm may be needed
more costly, design.
for the caisson-retained islands to stabilize the pro-
Wuebben (1995) reviewed the literature on ice
tective rubble ice zone.
effects on riprap, river hydraulics and scouring of
Some of the types of slope protection armor that
river beds. He pointed out that nonuniform or un-
have been used are sandbags (Gadd 1988), con-
steady flows during ice jams can cause serious
crete mats (Leidersdorf 1988), riprap and armor
scour and degrade riprap performance. Ice jams
stones (McDonald 1988), and large precast con-
can cause stages comparable to rare open water
crete armor units (Collins 1988). In Appendix B,
events, and it is necessary to consider protection
we discuss the experiences with each of these types
of upper limits of banks.
of slope protection armor.
Ice can damage a slope protection system by
Matheson (1988) surveyed the performance of
direct impact and shoving action. Even when the
riprap slope protection of Canadian and other
damage caused by direct ice action on an offshore
dams. He found that the dominant mechanism of
artificial island may not be extensive, it can be
riprap deterioration was freeze-thaw action on
exacerbated by the wave action during storms in
riprap consisting of bedded sedimentary rocks,
the summer months (Gadd 1988). In a similar way,
resulting in delamination and breakdown. While
river ice can damage slope protection armor at a
the sedimentary limestone and sandstone were
few places, and this damage can be extended over
prone to breakdown, such degradation was found
a large area by high water levels and currents fol-
to be negligible in the case of igneous and meta-
lowing the breakup of ice.
morphic rocks, e.g., granite. He described the fol-
When an ice sheet moves against a sloping bank
lowing ice-related mechanisms that cause riprap
protected with a riprap armor, the ice may ride up
damage: 1) "plucking" of riprap by rising and fall-
or pile up. Sodhi et al. (1983) conducted small-scale
ing water levels, and 2) ice shoving. He found that
experimental studies to understand the factors that
the plucking action can affect riprap with rock size
lead ice to pileup and found that ice pileup occurs
up to approximately the ice thickness, and that the
more often on a rough sloping surface than on a
riprap damage due to winter drawdown was sig-
smooth surface. At present, the set of conditions
nificant for steeper slopes (1 vertical:1.75 hortizon-
or parameters that can predict a ride-up or a pile-
tal) and minimal for shallow slopes (1V:3H).
up of an ice sheet is not exactly known. Extensive
Doyle (1988) described the damage to public
ride-up of an ice sheet is not preferred because it
and private property, including the destruction of
may damage onshore facilities. A rough sloping
a bridge and several riprapped banks, from a sud-
surface may promote an ice pileup because the
den river ice breakup in January 1984 on the Nicola
forces required for continued ride-up of an ice
River and its two main tributaries in British Co-
sheet become large. Croasdale et al. (1988) dis-
lumbia, Canada. He attributed all damage either
cussed the strategies to be employed in the de-
to the severe ice run within the channel or to flow
sign of a sloping surface to promote an ice pileup.
forced out over the floodplain by ice jams. The ice
McDonald (1988) discussed the ice forces on a
run, caused by runoff generated by warm weath-
coastal structure caused by a moving ice floe or
er moving into the watershed after extremely cold
by thermal expansion of a restrained ice sheet. The
weather, damaged seven riprapped bends in the
global force exerted by an ice sheet against a large
Coldwater and Nicola Rivers. The riprap protec-
structure is not uniformly distributed across the
tion on 1V:2H slope banks consisted of a single
width of the structure. The force (or pressure) that
layer of well-graded angular rocks having a max-
may develop on an individual armor unit may be
imum (D100) and median (D50) diameter of 750
very high at one place and almost negligible at
900 mm and 500600 mm, respectively. His field
another place. This is attributed to nonsimulta-
evidence suggests that the running ice took out
neous local ice failure, resulting in variation of ice
individual rocks from riprap-protected banks,
pressure with respect to time and location. The
causing total destruction of the protective works.
normal component of this force may cause a pres-
The damage sustained by trees in the path of run-
sure buildup to the ice crushing pressure, which
ning ice on the floodplains also would tend to cor-
is less than that sufficient to damage most stones.
roborate the conclusion that running ice was able
The shear component of ice force may act on an
to remove moderately heavy rocks. Because such
individual stone protruding into the ice mass, and
4
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