channels and harbors. Further, structures designed
1980, with any visible signs of recent erosion noted.
for commercial use are typically able to withstand
Banks were found to be eroding along 21.5 miles
forces in excess of those generated by the above
(6.2%). The erosion along approximately 15 of the
mechanisms. This leaves privately owned struc-
21.5 miles (70%) was occurring along reaches not
tures, such as docks, boathouses, boat hoists, etc.,
bordering winter navigation channels. The results
as the structures most likely to experience dam-
of the twice-yearly surveys did not conclusively
age. The majority of these private structures are
indicate whether or not winter bank erosion was
of lightweight construction sufficient to serve their
more or less than that occurring during the sum-
summertime function but not necessarily engi-
mer. Along most of the reaches, the degree of ero-
neered to withstand the load potential of winter
sion appeared to remain the same over the win-
ice.
ter and summer.
Ice conditions within the Great Lakes system,
On the St. Lawrence Seaway a study was con-
even without winter shipping, have always sub-
ducted to determine the nature and extent, if any,
jected these small structures to forces capable of
of shoreline erosion during the winter season to
damage, but over time, construction techniques
serve as a database in the event that the naviga-
evolved to provide structures that were generally
tion season was extended there (Palm 1977a,b,
competent to withstand the local ice conditions.
Palm and Cutter 1978). They developed a classi-
The degree to which the shore structures of the
fication system to define the potential for shore-
Great Lakes system may be damaged by ice varies
line erodibility based on soil type, slope, vegeta-
greatly according to the manner of ice action. Win-
tion and potential for ice action. They estimated
ter navigation, by disrupting the normal ice cover
that 28.6 miles of shoreline could be impacted by
characteristics, may aggravate any natural ice-re-
an extended navigation season, or 7% of the shore-
lated damage. Ice effects on structures typically
line length evaluated. Of 8250 ft considered to
fall into one of the following categories:
have a potential for high impact, 6200 ft was classed
Static ice forces, which arise from a struc-
as highly erodible. They also monitored 12 shore-
ture in contact with an ice sheet subject to
line sites to document any ongoing erosion. Al-
thermal expansion and contraction or steady
though some slumping of bluffs was noted, no
wind or water drag forces;
general recession of the shore profiles was evi-
Dynamic horizontal ice forces, which arise
dent during the winter season.
from ice sheets or floes that move against a
In summary, although various analyses of vessel
structure due to water currents or wind; or
effects have concluded that there is a potential
Vertical ice forces, which arise from a change
for shoreline erosion, field surveys and reviews
in water level and require the adhesion of
of historical records have not supported that con-
floating ice to structures.
clusion. For the most part, erosion rates due to
For small structures within the connecting chan-
any cause have been minor, and a comparison of
nels, dynamic horizontal and vertical forces are
erosion rates during years with and without winter
typically the critical modes of ice action.
navigation shows no appreciable difference. Both
one- and two-dimensional models have been de-
veloped to examine sediment transport caused
Dynamic horizontal ice forces
Depending on the size and strength of an ice
by drawdown and surge, and the two-dimensional
floe, the horizontal force exerted on a structure
treatment also considers propeller-induced trans-
may depend on the strength of an ice sheet and
port. All of these models have a strong empirical
its failure mode (bending, crushing or shear) or
component due to the complexities of vessel ef-
the magnitude of the force driving the ice sheet
fects and their interaction with details of the river
(wind or water current). With a vertical pile or
channel geometry and flow.
structure face, failure of the ice sheet usually oc-
curs by crushing. Current Association of State
Shore structure damage
Highway Transportation Officials standards em-
Damage to shore structures can occur due to
ploy a crushing strength of ice of 400 psi, while
water currents, water level fluctuations or ice ac-
the Canadian bridge design code provides for
tion, either alone or in combination with vessel
"effective ice strength" values ranging from 100
traffic. Since ship effects extend over a limited
to 400 psi. Thus, if there is sufficient driving force
area surrounding the ship, the potential for ves-
for the ice sheet, a pile subjected to horizontal ice
sel-related damage is primarily limited to areas
loads would have to be strong indeed.
near the shipping tracks, such as the connecting
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