found to be more resistant but not immune to
the 12 docks monitored were determined to have
damage. Improved structure designs and proper
been displaced vertically and/or horizontally, but
control of vessel speed could minimize damages.
most displacements were on the order of 0.1 ft
with a maximum of 0.2 ft. Although it was stated
Summary
that the measurements were accurate to 0.001 ft,
Although various analyses of vessel effects have
the reported displacements may actually reflect
concluded that there is a potential for shoreline
only the true accuracy of the survey. It is particu-
erosion, field surveys and reviews of historical
larly interesting to note that all recorded vertical
records have not supported that conclusion. For
displacements indicated that the structures were
the most part, erosion rates due to any cause have
sinking as the winter progressed. This is contrary
been minor, and a comparison of erosion rates
to the typically reported lifting or "jacking" of
during years with and without winter navigation
pile structures by vertical ice forces.
shows no clear trend. High water levels, how-
During 1978, their second winter season, 8 of
ever, have been associated with periods of in-
the 18 structures monitored during the first sea-
creased shoreline recession. There is a definite
son showed vertical and/or horizontal displace-
potential for sediment transport during vessel
ments in excess of 0.1 ft at one or more locations.
passage, and it appears that this material is often
Of six additional structures monitored during the
transported offshore towards the channel. Since
second season only, five exhibited measurable
ambient sediment transport in the connecting
movement. Structures not experiencing damage
channels is quite low, this would constitute a
were supported by concrete or substantial tim-
permanent (though apparently small) net loss of
ber cribs. Of the docks experiencing displacement,
soil from the nearshore zone. The transport could
one was pile supported and the remainder were
also have biological ramifications through in-
timber cribs. For the survey points monitored on
creases in suspended solids and turbidity; these
these docks, 109 displacements in excess of 0.1 ft
topics will be discussed later.
were recorded. For events with vertical displace-
Information from periods without navigation
ment, 13 were up and 19 were down. For hori-
revealed little or no ice-related damage to even
zontal motion events, 6 displacements were on-
poorly constructed structures. There was a docu-
shore and 20 were offshore; 26 were upriver and
mented potential for damage to private shore struc-
22 were downriver. The maximum vertical dis-
tures due to vessel passage in ice, particularly the
placements were about 0.6 ft and occurred in both
light-duty pile-supported docks constructed by
directions. The maximum horizontal displacement
recreational boaters. Because there are thousands
was 0.9 ft onshore and 1.8 ft downstream. One
of these structures on the connecting channels,
incident of total destruction was cited, but no
the previous damage estimates have run into
description of cause was provided.
millions of dollars. Well-designed structures were
In summary, there is a documented potential
found to be significantly more resistant to dam-
for shore structure damage due to vessel passage
age but not immune. However, numerous stud-
in ice. While little or no damage was observed
ies cited in the text have pointed out the strong
during periods without winter navigation, exten-
relation between the magnitude of vessel effects
sive damage or complete destruction of a num-
and vessel speeds. Speed limits developed with
ber of structures was documented on the St. Marys
regard to sediment transport and dock damage
River during the demonstration program. Because
could essentially eliminate measurable damage.
it is necessary to consider details of construction
In most cases existing limits were found adequate.
techniques, ice conditions, ship passage charac-
teristics and site layouts, quantitatively predict-
ing damage for the thousands of privately owned
OIL AND HAZARDOUS
structures on the Great Lakes connecting chan-
SUBSTANCE SPILLS
nels is a nearly impossible task. The most com-
prehensive analysis conducted so far relied on
Spills of oil or hazardous substances from tran-
probabilistic estimates of damages based on broad
siting vessels during the season extension pro-
categories of structure types, their values and ice
gram represent a potential adverse impact to the
conditions in the general area. Lightly constructed,
environment. Because of the combination of cold
pile-supported structures were found to be most
weather and ice, spills in winter present additional
susceptible to damage. More substantial pile struc-
difficulties in tracking, recovery and mitigation.
tures or other designs (such as timber cribs) were
Because of this, the potential for spills during ex-
20
Previous Page
