able limits for an arch to form, joining the intact
cross the river. Alterations of the streambed could
ice sheets from both riverbanks and allowing the
not be made to anchor the structure to the bed
incoming floating ice to be incorporated into this
because of strict environmental regulations. There-
newly formed ice sheet.
fore, four shore anchors were used. One wire rope
When frazil flocs are present, the situation is
is attached to each anchor and joined at a junction
more complicated. Although a 100% ice cover with
plate at the approximate centerline of the river.
solid ice blocks would indicate a definite arching
Floats support the weight of this junction plate and
condition, frazil floes can be compressed, even
the two wire ropes to which the pontoons are
though the surface concentration may be consid-
attached.
ered 100%. This means that cohesion and strength
must also be considered when calculating arch-
Oil Creek
ing probability. The compacting of the frazil discs
Single-sag ice booms were tested at two sites
on Oil Creek, for three successive winters (1981-
increases in proportion to the force exerted on it.
1984). The intent of the ICS is to encourage a sta-
If there is little compression of the frazil before it
ble ice cover to form upstream of the ICS, both to
reaches a structure, the ice floe may separate eas-
capture transported surface ice and to suppress
ily. Frazil floes may also develop strength due to
ice production upstream. Unfortunately a floating
the freezing of the surface layer exposed to the air.
boom is effective in forming a stable ice cover only
The combination of compression and freezing of
if the surface velocity at the boom is less than a
the floe determines its final behavior when it meets
critical velocity. This critical velocity is determined
a resistant ice cover.
by the properties of the ice floes arriving at the
boom: thickness, shape, overall density and
BOOM TESTS
strength. If the surface velocity at the boom is
greater than this critical velocity, ice floes arriving
Allegheny River
at the boom will tend to underturn and pass un-
Oil City, located in northwestern Pennsyl-
der the boom (Gooch and Daly 1994). These con-
vania, has been plagued by ice jam flooding since
ditions typically result in Froude numbers above
the mid-1800s. In February 1979, Oil Creek flood-
0.10.
ed the downtown business district, causing an
During the first winter the river site was 58 m
estimated 0,000 in damages. As a result of this
flood the U.S. Army Engineer District, Pittsburgh,
wide and 0.6 m deep, with an average velocity
across the river of 0.4 m/s and a pool length of 98
m. Ice collection screens hanging below each boom
Oil Creek. Deck and Gooch (1981) concluded that
unit (Fig. 5) captured frazil and resulted in a 0.3-m
controlling the production of frazil ice on the Alle-
increase in water level and a 200-ft ice cover
gheny River and Oil Creek would eliminate
upstream (Fig. 3). The upstream open-water area
future flooding.
resembled a funnel, forcing the majority of the
In the winter of 1982-83 an ice boom was in-
floating frazil toward the center of the structure
stalled on the Allegheny River upstream of the
and allowing nearly all of it to pass. As the
confluence with Oil Creek to alleviate ice jam
flooding. The configuration, design load and gen-
eral design criteria for the ICS were furnished by
CRREL to the U.S. Army Engineer District, Pitts-
burgh. They developed the anchor and detailed
structural design and awarded the contracts to
fabricate and install the structure prior. The cost
of these contracts was about 0,000 (Deck and
Gooch 1984).
The Allegheny River ice boom has a multiple-
sag design (Fig. 1b) and is located about 1600 ft
(487 m) upstream of the mouth of Oil Creek at the
downstream end of a pool in the river. The ice
boom consists of 20 floating steel pontoons. Each
pontoon is attached by chains to a 60-mm-diam.
wire rope and is 613 cm long, 91 cm wide and 40
cm deep. Two 77-m spans of wire rope are used to
Figure 5. Oil Creek ice boom screens.
4
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