upstream ice approached the structure, the ice vel-
ice boom and the failure of the single-sag Oil Creek
ocity increased because of the constriction caused
boom initially spurred interest in capture efficiency
by the ice blockage in the channel, reducing the
as a function of boom geometry. The Allegheny
structure's ability to create an arch as the floating
River's 650-ft (198-m) channel width required a
ice made contact with the intact ice cover. The fra-
double-sag boom design to position the boom in
zil flocs would shove against each other and com-
the low-velocity reach of the river without using
press, decreasing their diameter and allowing
riverbed anchors. This design directs the very large
them to pass through the constriction. At this point
flocs or floating frazil ice pans to the shore, where
the structure could no longer capture ice or influ-
the pans accumulate and thicken due to the shov-
ence the ice cover progression upstream. This
ing action created by the increase in hydrostatic
could explain the poor performance of the first Oil
forces acting on the ice accumulation. The ice cap-
Creek structure. Since frazil is difficult to capture
ture efficiency is increased, and the load on the
if the river velocity is above 2.0 ft/s (0.6 m/s) and
structure is reduced as the ice comes in contact
the water depth is less than 1.0 ft (0.3 m), the use
with the shore and riverbed. This concept com-
of a boom alone to initiate the formation of an ice
bines the direction feature of the shear boom and
cover incorporating frazil ice may be ineffective
the collection and thickening capability of the for-
without some source of hydraulic control to re-
mation boom. As a result of this geometry, the ice
duce the velocity (Deck 1984).
thickening process increases the upstream water
In December 1983, researchers selected a site
level, reduces the flow velocity upstream and nar-
with a water depth 0.8 ft (0.2 m) deeper than the
rows the open-water channel. This eventually al-
previous location but with only a 76-m pool length.
lows ice to arch more easily, joining the right- and
By increasing the depth the resulting velocity
left-bank ice sheets into one solid ice cover (Deck
would be reduced, thereby improving the ice for-
and Gooch 1983).
mation potential. A stable ice cover formed in four
hours, with 70% of the cross-sectional flow area
Salmon River
restricted. All additional incoming frazil ice was
Two single-sag ice booms were tested on the
submerged, however, and transported down-
Salmon River in Salmon, Idaho. Ice jams on the
stream. The initial capture efficiency was excellent,
Salmon River have caused millions of dollars in
but the length of the pool and the stage rise were
damages in recent years. The Salmon River is char-
inadequate to allow the ice cover to advance
acterized by a series of rapids and pools. The bed
through the steeper upstream reach. As a result,
slope is 0.003 in the study reach and therefore very
the ice boom failed to effectively capture frazil ice
steep. Research efforts for a structural solution to
and cause significant ice cover formation.
ice jam flooding in Salmon have been the major
In 1984 a more conventional and costly .2-
focus. Because of cost and environmental con-
million, 306-ft (93-m) fixed-crest concrete weir with
straints, the most favorable ice capture and freeze-
a 45-ft (13.7-m) bascule gate was designed by the
up structure design with the least impact on the
Pittsburgh District. The Oil Creek ICS, which was
hydraulic conditions in the river is an ice forma-
completed in 1988, provided the hydraulic con-
tion boom.
trol needed to form an ice cover. The gate is raised
The site chosen for an ICS was located about
by mid-December, creating a 5.0-ft- (1.5-m-) deep
nine miles upstream from the city of Salmon. At
pool, lowering the Froude number from above 0.10
this point the river is about 85 m wide and 1 m
to 0.04 and increasing the length of pool upstream
deep, with a pool length of 450 m. Surface veloci-
of the structure by a factor of 10 from previous
ties are 2.03.0 ft/s (0.60.9 m/s) at the expected
winter flows of 25.037.0 m3/s (8831300 cfs). This
test sites (Table 1). A timber single-sag boom de-
ployed approximately 200 ft upstream of the struc-
site has a Froude number of 0.20 and would be
ture captured ice to form a stable ice cover during
considered unsuitable by the current Froude cri-
high flows. The ice boom is no longer used be-
terion for ice cover formation (Fig. 6). A single-
cause of continual cable failures. However, the
sag formation boom was tested at this site during
weir by itself provides the hydraulic control need-
the winter of 1989-90. Field measurements and
ed to form an adequate ice cover each winter. There
observations at the site included river cross-sec-
has been no ice jam flooding on Oil Creek since
tion geometry, ice boom loading, orientation and
the installation of the Allegheny River ice boom
river velocity distribution. The ice boom failed,
in 1982-83.
allowing the incoming ice to pass under the struc-
The success of the double-sag Allegheny River
ture and continue downstream. This indicated that
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