form. Ice nets were successfully deployed in the
in the floodplain, erosion protection must be pro-
winter of 1989-90 in front of the intakes at the
vided, particularly where the flow exits from and
Stornorrfors hydrostation on the Ume River in
returns to the main channel. A weir is usually
Sweden. In their few applications to date, frazil
needed if relief flow is to pass under a stable
nets and lines have promoted ice cover growth in
floating ice accumulation in the main channel,
channels with surface velocities as great as 3 ft/s,
since design velocities must be low enough, and
compared to 2.5 ft/s, the upper velocity limit for
the depth of flow great enough, to avoid exces-
other sheet ice retention structures.
sive thickening. These issues are further illustrat-
ed in the following sections on breakup ice con-
trol structure purposes, types and examples.
BREAKUP ICE CONTROL STRUCTURES
Purpose
Many of the previous examples illustrate the
The purpose of a breakup ice control structure
difficulty in categorizing sheet ice retention
may be simply to retain the breakup ice run at an
structures separately from structures to control
undeveloped location upstream of the historic ice
breakup ice, since many perform both roles. The
jam problem site, reducing the flood threat to set-
next section will describe structures whose main
tled areas. River towns at transition points from
function is breakup ice control.
steep to mild slope pose a particularly severe ice
The technology for breakup ice control is less
jam problem, since their location not only favors
developed and less well documented than sheet
the deposition of frazil but provides a likely stop-
ice retention technology. In many ways, the
ping place for the breakup ice run. These changes
problem is more complex. A breakup ice control
in slope often coincide with river confluences. As
structure may be designed to cause an ice jam at
mentioned in the previous section, many breakup
a desired location. Forces on a breakup ice con-
structures such as weirs have the dual purposes
trol structure are typically much greater than on
of creating an impoundment to capture and store
a sheet ice retention structure. On steep rivers
frazil during the course of the winter, as well as
with dynamic breakups, forces on the ice accu-
retaining the breakup ice run.
mulation may be sufficient to cause internal fail-
ure and thickening of the ice accumulation by
Types
shoving, rather than by juxtaposition, as with
Wire rope breakup structures have been used
sheet ice retention. Forces resulting from mo-
on small rivers in New England with limited suc-
mentum transfer, both from within the ice accu-
cess. If the intent is to create a grounded jam, a
mulation and from direct impact of ice pieces on
breakup ice control structure may be as simple as
the structure, are much greater than in the sheet
a line of boulders or piers, spaced at intervals
ice retention case. A breakup ice control structure
across a river channel. Weir structures and weirs
may cause the ice to thicken to the point where
with piers have successfully retained floating ice
flow is impinged along the bed or banks, result-
accumulations, reducing ice jam severity at
ing in scour. For this reason, a significant part of
downstream locations. In addition to their value
the cost of the structure may lie in bed and bank
in trapping and storing frazil, large dams are ex-
protection. Discharges associated with breakup
tremely effective barriers for breaking up ice
often reach flood levels, in contrast with the base
runs. Some unique structures prevent breakup
flow levels commonly associated with the freeze-
ice from passing dam spillways. Finally, struc-
up period. The design of a breakup structure
tures designed to withstand the forces generated
must address the issues of ice supply, ice storage,
by pack ice off the northern coast of Japan might
flow relief and ice accumulation stability. If the
be applied to breakup ice problems on major U.S.
breakup and annual peak flows coincide, as is of-
rivers.
ten the case, the breakup structure must be de-
signed to retain the upstream ice while passing
Examples
the flood flow. This may be achieved either by
storing ice behind a grounded jam in the main
Wire rope structures
channel while bypassing the flow in the over-
A military surplus submarine net was installed
bank, or by storing the bulk of the ice in the
on the Israel River 1 mile upstream of Lancaster,
floodplain areas while routing the flow under a
New Hampshire, in the early seventies to retain
stable, floating ice accumulation in the main
breakup ice. According to field observers, during
channel. For the grounded jam with bypass flow
ice runs the structure fills with ice pieces to act as
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