Perhaps the best strategy for reducing flood
location. By developing low overbank areas where
losses is to keep people and property out of the
ice can easily leave the channel during breakup,
floodplains. Proper land-use planning would dra-
perhaps supplemented by dikes or booms to redi-
matically reduce the flood damage potential. This
rect ice movement, the volume of ice passing
is particularly valuable in areas which experience
downstream can be substantially reduced. The
chronic flooding. If such land-use regulation is
ice left behind settles in side channels, on flood-
not feasible, floodproofing should be considered.
plains or on the riverbanks.
There are four basic types of floodproofing to min-
Diversion channels can be built to bypass ice
imize damage to individual structures during
and water flow around the normal jamming sites,
floods: raising or relocating a building, barrier
lowering the upstream stage. If such diversion
construction, dry floodproofing and wet flood-
channels are used, they should be designed to
proofing. Specific techniques of floodproofing are
remain dry except during flood events so they
presented in the U.S. Army Corps of Engineers
will be available to function as open-water chan-
manual on floodproofing, Flood Proofing Techniques,
nels and not add to the ice supply reaching down-
Programs and References (USACE 1991).
stream ice-jam sites.
Ice-retention structures are used to control
freeze-up or breakup jams by promoting the initi-
Advance measures
Mitigation measures that are put into place in
ation of an ice jam at a suitable location where
anticipation of actual ice-jam flooding are known
flooding will create little or no damage. Fragment-
as advance measures. Some emergency measures,
ed ice is captured and retained upstream from the
such as ice removal, may also be initiated in ad-
retention structure to create the ice jam. A variety
vance of flooding. Monitoring and detection
of structures can serve as ice-retention structures,
should be addressed in any ice-jam mitigation
ranging from suspended structures such as a sub-
effort. Ice-cover breakup, jam formation and jam
marine net, to structures that project from the
progression are particularly important processes
streambed, such as concrete piers placed at regu-
to monitor. Simple remote gages to collect data on
lar intervals across the stream. Provision for ice-
river-ice movement and breakup are useful.
storage zones or diversion channels may also be
Water-level gages can be monitored to detect rap-
required to limit the rise in upstream water level
id increases in river stage, which often precede
and corresponding load on the structure elements
ice breakup. Automated temperature sensors help
as well as the upstream flooding potential. Sus-
to verify whether conditions are conducive to ice-
pended structures are generally more suited to
jam formation or breakup.
smaller rivers and streams. Bed and bank scour
Ice motion detectors can be imbedded in intact
must be considered in the design of ice-retention
ice covers prior to breakup to warn of the initia-
structures.
tion of breakup upstream from a likely jam site.
Thermal control of ice jams uses an existing
These can be connected to inexpensive burglar
source of warm water to melt or thin a down-
alarm units that will automatically dial preselect-
stream ice cover or ice jam. Water even a fraction
ed telephone numbers when the unit detects ice
of a degree above freezing can be quite effective
motion (Fig. 22), or existing USGS gages can be
in melting ice over a period of days or weeks
augmented with telemetry transmitters that send
(Wuebben and Gagnon 1995). External heat sourc-
ice motion data directly to a local monitoring
es include cooling water effluent from thermal
center or to State and Federal agencies (e.g., NWS
power plants, wastewater treatment plant efflu-
or USGS) either through telephone, radio or satel-
ent and groundwater. The thermal reserve pro-
lite (Zufelt 1993).
vided by water in nearby lakes and large reser-
Ice cutting involves creating slots in the ice,
voirs may also be a source of warm water for
either mechanically, using a chainsaw, trenching
thermal control. Because water reaches its maxi-
mum density at about 39F, colder water in lakes
machine, backhoe or other convenient device, or
thermally, using warm water or a substance that
tends to stratify above warmer water. An ice cov-
reacts chemically with the ice, melting it. The slots
er can form on the water surface even though the
can be made to a partial depth or can extend the
water at depth is still well above freezing. Warm
full depth of the ice. They may follow the natural
water can be brought to the surface using air bub-
thalweg of the river channel, be cut along the
blers, pumps or flow enhancers, or a low-level
edges of the channel to facilitate movement of the
outlet in a dam may be used to release warm
ice sheet, or be cut into the ice in a pattern de-
water.
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