every ice-jam location within the study area. The
techniques within the study area requires more
ice monitoring conducted under the aegis of the
detailed analysis than is possible within the scope
Nebraska NRC should prove useful in the early
of the present study. The available information
detection of ice breakup. The use of ice-motion
suggests that levees could prove effective in the
detectors will provide warning at a number of
Big Island area. The existing levee at Leshara
locations in the study area. For example, a series
should be evaluated to see what improvements
of ice-motion detectors on the Loup River up-
might increase its level of protection. An ice boom
stream from Columbus will provide information
located just upstream from the Loup River Power
on the timing and location of ice breakup, which
Canal headworks could provide frazil-ice control,
can contribute to damaging jams at Columbus.
which would decrease somewhat the amount of
Knowledge of ice conditions on the Loup and
ice available to downstream jams and might re-
Platte Rivers downstream from Columbus and
sult in more efficient operation of the canal. The
estimates of broken ice volumes could be used to
effects of the canal operation must be studied in
assess the damage potential of a jam. The addi-
detail before any conclusion can be reached as to
tional lead time provided by ice-motion detectors
the impacts of fluctuations in stage and discharge
will increase the effectiveness of early warning
on the Loup River.
and other advance measures, such as sandbag-
ging or evacuation. Emergency measures such as
Ice dusting as a
blasting or ice removal are most effective if they
mitigation alternative
are begun soon after a jam has formed, so early
Ice dusting refers to the practice of spreading
detection is very important if such measures are
solid material on top of the ice surface to acceler-
contemplated. Particular locations that would ben-
ate melting and decay of the ice. Dusting was
efit from the use of ice-motion detectors include
performed on the Platte River in 1979 at four loca-
the Loup River near Columbus and the Platte River
tions (USAED, Omaha 1979). The dusting opera-
and lower Elkhorn River in the vicinity of their
tion was done under the direction of the Nebras-
confluence.
ka Civil Defense Office and the Omaha District
Water-stage recorders can also be used to alert
Corps of Engineers; the Cold Regions Research
authorities when the stage reaches a certain level
and Engineering Laboratory (CRREL) provided
or increases rapidly. Water-stage recorders could
technical assistance. The ice on the Platte River
be used singly or in conjunction with ice motion
just prior to breakup was as much as 3 ft thick in
detectors to provide early warning. Placing wa-
1979, and the snowpack water equivalent was es-
ter-stage recorders both upstream and down-
timated at 2.53 inches in late February. The re-
stream from a known ice-jam location will allow
sults of the dusting were inconclusive because of
the observer to detect the formation of a jam which
the insufficient time between dusting and break-
causes backwater. If such an installation is not
up, especially at the mouth of Salt Creek, where
possible, jam formation can also be detected by
breakup occurred two days after application. The
combining discharge data from USGS gages and
report noted that there may have been some posi-
information from a single water-stage recorder
tive effect in that no significant jamming prob-
upstream from a known jam location. Two likely
lems occurred at the dusting sites, which had his-
sites for water-stage recorders are Big Island and
torically been problem areas, and jamming
Columbus.
occurred at other sites that were not dusted. The
Other nonstructural ice-jam mitigation mea-
report did conclude that dusting will reduce the
sures that are suitable for application within the
strength of river ice and reduce the problem of
study area are dusting, icebreaking, mechanical
jamming if an adequate time elapses between dust-
removal and blasting. The formation and progres-
ing and breakup.
sion characteristics of a particular jam must be
Materials. The material used in 1979 for dust-
known in order to design an effective plan of ap-
ing was black-bottom slag from the Nebraska Pub-
plication. Dusting is best used when the ice jam is
lic Power District power plant at Hallam, Nebras-
caused by a competent ice sheet that impedes the
ka. This material was chosen because of its dark
movement of broken ice. Icebreaking, mechanical
color and relatively low cost. The target applica-
tion rate was 0.5 lb/yd2, and it was applied using
removal and blasting can be used to create a chan-
airplanes equipped with crop-dusting equipment.
nel through a competent ice cover or through a
The material was chemically analyzed for envi-
jam to increase flow and ice conveyance.
ronmental reasons (Table 18). If this material is
The application of structural ice-jam mitigation
42
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