Table 5. Ice jam potential rating factors.
Indicated potential
Term
Range
Low
High
Fahrenheit freezing
918 / 3300
<1700
>2600
degree-days
FDDmax
135 / 180+
<150
>165
Qmax
145 / 175
<155
>170
Qmax FDDmax
13 / +30
<8 or >+10
>5 or <+7
Qb (K cfs)
17 / 124
<25 or >90
>30 or <70
Garrison stage (ft)
1798 / 1844
<1835
>1840
Total snowfall (in.)
5 / 60
<20
>40
Snowfall timing
N.A.
<5 in. after
>10 in. after JD =90
JD=90
> 5 in. after JD =120
runoff sufficient to cause breakup came when the
Based on these criteria, values for the various
ice was still thick and strong. Because of the low
terms that would indicate a high correlation with
topographic relief within the Irrigation District,
ice jamming are indicated by a "+" in the table,
relatively small jams will cause overbank flooding,
while values associated with a lesser risk are de-
as the overbanks are generally within 5 ft or so of
noted with a "." Note that three terms, FDD,
normal winter stages.
ble weighted.
Techniques for mitigating ice-jam-related floods
In the final three columns the number of "+"
can be grouped into those measures that are de-
and "" symbols have been added and a ratio cal-
signed and implemented to alleviate future prob-
culated. Values less than one would indicate lesser
lems and those that take place in response to a jam
ice jam flooding potential, while values greater
already in existence. Advance measures range
than one would indicate a greater potential. The
from alteration of river ice formation processes to
lowest ratio for a known ice jam is 1.5 in 1978, but
provisions for structural containment of ice and
three years in which no jams were reported (1974,
flood waters. Although the mild ice conditions en-
1979 and 1984) have values greater than 1.5. The
countered during 1991-92 required no response for
highest ratio, 5.00, occurred in 1984, which had no
mitigation of ice-related flooding, short-term ad-
vance measures would appear to be technically
reported jam. However, the breakup period dis-
feasible. Since there are a relative few problem jam-
charge was only 19,500 cfs, less than the flow re-
ming locations in the study area, and in view of the
quired to initiate ice cover breakup. While these
smooth ice cover and favorable weather condi-
criteria are empirical and approximate (there is no
tions, structural weakening of the ice (either me-
precedent for this prediction scheme), their corre-
chanically or by dusting) could ease the passage of
lation with past ice jam events is quite clear. More
the ice run through these resistant reaches. Had the
importantly, tabulating these or comparable terms
advance preparations failed and ice jams formed,
during future winter seasons should give a useful
emergency flood-fighting measures would also
indication of the potential for ice-related flooding
have been possible.
in the spring.
The smooth, single-layer ice sheet observed on
the river during the winter of 1991-92 indicates
MITIGATION TECHNIQUES
that ice growth (and hence the total ice volume
While the winter of 1991-92 was warmer than
available to form a spring breakup jam) is domi-
normal and had less snowfall and no significant
nated by thermal growth once the river is ice cov-
ice jamming, the ice breakup observed may still be
ered, rather than by accumulation or jamming pro-
indicative of breakup processes in a majority of
cesses during the freezeup period. Based on the
years. We were able to identify only six significant
observations of 1991-92 and the information col-
ice jam events in the last 40 years, although more
lected in the historical review, it is clear that long-
events may have occurred. For those years in
term mitigation techniques will have to focus on
which jams did occur, the weather was colder
the spring breakup period rather than the control
and/or snowier than average, and an increase in
of ice formation. Basic flood control alternatives
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