5
10
10 4
Figure 13. Average daily discharge at
the time of the maximum ice stage vs.
Julian Day. The line represents the rela-
3
10
0
40
80
120
160
200
tionship Q = 0.39 (JD)1.96.
Julian Day (1 Oct is day 1)
3
11 x 10
10
9
8
7
6
Figure 14. Minimum discharge required
5
0
50
100
150
200
for ice jam vs. Julian Day.
Julian Day (1 Oct is day 1)
ferent from the actual event) or that an ice event
recorded maximum ice stage. The third column
would occur and one did not occur. Only once (in
indicates whether the criteria predicted an ice
water year 1961) did the criteria predict that no
event within one week of the actual recorded max-
ice event would occur and one actually did occur.
imum ice stage. A false prediction is defined as
The ice-related stage of 6.93 ft in 1961, the second
predicting a date farther than one week from the
smallest ice-related stage in the 26 years with ice
actual date, or predicting an event and none oc-
events, was more than a foot below flood stage
curred (such as in 1957). A negative prediction is
and may be considered a nondamaging ice event.
defined as predicting no ice event and none oc-
Considering the lack of detailed ice data, the
curred. A false negative prediction is defined as
model performed quite well in hindcasting ice
predicting no ice event and an ice event did occur.
events. This is particularly true if one considers
For years when a maximum ice stage was re-
that the model may have accurately predicted ice
corded, the criteria identified 65% of the ice events
events for years where ice data at North Bend
within one week of the time they actually occurred.
was particularly sketchy, such as 1966 and 1969,
For the whole period the criteria predicted 55% of
when serious ice jams occurred elsewhere in the
the ice events within one week of occurrence or
study area (Table 13). The model overpredicts the
correctly predicted that no ice events would oc-
occurrence of ice jams at North Bend. However, it
cur. Also, 41% of the time, the criteria either pre-
does provide good initial predictive model capa-
dicted the wrong date (more than seven days dif-
28
Previous Page
