200
150
100
0
50
100
150
200
100
150
200
Julian Day of Maximum AFDD
Julian Day of Maximum Discharge
b. Date of occurrence of the maximum ice-related stage
a. Date of occurrence of the maximum ice-related stage vs.
vs. the date of the maximum AFDD.
the date of the maximum discharge.
2
1
0
20
0
20
40
60
40
Date of Maximum Discharge Date of Maximum AFDD
c. Normalized stage vs. the difference between the date
of maximum stage and the date of the maximum AFDD.
Figure 10. Relationships among dates of occurrence of maximum ice-related stage, maximum discharge and
maximum AFDD for January through March, water years 1950 through 1990 (October 1 = Julian Day 1).
(Missouri River Basin Commission 1975), the av-
plus or minus 50% of the flood stage at a given
erage Platte River water slope decreases from 4.6
location (normalized stage 0.51.5). However, ice
ft/mile upstream of the Elkhorn confluence to 3.9
events may result in stages that are almost double
ft/mile downstream of the Elkhorn to the mouth.
flood stages under worst-case conditions, such as
The change in slope corresponds closely with his-
those experienced on the Platte River at North
toric jam locations and may be a contributing fac-
Bend in 1978. In general, ice events occur only
tor in the formation of breakup jams downstream
after at least 400 FDD have accumulated.
of the Elkhorn confluence. A second marked
change in slope occurs at a point approximately
Hydraulic analysis
2.5 miles upstream of the Highway 64 bridge,
Hydraulic analyses of ice-affected rivers can
where the stream slope decreases from 5.1 to 4.6
often reveal possible causes of jam formation at
ft/mile. This location also corresponds closely
particular locations. According to profiles devel-
with the observed jamming location along Union
oped for the Platte River Basin Level B Study
20
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