14 10 3
station, say Yankton, and a given month, say
January 1970, all of the mean daily flows were
1%
12
averaged to come up with a mean monthly
daily flow. The mean flows for all months of
10
January from 1955 through 1988 at that station
5%
were likewise determined, and then averaged
8
to determine a mean of the mean monthly
10%
discharges for all January flows at Yankton.
6
The starting year of 1955 was selected, as that
20%
4
was the year storage behind Gavins Point
Dam was first available for regulation of
2
downstream flows. Monthly mean dis-
50%
charges prior to 1955 were normally much
0
0
100
200
300
400
500
600
700
lower during winter months.
AFDD
This averaging process was repeated for all
Figure 19. Discharge deficit risk associated with AFDD prob-
months between October and March and for
all 10 gaging stations. The results of this pro-
cess for the months of December through
March are presented in Table 8. This information is
CORRELATION OF
also presented graphically in Figure 20 in terms of
ICE-IMPACTED DISCHARGE
discharge variation with drainage area below
AND RIVER STAGES
Gavins Point Dam. Discharge measured at Yankton
is considered equivalent to releases from the dam.
Baseline flows
For comparison, an actual, single-day flow distribu-
While the objective of the study is to determine the
tion from a low flow period in January 1970 that had
required releases from Gavins Point Dam to ensure
the same Gavins Point release as the long-term mean-
adequate flows for water intakes downstream, the
of-means is included in both Figure 20 and Table 8. A
performance of those intakes is dependent not only
comparison of this single-day flow distribution with
on those releases but also the incremental discharge
that of the January mean-of-means distribution
entering the river from tributary sources. When de-
termining an appropriate release in anticipation of a
shows good correlation.
cold weather period in the near future, decisions can
Since the release from Gavins Point Dam is not
be based on the water levels and flows in the river at
directly coupled to inflows elsewhere in the river
the time of its occurrence. However, long-term plan-
downstream, those releases have been assumed to
ning efforts must rely on predicted flow distribu-
constitute an independent variable, distinct from
tions.
inflows elsewhere in the system. Under this assump-
tion, whether 1,000 or 30,000 ft3/s was to be released
For each of the 10 discharge gaging stations listed
in Table 1, mean daily flow information was sorted
from Gavins Point, downstream incremental in-
by month to determine representative variations in
flows would remain unchanged. Therefore, in Table
flow along the river. That is, for a given gaging
9 the Yankton discharge for each month shown in
Table 8. Mean monthly discharge distributions.
Discharge (ft3/s), mean of means
Q (ft3/s)
River
Station
mile
December
January
February
March
Jan 70
Yankton
805
17,500
15,000
14,600
18,600
15,000
Sioux City
732
18,400
15,600
15,700
22,000
16,500
Omaha
616
20,000
16,700
18,700
26,900
17,000
Nebraska City
562.6
24,500
20,200
25,500
37,500
19,000
Rulo
498
26,000
21,300
27,100
40,700
19,700
St. Joseph
448
28,200
23,100
30,000
45,000
20,750
Kansas City
366.1
33,200
27,000
36,500
54,525
25,000
Waverly
293.5
34,100
27,900
37,600
55,000
25,500
Booneville
196.6
41,000
33,200
46,500
69,500
34,800
Hermann
97.9
58,100
46,400
64,600
96,000
40,000
17
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