interval events the user would follow
Table 12. Minimum releases from Gavins Point Dam for ice-impacted
the 20% chance line to come up with
flows.
minimum recommended releases.
December
January
February
Similar information for the primary
Risk
115
1531
115
1531
115
1528
ice event months of December
Open water
7,100
7,100
7,400
7,400
6,900
6,900
through February is also provided in
Table 12.
50%
7,200
7,500
9,800
8,200
7,100
7,000
This approach could also be
20%
7,400
9,000
12,800
10,400
8,800
7,900
modified to address releases during
a specific year. It would still be based
10%
8,200
11,300
14,300
11,400
9,700
8,900
on long-term weather statistics, but
5%
10,500
13,500
15,600
12,400
10,300
9,600
could allow consideration of years
that are relatively wetter or drier than
normal. If information was available
on expected tributary inflow rates, the monthly mini-
further assuming that it is 3 January, we could esti-
mum release flow distributions, such as those pre-
mate that there is a 10% risk of discharge deficit of
6,900 ft3/s or greater occurring. Since the discharge
sented for long-term averages in Table 11, could be
in reach 2 would be 10,600 ft3/s, and the minimum
calculated for specific time periods. Combining this
required reach flow is 8,000 ft3/s, the discharge from
with the discharge deficit probability information
contained in Figure 15 would provide recom-
Gavins Point would have to be increased by 4,300
ft3/s to avoid difficulty in that reach. Further, since
mended release information similar to that in Figure
24, but tailored more to the inflow conditions ex-
the water travel time from Gavins Point to reach 7 for
pected for a given year.
January 1970 flow rates is on the order of 8 days and
the wave travel time about 6.5 days, the release
would have to begin before the normal 3- to 5-day
Short-term approach
This approach would be applicable for near-term
weather forecast period would provide reliable fore-
modification of the planned winter release schedule
casts. Should a cold weather system be anticipated,
in anticipation of an approaching cold weather pe-
the response would need to be based on known
riod. As in the long-term approach just discussed, the
water discharge distributions and a risk-based esti-
estimation of required releases from Gavins Point is
mate of cold snap severity.
again based on an acceptable level of risk. Since the
Alternately, if the accumulated freezing-degree-
travel time of releases from Gavins Point Dam to
days (AFDD) have been tracked as described earlier,
a discharge deficit estimate can be made that ac-
many of the water intakes is significantly longer than
counts for the severity of the current winter. In this
the time period of reliable weather forecasts, winter
case, the user would check the current AFDD tabu-
releases must be based on probabilistic, conservative
lated since 1 December, enter Figure 19 within that
estimates of required flows.
AFDD range and read the discharge deficit magni-
To use this approach it is first necessary to esti-
tude corresponding to the preselected level of risk. If
mate the potential severity of the event. The probable
a cold snap is anticipated that would increase the
level of discharge deficit can be determined from
AFDD to the next range in Figure 19, then values in
either Figure 15 or Figure 19. The most straight-
each range should also be considered in determining
forward approach is to use Figure 15, which provides
an appropriate response.
the probable level of discharge deficit corresponding
For example, if the January 1970 discharge sce-
to a selected level of risk based on calendar date.
nario is assumed along with 180 AFDD, a 10% level
Thus, a 10% chance of exceedence on, say, 23 Decem-
of risk would correspond to a discharge deficit of
ber would give an estimated discharge deficit of
3/s, whereas on 23 February it would be only
6,800 ft3/s. However, if it is 3 January we could
4,200 ft
2,000 ft3/s. This estimated deficit could then be
estimate (using Fig. 8a) that there is a 50% risk of a 5-
coupled with current known (or projected) levels
day cold snap exceeding 80 FDD and a 10-day cold
and flows to determine whether such a deficit might
snap exceeding 140 FDD. In that case we would
create a problem and if so, how much the Gavins
expect that in one out of two years we could go from
Point discharge should be increased.
180 to 260 AFDD within the next 5 days and to as
For example, using the long-term monthly mean
much as 320 AFFD within 10 days. Thus, the ex-
incremental discharge distributions in Table 9, an
pected discharge deficit might actually decrease
3/s, and
from 6,800 ft3/s to 6,400 and then 5,300 as the cold
assumed Gavins Point discharge of 10,000 ft
23
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