cant open water areas. The AFDD totals should be
snap continued. Of course, for the immediate future
monitored for several stations along the river and
3- to 5-day period, the weather forecast should be
estimates of the potential discharge deficit calcu-
more reliable, but longer term operations must be
lated for each to determine the appropriate Gavins
dealt with in a statistical framework.
Point release response.
Using this method, if the weather forecast pre-
dicted the approach of a major cold front such that
a significant discharge deficit event might be
expected, plots corresponding to Figures 8 and 9
could be employed to determine probable cold
This report has provided two basic approaches to
snap severity and duration for the current time
the regulation of Gavins Point Dam releases during
period for the various river reaches. The onset of a
the winter ice season: a long-term seasonal statistic-
significant discharge deficit event typically in-
based method and a short-term response method
volves a 3-day or longer cold snap with tempera-
tures reaching 12C (10F) or below. If such an
based on expected weather severity. Both methods
are statistically based, but the second approach takes
event is anticipated, then the current AFDD totals
into account the severity of the weather preceding
for the various reaches along the river, coupled
the date of analysis. Each method might be used on
with these estimates of near-term additional
either a full-time basis throughout the winter or only
FDDs, would allow an assessment of expected
when significant cold weather periods are antici-
discharge deficits at various levels of risk.
Note that the coldest weather does not always
correspond to the greatest discharge deficit levels.
Initially, as AFDDs increase the magnitude or the
probable discharge deficit event also increases.
However, as the total AFDD continues to increase,
Ashton, G.D. (1988) Ice cover formation down-
the magnitude of a discharge deficit event begins
stream of a large reservoir with variable release. In
to decrease in response to water surface area al-
Proceedings of the 9th International Symposium on Ice,
ready being covered by ice such that the potential
Sapporo, Japan, August 2327, International Asso-
or additional water storage begins to decrease.
ciation for Hydraulic Research, vol. 2, p. 189198.
Further, the location where the discharge re-
Carey, K.L. (1966) Observed configuration and
sponds to a cold snap can vary through the winter.
computed roughness of the underside of river ice,
Figure 25 shows probable cumulative freezing-
St. Croix River, Wisconsin. U.S. Geological Sur-
degree-day totals in 2-week time intervals. For
vey, Professional Paper 550-B, p. B192198.
example, at the 50% risk level, the 180 AFFD total
USACE (1990) Missouri River master water con-
used in the January 1970 example above would be
trol manual review and update--Draft phase 2
reached in late December at Yankton, late January
project management plan. U.S. Army Corps of
at Nebraska City, and not at all at Booneville.
Engineers, Missouri River Division, September.
Thus, depending on the pattern of an indi-
Wuebben, J.L. (1986) A laboratory study of flow in
vidual winter's weather, we might find that the
an ice-covered sand bed channel. In Proceedings of
magnitude of a discharge deficit event (at a given
the IAHR Symposium on Ice, Iowa City, Iowa, August
risk level) is greater in reach 1 early in the season,
1822, International Association for Hydraulic Re-
but greater in reach 4 at some later date when reach
search, vol. 1, p. 314.
1 is largely ice-covered and reach 4 still has signifi-