14
ERDC/CRREL TR-02-14
Ice formation
With Fort Peck Reservoir as the source of flow, the river begins forming its
ice cover at several locations well downstream of the dam. The flow rate and
temperature of the water released from the reservoir influence where and how ice
covers form on the lower Missouri. The river's sinuous-point-bar and sinuous-
braided morphologies ensure that the flow through the Fort Peck reach is well
mixed over the flow depth, so its temperature and thermal energy are practically
homogeneous over its depth. Those morphologies also produce substantial varia-
tions in flow velocity within reaches, and they provide numerous locations where
frazil ice slush and pans can congest and form an ice cover or initiate an ice jam
as thin, unstable ice covers break up. Once a cover is initiated, it propagates
upstream.
The flow rate and temperature of the water influence where ice first appears.
The higher the flow rate and temperature of the flow released into the river, the
greater the distance of flow before ice cover formation begins. The local channel
morphology influences where the river actually begins forming its ice cover. The
river's mild slope (about 1.6 104 to 1.8 104) and moderate average velocities
facilitate the formation of an initial cover of juxtaposed ice pans and floes over
most reaches. Average velocities at Wolf Point and Culbertson are about 2.3 and
2.0 ft/s, respectively (Pokrefke et al. 1998). At some sections and flow rates,
velocities may be high enough (nominally in excess of 6 ft/s) that ice pans get
subducted under the ice cover front, causing a thickened accumulation to
develop. The average flow velocities seem to remain low enough that a severe
freeze-up jam does not occur. The velocities are such that frazil slush may accu-
mulate under the cover. On the whole, bulk flow velocities increase upstream,
with the likely consequence that cover formation becomes progressively more
difficult at upstream locations.
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