RIVER ICE INFLUENCES ON FORT PECK REACH, MISSOURI RIVER
17
By reducing the sediment transport capacity of a reach, river ice redistributes
bed sediment along the channel. Whatever local-scale effects river ice may exert
in accentuating erosion, river ice reduces the channel's overall capacity to convey
the eroded sediment away from the erosion location. Consequently bars may
develop in response to flow conditions under river ice but be washed out shortly
after the cover breaks up. In situations where a significant load of bed sediment
enters a long reach, river ice may tend to cause mild aggradation of the channel it
covers.
Congestion or jamming of river ice at one channel (or subchannel) location
may divert flow either into an adjoining channel, which then enlarges (channel
anabranching and thalweg avulsion), or over the bank, which may result in a
channel cutoff (avulsion).
Ice cover influence on the local cross section of flow
An ice cover imposes an additional flow-retarding boundary that decreases
the flow-conveyance capacity of a channel and redistributes flow vertically and
laterally. The vertical redistribution of flow is marked by flow depth increase
(usually) and by a flow velocity of zero at the cover underside. The lateral redis-
tribution of flow, though, depends on how the ice cover forms, is attached to the
channel banks, and thickens. It can be explained using a flow resistance equation,
such as the Darcy-Weisbach equation, that combines the appropriate variables:
Qo = Yo B (8gRoSo / fo) = KoSo
(2)
where Qo
=
flow rate per unit width of channel
Yo
=
flow depth
B
=
flow width
fo
=
Darcy-Weisbach resistance coefficient
Ko
=
conveyance per unit width.
The subscript "o" refers to open water flow. The presence of an ice cover
may redistribute the flow laterally or concentrate it in accordance with lateral
variations in flow depth and/or ice cover thickness. This influence can be illus-
trated using a simple, idealized channel comprising two bottom elevations of
equal width (Fig. 9). It can be described approximately in terms of two convey-
ance components, K1o and K2o, where K1o is associated with the deeper water.
For constant flow, a free-floating, uniformly thick ice cover reduces the
magnitudes of the two conveyance components. It smears flow over the full
channel width, as K1i/K2i < K1o/K2o (Fig. 9b), where the subscript "i" denotes ice-
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