16
ERDC/CRREL TR-02-14
σg = geometric standard deviation (a measure of sediment size distribu-
tion)
ρs = density of sediment
g∆ρ = submerged unit weight of sediment
B = channel width
So = channel slope.
Other dependent variables of practical interest are channel width, average
depth, shape, sinuosity (ζ), flow energy gradient (S), and sediment transport
capacity (Qsc). Significant changes in any of the independent variables in eq 1
may alter R, ζ, or Qsc and may destabilize the alluvial reach. The greatest natural
disturbances typically result from changes in Q or Qs, which usually vary season-
ally.
The seasonal appearance and disappearance of river ice expands and modi-
fies the set of hydraulic variables in eq 1 in a periodic manner. The extent to
which it affects important dependent variables, such as R, ζ, or Qsc, is unclear for
alluvial channels. Several qualitative aspects of river ice are clear, however.
River ice modifies flow resistance. It exerts hydraulic and geomechanical influ-
ences that act over a range of scales in space and time. And, not surprisingly, its
influence increases as channel stability under open water conditions decreases.
A relatively long, level floating ice cover, for instance, practically doubles
the wetted perimeter of flow in a channel and thereby significantly increases the
boundary resistance exerted on the flow. Ice accumulated as an ice jam increases
the flow resistance by locally constricting flow. Increased flow resistance typi-
cally results in increased flow depth, altered flow distribution, and reduced flow
drag on the bed--at least for fixed-bed channels. For a given channel, ice impacts
on the channel bed and banks increase in significance as water discharge, Q,
increases. Sediment entrainment and transport increase with increased flow in an
ice-covered channel as with open water flow. Increased flow also increases the
velocity of moving ice and increases the possibility of overbank flow. River ice
influences likely become more significant when the water discharge fluctuates
appreciably; then the prospects for other ice influences increase, such as ice
cover breakup followed by ice jamming.
Through its effects on the lateral distribution of flow resistance, and, thereby
flow and boundary drag, river ice may modify channel cross-sectional shape
developed under open water flow conditions. By imposing additional flow resis-
tance, river ice diminishes the effective gradient of flow energy available for
sediment transport and alluvial channel shaping. It may consequently alter
channel-thalweg alignment.
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