50
Qr ≈ 1.53: β = 5.51 + 0.366α, r = 0.96
Qr ≈ 1.02: β = 6.98 + 0.297α, r = 0.95
40
Qr ≈ 0.53: β = 3.29 + 0.262α, r = 0.82
30
Definition
20
Sketch of β
Avalanche
Line of
α
Face Edge
Maximum
10
β
Scour
0
0
20
40
60
80
100
120
Confluence Angle, α
Figure 7. Relationship between the orientation of the maximum scour depth,
β, confluence angle, α, and discharge ratio, Qr = Q1/Q2. (After Best 1988.)
Bathymetry of discordant bed confluences
The literature on the bathymetry of confluences of discordant bed channels is
quite extensive. The bathymetry typically features sediment depositional patterns,
such as deltas, alluvial fans, and alluvial cones. These features are described by
ASCE (1975), Richards (1982), Simons and Li (1982), and Chang (1988). They are
accumulations of sediment deposited in approximately a fan-shaped pattern
formed where a river or stream enters an area of flatter slope or emerges into a
wider body of water. They commonly occur where a tributary draining steeper
terrain enters a larger river.
In their study, Biron et al. (1996) found that bed level discordance changes the
flow dynamics in three principal ways:
The shear layer between the two confluent flows is distorted towards the shal-
lower tributary and causes upwelling from the deeper channel into the shal-
lower channel.
Bed-level discordance and the rapid increase of flow area at the mouth of the
lesser channel greatly reduce flow acceleration near the bed within the conflu-
ence.
Bed-level discordance may eliminate mainstream flow deflection near the bed.
The shift of the shear layer and dividing streamline toward the smaller inflow
channel may hamper ice movement from the smaller channel. The changes to the
flow field, though, primarily affect sediment movement through the confluence
and sediment depositional zones. They typically result in the development of a
depositional cone, such as illustrated in Figure 5b. In terms of ice movement through
such confluences, the main effect is ice grounding on sediment depositional fea-
tures during periods of low flow.
KEY PARAMETERS
It is useful to characterize flow, channel, and ice movement processes at conflu-
ences in terms of nondimensional parameters. A potentially large number of them
may be needed for this purpose. A useful simplification is to consider the conflu-
15
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