Figure 3.4. Angle of repose of non-cohesive materials (Simons 1955).
The interaction between the flow of the water-sediment mixture and the sandbed creates
different bed configurations which change the resistance to flow and rate of sediment
transport. The gross measures of channel flow, such as the flow depth, river stage, bed
elevation and flow velocity, change with different bed configurations. In the extreme case,
the change in bed configuration can cause a three-fold change in resistance to flow and a
10-to-15 fold change in concentration of bed sediment transport. For a given discharge and
channel width, a three-fold increase in Manning's n results in a doubling of the flow depth.
The interaction between the flow and bed material and the interdependency among the
variables makes the analysis of flow in alluvial sandbed streams extremely complex.
However, with an understanding of the different types of bed forms that may occur and a
knowledge of the resistance to flow and sediment transport associated with each bed form,
the river engineer can better analyze alluvial channel flow.
3.3.1 Regimes of Flow in Alluvial Channels
The flow in alluvial channels is divided into lower and upper flow regimes separated by a
transition zone (Simons and Richardson 1963, 1966). These two flow regimes are
characterized by similarities in the shape of the bed configuration, mode of sediment
transport, process of energy dissipation, and phase relation between the bed and water
surfaces. The two regimes and their associated bed configurations shown in Figure 3.5 are:
(1) ripples; (2) dunes with ripples superposed; and (3) dunes
Lower flow regime:
The bed roughness ranges from dunes to plane bed or
Transitional Flow Regime:
antidunes.
(1) plane bed; (2) antidunes, a) with standing waves, b) with
Upper Flow Regime:
breaking antidunes; and (3) chutes and pools.
3.13
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