Sohlenformen

Influence of bed forms on stage discharge relations in compound channels

T. Branß, J. Aberle & F. Núñez-González

Abstract

The relationship between stage and discharge can be rather complex in compound channels due to the influence of numerous factors such as channel geometry, vegetation, or varying bed roughness. The latter aspect is of particular interest for sand rivers with bed forms, as the bed roughness changes with the bed form dimensions depending on flow depth and bed shear stress. However, the particular impact of the varying size of lower regime bed forms on the stage-discharge relation in compound channels has not yet been investigated in detail. This work addresses this issue by presenting results of a flume study using a cross-section of a half trapezoidal compound channel and two different floodplain roughness. The experiments were carried out with fixed and mobile main channel bed and the experimental data were used to derive stage-discharge curves highlighting the effect of main channel bed forms on the stage-discharge relationship in compound channels.

Compound Channel
Upstream view into the model section

Boundary conditions

*all experiments conducted with quasi uniform flow conditions
Experimental flume Model:
30 m length 20 m length
2 m width 1 m floodplain width
0.8 m height 0.2 m floodplain height
0.0025 slope 0.8 m channel bed width
  0.85 mm sand grain size

Compound channel geomtry

cahnnel cross section
Investigated compound channel geometry and maximal water level. Dimensions are given in meters.

Investigated set ups

smooth floodplain rigid bed
Experimental setup (SF): Smooth floodplain, Fixed main channel bed
rough floodplain rigid bed
Experimental setup (RF): Rough floodplain, Fixed main channel bed
smooth floodplain mobile bed
Experimental setup (SM): Smooth floodplain, Mobile main channel bed
rough floodplain mobile bed
Experimental setup (RM): Rough floodplain, Mobile main channel bed

Results

The presence of bed forms revealed a high impact on the stage discharge relation in the tested compound channel model.
For example, bed forms reduced the conveyance capacity of the channel by 45% for bank full conditions. Whereas at the highest investigated water stages bed forms reduced the conveyance capacity by:

  • 32% in case of a rough floodplain (RF)
  • 45% in case of a smooth floodplain (SF)

 

Timelapse of bedform formation (rough floodplain, fixed water level of 32 cm, required discharge 238 l/s)

Snapshots bed form formation

Impact of bed forms on the water level at fixed discharge. (Additional experiment conducted beyond the scope of the extended Abstract)


This work was funded by the Lower Saxony Water Management, Coastal Defense and Nature Conservation Agency (NLWKN), Germany.