1 Introduction
1.1 River widening
1.2 Sediment motion
1.3 Velocimetry
2 Experimental setup
2.1 Study perimeter
2.2 Similarities
Parameter | Scaling ratio |
---|---|
Length | λ = 52 |
Velocity/time | λ1/2 = 7.21 |
Discharge | λ5/2 = 19′499 |
Roughness | λ−1/6 = 0.52 |
Prototype scale (mm) | Model scale (mm) | |
---|---|---|
d25 | 26 | 0.5 |
dm | 43 | 0.8 |
d90 | 86 | 1.6 |
2.3 Scenarios
Discharge | Solid discharge | |||||||
---|---|---|---|---|---|---|---|---|
Rhone river | Dranse river | Rhone river | Dranse river | |||||
Prototype scale | Model scale | Prototype scale | Model scale | Prototype scale | Model scale | Prototype scale | Model scale | |
(m3/s) | (l/s) | (m3/s) | (l/s) | (kg/s) | (g/s) | (kg/s) | (g/s) | |
Q10 | 669 | 34.3 | 95 | 4.9 | 80 | 3.4 | 64 | 2.7 |
Q50 | 885 | 45.4 | 99 | 5.1 | 113 | 4.8 | 70 | 3.0 |
Q100 | 1260 | 64.6 | 144 | 7.4 | 169 | 7.2 | 152 | 6.6 |
Extreme flood | 1600 | 82.1 | 204 | 10.5 | 220 | 9.3 | 310 | 13.4 |
2.4 Initiated morphology
2.5 Instrumentation
Parameter | Value | Unit |
---|---|---|
Cycle number | 8 | – |
Repetition number | 256 | – |
Number of profiles | 100 | – |
Sampling rate | 6.3 | Hz |
Voltage | 60 | V |
Gain | 3–6 | – |
Start point | 4.07 | mm |
Channel distance | 2.96 | mm |
End point | 149.11 | mm |
Number of channels | 50 | - |
Maximum depth | 456.58 | mm |
Maximum velocity | ± 297.5 | mm/s |
Velocity resolution | 2.342 | mm/s |
PRF | 1.62075 | kHz |
3 Data processing
4 Experimental results
4.1 UVP velocity profiles
4.2 Statistics of velocity variation
4.3 Bed shear stress
5 Discussion
5.1 A structured flow
5.2 Macroshape effects
5.3 Scale and steady condition influence
5.4 The preferential channel: area with conditions enabling grain motion
Measurement N° | 23 | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | 14 |
---|---|---|---|---|---|---|---|---|---|---|
τcr,dm | 0.013 | 0.100 | 0.033 | 0.044 | 0.084 | 0.214 | 0.127 | 0.060 | 0.065 | 0.209 |
τcr,d90 | 0.006 | 0.044 | 0.014 | 0.020 | 0.037 | 0.095 | 0.056 | 0.027 | 0.029 | 0.093 |
6 Conclusions
-
A structured flow is observed upstream and downstream of the widening with higher and steady velocities in the preferential channel compared to velocities above the deposits.
-
Dunes strongly limit the application of the methodology presented in this paper. As shown, dunes are also moving, which indicates grain motion. However, these bedforms are retained while scaling up, even though they might not occur in reality. Moreover, the steadiness of the tests reinforces their presence.
-
Shear stresses are higher in the preferential channel than the critical shear stresses for dm and for d90 in most places along the preferential channel. This means bed particles are likely to move.
-
Some grain sorting might occur upstream the observed model reach, leaving some of the larger grains deposited upstream.
-
Dimensionless shear stress lower than the presumed critical value of 0.055 corresponds already to partial transport, therefore larger grains can be transported, especially within the already fully moving smaller particles.
-
The bathymetric survey shows insignificant changes in the preferential channel geometry during the measurement campaign with the bed in constant motion. The main explanation is that a sediment "conveyor belt" layer at the bottom is formed, ensuring favorable conditions for the motion of larger grains.