Lateral and vertical sorting in an annular flume
S.A.H. Weisscher, A.W. Baar, W.S.J. Uijttewaal, M.G. Kleinhans
s.a.h.weisscher@students.uu.nl; a.w.baar@uu.nl; w.s.j.uijttewaal@tudelft.nl; m.g.kleinhans@uu.nl;
Problem description
Spatial sorting is observed on transverse slopes in natural meanders where the outer bend is coarser than the inner bend. Well developed lateral sorting just upstream of a bifurcation can have major consequences for large‐scale morphology of the downstream branches. This is, since the grain size dependent transverse bed slope effect influences e.g. the degree of braiding and meander wavelength. Yet, previous research focused mainly on transverse slope development using uniform sediment.
So, there is a demand for better understanding of how spatial sorting in river bends comes about.
Objective
Experimentally determine the effect of transverse bed slope and sediment mobility on lateral and vertical sorting in meanders.
We hypothesise that spatial sorting becomes more pronounced with steeper transverse slopes and lower sediment mobility.
Methods
Annular flume experiments
• Lid rotation steers flow determines helical flow intensity and flow velocity
• Counter‐rotation of flume itself introduces a centrifugal force on lower part of the water column, weakening secondary flow intensity.
• Enables isolation of effect of transverse bed slope and sediment mobility
• A near‐unimodal mixture with , mm.
close to equimobility.
Conclusions
• More distinct bend sorting with steeper transverse slope
• Mobility effect is insignificant
• Longitudinal variability bend sorting due to dunes
Sediment sampling
Results – lateral sorting
Impression of the bed
Results – vertical sorting
References
Booij, R., & Uijttewaal, W. S. (1999). Modelling of the flow in rotating annular flumes.
Ikeda, S., Yamasaka, M., & Chiyoda, M. (1987). Bed topography and sorting in bends. Journal of Hydraulic Engineering, 113(2), 190‐204.
Funding: NWO‐STW Vici grant to MGK 15 cm
OUT
IN
0.47 m 0.3 m
1.85 m lid
floor
Top view
Side view
IN
OUT
Brinkpoint ripple
Brinkpoint collapsing ripple Sheet ripple
Flume wall
Brinkpoint/rim dune
Legend
Sediment mobility
Tr an sv e rs e bed slope
IN OUT IN OUT
0.96 1.00 1.04 1.08 r/rc
0.92 0.96 1.00 1.04 1.08 r/rc
1.00
0.75
0.50
0.25
0.00 1.00
0.75
0.50
0.25
0.00
Fraction [‐]Fraction [‐]
Top Trough Bulk
Ikeda et al. (1987) d
50/d
50,in= (r/r
c)
10.90 0.95 1.00 1.05 1.10 r/rc
2.5 2.0
1.5
1.0
0.5 d 50/d 50,in[‐]
2.5 2.0
1.5
1.0
0.5 d 50/d 50,in[‐]
IN OUT
coarse
fine
5‐95%
16‐84%
average slope
IN OUT
‐0.10 ‐0.05 0.00 0.05 0.10 Distance relative to channel axis[m]
0.25
0.20
0.15
0.10
0.05
0.00
Height [m]
d
50[mm] d
50[mm]
σ/σ
in[‐] σ/σ
in[‐]
Re la ti ve dune heigh t (1 = to p ; 0 = base)
Gentle transverse slope Steep transverse slope
Normalised ST D Gr ain siz e
IN OUT
0.25 0.50 1.00 2.00 0.25 0.50 1.00 2.00
1.0
0.5
0.0 1.0
0.5
0.0
1.0
0.5
0.0 1.0
0.5
0.0
Steven Weisscher
