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Analysis of the grain sorting of the armoring layer
30
Characteristic grain diameter [mm]
25
20
15
10
5
upstr, outside, dm
upstream, inside, dm
downstr, outside, dm
downstream, inside, dm
Outlet, dm
d m,ini = 8.5 mm
0
B1 B2 B3 B4 C1 C2 C3 C4 D1 D2 D3 D4 E2 E3 E5
Test ID
Figure 6.20: Characteristic average grain size diameter for the different test series (average over the 3 discharges)
The presence of macro-roughness does not seem to influence the grain sorting process in a significant
way. The finally obtained grain size distributions are of the same order of magnitude as
without ribs. A strange phenomenon occurs during test B2. The sediments in both scour holes are
much coarser than the ones during all other tests. Since the samples taken at the outlet were also
somewhat coarser than the other ones (Figure 6.20), this could be due to an irregularity in the sediment
feeding with a too coarse mean diameter.
Once scouring has started, the transversal sorting of grains in the bend is almost independent of
the discharge and the initial longitudinal bed slope (Table 6.5). Analyzing all the tests, the following
approximate relationship of the sorted mean grain sizes at different locations and the mean
grain size of the initial mixture (bed material) can be given (see also SCHLEISS & HERS-
BERGER, 2001):
d m
outside (scour)
inside (point bars)
upstream
downstream
d ma, 1o
d ma, 2o
=
=
1.93 ⋅ d m
1.80 ⋅ d m
d ma, 1i
d ma, 2i
= 0.68 ⋅ d m
= 0.74 ⋅ d m
Table 6.4: Formula for the determination of the grain size in the scour holes and on the point bars
Since all tests were performed with the same initial sediment mixture, the influence of the with of
the grain size distribution is not found in Table 6.4.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 135