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Chapter 6 - Analysis of the test results
6.5 Analysis of the grain sorting of the armoring layer
As described in paragraph 4.4.6 e), sediment samples were taken at the end of each test at the two
scour locations. The grain size distributions of the armoring layer after the tests are given in
Appendix 9 (see also Table 6.5). It can easily be seen on Figure 6.20 that by using a wide grain size
distribution, as observed in natural rivers, a significant transversal grain sorting process occurs.
The coarse grains are left or deposited along the outer wall in the scour hole, whereas the fine
sediments are moved towards the inner side forming a point bar. An armoring layer is formed by
the coarse grains in the scour holes, which limits their maximum depth (see also SCHLEISS &
HERSBERGER, 2001).
Computing the grain size distribution of the armoring layer based on the initial grain size distribution
(GESSLER, 1965, 1970, 1990), the following result is obtained (Figure 6.19). It can be seen that
the predicted order of magnitude fits quite well with the armoring of the second scour. The conditions
in the first scour do not seem to be exactly the same, leading to a more uniform armoring
layer. This method allows to get an estimation of the order of magnitude of the mean diameter,
but the shape of the grain size distribution of the armoring layer in a bend is not easily predicted.
The mean diameter of the predicted armoring layer is of 11.5 mm, which is too fine compared to
the measured one (15.6 and 15.8 for the upstream scour and 17.3 for the downstream scour).
100%
Grain size distribution of the armoring layer
80%
Cumulative weight [%]
60%
40%
20%
Initial grain size distribution
Computed armoring
+ standard deviation
- standard deviation
Measured armoring: Jf = 0.50%, Q=212 l/s, upstream
Measured armoring: Jf = 0.50%, Q=210 l/s, upstream
Measured armoring: Jf = 0.50%, Q=210 l/s, downstream
0%
0 5 10 15 20 25 30 35 40 45 50
Sieve openings [mm]
Figure 6.19: Comparison of predicted and computed armoring layer (computed with Gessler, 1965, 1670, 1990)
The width of the grain size distribution σ of the final armoring layer is reduced and we find an
almost uniform grain size on the point bar and in the scour hole (see Appendix 9 for the plots).
The grain sorting process is somewhat more significant in the region of the first, upstream scour
hole, which leads to coarser sediment at the outer bank and finer grains on the depositions
(Figure 6.20).
page 134 / November 9, 2002
Wall roughness effects on flow and scouring