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Chapter 6 - Analysis of the test results
0.0
h m
0.1
S
Water depth [m]
0.2
h
B/6
B/2
0.3
C01b
C01c
C01d
0.4
6.5
6.3
6.1
5.9
Radius from center of the bend [m]
Figure 6.4: Approximation of the real cross-section profile by a simplified one
Other parameters having an influence on the scour depth are the grain size and the grain size distribution.
These parameters determine the roughness of the bed and the possibility to get an
armoring layer. They will be discussed in section 6.5.
Finally, the macro-roughness of the outer bank has a significant influence on the scour depth. If
vertical ribs are placed with an appropriate spacing (2° and 4° for the performed tests) on the
outer side wall, the following observations can be made:
• The erosion is well distributed over the whole bend and almost no prominent scour holes
can be found (Figure 6.8 and Appendix 4).
• The maximum scour depth is significantly reduced, especially for high bed slopes and high
discharges (Figure 6.5), where the reduction of the scour depth can reach up to 38%.
But it is important to use an optimum rib-spacing. GAIROLA (1996) (see also § 6.6 c) investigated
vertical ribs of different shape in a straight channel. He observed the highest impact on the flow
(in terms of head losses) if the ratio of rib-depth to rib-spacing is of about 12. This order of magnitude
is confirmed by our tests, but the optimum ratio can also be somewhat bigger
( ⁄ = 10 ÷ 20 ), depending on the tangential bed slope and on the curvature.
e s
e d
5.7
5.5
page 118 / November 9, 2002
Wall roughness effects on flow and scouring