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Chapter 6 - Analysis of the test results
6.7.2 Influence of the macro-roughness on the scour process
a) Bed topography
By placing verical ribs on outer side walls with an appropriate spacing, the scour depth can be
reduced up to 38% (Figure 6.5), especially for high discharges and steep slopes (§ 6.2.1 a).
The first scour location shifts in the downstream direction in the presence of macro-roughness by
15 to 40° (Table 6.2). It is interesting that the second scour remains more or less at the same place
(§ 6.2.1 b). The first point bar moves in the downstream direction (by up to 20°), too. But as for
the second scour, the downstream point bar remains at about the same position (§ 6.2.1 c).
For an optimum rib spacing the shape of the two scour holes remains the same as without macroroughness.
The difference concerns essentially the length of the first scour covering a larger segment
of the bend and being of reduced depth. The second scour (if there is one) has about the
same extend (surface) as without ribs (§ 6.2.1 e).
Concerning the shape of the cross-section (in radial direction), the same line bend can be
observed; there is no significant difference (§ 6.2.1 f).
b) Water surface
The stationary waves on the free water surface (without ribs) are replaced by shock waves emerging
at the outer bank and spreading out over more than half of the channel. The maximum amplitude
of these waves is significantly smaller than the one of the stationary waves. It is reduced by
50% to 0.25% of the mean water depth h m . The inclination of the free surface shows no significant
modification due to the presence of macro-roughness (§ 6.2.2).
An important influence of the ribs is the head loss they create in the bend, which induces an
increased water level in the inlet reach 1 just upstream the bend.
Return currents behind the point bars were also observed with macro-roughness if important
point bars formed. Since the first point bar was in general much smoother, the cross-section was
better used for the flow and the return currents were less frequent.
Alike the case without macro-roughness, floating objects introduced upstream the bend quit the
channel at the center not depending on the radial point of introduction (§ 6.2.2).
c) Evolution of the scour
An important effect of the ribs is the smoothing of the scour process. Instead of having important
oscillations (especially in the second scour) during the development of the scour, the scour develops
more regularly. The oscillations are reduced by 50% to 0.25% of the mean flow depth for
small discharges and they even disappear for higher discharges (180 and 210 l/s) (§ 6.3).
1.The increase is of about 10% of
needs to be considered (§ 6.4).
h m
. For constant sediment feeding rates, an additional increase
page 144 / November 9, 2002
Wall roughness effects on flow and scouring