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Chapter 6 - Analysis of the test results
• The prominent local scour holes disappear and make place to an elongated scour hole.
Sometimes, the first and second scour cannot be distinguished.
• Important oscillation of the scour depth, observed especially in the second hole, are significantly
reduced by the presence of macro-roughness. They even disappear for high discharges.
• The maximum water levels in the bend is not subject to important changes since the
increase of the average water level goes along with the reduction of the surface wave amplitude.
• At the upstream end of the bend, the water level is increased by about 10% due to the head
losses in the bend 1 .
• The transport capacity is considerably reduced by the macro-roughness (to 35 to 50%). In
natural rivers, this reduction is compensated by a steepening of the channel slope (mainly
realized by depositions in the upstream reach). The channel slope was increased by about 40
to 50% of the initial bed slope for the perliminary tests performed at constant sediment
feeding rate.
• The grain sorting process is not significantly influenced by the ribs, beside the fact that
about 50% of the channel width are armored compared to 25% without ribs.
• The flow field is considerably modified by the macro-roughness. The highest tangential
velocities remain closer to the free surface than without ribs. If they shift towards the bottom,
they remain at about the mean flow depth from the outer wall.
c) Optimum rib spacing
GAIROLA (1996) indicated the length of the separation zone behind the ribs to be 12 times the rib
depth for a straight reach. In a bend, we can expect this separation zone to become shorter
since the flow in the bend “squeezes” the zone to the side wall (especially in the upper part of the
bend). Since it is this separation zone which is responsible for the head losses in the bend, the rib
spacing optimum rib spacing corresponds quite well to the length of this separation bubble.
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Adapting Gairolas observations to our tests, this would indicate an optimum rib spacing between
2° and 3° (corresponding to ⁄ ratios between 10 and 12). In the present chapter it was
observed that the reduction of the scour depth was the most effective for spacings between 2° and
4° (Figure 6.5). Since the spacing of 4° gives better results especially for high velocities, the optimum
spacing is probably comparable to the one in a straight reach. Therefore a spacing of about
12 times the rib-depth can be proposed for constructions projects.
1. It needs to be mentioned that this was observed during the tests for which the bed slope of the
inlet reach was maintained constant, by adjusting the sediment transport feeding. For a constant
sediment transport rate, the bed level will rise upstream the bend, leading to higher water elevation,
too.
page 148 / November 9, 2002
Wall roughness effects on flow and scouring