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Chapter 6 - Analysis of the test results
nel width cannot explain such an important increase in scour depth. Based on the shape of the
scour hole (like a drop with an elliptic center) and on the location of the scour compared to the
point bar (downstream the first point bar), the hypothesis that the scour hole corresponds to a
combination of the first and second scour seems to be confirmed. Furthermore the flow field
shows many secondary cells indicating a highly perturbed flow field (Figure 6.24). Behind the
point bar, the inner bank cell is considerably amplified. The big amount of secondary cells indicates
that the velocity fluctuations are quite important in this cross-section.
Distance to free water surface (mm)
0
100
200
300
outer bank cell
residual main cell
B04−Q210−F09
main cell
inner bank cell
200 mm/s
“main” cell
behind point bar
400
0 100 200 300 400 500 600 700 800 900 1000
Distance to outer bank (mm)
Figure 6.24: Flow field after the first point bar for a rib spacing of 1° (at 85°)
And since the scour depth depends on
• the tangential velocities - being comparable or reduced compared to the other cases,
• the secondary currents - which are not more important than for the other cases, and finally
• the velocity fluctuations (turbulence)
the last ones have to be the key element for the heavily increased scour depth for this rib configuration.
Since these fluctuations seem to be provoked by the discontinuity after the point bar, we
see that the scour hole is submitted to an influence of the point bar as it is usually only the case for
the second scour hole.
page 146 / November 9, 2002
Wall roughness effects on flow and scouring