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Analysis of the flow field
increases. But if the rib spacing is too dense (1°), it decreases again, but does not disappear (see
55°). BLANCKAERT (2001) observed the highest tangential velocities at the interface between the
two cells. If we compare Appendix 11.4 with 11.5 we find high velocities at the interface, but the
highest velocities occur inside the main secondary cell either towards the free water surface or near
the outer bank.
The intensity of the radial velocity components on the ground is of the same order of magnitude
as without macro-roughness for all rib spacings. This leads to the conclusion, that the radial velocity
components cannot explain the difference in the scour depth. And if the radial components
cannot explain this difference, the secondary current cannot explain it either, even if it contributes
to the erosion process. But if we look at the tangential velocities, wee see a significant reduction of
the near bed velocities due to the presence of vertical ribs along the outer bank. Furthermore the
tangential velocities are about 10 times bigger than the radial ones. Therefore we may conclude
that the modification of the tangential flow field seems to be determinant for the reduction of the
scour depth due to macro-roughness.
c) Velocity fluctuations and discussion
The extent of scour will be governed not only by the magnitude of the mean velocity but also by
the velocity fluctuations, since the force (drag or shear) acting on the particles will be a combined
effect of both.
Studies on the flow past rectangular obstructions on a flat bed (GAIROLA, 1996) indicate that the
flow separation at the obstruction reattaches the wall a certain distance downstream. According to
Gairola, the length (L d ) of the so formed separation bubble is a function of the Reynolds number
and the ratio length ( e θ
) to depth ( e d ) of the rib. The ratio L d ⁄ e θ
for e θ
⁄ e d ratios of the order
of 1 is about 12.0, for turbulent flows. In the present case, since the macro roughness is on a
curved bank, it can be expected that the length of the separation bubble may be somewhat smaller.
However, it is quite clear that a zone of separation exists which will reduce velocities in the neighborhood
of the outer bank. Further the turbulence intensities will be higher in the vicinity of the
separation zone and decrease as one moves away from this zone. It is therefore expected that the
flow will undergo a modification in the following manner (with macro-roughness):
• Mean velocities will reduce near the outer bank due to separation.
• Turbulence intensity will be maximum near the shear layer, where the mean flow velocity is
quite low.
• There will be an increase in velocity towards the center of the channel.
• The strength of the secondary circulation may decrease.
The effect of the macro roughness on the scour will be a combined effect of all these factors i.e.
the mean velocity, the turbulence intensity and the strength of the secondary circulation. The
observations made in the present study generally confirm the expected flow pattern. The strength
of the secondary circulation decreases. In addition to this, the main velocity reduces and the turbulence
intensity distribution is modified, which results in a reduced scour depth.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 141