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Summary and discussion
6.7.3 Discussion
a) Scour mechanism
It has been shown in this section that the scour formation and development depend on many factors
acting in a combined way:
• the tangential velocity components play a major role in the scour process,
• the secondary currents are important; but they explain only in part the modification of the
flow field due to the presence of macro-roughness,
• the velocity fluctuations contribute to the scour process, too.
The first scour formed between 30° and about 40° in the tests. At the beginning of the bend, the
secondary current starts and grows towards this first maximum scour at the outer bank. Since the
point bar towards the inner channel wall is located after the maximum scour location, the flow is
not particularly perturbated and the velocity fluctuation can be assumed to be less important than
in the second scour. But the highest tangential velocities are located close to the bed surface
inducing an important solicitation of the bed.
Already at the first scour location, an additional secondary cell (inner bank cell) emerges. This cell
hinders the main cell to grow, since it has the same direction.
Finally the main secondary cell grows again towards the second scour. But the intensity of the circulation
is weaker than in the first scour. This second scour is located after the second point bar.
Therefore the flow field is perturbed and the velocity fluctuations are getting important for the
scour process. The main (tangential) velocities are of about the same intensity close to the bed surface.
Comparing the two scour holes, it can be concluded that the first one is dominated by the flow
field and especially by the secondary currents developing their strongest intensity in the first maximum
scour. The second scour, is most likely dominated by the velocity fluctuations induced by
the discontinuity created behind the second point bar.
Both scour holes stabilize, once the bed surface armored. It takes a few hours to armor the first
scour hole, but about twice as long for the second one. This armoring layer formed of coarse sediments
(about twice the mean diameter of the substrate) can be observed in the outer part of the
bend, whereas the point bars are formed of fine sediments (about two third of the mean diameter).
b) Influence of the macro-roughness
Vertical ribs placed with an optimal spacing on the outer side wall have the following influences
on the scour:
• The maximum scour depth is considerably reduced (up to 38% for the highest discharges
and slopes).
• The first scour location shifts in the downstream direction (by 15 to 40°), but the second
scour remains about at the same place.
• As for the scour, the first point bar moves in the downstream direction; but for the second
one it remains at the same position.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 147