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Summary and conclusions
bar. The upstream scour is essentially influenced by the action of the main velocity (along the
channel axis) on the outer bank and the induced secondary flow. Downstream, the scour is governed
by another phenomenon. The point bar upstream of the scour induces important velocity
fluctuations which are at the source of this scour.
The flow field in the channel undergoes important changes over the whole bend. At the inlet, the
maximum main velocity is located close to the free water surface in the center of the channel.
Over the bend, this maximum velocity zone shifts towards the outer bank and finally plunges
down along the outer wall to reach the bed surface at the first scour location. The secondary current
develops from the beginning of the bend at about 10° to increase up to the first scour. In the
upstream scour hole, another secondary cell appears at the inner bank, hindering the growth of the
main secondary cell. In the downstream direction, the secondary current reduces and then
increases again towards the second scour. The intensity of the secondary cell is less important at
the second scour location than in the first one.
During the development of the scour, an important oscillation of the scour depth can be
observed, especially in the second one. The scour depth along the outer wall shows fluctuations of
about 50% of the average water depth. On the free water surface, stationary waves are observed.
Their amplitude is of about the same order of magnitude as the oscillations of the scour depth.
A considerable grain sorting, due to the wide grain size distribution, can be observed over the
cross-section. At the outer bank, coarse sediments are found in the scour holes and, on the inner
banks, fine material is deposited.
Influence of the macro-roughness on the outer side wall
The presence of vertical ribs on the outer side wall placed with appropriate spacing, has a major
impact on the scour process and the flow field. The maximum scour depth is significantly
reduced, specially for high bed slopes and discharges (Figure 6.5 on page 119). The reduction of
the scour depth can be of up to 40%. With increasing wall roughness, the first scour is shifted in a
downstream direction, but the second one remains at about the same position. The prominent
scour holes disappear and frequently, the two scour holes cannot be distinguished from one
another.
Important oscillations of the scour depth observed during the formation of the second scour hole
are reduced by half for small discharges. They even disappear completely for the highest discharges.
The maximum level of the water surface is not significantly influenced. The ribs create a
head loss in the bend inducing an increased mean water depth; but at the same time, the ribs avoid
the important stationary surface waves by inducing smaller shock waves. Superposing the two
phenomena results in the same maximum water level as observed without macro-roughness. Furthermore,
it is interesting to observe that the amplitude of the water surface oscillations is of the
same order of magnitude for the tests, both with and without macro-roughness.
By deflecting the flow away from the outer wall and inducing additional wall roughness, the ribs
modify the velocity field in the bend. The highest velocities remain longer next to the free water
surface. Once they shift bottom-wards, the highest velocities occur at a distance of about the
mean water depth from the outer wall. In this way, the impact of the high tangential velocities is
directed away from the outer wall foundations.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 195