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Summary and discussion
c) Evolution of the scour
The scour process starts in the downstream part of the bend and moves quickly in the upstream
direction. At the same time, the scour deepens. The first scour armors rather quickly and gets stable
after a few hours. The second one needs much more time to get armored and stabilized. The
formation of the scour, especially the second one, goes along with important fluctuations in the
maximum scour depth. These oscillations have an amplitude up to 50% of the mean water depth
(§ 6.3).
d) Sediment transport
The sediment transport rate during the test decreases with an exponential function. For the higher
discharges, this decreasing function was not observed, but this is due to the fact that the test at 180
and 210 l/s were performed without initial bed levelling but continued with the bed topography of
the previous test (§ 6.4).
e) Grain sorting
Due to a wide grain size distribution - as it can be found in natural mountain rivers - a significant
grain sorting process can be observed over the cross-section. At the outer bank coarse sediments
form an armoring layer and at the inner bank, fine sediments are deposited. The mean grain size
diameter of the sediments along the outer bank is about twice as big as the mean diameter of the
substrate ; at the inner bank it is about two third of (Table 6.4 and § 6.5).
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f) The flow field
In the bend, the flow field undergoes some modifications. Instead of the typical log-velocity profile
of a straight channel flow, the observed profile has its maximum value shifted from the free
surface towards the ground.
An analysis of the evolution of the tangential velocities (in direction of the channel axis) reveals
that the maximum velocity is located close to the free surface at the inlet. In the bend, the velocity
first shifts toward the outer side wall and then plunges down towards the bed surface. At the
maximum scour location, high velocities are found next to the ground (§ 6.6 a).
The velocity plots in the cross-sections show the secondary currents. At the beginning of the
curve a secondary cell starts developing, growing up to the first scour. At the first scour location,
another secondary cell forms at the inner bank. Since this new cell has the same rotation direction,
it hinders the primary cell to grow. The primary cell is first reduced in the downstream direction
and then growing again towards the second scour. The secondary current is less important in the
second scour than in the first one (§ 6.6 b).
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EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 143