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6.3 Analysis of the evolution of the scour
Analysis of the evolution of the scour
As mentioned in the discussion of the preliminary tests (§ 5.2.1 2) a), the erosion first starts almost
at the end of the bend at a discharge of about 120 l/s (for a bed slope of 0.5%); then it moves
quickly in the upstream direction with increasing discharge to stabilize or rather oscillate around
its final position at about 30° to 35°.
Considering the evolution of the scour on the outer side wall without ribs (Figure 6.15), it can be
seen that the first scour hole armors quickly and gets stable. But the second one needs much more
time. The scour deepens and is filled again, until the scour finally stabilizes at the end of the test.
This is probably the moment when the armoring layer is well formed in the second scour, too. The
phenomenon is the same, independent from the discharge.
Analyzing the same cases but with macro-roughness (Figure 6.16) reveals that the first scour takes
some more time to be formed, but without oscillations. The second scour shows some oscillations,
but with a significantly reduced amplitude (reduction of about 50%).
The study of these plots (Figures 6.15 and 6.16) rises a question: Is the final scour the maximum
scour ? In order to answer this question, we have to remind ourselves that the maximum scour is
in general not located directly at the outer side wall but at about 10 to 20 cm from the wall. Some
sediments remain just next to the outer bank. The height of these sediments was frequently twice
the maximum grain size diameter d max . During the development of the scour, this depositions
became sometimes like a steep outer bank that collapsed from time to time. This can in part
explain the oscillations along the outer side wall. The amplitude of the oscillations in the second
scour is up to 3 times d max (or 50% of h m ) for the tests without macro-roughness and up to 1
times d max (or 15% of h m ) with ribs. With vertical ribs the oscillations can only be observed for
the lowest discharge (Q = 150 l/s). At higher discharges, the oscillations seem to disappear.
Turning back to our question, we see that the given final scour depth may be about one maximum
grain size diameters too small for the measurements without macro-roughness, but this is a part of
the uncertainty: if only one big sediment grain moves, the scour will locally increase by this order
of magnitude. For the tests with macro-roughness, it can be assumed that the final scour depth
corresponds to the maximum scour, since the deepest scour is not found at the outer bank, and
since the oscillations (for the smallest discharge only!) are less that the maximum grain size diameter;
for higher discharges they were too small to be measured (if there were any).
The other rib-spacings (4° and 1°) that are not given on Figures 6.15 and 6.16 behave like the
mentioned case (2°) with macro-roughness.
In conclusion, it is evident that the macro-roughness contributes to a smoother development of
the scour holes, reducing considerably (for small discharges) and even suppressing (for higher discharges)
the oscillations of the scour depth, especially in the second scour hole, where the phenomenon
is more pronounced.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 129