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Preliminary tests
2) Results
a) Location of the scour holes
The first movements of the finest fractions were observed at a discharge of about 100 l/s. But still
higher discharges were necessary to initiate the scour process. Appendix 3.1 gives the situation
plots of the bed topography after the tests with different discharges. The plots show clearly that
the bed topography starts changing at a discharge between 120 and 140 l/s. The formation of a
point bar can be observed in the lower part of the bend between 70 and 90°. Parallel to the development
of this point bar, the scour process starts at almost the same location. This process can
also be seen on Appendix 3.3.1 (difference plots of the transversally averaged / minimum / maximum
bed profile) and on Figure 5.1. The longitudinal profiles (Appendix 3.3) show the minimum
respectively the maximum bed level of each cross-section, corresponding to the maximum scour
and the maximum point bar elevation. These extreme values are very close to the profiles along
the outer and inner walls. The average bed level is the arithmetic mean value of the measured
points in the cross-section.
These plots further show that the scour starts towards the end of the bend. The first signs of a
scour hole become visible in the cross-section at 90° (17 m at Q=120 l/s). With an increasing discharge
the hole moves in the upstream direction to 75° (15.5 m, Q=130 l/s) and 65° (14 m,
Q=140 l/s) to reach its final position at 35° (11 m) for the discharges of 170 and 212.5 l/s (see
arrow on Fig. 5.1 and Appendix 3.3.1).
0° 90°
50
Difference to initial bed level [mm] .
0
-50
-100
-150
-200
-250
min Q=120 l/s
min Q=130 l/s
min Q=140 l/s
min Q=170 l/s
min Q=212.5 l/s
Bend
upstream shift of the scour hole
with increasing discharge
0 5 10 15 20 25
Distance to the inlet of the channel [m]
Figure 5.1: Evolution of the scour (Thalweg) for the preliminary test without macro-roughness (A1)
b) Comparison of the scour holes
A comparison between the final bed topography (after Q=170 and 212.5 l/s) and the initial bed is
given in Appendix 3.2. The Appendix further gives a comparison between the bed topography
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 103