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Analysis of the final scour
The macro-roughness has the advantage to reduce the oscillations of the free water surface long
the outer side wall. The wave amplitude is reduced by 50% from about 10 cm (~50% of h m ) to
about 5 cm (~25% of ).
h m
At the outer bank, the water surface is normally super elevated by 2 to 4 cm (~10 to 15% of )
due to the influence of the curve. With macro-roughness, the superelevation remains the same
order of magnitude or gets smaller (Appendix 7 and 8). The superelevation ∆h of the free water
surface can be calculated as 1 :
B⋅
R c
∆h = --------------- ⋅ -------
⋅ 2g
The measured and the computed water superelevation agree quite well.
R i
Since the ribs introduce a head loss in the bend, the water depth upstream the bend increases. For
the performed tests the water depth increased by 10 to 20% depending essentially on the discharge
(Appendix 7 and 8). In the bend, the mean water level remains about 10% higher than without
ribs.
Combining the different effects of the presence of macro-roughness - the reduction of the oscillations,
the increased mean water level and the superelevation due to the curve - the highest water
levels in the bend remain about the same as without ribs.
Another interesting phenomenon is the flow separation that can be observed when the depositions
are quite important (Figure 6.13). This separation is visible both at the first and the second
scour location.
Furthermore the flow at the water surface carries any floating object towards the center of the
channel. This is valid for all tested configurations. An object placed anywhere in the cross-section
upstream the bend, leaves the bend more or less in the center. The phenomenon remains the same
with and without macro-roughness. It can be explained with the velocity field concentrating at the
beginning of the bend in the center (attracting objects near the inner wall towards the center). Farther
downstream, the stationary waves or shock waves deflect floating objects towards the center.
R o
V m
2
h m
(6.2)
1. This formula applies for the case without macro-roughness. But, since the superelevation of the
free water surface in radial direction remains about the same with ribs, the same equation can be
used.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 127