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Analysis of the final scour
20%
1° 2° 4° without ribs
Scour reduction [%]
0%
-20%
-40%
S=0.5%, Q=150 l/s
S=0.7%, Q=150 l/s
S=0.5%, Q=180 l/s
S=0.7%, Q=180 l/s
S=0.5%, Q=210 l/s
S=0.7%, Q=210 l/s
0 15 30 45 60 75
Rib-spacing / rib-depth: e s /e d [mm]
Figure 6.5: Reduction of the scour depth due to macro-roughness (e d = 20 mm)
b) Location of the scour holes
As already observed by other authors (e.g. GARBRECHT, 1953, PETER, 1986), two scour holes
occurred systematically in the bend. The deepest point of the upstream hole was located at the
beginning of the bend (Table 6.2) at about 30° (no ribs) and at the exit of the bend for the downstream
scour. For higher slopes and increasing discharges, the scour moved slightly downstream.
For the estimation of the location of the scour holes, PETER (1986) proposed the following formulae
based on the standard deviation σ of the grain size distribution of the substrate:
upstream scour: α 1
= 26.3 ⋅ σ + 11 ; downstream scour: α 2
= 104 + 5.66 ⋅ σ (6.1)
For the used sediment mixture (σ = 1.82), the first hole should develop at 46°, the second at 114°,
which is too far downstream compared to the observed scour holes. It needs to be mentioned that
both equations (6.1) have a rather poor correlation (less than r 2 = 0.5 ).
Maximum scour location no mr e s = 8 ° e s = 4 ° e s = 2 ° e s = 1 °
1st scour - erosion start
1st scour - max. depth
1st scour - min. thalweg
2nd scour - erosion start
2nd scour - max. depth
2nd scour - min. thalweg
Table 6.2:
10°
30°
50...60°
70°
90°...1 m
2...(3) m
10...20°
45...50°
60...70°
70°
90°
2m
10°
45...55°
70...80° *
70...80°
90°...1 m
2...3 m
Overview of the observed scour positions
* Often the thalweg does is not reduced between the two scour holes
** The second scour is very small
10°
50...60°
70...80°
70...80°
90°...1 m
2...3 m
10°
60...70°
80°
90° **
1...2 m
3...4 m
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 119