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Chapter 6 - Analysis of the test results
6.2.2 Analysis of the water surface
Without macro-roughness, stationary waves can be observed at the outer side wall (Figure 6.11,
Appendix 6 and 8). The amplitude of these surface waves is about 10 cm and their length about
60 cm to 1 m. For the highest channel slope (S 0 = 0.70%), the one or two last waves after the
bend showed small surface rollers (Fig. 6.12, left).
no MR e s =4° e s =2° e s =1°
Figure 6.11: Water surface views for tests C1 to C4 at Q = 210 l/s
With vertical ribs, the previously observed stationary waves are replaced by shock waves, resulting
from the reflection of the water on the macro-roughness elements. They emerge about 0.5°
upstream the ribs (Figure 6.14) and redirect the flow towards the center (with a deviation angle
from the channel axis of about 50 to 55° depending on the local Froude number 1 ). The waves can
be observed over the outer 70% of the channel for a rib spacing of 4° and over the outer half of
the channel for 2°-spacing. If even more ribs are added (1°), these shock waves disappear and the
ribs no longer work as isolated elements but rather as continuous roughness on the outer wall.
The roughness is too important to observe the stationary shock waves that were observed without
ribs.
Figure 6.12: Stationary wave at the end of the bend with surface roller (left) and a common one (right). Picture
taken across the outer side wall
1. The local Froude numbers are of about 1.2 to 1.4, indicating supercritical flow conditions along
the outer side wall.
page 126 / November 9, 2002
Wall roughness effects on flow and scouring