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Chapter 8 - Summary, conclusions and recommendations
in the second scour hole. These oscillations are reduced by half by the presence of vertical ribs on
the outer wall for low discharges and they even disappear for high discharges.
All the proposed formulae should be used within the domain for which they were established. The
tests were performed at rather high Froude numbers (> 0.3) but at subcritical flow regimes. The
radius of curvature to width ratio ( R c ⁄ B ) ranged between 2 and 6 and the flow depth to mean
diameter of the substrate ( h m ⁄ d m ) was between 6.5 and 39. The study of the influence of the
macro-roughness was performed at R c ⁄ B ratios of 6 and h m ⁄ d m ratios between 18 and 24, but
the positive influence of the macro-roughness on scour and flow in bends can certainly be
observed in a much wider range of parameters.
c) The optimum rib spacing
A rib spacing to rib depth ratio of 10 to 15 is recommended. The depth of the ribs was chosen in
the present research study at about 2.5 to 5 times the mean diameter of the initially built-in bed
material (substrate). Almost no local scour was observed at the bottom of the ribs having a depth
of 2.5 times the mean diameter, whereas for the deeper ribs, some local scour was observed.
Therefore a rib depth of about 2.5 times the mean diameter (or about one time d 90 ) can be
recommended for engineering applications.
It is important to choose an appropriate rib spacing, since the scour can be even amplified by a
too small a rib spacing.
In order to obtain a smooth transition with the upstream and downstream reaches next to the
bend, additional ribs should be placed in a transition zone towards the bend. The laboratory tests
were carried out with a transition zone having a length of about 10 times the mean flow depth.
The ribs were placed with increasing spacing (factor 2 compared to the previous spacing,
Fig. 5.3 on page 111) for an increasing distance from the bend with good results.
page 200 / November 9, 2002
Wall roughness effects on flow and scouring