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Summary and discussion
d) Sediment transport
The sediment transport rate during the test decreases with an exponential function, too. The
behavior is like the one without ribs, except that the transport capacity is considerably reduced.
For the performed tests the reduction of the transport capacity was up to 50 to 65%. In a natural
river, the upstream reach will continue bringing sediments to the bend. But since the transport
capacity in the bend is insufficient, the sediment will deposit in a first stage upstream the bend and
increase in this way the overall bed slope. With an increased slope, the transport capacity is
increased again, and the entire bedload transits through the bend. The observed necessary increase
of the overall bed slope was the same order of magnitude as the reduction of the transport capacity.
Since the observations on the steepening of the slope are based on one test series (the preliminary
tests), these results should be confirmed by additional research (§ 6.4).
e) Grain sorting
The macro-roughness has no significant influence on the grain sorting process, beside the fact that
the zone in the scour holes where a coarse armoring layer can be observed is extended to about
50% of the channel instead of 25% without ribs. But this is rather due to a flatter transversal bed
slope than to a modified behavior concerning grain sorting (§ 6.5).
f) The flow field
Ribs on the outer wall influence the tangential velocity field considerably. If the ribs density is 4°,
the maximum velocities remain close to the free surface all over the bend. With additional ribs, the
maximum velocity shifts towards the bottom but at a distance of about the average flow depth h m
of the outer wall (§ 6.6 a).
In the presence of macro-roughness the secondary current does not seem to undergo an important
modification. Only in at the first scour location a slight decrease in the intensity can be
observed. But the near bed velocities in radial direction remain about the same as without ribs.
An interesting phenomenon which was not yet visible for the case without macro-roughness 1 is
the small secondary cell on top of the outer wall (Figure 6.22). This cell is of about constant intensity
all over the bend. If the rib spacing becomes very small (1°), the intensity of this cell decreases
again (§ 6.6 b).
g) The special case of the smallest rib spacing of 1°
In the special case of the smallest rib spacing of 1°, as the ribs are very close to each other, the isolated
effect of each roughness is lost to the profit of a continuous roughness along the outer wall.
The first scour location shift even farther downstream and the scour depth increases again. This
indicates that the ribs loose their efficiency. It is obvious that if the separation zone cannot reattach
at the outer wall before hitting the next vertical rib, the energy dissipation will not be optimal.
But this does not yet explain why the scour gets deeper than without macro-roughness. It could be
mentioned that the channel width is reduced due to the ribs. But the decrease by 2% of the chan-
1. ...since the phenomenon was too weak and the measurements too far away from the outer wall.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 145