pdf, 12 MiB - Infoscience - EPFL

Chapter 8 - Summary, conclusions and recommendations
On the water surface at the outer wall, an outer bank secondary cell can be observed. This cell has
been identified as contributing to bank protection. For an optimum spacing, this cell shows the
greatest degree of extension.
Due to the increase of the water level in the bend, and the reduction of the near bed main velocities,
the sediment transport capacity is significantly reduced (about 40 to 50%). In a natural river,
the bed slope steepens by the same order of magnitude (40 to 50%) as a result of depositions
upstream of the bend, if the bedload is maintained at the same rate.
An influence of the ribs on the grain sorting process is not found, except with respect to the size
of the zone where the coarse sediments are found in the scour holes. A marked armoring layer is
found over approximately the outer half of the cross-section, compared to a quarter without ribs.
But this is rather due to a smoother radial bed slope than to a modified pattern in the grain sorting
process.
At the extremities of the bend, the macro-roughness induces some modifications: in the upstream
reach, the water surface is increased by about 10% due to the head losses in the bend 1 . In the outlet
reach, some additional erosion can be found due to the increased velocities in the end of the
curve. This erosion is not found next to the outer bank, but towards the center of the channel,
where usually no important civil engineering structures are located.
Optimum rib dimensions
Based on the numerical investigation of GAIROLA (1996) and the performed tests, an optimum rib
spacing can be recommended. To obtain the most important energy dissipation along the outer
wall, the separation bubble behind the roughness element needs to reattach itself to the wall
before meeting the next rib. The length of this bubble is about 12 times the depth of the roughness
element in a straight reach. The tests showed that the optimum length is of the same order of
magnitude in a bend. Therefore, the optimum rib spacing can be indicated with 10 to 15 times the
rib depth.
The depth of the ribs was chosen about 2.5 to 5 times the mean diameter of the initially build-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.
Empirical scour assessment formulae
By means of a dimensional analysis (§ 7.2) the main parameters influencing the scour process were
identified ( S eall , , h m ⁄ B , V⋅ R h ⁄ g⋅
B 3 , R c ⋅ h m ⁄ B 2 ). The influence of the macro-roughness
on the scour process seems to be essentially determined by the spacing of the ribs .
In a first step, existing scour formulae were enhanced (§ 7.3.2 and Table 7.6). To the formula of
KIKKAWA ET AL. (1976) frequently used in practice, additional parameters were introduced with
quite good results. BRIDGE’s (1976) formula, giving the smallest error on the predicted average
e s
1. In addition to this, the steepening of the bed slope due to the reduced sediment transport capacity
in the bend needs to be considered.
page 196 / November 9, 2002
Wall roughness effects on flow and scouring