pdf, 12 MiB - Infoscience - EPFL

Abstract
By applying vertical ribs on the outer sidewall, an important impact on the scour process and on
the flow field can be observed (§ 6). The macro-roughness has the following effects:
• The scour depth along the outer sidewall is significantly reduced and the prominent scours
almost disappear.
• With increasing wall roughness, the first scour hole shifts in the downstream direction
whereas the second one remains at about the same position.
• Significant oscillations of the water surface and the scour depth, observed without macroroughness,
are reduced by about 50% and the scour develops in a “smoother” way.
• The flow field undergoes a pronounced modification: the highest velocities are kept away
from the outer sidewall, reducing the scour at the bottom of the outer wall foundations.
• Along the outer wall at the free surface, an secondary flow cell at the outer bank can be
observed. The importance of this cell shows an important correlation with the bank protection
effect.
• Optimum rib spacing is essential since an inappropriate spacing may lead to an important
increase in scour depth.
• The transport capacity in the bend is reduced. In natural rivers, this phenomenon is compensated
by a steepening of the bed slope.
• The upstream and downstream extremities of the bend are influenced by the ribs: upstream
of the bend, the water depth increases due to the head losses in the bend, and downstream,
some additional erosion is found in the center of the channel.
• The grain sorting process is not significantly influenced by the presence of the ribs beside an
increase of the area of the coarse sediment zone.
This report furthermore presents a new empirical scour formula for the estimation of the maximum
scour depth for mountain rivers with wide grain size distribution (§ 7), established with
physically based parameters, which are the ratio mean water depth to channel width, a dimensionless
ratio combining the mean velocity with the hydraulic radius and finally the friction angle of
the bed material. The lateral bed profile in the maximum scour cross section can also be computed
with this equation.
An estimation of the maximum scour depth in the presence of macro-roughness on the outer wall
can be obtained with a formula depending on the rib spacing, the hydraulic radius, the Froude
number and the difference between the dimensionless shear stress and the critical Shields parameter
(§ 7).
Finally recommendations for hydraulic engineers (§ 8.3) are given to facilitate the application of
vertical ribs on outer banks serving as macro-roughness.
Keywords:
Sediment transport in curved channels, scouring in bends, mountain rivers, laboratory tests, grain sorting process,
coarse gravel bed, wide grain size distribution, armoring, mean velocity flow field, macro-roughness, vertical ribs
page iv / November 9, 2002
Wall roughness effects on flow and scouring