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Abstract
Abstract
Wall roughness effects on flow and scouring in curved channels with gravel bed
In the narrow valleys in Alpine regions, rivers frequently flow across constructed zones, passing
through villages and cities. Due to limited space, the protection from high floods often needs to
be ensured by protection walls. During floods, these protection walls may be endangered by scour
phenomena, especially if they are located in bends. In the past, the potential danger of underscoured
structures was reduced by sufficient foundation depth.
By providing roughness elements such as vertical ribs at the surface of walls located at the outer
side of river bends, the local erosion along the foot of the wall can be considerably decreased,
reducing the cost of wall foundations. Such observations were made during the review and optimization
of several flood protection projects with hydraulic model tests in Switzerland. A literature
review showed, that no systematic study of the influence of these ribs on scour and flow in bends
was performed up to this day (§ 2). This research project covers this gap by investigating the
development of the scour as a function of main parameters.
The study is based on an experimental investigation in a 90° bend with a radius of 6 m including
measurements of the velocities, the water level, the bed topography, the sediment characteristics,
the grain size distribution of the armoring layer, the discharge and the bed load (§ 4 and 5). The
large set of tests covers a wide range of discharges, bed slopes, rather high Froude numbers, but in
subcritical regimes, many rib spacings and depths. The formation of two scour holes was
observed. Without macro roughness, a first scour hole occurs in the prolongation of the inner
sidewall of the entry reach at the outer wall and a second scour hole appears at the end of the 90°
bend.
The analysis of the performed tests results in the following conclusions:
• Most existing scour formulae considerably underestimate the scour depth in mountain rivers
with coarse gravel bed (§ 3).
• Significant oscillations, both of the free water surface (stationary waves) and of the scour
depth were observed, especially for the second scour hole.
• The two scour holes have different reasons. The first one is essentially due to the change of
the main flow direction (impact on the wall) and the induced secondary current, whereas the
second one is mainly due to increased velocity fluctuations after the point bar formed at the
inner side of the channel (§ 6).
• In the average flow field in a bend, the highest main velocities are shifted from the centerline
close to the surface toward the outer wall and then towards the bottom (§ 6). At the first
scour, the highest main velocities are found next to the ground.
• A significant grain sorting process is observed over the cross-section due to the use of
coarse gravel mixture, resulting in the accumulation of coarse sediments at the outer wall
and depositions of fine material on the inner bank.
EPFL Ph.D thesis 2632 - Daniel S. Hersberger
November 9, 2002 / page iii