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Recommendations for practical engineers
8.3 Recommendations for practical engineers
a) Advantages and inconveniences of macro-roughness on outer walls
Vertical ribs placed on the outer side wall with an optimum spacing have the following advantages:
• The maximum scour depth is significantly reduced (20 to 40% for an optimum rib-spacing).
• Prominent scour holes almost disappear.
• Important oscillations of the bed level during development of the scour are considerably
reduced or even suppressed for high discharges.
• The amplitude of the stationary surface waves is diminished by half. But at the same time,
the mean water level increases, resulting in about the same maximum water levels in the
curve as without ribs.
• High main velocities are shifted from the outer wall to a distance of about an average water
depth from the outer bank.
• The armoring layer zone next to the outer wall due to grain sorting is doubled, now covering
about half of the channel.
But the macro-roughness on the outer wall has also some negative effects, such as:
• A significantly reduced transport capacity in the bend. This reduction is compensated in
natural rivers by a steepening of the bed slope obtained by depositions upstream of the
curve (see also preliminary tests).
• Some additional erosion can be found after the bend in the center of the channel. But with
the possible exception of bridge piers, no civil engineering structures are endangered by this
limited local erosion.
b) Estimation of the maximum scour depth, transversal bed slope and scour locations
If existing scour formulae are applied to mountain rivers, attention needs to be paid to the fact
that most of them underestimate the maximum scour depth by a factor of up to 2.4. In order to
adjust the maximum scour obtained using existing equations, the result can be multiplied with a
correction factor given in Table 3.7 on page 64 (in general R 2 < 0.8 , except for Peter’s formula
with a higher correlation).
Without macro-roughness, equation 7.7 can be used to compute the maximum scour depth and
the transversal bed slope. This formula combines both, a good prediction of the maximum scour
depth and a good fit with the cross-section at the maximum scour locations.
It is difficult to compute the reduction of the scour depth directly. Therefore formula 7.63 is proposed
to compute the scour depth in the presence of vertical ribs.
The location of the two scour holes can also be predicted by two relations (equations 7.54 and
7.57). The prediction of the first scour gives quite good results ( R 2 = 0.83 ), but the forecast of
the second one is more difficult ( R 2 = 0.60 ).
During the development of the scour holes, important bed fluctuations were observed for the
tests without macro-roughness. They can reach amplitudes of up to 50% of the mean water depth
EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 199