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Chapter 3 - Theoretical considerations
3.5.5 Conclusions
There exists a fair number of scour formulae. The comparison of scour formulae with laboratory
tests show that most of them underestimate the scour depth in an mountain rivers with a coarse
sediment mixture. The formulae of PETER and REINDL overestimate the scour and the formula of
BRIDGE gives on average a good prediction of the scour, yet with a significant scatter. If the maximum
scour depth computed with existing scour formulae are corrected with a correction factor
F C , good results can be obtained with the following equations: Peter, Zimmermann, van Bendegom,
Falcon & Kennedy and Odgaard who all obtained correlations of more than R 2 = 0.70 .
The correlations of the other formulae are smaller than R 2 = 0.60 . The correction factors as well
as the correlations of the different formulae are given in Table 3.7.
Despite a very good correlation, the formula of PETER should be used with caution, since it does
not take into account the grain size. It is interesting that the frequently preconized formula of
KIKKAWA ET AL. shows an important lack of prediction capability when applied to mountain
rivers.
The formula of BRIDGE needs no correction factor (1.0). This shows that, over a large number of
tests, the average scour depth is quite well predicted. But the scatter of the different individual
measurements is considerable and therefore the correlation is only 0.3. This shows that some
essential parameters are neglected, despite good theoretical bases.
Most formulae assume an exponential cross-section profile. But the tests show that the bed topography
in radial direction is rather s-shaped.
page 66 / November 9, 2002
Wall roughness effects on flow and scouring