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Chapter 8 - Summary, conclusions and recommendations
8.1 Summary and conclusions
Analysis of existing scour assessment approaches
An extensive literature review (§ 2) showed that little work has been done on the effect of macroroughness
of the side walls on the flow and scouring in curved channels. The little known research
that was done was limited to general bank roughness, macro-roughness in straight reaches
(numerical studies) or isolated “roughness-elements” like bridge abutments. The only known work
on macro-roughness in bends compared different types of wall roughness with each other. There
is no known work which systematically studied the influence of macro-roughness placed as vertical
ribs on scouring and flow in river bends.
Performing a detailed study of existing scour formulae (§ 3) revealed that most known equations
considerably underestimate the scour in mountain rivers with a coarse gravel bed. The scour formulae
of PETER (1986), ZIMMERMANN (1978), VAN BENDEGOM (1947), FALCON & KENNEDY
(1983) and ODGAARD (1986) obtained fairly good correlations ( R 2 > 0.70 ) 1 . But the predicted
mean value is substantially underestimated. The maximum scour depth computed with the above
equations needs to be multiplied by a correction factor between 1.8 and 2.4 to get an accurate prediction
(see Table 3.7 on page 64), except PETER’s formula (1986), which overestimates the scour
by 15%.
The best prediction of the average scour depth was obtained by BRIDGE’s formula (1976) (needing
no correction factor), but with a poor correlation ( R 2 = 0.3 ) due to a wide scatter around the
average value. This shows that essential parameters are neglected in this formula.
Laboratory tests
Laboratory tests (§ 4 and 5) were performed in a 23 m long and 1 m wide channel with a 90° bend,
6 m radius bend on a mobile bed composed of coarse gravel with a wide grain size distribution as
found in natural mountain rivers. The bed topography, the water surface, the 3D-velocity field, as
well as the sediment transport rate and the grain size distribution of the armoring layer, were measured.
The tests covered a wide range of different parameters: discharges between 150 and
210 l/s, bed slopes from 0.44% to 0.79% and Froude numbers between 0.68 and 0.97. The
macro-roughness was analyzed for rib spacings of 8°, 4°, 2° and 1° (about 80, 40 20 and 10 cm)
and rib depths of 20 and 40 mm. The longitudinal bed slope was maintained constant in the inlet
reach during the tests, by adjusting the sediment transport rate.
Flow and sediment transport in bends
The tests showed that the scour process is principally influenced by the combined action of the
main flow (tangential velocity), the secondary flow and the velocity fluctuations. Two scour holes
can be observed in the bend; a first one between 30° and 40° and a second one at the end of the
bend. The first scour is located upstream of the point bar formed towards the inner wall of the
channel by the deposition of fine sediment, and the second one downstream of the second point
1. The square of Pearson’s correlation factor r 2 quantifies the extent of a linear relationship
between two data sets and not the error between measured and computed data!
page 194 / November 9, 2002
Wall roughness effects on flow and scouring