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Chapter 2 - State of the art
complex flow structure (Figure 2.1). On the upstream face of the abutment the flow plunges down
towards the bottom, where the principal vortex along the scour hole is located. Behind the abutment,
the wake vortex can be found. Some small surface rollers can be seen upstream of the
obstacle.
MELVILLE & RAUDKIVI (1984) summarized studies performed at the University of Oakland. An
important result of their study is the conclusion that the abutment scour depth can be up to 2 to 5
times the mean flow depth.
Semi-empirical formulae to estimate the equilibrium scour depth are given by INGLIS (1949),
AHMAD (1953), LIU ET AL (1961) and many others. Inglis found the scour depth to depend on the
flow characteristics (mean water depth and discharge) and the with of the channel. Ahmad introduced
correction factors depending on the channel bend, the shape of the abutment structure, the
angle of attack and the porosity of the abutment. Liu et al. found the abutment scour to depend
also on the Froude number. A recent review of local abutment scour equations is presented in
PRZEDWOJSKI ET AL. (1995).
MOLINAS, KHEIRELDIN & WU (1998) determined the necessary rip-rap size for protection of
bridge abutments. The phenomenon at the downstream side of such a narrow cross-section can
be compared to the one observed behind an element of macro-roughness. From their results, it
can be supposed that the elements of macro-roughness might have a significant influence on the
flow structure. However, it is important to note that these experiments were not carried out in a
bend.
c) Bed sills
GAUDIO ET.AL. (2000) performed experiments on scour downstream of bed sills in uniform sediment
and obtained empirical formulae to predict the dimensions of the scour hole. An important
result is that the Froude number does not seem to influence the dimensions of the scour hole.
2.3.3 Bridge scour
An overview of the phenomena influencing bridge scour related formulae can be found in textbooks
like BREUSERS & RAUDKIVI (1991), MELVILLE & COLEMAN (1999). The large number of
studies on bridge scour identified the following main parameters to influence the maximum scour
depth: the particle Reynolds number, a particle Froude number, the ratios mean water depth to
channel width and characteristic grain size to channel width and the ratio of sediment to water
density. Furthermore the pier dimensions and placement in the flow field and the sediment grading
play a role.
Some recent studies on scour in cohesive material were undertaken by TING, BRIAUD,& CHEN
(2001) who found that the equilibrium scour depth in cohesive material is of the same order of
magnitude as the one in a sand bed.
page 14 / November 9, 2002
Wall roughness effects on flow and scouring