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Chapter 2 - State of the art
2.1 Flow in bends
The water particles in the bend near the surface of an open channel are moved by acceleration
forces, which are most significant near the surface, towards the outer side of the bend. Consequently
the water surface at the outer side of the bend is higher than at the inner side. This difference
in pressure head induces a flow near the bottom of the channel towards the inner side of the
bend. This secondary flow in lateral direction is superimposed to the main flow in longitudinal
direction. A spiral flow is created by these two flow components. Such a flow differs considerably
compared to a flow in a straight channel in view of flow resistance and sediment transport. Therefore
the following aspects were investigated in various research programs:
• inclination of water surface in lateral direction,
• spiral flow,
• head losses in bends,
• sediment transport in bends.
One of the oldest investigations was carried out by SHUKRY (1950), who measured in an rectangular
deep channel with fixed bed the three flow components, the water surface elevation in lateral
sections and the energy gradient in the curvature.
A different approach of the description of the flow was developed by GARBRECHT (1953); he
explained the phenomenon by local impact and deviation of the flow. His experiments were performed
at relatively steep slopes and sharp curvatures.
On the other hand the classical work of ROZOVSKII (1957) is based on a potential flow theory,
because his measurements were carried out in channels and rivers with very low bed slopes.
DE VRIEND (1976, 1981) developed a numerical model for bend flow. Based on a logarithmic
main velocity profile and its acceleration he derived the vertical and radial velocity components of
the secondary flow.
More recent research concentrated mainly on the interaction between the flow and the bed topography
(YEN, 1970 and HECKEL, 1978). The head losses in the bend were investigated in detail by
ONISHI ET AL. (1976).
BATHURST ET AL. (1979) carried out field measurements of secondary currents and boundary
shear stresses at bends of rivers with coarse alluvial beds. Other field experiments in a river bend
were undertaken by HABIB (1986). He measured the velocity components in longitudinal, lateral
and vertical direction and determined the bed topography as well.
A recent literature review on the research works on flow in bends has been given by BLANCKAERT
& GRAF (2001). Blanckaert performed laboratory experiments in a strongly curved 120° bend. He
observed an secondary cell at the outer bank close to the water surface. This outer-bank cell and a
reduced turbulent activity are shown to have a protective effect on the outer bank.
page 6 / November 9, 2002
Wall roughness effects on flow and scouring