Geoscience Reference
In-Depth Information
Entrainment Threshold of Loose Boundary
Streams
Subhasish Dey
1
Introduction
When a turbulent flow interacts with a loose boundary composed of noncohesive
sediments, hydrodynamic forces are exerted on the sediment particles forming the
boundary (henceforth bed). With an increase in flow velocity, the sediment particles
on the bed surface are intermittently entrained at a random rate if the magnitude of
the induced hydrodynamic forces acting on the sediment particles exceeds a certain
threshold value. The condition that is just adequate to initiate sediment motion is
termed
threshold
or
critical condition of sediment entrainment
. Importantly, the
induced boundary shear stress of the stream flow in excess of that of the stream flow
in threshold condition governs the sediment transport rate.
The doctoral research study by A.F. Shields (
1936
) on sediment movement
conducted in the Technischen Hochschule Berlin was a phenomenal contribution
(Kennedy
1995
). His major finding was his diagram, well known as Shields
diagram, that represents the variation of nondimensional threshold boundary
shear stress (or threshold Shields parameter) with shear Reynolds number
corresponding to the threshold of sediment entrainment. It is considered to be the
reference of any sediment transport research. His pioneering work which is
widely applied to the fields has inspired numerous researchers conducting further
studies. However, not many attempts were made before Shields (
1936
), but
they were mostly empirical with limited applicability. Despite the fact that the
Shields diagram is widely applied, even as of today, researchers have expressed
some dissatisfactions (Mantz
1977
; Miller et al.
1977
; Yalin and Karahan
1979
;
Buffington
1999
), since the diagram less complies with the experimental data plots
in the smooth and rough-flow regimes (Yalin and Karahan
1979
). Thus, further
attempts have so far been made to modify the Shields diagram, conducting addi-
tional experiments and analyzing the problem theoretically based on deterministic
