Geoscience Reference
In-Depth Information
and probabilistic approaches. Miller et al. (
1977
), Buffington and Montgomery
(
1997
), Paphitis (
2001
), and Dey and Papanicolaou (
2008
) presented a survey on
this topic. However, after the discovery of the bursting phenomenon in turbulent
flows (Kline et al.
1967
), it has created a new look to further explore the sediment
entrainment problem. The turbulence is so far introduced as an average like
Reynolds shear stress. The conditional statistics towards the bursting events can
be the obvious treatment of the sediment entrainment problem, as the most impor-
tant turbulent events remain implicit with an averaging process. Therefore, the
merger of turbulence with a sediment entrainment theory demands its way in
between a deterministic and a probabilistic approach. It leads to an open question
that to what extent the micromechanical process can be studied in a deterministic
framework and when the results can be determined by a probabilistic approach.
A brief perspective review of the important laboratory experimental and theo-
retical studies on entrainment threshold of sediments under steady stream flows is
presented, highlighting the empirical formulations and semitheoretical analyses.
Special attention is given towards the role of the turbulent bursting on sediment
entrainment.
2 Definition of Entrainment Threshold of Sediments
It is always difficult to set a clear definition of the threshold of sediment entrain-
ment. First type of definition corresponds to the sediment flux. Shields (
1936
)
suggested that the boundary shear stress has a value for which the extrapolated
sediment flux vanishes. On the other hand, USWES (
1936
) put forward that the
tractive force is such that produces a
general motion
of bed particles. For the
median diameter of sediment particles less than 0.6 mm, this concept was found
to be invalid. Thus, the general motion was redefined that the sediment in motion
should reasonably be represented by all sizes of bed particles, such that the
sediment flux should be greater than 4.1
10
4
kg/sm. Paintal (
1971
) suggested
from stochastic viewpoint that due to the fluctuating mode of the instantaneous
velocity, there is no mean boundary shear stress below which there is no flux. With
this consideration, the threshold condition was defined as the boundary shear stress
that produces a certain minimal amount of sediment flux.
Second type of definition corresponds to the bed particle motion. Kramer (
1935
)
defined four types of boundary shear stress conditions for which: (1) no particles are
in motion, termed
no transport
; (2) a small number of smallest particles are in
motion at isolated zones, termed
weak transport
; (3) many particles of mean size
are in motion, termed
medium transport
; and (4) particles of all sizes are in motion
at all points and at all times, termed
general transport
. However, Kramer (
1935
)
expressed the difficulty in setting a clear demarcation between these regimes, but
defined threshold boundary shear stress to be the stress that initiates a
general
transport
. Vanoni (
1964
) proposed that the sediment threshold is the condition of
particle motion in every 2 s at any location of a bed. Different threshold definitions
