Geology Reference
In-Depth Information
Fig. 2.5
Schematic illustration of time-velocity asymmetry. Because the bedload transport rate is proportional to velocity cubed,
much more sediment is transported in the direction of the greater velocity, which results in a net transport toward that direction
suspended sediment transport is an important mode of
transport.
From Eq.
2.16
, suspended-load transport is strongly
influenced by the shapes of the current and sediment-
concentration profiles, which are controlled by the
intensity of fluid and sediment mixing. Active mixing
by highly turbulent flow results in a more homoge-
neous concentration profile throughout the water col-
umn. Weak mixing results in a profile with rapidly
decreasing concentration upward. A general under-
standing is that the turbulence that is responsible for
the mixing of sediment through the water column is
generated at the sediment-fluid interface. A mixing
coefficient (H
s
) is developed to parameterize the sedi-
ment mixing (summarized by Van Rijn
1993
). Sediment
concentration profiles can be obtained by solving the
sediment convection and diffusion equation:
experiments. Two of the commonly used sediment
concentration profiles solved from Eq.
2.17
are:
¤ ³
¤ ³
w
za
cz
()
a
(2.18)
¥ ´
¦ µ
¦ µ
¥ ´
1
bk
u
h
e
*
c
a
¤ ³
¥
¦ µ
w
a
cz
()
a
¤³
¥
¦µ
2
bk
u
*
(2.19)
c
z
a
where
c
(
z
) = suspended sediment concentration profile,
c
a
= reference concentration, and D
1
, D
2
, and E are empir-
ical coefficients. Equation
2.18
describes a logarithmic
decrease of sediment concentration upward through the
water column, solved assuming the mixing coefficient is
constant throughout the water column. Equation
2.19
describes a power-function decrease of sediment con-
centration upward, solved assuming the mixing coeffi-
cient is a linear function of depth. Both Eqs.
2.18
and
2.19
show that suspended sediment concentration
decrease rapidly upward through the water column. The
reference concentration, (a maximum concentration
near bed) is determined largely based on field and
laboratory data and is the subject of active research.
dc
cw
e
0
(2.17)
s
s
dz
Equation
2.17
is valid where sediment concentration is
low and fall velocity is largely constant. It can be
solved analytically with a known mixing coefficient
which is determined based on field and laboratory
