Geoscience Reference
In-Depth Information
SEDTRA module (Garbrecht et al., 1995)
The SEDTRA module (Garbrecht
et al
., 1995) calculates the fractional sediment trans-
port rates using three established transport formulas: the Laursen (1958) formula for
size classes from 0.01 to 0.25 mm, the Yang (1973) formula for size classes from 0.25
to 2.0 mm, and the Meyer-Peter-Mueller (1948) formula for size classes from 2.0 to
50.0 mm. The total concentration of sediment
C
t
∗
is calculated by
C
t
∗
=
p
k
C
t
∗
k
(3.115)
k
where
C
t
∗
k
is the sediment concentration of size class
k
; and
p
k
is the fraction of the
k
th size class of available sediment, usually set as the bed-material gradation.
In order to account for the hiding and exposure effect in non-uniform bed material,
the sediment size
d
ek
, used to calculate the critical flow strength for the incipient motion
of each size class, is adjusted using the following equation (Kuhnle, 1993; Wilcock,
1993; Garbrecht
et al
., 1995):
d
k
d
k
d
m
−
x
d
ek
=
(3.116)
where
d
m
is the mean diameter of bed material; and
x
is an empirical parameter,
determined by
x
=
1.7
/
B
m
, with
B
m
being a bimodality parameter (Wilcock, 1993):
d
c
d
f
1
/
2
p
m
B
m
=
(3.117)
where
d
c
and
d
f
are the representative diameters of coarse and fine modes, respectively;
and
p
m
is the portion of the sediment mixture contained in the two modes.
When
B
m
is less than 1.7,
x
1, and for high values of
B
m
,
x
approaches zero.
Table 3.6 lists the values of
x
recommended by Kuhnle
et al
. (1996). The mixture
names for Wilcock and Southard's (1988) data refer to the standard deviation of bed
material, and those for Kuhnle's (1993) data refer to the percentage of gravel in bed
material, e.g., SG25 for the mixture with 25% gravel and 75% sand.
The SEDTRA module takes the advantages of the three formulas used and thus
performs well in general; however, these formulas may not transit smoothly in the
=
Table 3.6
Values of
x
recommended by Kuhnle
et al
. (1996)
Mixture name
Reference
d
m
(mm)
Mixture type
B
m
x
SG10 (lab.)
Kuhnle (1993)
0.616
Bimodal
2.49
0.7
SG25 (lab.)
Kuhnle (1993)
0.927
Bimodal
2.60
0.7
SG45 (lab.)
Kuhnle (1993)
1.454
Bimodal
2.73
0.6
1/2
ψ
(lab.)
Wilcock & S. (1988)
1.82
Unimodal
0.67
1.0
ψ
(lab.)
Wilcock & S. (1988)
1.85
Unimodal
0.37
1.0
Goodwin Creek
Kuhnle (1993)
1.189
Bimodal
3.10
0.5
