Geoscience Reference
In-Depth Information
submerged plants as the blockage area increased with depth until the plants were sub-
merged. The transition between submerged and partially submerged flows occurred
at a depth of about 80 percent of the undeflected plant height. Freeman
et al
. (2000)
obtained the regression equation for theManning
n
in the case of submerged vegetation
(
h
>
0.8
h
v
):
0.183
EA
s
ρ
0.183
h
v
h
0.243
0.273
U
∗
R
ν
−
0.115
R
1
/
6
√
g
n
=
(
N
a
A
v
)
A
v
U
2
∗
(10.33)
where
A
s
is the total cross-sectional area of the stem(s) of an individual plant, measured
at
h
v
m
−
2
); and
A
v
is the frontal blockage
area of an individual plant, which is approximated by an equivalent rectangular area
of blockage by leaves. It is important to note that the plant characteristics
h
v
,
A
s
, and
A
v
are the initial characteristics of the plants without the effect of flow distortion.
The regression equation for the Manning
n
in the case of partially submerged
vegetation (
h
/
4;
E
is the modulus of plant stiffness (N
·
<
0.8
h
v
)is
0.00003487
EA
s
ρ
0.150
N
a
A
v
0.166
U
∗
R
ν
0.622
R
1
/
6
√
g
=
n
(10.34)
A
v
U
2
∗
where
A
v
is the blockage area of the portion of the leaf mass submerged.
The experiment conditions for the data used to develop Eqs. (10.33) and (10.34)
were: flow depths from 0.4 to 1.4 m, average flow velocities from 0.15 to 1.1 m
s
−
1
,
n
from 0.04 to 0.14, plant heights from 0.20 to 1.52 m, plant widths from 0.076 to
0.91 m, plant densities from 0.53 to 13 plants
·
m
−
2
, plant moduli of stiffness from
·
10
6
.
In addition, for vegetation submerged in intermediate flow, Ree and Palmer (1949)
presented a set of curves for the Manning
n
as a function of
UR
. For both submerged
and emergent vegetation, Wu
et al
. (1999) related the drag coefficient and theManning
n
to the Reynolds number and the channel (or friction) slope. The obtained relations
of
n
10
7
to 4.8
10
9
N
m
−
2
, and Reynolds numbers from 1.4
10
5
to 1.6
5.3
×
×
·
×
×
vary with vegetation species. These relations can be used to
determine the roughness coefficient in vegetated channels.
∼
UR
or
n
∼
(
Re
,
S
)
10.1.3 Sediment transport capacity in vegetated
channels
How vegetation affects sediment transport is an important issue of concern. Jordanova
and James (2003) experimentally investigated the bed-load transport in a flume cov-
ered with uniformly distributed, emergent, rigid cylindrical metal rods. The rods were
arranged in a staggered pattern, and the median grain size of sediment was 0.45 mm.
Jordanova and James used the method of Li and Shen (1973) to determine the effective
bed shear stress and proposed the following formula for the bed-load transport rate
(kg
s
−
1
m
−
1
) in vegetated channels:
·
1.05
q
b
=
0.017
(τ
b
−
τ
)
(10.35)
c
m
−
2
).
where
τ
b
is the effective bed shear stress (N
·
