Geoscience Reference
In-Depth Information
is measured and then used to calculate the parameter
MEI
from the standard
calibration curves (Kouwen, 1988).
Based on laboratory experiments, Kouwen (1988, also see Temple, 1987) related
the parameter
MEI
with the vegetation height for growing and dormant grass species,
respectively, as
319
h
3.
v
,
MEI
25.4
h
2.26
v
MEI
=
=
(10.28)
Application of Eq. (10.28) should be restricted to those grasses that have been
tested, including alfalfa, Bermuda grass, buffalo grass, blue grass, weeping love grass,
Kentucky grass, Serica lespezeda, Sudan grass, and Rhodes grass. Experimental data
encompass stem heights in range of 0.04-1.0 m,
MEI
values in range of 0.007-212
N
m
2
, and stem densities (M) in range of 140-11,600.
Apparently Eq. (10.25) is only applicable to submerged flexible vegetation, such
as grasses. Kouwen and Fathi-Maghadam (2000) conducted flume experiments using
coniferous tree sapling and air experiments using large coniferous trees to investigate
the relation between the friction factor andmean flow velocity, and proposed a method
to estimate the friction factor for emergent woody vegetation:
·
4.06
U
−
0.46
h
h
v
λ
=
(10.29)
ξ
E
/ρ
where
accounts for all aspects of deformation of the plant as a result of an increasing
flow velocity. The parameter
ξ
E
is called the “vegetation index”, which is obtained
from the resonant frequency, mass, and height of a tree using a mathematical model
based on the works of Niklas and Moon (1988) and Fathi-Maghadam (1996):
ξ
m
s
h
v
Nf
1
ξ
E
=
(10.30)
where
m
s
is the total mass, and
Nf
1
is the natural frequency of the tree.
Nf
1
is measured
by rigidly mounting the tree, followed by flexing the top of the tree sideways prior to
releasing the tree to swing freely. An accelerometer is attached part way along the stem
to record the frequency of the swings. Table 10.1 lists the average vegetation indices
for four species of coniferous trees measured by Fathi-Maghadam (1996).
Table 10.1
Vegetation indices for coniferous trees
(Fathi-Maghadam, 1996)
Species
ξ
E(N
·
m
−
2
)
Cedar
2.07
Spruce
3.36
White pine
2.99
Austrian pine
4.54
