Geoscience Reference
In-Depth Information
significant deposition, the bed material in the reservoir gradually becomes finer, and
thus the Manning
n
of the channel bed decreases with time. This can be described
using the movable bed roughness formulas introduced in Section 3.3.3, or using the
following relation proposed by Han
et al
. (1986):
n
3
/
2
0
n
3
/
2
n
3
/
2
e
n
3
/
2
e
1
/
4
=
+
(
−
)(
1
−
a
/
a
e
)
(5.10)
where
n
0
,
n
, and
n
e
are the Manning roughness coefficients in the beginning, tran-
sitional period, and equilibrium state of reservoir deposition, respectively;
a
is the
deposition area accumulated with time at a cross-section; and
a
e
is the final deposi-
tion area when the reservoir reaches equilibrium. The values of
n
e
can be determined
by referring to those in the downstream alluvial channels with flow and sediment
conditions similar to the equilibrium state of the reservoir.
5.1.1.4 Composite hydraulic properties
If hydraulic properties, such as roughness and conveyance, are non-uniform across the
channel, their composite values need to be computed. The often used methods include
the alpha method, hydraulic radius division method, energy slope division method,
and conveyance method, which are described below.
Alpha method
In the alpha method, the cross-section is divided into panels between coordinate points
(stations), as shown in Fig. 5.3. The divisions between the panels are assumed to be ver-
tical. The cross-section is not distinguished between the main channel and overbanks
in this method.
The flow area
A
j
, wetted perimeter
χ
j
, hydraulic radius
R
j
, and conveyance
K
j
of
panel
j
are calculated by
A
j
=[
z
s
−
0.5
(
z
b
,
j
+
z
b
,
j
+
1
)
]
y
j
(5.11)
2
y
j
χ
=
(
z
b
,
j
−
z
b
,
j
+
1
)
+
(5.12)
j
R
j
=
A
j
/χ
(5.13)
j
Figure 5.3
Representation of cross-section in alpha method.
