Geoscience Reference
In-Depth Information
In the case the diversion takes out only water, the main channel will carry the same
load with reduced discharge, which may result in aggradation.
In river morphology models, the bifurcation problem is usually approached by
defining nodal-point relations in which the sediment distribution is a function of the
discharge distribution (Fokkink and Wang
1993
). However, the sediment distribu-
tion in a bifurcation is locally defined and strongly influenced by the local flow
field. For this reason, nodal-point relations can only be found using prototype
measurements, physical model studies, or higher order (three-dimensional) numer-
ical computations.
Some of the nodal-point relations used in 1D morphological computations are
given below.
S
1
S
2
¼
Q
1
Q
2
(1)
sediment inflow into branch 1 and branch 2 (m
3
s
1
),
Q
1
,
Q
2
¼
with
S
1
,
S
2
¼
discharges in branches 1 and 2 (m
3
s
1
).
Equation (
1
) assumes proportionality between the sediment transport and the
discharge in the bifurcation. This relation is quite plausible for highly suspended
material that is nearly uniformly distributed over the channel cross section, such as
fine sediment and silt. If there are significant vertical concentration gradients, for
example, in channels with mainly bed-load transport, the sediment distribution is
highly influenced by the local flow field and not necessarily proportional to the
discharge distribution.
S
1
S
2
¼ a
Q
1
Q
2
þ b
(2)
The formulation (
2
) is used in the 1D-modeling system (Vermeer
1993
), with
user-defined values for
.
Wang et al. (
1993
) propose the following configuration of nodal-point relations:
a
and
b
m
n
S
1
S
2
¼
Q
1
Q
2
B
1
B
2
(3)
with theoretically
n
m
.
For practical use, the exponents
m
and
n
in (
3
) are to be determined empirically
and are dependent on local conditions. In alluvial channels, the inflow of sediment
into a diverting branch can be in disharmony with the transport capacity further
downstream, causing a change of the channel bed. This change can, as a second-order
effect, again influence the sediment distribution. Alluvial bifurcations can be stable or
unstable. Both types can be recognized in nature. If a bifurcation is unstable, one of
the diverting branches eventually disappears. Examples of unstable bifurcations are
branches in breeding rivers that mainly carry suspended load.
The experimental results of the distribution of sediment at channel bifurcations
suggest that this distribution can be expressed by the general nodal-point relation
¼
1
