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and the river outlet and coastline change.
4.1 Coastal Fluvial Morphologic Change
It includes channel shifting change, channel
geometric change, and channel pattern change.
Both banks have been examined separately their
shifting and erosion changes. The north bank showed
extremely unstable by shifting northward and
southward alternately during the first 66 months
(May 1976 - Nov., 1981). The maximum shifting
distance amounted to 6,458 m (northward) within 17
months for Q4. It was becoming docile with the
declining of shifting rate after 1981. But it still
remained a moderate high migrating rate, ranging
from -150 to -500 m per year. The south bank
showed a similar change with the north bank during
the first 66 months. The maximum shifting distance
amounted to -3,728 m (southward) within six months
for Q7. For the stage of 1982 - 1987, it tended to
decrease its migrating rate. But it is becoming
extremely unstable with the escalating moving rate
after 1987. The difference of the shifting change
between both banks after 1987 possibly results from
the differential embankments. The northern
embankment is stronger than that on the south bank.
The channel thalweg is examined its change in both
the (metric) shifting rate and the all-round spatial
arrangement and orientation changes. The lateral
shifting of the channel thalweg shows a similar trend
in the south bank. The lineament analysis is used to
characterize the orientation, sinuosity, and river fluid
potential changes.
Some aspects of the channel geometric change are
examined. The channel length shows an increasing
trend with an average of 1,650 m per year. But the
increment tends to slow down. The channel tends to
decrease its width and is becoming approximately
equal wide as a whole.
Channel pattern changing from braiding, straight, to
slightly meandering has been well documented by the
accompanying maps (Figure 2). As a whole, the
channel bifurcation index" tends to decrease, while
the channel sinuosity tends to increase. Two major
stages can be well defined. The second stage can be
further subdivideded, based on the degree of
xs Bifurcation index is the ratio of total branching channel(s)
length to the length of the river axis.
artificial intervention.
Braiding Stage (1976 - 1980): During this period, the
river channel system was extremely unstable,
showing the most significant change. The channel
system changed from disorderly braiding, well-
organized braiding, to generally straight. The
channel system became relatively straight, with a
decreasing trend of channel sinuosity. The channel
bifurcation index tended to reduce and so for the
longitudinal profile gradient. The channel stability
index™ shows an obvious decreasing trend, and the
channel tended to be more stable.
Straight - Slightly Meandering Straight Stage I (1981
- 1987): During this period, the channel stability
index tended to increase, and the channel became
less stable. Due to the rapid vertical progration, the
river longitudinal profile gradient became much more
gentle. The capacity of the flow to transport its
suspended bed-load diminished. This resulted in
severe silting which caused the channel unstable and
successive flooding events (twice within 1987). The
situation in late 1986 - 1987 evidenced that the
development of the channel entered the stage of
wither or decay under a natural condition.
Straight - Slightly Meandering Straight Stage II (after
1987): As the channel sinuosity was increasing, the
channel was gradually changing from straight to
slightly meandering straight. The channle as a whole
tended to become more stable. Human intervention
was highly successful, especially during the first two
years (1988-1989), in strengthening the mainstream
to improve the capacity of the flow to transport its
suspended bed-load. However, this may maintain the
actual running state of the channel for several years,
on the long run, a well-organized (artificial)
diversion has to be carefully considered.
4.2 River Outlet and Coastline Dynamic
Change
The temporal migrating change of the river outlet is
presently examined (Table 1). The outlet
displacement, a scalar without direction, represents
the distance between two outlets. The outlet shift, a
vector with the direction (negative value means a
westward shift for x-shift, or a southward shift for a
y-shift), characterizes the migrating change over
Ex Channel stability index — B'?/M, where B is the width of
main channel, and M is the average depth of river channel.
803
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996