Floodplains are an important part of the LYR, and the reasons for
floodplain erosion can be summarized as follows. (a)
Human activities, which affect
fluvial systems in many ways (Xu,
2003). i) In recent
decades, with increasing population
and rapid urbanization, economic construction and land use have entered
a period of rapid development, including agricultural production and
building construction. ii) The residents of the floodplain domain have
constructed many farm dykes on both sides of the main channel to allow
agricultural production and to prevent small to moderate floods. iii)
The construction of reservoirs has played an important role in blocking
sediment, especially the operation of the XLD reservoir since 2000, and
the sediment entering the LYR has dramatically decreased with an average
annual sediment load of 1.03×108 t/yr during
2000-2017, which is 83% lower than that from 1986 to 1999 (see Figure
3). Additionally, the XLD reservoir has efficiently controlled
large-scale floods, which has reduced the risk of flooding in the
floodplains. (b) Climate changes. A
change in the rainfall regime may change the erosion and transport
capability of storm runoff (Xu, 2003), especially extreme rainfall
events, which are a determining factor in hydrological soil erosion
(Nunes, Seixas, Keizer, & Ferreira, 2010).
Owing to the thick layer and short
depositional time of sediments in the floodplains of the LYR, loose
sediment particles are vulnerable to erosion by rainfall and runoff,
resulting in a decrease in the surface elevation.
4.2 Morphology changes
The morphology of natural river channels can be divided into the
cross-sectional form, longitudinal profile and plan form. The
quantitative characteristics of the cross-sectional form include the
width, depth, area, wetted perimeter and width-to-depth ratio. The
longitudinal profile is mainly reflected in the changes in the riverbed
slope along the channel, while the quantitative features of the plan
form include the bending coefficient, curvature radius, bending distance
and swing amplitude. In this research, we mainly consider adjustment
regulation of the riverbed morphology from three aspects: the
longitudinal profile, the horizontal
migration of the thalweg and the cross-sectional forms.
4.2.1 Changes in the longitudinal profile and the thalweg
point
The longitudinal profile, which is a graph of height (H ) against
distance downstream (L ) expressed by H = f (L ), is
an important element of river geomorphology. From 1986 to 1999, the
longitudinal profile of the LYR displayed a trend of continuously rising
riverbed elevation, in which the average riverbed elevation of the
HYK-JHT reach increased by approximately 2.6 m, and the average riverbed
elevation of the JHT-GC reach increased by approximately 3.0 m (Shen,
Zhang, Li, Shang, & Pan, 2000). Since 2000, the river channel in the
LYR has experienced continuous scouring. Figure 9(a) shows the
longitudinal profiles of the riverbed in the HYK-GC reach for different
years (2000, 2005, 2011 and 2017) by using the mean riverbed elevation
points of the channel. The average riverbed gradient basically remained
at approximately 0.18‰ during 2000-2017, which indicates that the
longitudinal profile of the riverbed underwent parallel downcutting
during scouring. The average bed elevation during the pre-flood season
in 2017 was 2.78 m lower than that during the pre-flood season in 2000.
The reach between HYK and GC in the LYR is a typical braided reach, with
complex changes in the river regime, and it is easily affected by the
water discharge and sediment load from upstream. In addition, the main
flows swing frequently and continually erode the floodplains, which
results in frequent shifts between the main channel and the floodplain
and places certain pressure on flood control works (Hu, 2003). Here,
selecting the thalweg points during the pre-flood season in May 2000 as
reference positions, the horizontal migration distances of the thalweg
points in different years are calculated (see Figure 9b). A negative
value indicates that thalweg points
moved towards the left bank, and a positive value indicates that thalweg
points moved towards the right
bank. From a comparison of the different periods, large variations are
observed to occur in the thalweg migration amplitude in the different
years. Among them, the average annual migration rates along the three
hydrological sections are 1,480 m, 1,300 m and 343 m. Moreover, we find
that the average annual migration rates of thalweg points in the HYK and
JHT cross-sections are four times higher than that in the GC
cross-section. The amplitude of thalweg migration gradually decreases in
the downstream direction, and the limiting effect of the channel form on
thalweg migration is strengthened.