Figure 7 Erosion/deposition masses for
the main channel and floodplain of the LYR during 2000-2017.
Here, we illustrate the change in
the masses of erosion and deposition in the main channel and the
floodplain for the different reaches (see Figure 7). During the period
from 2000-2017, the mass of eroded
sediment in the main channel was approximately
10.33×108 t,
with an average annual erosion rate
of 0.61×108 t/yr, while the mass of eroded sediment in
the floodplain reached approximately 3.46×108 t, with
an average annual erosion rate of 0.20×108 t/yr. The
main channel erosion has been caused mainly by low sediment flow, and
floodplain erosion may have been caused by human activities such as sand
mining and land use.
4.1.2 Floodplain erosion and
deposition
The total area of floodplains in the braided reach (between HYK and GC)
is nearly four times that of the main channel, but the intensity of
topographic adjustment of floodplains is lower than that of the main
channel. Figure 8 shows the annual deposition and erosion volumes of the
main channel and floodplains between 2000 and 2017. The main channel was
fully scoured, with a total erosion volume of 8.38×108m3 and an average annual erosion volume of
49×108 m3/yr. However, sedimentation
was the main mode in the floodplain before 2002, and the floodplain was
dominated by erosion after 2002, with the exception of a few areas where
sediment deposition occurred; the other areas of floodplains were
scoured.
During the period from 2000 to 2002, the cumulative depositional volume
in the floodplain between HYK and GC was approximately
2.65×108 m3, with an average annual
deposition of 1.32×108 m3/yr. In
addition, the average annual deposition of each reach was approximately
0.09×108 m3/yr; the right floodplain
deposition rate (0.12×108m3/yr) was larger
than that of the left floodplain
(0.07
×108m3/yr). During
the operation of the XLD reservoir and the implementation of the WSRS,
the cumulative erosion was 5.73 ×108m3 in the floodplain between 2002 and 2017, with an
average annual erosion value of 0.34 ×108m3/yr, of which 0.11 ×108m3/yr occurred in the left floodplain and 0.23
×108 m3/yr in the right floodplain.