DATA AND METHODS
3.1 Data collection
With the operation of the XLD
reservoir, the sediment entering the LYR has dramatically decreased, and
the geomorphology of the LYR has changed considerably. To investigate
the temporal-spatial distribution of sediment and the geomorphological
evolution of the braided reach between HYK and GC, hydrological data
from 1960-2017 and bathymetry data sets of various cross-sections from
2000 to 2017 were collected.
The hydrological data on this study reach, including the annual runoff
and sediment concentration, were measured at the HYK and GC stations
from 1960 to 2017. The cross-sections lie between opposite primary
levees of the LYR, with the distance between neighbouring cross-sections
ranging from 0.5-5 km along the channel direction. The Beijing 54
geodetic coordinate system was adopted for all the bathymetric data sets
to obtain the elevation of the riverbed in reference to a fixed level
across the whole region. The Yellow Sea 1985 datum was applied as the
reference level in this research.
3.2 Methods
The spatial interpolation method has
been an active research topic for many years in various fields. This
method has been applied to examine changes in sediment deposition and
erosion in different areas, such as the tidal reach of the Yangtze River
(Yuan, Lin, & Sun, 2020) or the Yellow River Delta (Fan, 2019; Jiang,
Pan, & Chen, 2017). Similarly, the numerical simulation of river
hydrodynamics requires spatial interpolation of discrete bathymetric
data to determine the elevations at the nodes of the computational grid.
There are many spatial interpolation methods for river channel
topography, such as the inverse distance weighted, natural neighbour,
kriging, spline, and other methods, which have their own advantages, and
a reasonable interpolation method should be selected according to
different characteristics of the river channel topography.
The braided reach of the LYR
contains a broad floodplain with secondary perched compound
cross-sections. According to the distribution characteristics of the
measured cross-section sets, the floodplain and main channel are treated
separately to obtain high-resolution DEMs from the scattered data.
Merwade et al. (Merwade, Maidment, & Goff, 2006) showed that in a
flow-oriented coordinate system, anisotropic
spatial interpolation methods are
clearly better than isotropic methods for interpolating river channel
bathymetry. In this research, the mesh generator of the MIKE software
(Amidror, 2002) was employed for mesh generation and interpolation
purposes. A curvilinear grid was constructed covering the main channel,
and the distance along the channel direction s was divided by a
factor larger than 1 (3 was adopted). An unstructured triangular mesh
was applied in the floodplains with natural neighbour interpolation in
regard to the bathymetry, as shown in Figure 4.