This reach is generally quite straight with a sinuosity
coefficient of 1.15. In addition, the braided reach is composed of many wide
and narrow stretches. In a wide stretch with a long stream length, there are
numerous sand bars and complicated branches appearing at low water stages. This
kind of planform results in flow diverging and frequent shifting of the main
streamline. The channel bed is composed of fine sediment, with non-uniform size
distribution ranging from 0.002 mm to 0.18 mm.
The
physical model uses a horizontal scale of 1:800, vertical scale of 1:60. Fine
residual of burnt coal ash from Zhengzhou Thermal Power Plant was used as the
model sediment, with dry bulk density of 0.66 ~ 0.68 t/m3, wet bulk
density of 2.1 t/m3. The initial condition of bathymetry was chosen as
the field measured data in 2002. The inflow condition was provided by Yellow
River Engineering Company of YRCC as a 15-year flow-sediment series from multi-year
regulation rules of the Xiaolangdi Reservoir. The first five years was the
initial operation period for Xiaolangdi Reservoir, only clear water was
released to flush the sediment. The last 10 years was the multi-year sediment
regulation when sediment was released only at certain right time during flood. In
the proposed river training plans, 14 existing dikes will be still used, plus constructing
of 250 new dikes. Each dike is around 1000 m long (200m of warping segment),
with the top of the dike being 0.5 m higher than the floodplain. The distance
between each dike is around 500 m to 800 m. The objective training width of the
reach is set to be 600 m in general. At some curvature locations, this value was loosening
to be 1000 m for flood protection purposes.
The physical model results showed
that with the regulation of Xiaolangdi Reservoir, around 80%-90% of the
sediment can be regulated to be transported when the flow discharges are
greater than 2,500 m3/s during flood season, especially for the year
with abundant flow and sediment. The sediment transport ratio can be as high as
90% and above for the man-made flood with the construction of dikes.
