Integrated River Management - River Dynamics and Integrated River Management - page 757

gullies is generally more than 40 times higher than the basin average. For example, the Xiaojiang River
is a tributary of the Jinsha River (as shown in Fig. 11.28), which is 138 km long and has a drainage area of
3043 km 2 . There are 107 debris flow gullies in this small river basin, among which the Jiangjia Ravine is
the most well-known because of its high frequency of debris flow events, and they transport huge
amounts of sediment into the Xiaojiang River every year. The sediment yield from the Jiangjia Ravine is
142,200 t/km 2 .yr, of which only 2% can be transported downstream by the Xiaojiang River.
The sediment storage in the upstream watershed was studied by comparing the size distributions of
sediments particles sampled from numerous tributaries, debris flow gullies, and the upper and middle
reaches of the Yangtze River (Wang et al., 2007). Bed load is a main type of sediment transportation in
mountain streams, which consists of coarse sand, gravel, cobbles, and even boulders. The size distribution
of bed load from the Diaoga River, a tributary of the Xiaojiang River, is shown in Fig. 11.43. Measurement
of the bed load and suspended load in the Diaoga River in the flood season from June to September 2006,
demonstrated that the ratio of bed load to the suspended load in the four months was 21% to 79%, much
greater than the ratio in the Yangtze River (2% to 98%). In fact, bed load transportation reduces from
mountain streams to tributaries and from tributaries to the main stem of the Yangtze River. In other words,
coarse particles are transported from mountains in low order streams and stop moving in high order
streams because the bed slope reduces with increasing stream order.
Figure 11.43 shows the size distributions of sediment deposits in debris flow gullies and tributaries in
the upper Yangtze River basin in comparison with the size distributions of suspended load at the Yichang
and Datong Hydrological Stations (Kang et al., 2004; Xu, 2005; Wang et al., 2001; Xie et al., 2004; Wan
et al., 2003). All the size distribution curves are averages of several tens of samples. Dividing the sediment
into n -fractions with diameter in the ranges D 1 ~D 2 , D 2 ~D 3 , D 3 ~D 4 , …in which the subscripts 1, 2, 3,…
are the order numbers, let ' be the percentage of diameter within the range D i ~D i+ 1 . The amount of

sediment depositing in the upper reaches, S di , is given by:
(

' ' (11.18)
in which the subscripts up and down represent the upper Yangtze River basin and the Yichang Station, e.g.
S up is the total sediment eroded from the watershed upstream from Yichang, and S down is sediment load at
the Yichang station.
A representative size distribution of the original sediment eroded from the upper Yangtze River basin
can be roughly estimated in the following way: (1) Make an average for the size distributions of sediment
deposits from the river beds and gully beds in the upstream basin, and denote it as Sa ; (2) Because a part
of the fine sediment has been transported as suspended load into the Yangtze River (23.6% of the total
eroded sediment), the representative size distribution of original eroded sediment, Sm , should be given by
taking this part into account and may be given by:
0.764

SSP S ) ( P )

di up i up down i down

(11.19)
where the coefficients are the ratios of sediment deposited and transported into the Yangtze River to the
total eroded sediment, and S 12 is the size distribution of suspended load at the Yichang station.
Figure 11.43 shows the representative size distribution curve Sm . At the Yichang station, gravel bed
load composes only 0.15% and sand bed load 1.7% of the total load, the size distribution curve of suspended
load can be approximately regarded as the size distribution of the total load. The amount of sediment
storage of different size fractions in the gullies and tributaries can be calculated by using Eq. (11.18) and
taking figures for the upper watershed from Sm and for the downstream section from the size distribution at
Yichang:
For the fraction coarser than 0.5 mm, S d = 2179 * 0.528 = 1151 million tons
For the fraction in the range 0.05 ~ 0.5 mm, S d = 2179 * 0.226 - 514 * 0.35 = 313 million tons

Sm Sa 0.236 12 S