Mountain Rivers and Incised Channels - River Dynamics and Integrated River Management - page 147

length of thalweg --

(3.4)

S 1

P

length of straight line -- -

The length-of-thalweg is the total length of the curved bed surface with boulders or gravel along the
thalweg, and the length-of-straight-line is the length of a straight line from the beginning point to the end
point of the measured bed section. For a flat bed with fine sediment, S P = 0. For beds with step-pools,
sand dunes, or other bed structures, S P is larger than 0.

Fig. 3.27 A step-pool system on a landslide dam in the Jiuzhai Creek in northern Sichuan

Wang et al. (2009) designed an instrument and measured the development degree of a step-pool
system, S P . The instrument, shown in Fig.3.28(a), consists of thirty measuring rods with a space of 5 cm
between them placed on a horizontal aluminum steel frame that may slide down onto the bed surface.
The upper ends of the rods describe the bed profile in front of a screen. Moving the frame along the
thalweg of the stream and each time taking a picture, the bed profile along the thalweg can be measured.
The S P value is then calculated by the following formula:

m

¦

2 2

( RR ) 5

i 1 i

S

i 1

1 (3.5)

P

2 2

[5( m 1)] ( R R )

m 1

in which R i is the reading of the upper end of the measuring rods on the screen in cm, and m is the total
number of the readings, which is generally larger than 300. Figure 3.28(b) shows the measurement of S P
with the instrument in the Yigong Tsangbu River in Tibet. For a stream without step-pools S P is smaller
than 0.1. If a step-pool system is well developed the value of S P may larger than 0.3. For extremely
developed step-pool on steep slope the value of S P can be as large as 0.5. Only for very huge step-pool
systems composed of huge stones with diameter of larger than 10-20 m the value of S P can reach 1.0.
With the specially designed instrument, the bed profile of the thalweg and the development degree of a
step-pool system, S P , in the streams were measured. Figure 3.29 shows the measured bed profiles of the
thalweg of Shengou Creek, Fork Gully, and the Jiangjia Ravine, which are located in the upper reaches
of the Yangtze River basin in southwestern China. Shengou Creek, the Jiangjia Ravine, and Fork Gully
are tributaries of the Xiaojiang River on the Yunnan-Guizhou Plateau in southern China, which flows
into the upstream reach of the Yangtze River. The elevation in Fig. 3.29 shows the relative elevation along
the course for each stream not the elevation above sea level. Figure 3.29 clearly shows that Shengou