Integrated River Management - River Dynamics and Integrated River Management - page 775

banks. There were no roads on the landslide deposit for any vehicles and machines. Only human labors
could be used for construction of the step-pool system. There were many large boulders on the bank slopes
of new Wenjiagou and they were moved from the slopes to the channel bed to become the keystones of
steps, as shown in Figure 11.63 (a). Several huge boulders of more than 500 tones in weight were left on
the channel bed after debris flows in 2008. Using the huge boulders as keystones more boulders of
0.5-1.5 m in diameter were placed around the huge boulders to form large steps of 2-4 m in height, as
shown in Figure 11.63 (b). Although the rainfall intensity in 2009 was higher than 2008 only a small
debris flows occurred and did not cause any loss of life and properties.

(a) (b)
Fig. 11.63 An artificial step-pool system was constructed in June 2009 for stabilization of the new drainage system
on the Wenjiagou Landslide: (a) Large stones with diameters larger than 1 m were removed from the banks of the
new gully for construction of steps; (b) Large stones were placed together to form the artificial steps. (See color figure
at the end of this topic)

Step-pool system created high resistance and dissipated most of the flow energy in water falls on the
steps and hydraulic jumps in pools. The flow was very turbulent with standing but broken waves and air
bubbles. The turbulence intensity and fluctuation of flow were intensive but the mean flow velocity was
not high because the kinetic energy of the flow had been transformed into turbulence. A lot of energy was
consumed at hydraulic jumps. The dissipation ratio of the total energy (the sum of potential energy and
kinetic energy) due to hydraulic jump may be calculated with the following formulas (Ni and Liu, 2008):
2

q

h

d 2

2 gh

K 1

d

(11.32)

hj 2

q

zh n D

1 1

2

2 gh

1

The parameters h d , h 1 , z 1 , and Dare given in Fig.11.64. q is the flow discharge per width, and g is
gravitational acceleration. No measurement was performed in the step-pool system, which was too
difficult and too dangerous. For torrential flood the parameters may be estimated based on observations:
q 2 m 2 /s, h d z 1 1.5 m, h 1 0.5 m and D 30 R
Thus, the energy dissipation ratio of each hydraulic jump is equal to 0.42.
As shown in Fig. 11.65 the flow energy dissipated at the steps as well. Very strong turbulence occurs at
the steps and generates a lot of bubbles and turbulent eddies. The energy dissipation at the steps was