Debris Flows and Landslides - River Dynamics and Integrated River Management - page 216

For large rivers with flood discharge higher than 30 m 3 /s, however, the stability of a landslide dam
depends not only on the development of a step-pool system, but also on the stream power of the flood
flow. The S P value of step-pool systems on the landslide dam for preserved, half-preserved, and failed
landslide dams, created by the landslides during the Wenchuan Earthquake or other landslides that occurred
a long time ago (10 years to 1,000 years), was measured with the instrument previously described. The
width of the channel, and the height and length of the landslide dams were measured with the GPS
receivers and laser range meters. The flood flow discharge was collected from the nearest hydrological
stations. Sediment samples were taken for size distribution analysis. In most cases the flow in the central
zone of the channel was not accessible or very dangerous to measure. The S P value was measured near
the banks. An experiment was conducted in a dry channel, where the water was diverted for power
generation. The measurements with the special instrument proved that the measured S P value near the
banks during the low flow season was very close to that in the central flow zone of the channel. The error
is less than 5% (Liu, 2009). Most of the measurements of the S P value were performed near the banks,
where the measurement was not so dangerous.
The stream power is given as follows:

PQs J (4.3)

in which q is the flood discharge, s is the bed gradient, and J is the specific weight of water. The
stream power per width is very useful, which is given as follows:

p s (4.4) J

in which q is the flood discharge per channel width.
Figure 4.29 shows the relation of the S P value of the step-pool system on the channel with the stream
power per width. Except for the landslide dams created by the Wenchuan Earthquake in 2008, the failed
and preserved landslide dams on rivers in the Qinghai-Tibet Plateau were also measured. The Yigong
Tsangpo landslide dam occurred in April 2000 and failed in June 2000. The landslide dam lake had a
huge capacity of about 3 billion m 3 . The dam failure flood resulted in an extremely high discharge. The
Taohuagou Landslide occurred on the Yarlong Tsangpo, which almost completely failed. The two
landslide dams have high, S P, values but the preservation ratio is low because the flow had very high
stream power.

Fig. 4.29 Development degree of step-pool system, S P , as a function of unit stream power, P , for stabilized channels
on landslide dams