Debris Flows and Landslides - River Dynamics and Integrated River Management - page 213

4.2.2.2 Preservation Ratio
No landslide dam has been completely removed by a dam break outburst flood and very few landslide
dams have been completely preserved. Naturally, as the water level in a quake lake reaches the lowest
part of the top of the landslide dam the water flow scours the loose material and forms a spillway channel.
The top of the landslide dam is scoured by the flow and, in the meantime, erosion-resistant boulders
overlap and construct a step-pool system on the spillway. The most important hydraulic feature of step-pool
systems is the extremely high bed roughness, which maximizes the resistance to the flow and causes the
highest energy consumption, and, thus, protects the bed from being scoured (Strom and Papanicolaou,
2007). Details of step-pool systems is given in Chapter 4.
The scouring process reduces the original landslide dam material to a smaller mass composed of boulders
and cobbles, stabilizing the dam and protecting the top of the initial deposit from further erosion. The
step-pool system is not easily moved in even large floods, and serves to maximize the roughness of the
channel (Wang et al., 2004; Maxwell and Papanicolaou, 2001). Finally, the spillway channel becomes a
narrow and steep reach cutting through the landslide dam, but with a step-pool system to consume the flow
energy. Thus, the height of the preserved part of the dam is lower than the original landslide dam height.
The preservation ratio of a landslide dam, R , is defined as:

H

preserved

R (4.2)

H

original

in which H preserved is the dam height after the formation of the spillway channel and scouring; H original is
the original landslide dam height. A landslide dam is regarded as preserved if the value of R is larger than
0.9; as half-preserved if the value of R is between 0.5 and 0.9; and as failed if the value of R is smaller
than 0.5. The ratio reduces in the first one or two years after the formation of the landslide dam and
quickly becomes stable after a major flood.
Three Diexi Lakes on the Minjiang River were created by landslides during the Diexi Earthquake
(Ms 7.5) in 1933. The lakes partly failed 45 days after the dam's formation. The dam failure flood killed
2,500 people (Sichuan Seismological Bureau, 1983). Nevertheless, about two thirds of the dams have been
preserved. They are half-preserved dams. Figure 4.25(a) shows the upper Diexi landslide dam, the spillway
channel, and the upper and lower lakes. The lakes have a maximum water depth of 98 m although quite a
lot of the lakes' capacity has been lost due to sedimentation in the past 76 years. Figure 4.25(b) shows the
Shibangou landslide dam formed during the Wenchuan Earthquake on the Qingzhu River in Qingchuan
County. The top part of the dam was removed by explosives in June 2008 in order to restore a highway and
mines in the quake lake. More than half of the dam height is preserved, therefore, it is a half-preserved dam.

(a) (b)
Fig. 4.25 (a) Spillway channel on the Diexi landslide dam, and the upper and lower lakes on the Minjiang River;
(b) Half-preserved Shibangou landslide dam on the Qingzhu River in Qingchuan County (See color figure at the end
of this topic)