Debris Flows and Landslides - River Dynamics and Integrated River Management - page 212

They act as a primary control on channel morphology and longitudinal river profiles, inhibiting incision
and further preventing the complete adjustment of rivers to regional tectonic forcing. The feedbacks
among hillslope processes are prevalent throughout this landscape and are characteristic of transient
landscapes on the eastern margin of the Qinghai-Tibet Plateau.
In general sedimentation begins to occur in quake lakes once the lakes are formed. Over time the quake
lakes will be filled with sediment. The river banks become more stable even during earthquakes. Figure
4.24(a) shows the bed profile of the Shenxi Ravine. Three landslide dams (N1, N2, and N3) formed more
than 1,000 years ago (by estimation with sedimentation rate) and the three lakes have been filled up. The
landslide dams were stable because they consisted of a lot of boulders and a step-pool system has
developed very well. The three barrier lakes lost their capacity due to sedimentation. There was only a 30
m section behind the N2 dam, which stored water with a depth of about 0.5 m. The river bed had been
raised by 200 m, which greatly reduced the risk of potential landslides. This stream is just at the Yingxiu-
Beichuan Fault. During the Wenchuan Earthquake on May 12, 2008, tectonic motion caused the left side
(northwestern side) of the Shenxi Ravine to rise by 4 m. Figure 4.24(b) shows a highway along the ravine,
which was tilted and broken by the earthquake. Nevertheless, no landslides or avalanches occurred on the
stream. The mountains are still green, which is very different from other stream basins. Within a 30 km
distance from the Longmenshan Fault, almost all mountains were bared due to landslides and avalanches
triggered by the earthquake (Wang et al., 2009a). The extraordinary stability of the Shenxi Ravine is
mainly due to the three preserved landslide dams.
Fig. 4.24 (a) Bed profiles of the Shenxi Ravine, showing three landslide dams and filled lakes; (b) Tilted and broken
highway along the Shenxi Ravine during the Wenchuan Earthquake (See color figure at the end of this topic)
The incising rivers can be seen as wounds on the surface of the Earth, and landslide dams and quake
lakes can be regarded as scabs formed to allow these wounds heal by themselves. The landslide dams
control channel bed incision and reduce the risk of new landslides, improve the stream ecology, and create
beautiful landscape. If the risk of dam failure is not high and the safety of humans is not threatened by
the quake lakes, the landslide dams and quake lakes should be stabilized rather than be removed. If a
landslide dam that persists for more than 10 years is assigned the “stable” status class, a statistical
inventory of 232 landslide dams and quake lakes showed that only 37% of all landslide dams appear to
have failed (Korup, 2004a and 2004b). This result indicates that preservation of quake lakes is not only
possible, but may also be a major strategy for quake lake management.