Debris Flows and Landslides - River Dynamics and Integrated River Management - page 217

All the measurements were performed on the stabilized channels of the landslide dams. In general, a
step-pool system developed following scouring and development of the spillway channel. Boulders and
cobbles were rearranged to form structures and reached maximum stability during the scouring. Finally
the step-pool system was so well developed that flood flows could not scour the bed material. The stream
power of the flow and the energy consumption by the step-pool system reached equilibrium, which is
represented by the curve in Fig. 4.29. If the S P value of the channel on a landslide dam was smaller than
the value of the curve, the channel would incise down. The S P value increased following the incision and
finally reached a value around the curve. If the S P value was higher than the curve the landslide dam
would be stable and the preservation ratio would be the final preservation ratio of the landslide dam.
The development of a step-pool system and the preservation ratio of landslide dams depend on a
number of factors, most important of which are the size of the original landslide deposit, the percentage
of large boulders within that deposit, and the geometry of the valley (Costa and Schuster, 1988). The
failure risk of the landslide dams soon after the earthquake depends on the size composition of landslide
materials, the width of the dam and the water head of the lake. If a landslide dam is mainly composed of
fine materials such as soil and fine gravel, it is likely that the materials are soon flushed away by the
overspill flow and the landslide dam collapses. If a landslide dam is composed of stones of different sizes,
including big boulders, a strong step-pool system may develop and the quake lake is stabilized. If a
landslide dam is composed of much fine materials, and also many big stones, the time needed for the
step-pool structures in the spillway to develop and stabilize the dam would be long, during which the risk
of dam failure is high. Figure 4.30 shows the size distributions of the original landslide deposits, in
which the symbols of preserved and half-preserved landslide dams are black solid points and those of
failed landslide dams are hollow points. All landslide dams with original material consisting of at least
10% of boulders larger than 1m have been preserved or half-preserved. For size distributions with a high
percentage of large boulders, a strong step-pool system may soon develop with a minimum incision of
the spillway channel; thus, the dam may be preserved. Therefore, the percentage of large boulders is of
the greatest importance for the final stabilization of the landslide dams.
Fig. 4.30 Size distributions of original landslide deposits of failed landslide dams (hollow points) and preserved
and half-preserved landslide dams (black solid points)