Integrated River Management - River Dynamics and Integrated River Management - page 782

Fig. 11.71 Variation of cross section 2 and cross section 7 in 2009 and 2010

Figure 11.72 shows the size distributions of the solid materials on the gully bed and bank slopes in 2009
and 2010, which were the average of size distributions from 6 sites along the gully. The size distributions
of debris flow deposits in 2008 and 2010 are shown in the figure as well. The solid materials in the gully
in 2009 and debris flow deposits in 2008 were coarser than the solid material in the gully and debris flow
deposit in 2010. A lot of large stones were on the top layer of the Wenjiagou landslide deposit. The debris
flows in 2008 were two-phase debris flow because the particles were very coarse. After the debris flows
in 2008 many large stones were left on the gully bed and bank slopes, which were used for construction
of step-pools. In 2010, however, all large stones were exploded into small pieces and used for construction
of 20 dams. The dams were removed by torrential floods. The solid materials were not very coarse and
the debris flows were not two-phase, but more or less, pseudo-one-phase, debris flows. Therefore, the
debris flows in 2010 were killing and disastrous. After the debris flows in 2010, the solid materials on the
gully bed and slopes became much finer than in 2009 and 2008.

Fig. 11.72 Size distributions of solid material on the gully bed and slopes and in debris flow deposits

The failure of dams does not mean that the dams are not effective for debris flow control. In many
cases check dams have successfully controlled normal debris flows. The focal point of designing check
dams should be energy dissipation rather than restoration. Although the check dams on Wenjiagou had
many windows the flow could not flow through the dam freely because stones carried by the debris flow
closed the windows. Water restored in the reservoirs, which resulted high pressure on the dam and caused
a high rising of ground water table. The loose solid materials were saturated with water and very unstable,
which was one reason for the large volume debris flow disaster. The check dams should have a low sill
allowing water flowing through but trapping large solid particles. Moreover, combination of check dams