Integrated River Management - River Dynamics and Integrated River Management - page 781

incised by more than 50 m because the step-pools were destroyed but the constructed dams could not
control incision and debris flows. The channel bed was still higher than the old Wenjiagou channel by
about 50 m. Nevertheless, the gradient of the channel bed became larger and the bank slopes became
larger and deeper than in 2009.

Fig. 11.69 Bed profile of Wengjiagou after the 2010 debris flows in comparison with the bed profiles in 2008 and 2009

As shown in Fig. 11.70, the remaining parts of failed dams were suspended on the banks. The upper
main dam was completely removed by the debris flows and the gully extended upward by 130 m. Figure 11.71
shows two cross sections of Wenjiagou in 2009 and 2010. The locations of the cross sections are shown
in Figure 11.59. The cross section 2 remained unchanged in 2009 because the step-pool system protected
the bed from erosion. Nevertheless, the bed was scoured down by 16 m and the gully was widened by 90
m in 2010 because the dams were not able to control the incision and slope erosion. At cross section 7,
the bed was scoured slightly in July, 2009 because the step-pools were partly damaged. The bed aggraded
again in August and September 2009 after the steps were repaired. The whole cross section remained
stable in 2009. In 2010, however, the bed was incised by 45 m and the gully was widened by 70 m.

Remains of dam

Upper main dam

(a) (b)
Fig. 11.70 (a) Suspended remaining part of a failed dam after bed incision; (b) The upper main dam was completely
removed and the gully extended due to headwater erosion. (See color figure at the end of this topic)