Environmental Engineering Reference
In-Depth Information
As a comparison more than 20 debris flow events occurred in the gullies with no step-pool systems
near Wenjiagou in 2009. Two debris flow events, each with a volume of more than 0.30 million m 3 , were
triggered at the Heidongya Gully (7 km upstream of Wenjiagou) by rainstorms. The two debris flows
dammed the Mianyuan River for two times. Another two debris flow events, each with a volume of 0.15
million m 3 , were triggered at the Yongjiagou Gully (2.5 km upstream of Wenjiagou) by rainstorms. Six
debris flows occurred in the Xiaogangjian Gully (7 km downstream of Wenjiagou) dammed the Mianyuan
River six times and cutoff the transportation from Mianzhu county to Qingping town to in 2009.
11.3.2.3 Check Dams in 2010
In 2010, however, the step-pool system was replaced with 20 check dams, of which two were main dams
at the lower and upper end. The dams were constructed on the new Wenjiagou from Feberary to July in
2010 with a budget of $1.42 million US dollars. The keystones of the 2009 step-pool system were broken
into small pieces of less than 0.5 m and used as building materials for the dams. The locations and lengths
of the dams are shown in Figure 11.59. Most of the dams have a height of 6 m except for one main dam
of 8 m high and two small dams of 3 m high. The base of the dams in the gully bed was 3 m. Figure 11.67
shows the design and construction of the dams. The lengths of the lower and upper main dams were 220
m and 84 m, respectively, and other dams had lengths of 20-27 m depending on the width of the channel.
The distances between small dams were 20-40 m and the distance from the lower main dam to nearest
small dam was 867 m and from the upper main dam to the nearest small dam was 68 m. The locations of
the dams are shown in Figure 1.
Fig. 11.67
Design and construction of the dams.
The dams were completed on July 28, 2010. On Aug.13 a rainstorm of intensity 98.5 mm/day at
Wenjiagou initiated a debris flow with a volume of 4.5 million m 3 . All the 20 dams failed and the gully
bed was incised by 50 m. The new Wenjiagou channel extended 130 m due to headwater erosion. As
shown in Fig. 11.68, the flow energy was not dissipated but concentrated at the dams. Water flowed over
the dams and scoured the base of the dams. Because the stones on the bed were removed and used for
construction of dams, the bed was scoured down by torrential flood, which resulted in suspension of
dams and caused dam failures. The bank slopes of the gully were at the repose angle of the loose solid
materials. Bed incision increased the slope angle and resulted in bank failures. A lot of solid materials fell
into the flow and mixed with water, and, thus, formed debris flows. Because the right bank of the gully
was near the rocky mountain slope. The flow scoured the left bank while incised the bed, and the main
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