Debris Flows and Landslides - River Dynamics and Integrated River Management - page 246

and dredged floodways to avoid or reduce debris flow disasters several hundred years ago. Engineering
measures to control debris flow are:

4.4.3.1 Diversion Works
Diversion channels and guide-ways are constructed to divert floods from the upstream of a debris flow
gully, and reduce discharge of floods and the kinetic energy of the flow so that debris flow is prevented
or reduced in scale. The Xinkang amianthus mine in Sichuan Province built a tunnel to divert water from
the upstream of the Dahong Gully to a river. The capacity of the tunnel was 190 m 3 /s, equivalent to the
20- year flood of the gully. The flood discharge of the gully was substantially reduced by the tunnel and
no debris flow has occurred in the past 20 years. Figure 4.71 shows the diversion channels at the Niwan
Gully and the Jiangjia Ravine. The Niwan channel diverts debris flow from the Niwan Gully to the Bailong
River and protects farmland from being buried by debris. The Jiangjia Channel diverts debris flow to a
debris siltation basin and then to the Xiaojiang River. Hence, damming of the Xiaojiang River is avoided.
Figure 4.72 shows debris flow-guiding channels in a debris flow gully in the Xiaojiang River basin,
which has successfully guided debris flows to the Xiaojiang River and protected the village and farmland
on the fan. The ribs on the guiding channel create resistance and consume the energy of debris flows, and,
therefore, reduce the destructive force of the debris flows.

Niwan gully

Guiding dikes

Hongshan Hill

Farm field

Old channel

Xi ao j i an g

(b)

(a)

Fig. 4.71 (a) Diversion channel at the Niwan Gully, which diverts debris flows and protects farmland. (b) A channel
diverts debris flow to a debris siltation basin, protecting the Xiaojiang River from damming

4.4.3.2 Dams and Dam Cascades
Dams and dam cascades on a debris flow gully have been built to trap debris and effectively check debris
flow. Daqiao Creek, a tributary of the Xiaojiang River was a very active debris flow gully. Five detention
dams were built over 7 km along the gully. These dams considerably reduced solid material transported
into the Xiaojiang River. Nowadays lattice dams are widely used because they can trap large boulders
carried by the debris flow and reduce harmfulness. Lattice dams also have much longer life spans because
they silt up at a low speed. Figure 4.73(a) shows the debris flow control dam cascade on the Houshan
Ravine of Heishui County, Sichuan Province (Kang, 1996). Each dam is 3-5 m high and 10-20 m wide.
They function to check debris flow and protect each other. Figure 4.73(b) shows the comb frame dam on
a debris flow gully in the Cho-Shui River basin in Taiwan, China. The dam trapped big stones but
released water and silt downstream.