Debris Flows and Landslides - River Dynamics and Integrated River Management - page 245

in 10 min P a is a factor of accumulated precipitation in the past 20 days and is given by the following
formula:
P a = P 0 + 0.8 P 1 + 0.8 2 P 2 + 0.8 3 P 3 + Ă + 0.8 20 P 20 (4.13)
in which P 0 is the rainfall just before the most intense 10-min. rainfall, P i is the rainfall on the day i-days
before the current day. Figure 4.69 shows that debris flow occurs if:
10.33 I 10 + P a > 62 mm (4.14)
The analysis also indicates that if P a is smaller than 30 mm and I 10 is larger than 4 mm, gusty and
short-time debris flow occurs, whereas if P a is larger than 60 mm and I 10 is less than 2 mm, low-viscous
debris flow occurs. By combining Eq. (4.14) and rainstorm forecasting, debris flow in that area can be
predicted. The same method and the criterion I 10 are applied for prediction of debris flow in other debris
flow gullies in China.
4.4.2.2 Warning System
Many detection and warning systems have been developed for protecting railways, highways, bridges,
factories and mines from debris flows. These detection and warning systems work based on different
principles. For instance, vibration detectors receive vibration induced by debris flow and transmit a
warning signal to the protected objects. Debris flow level detectors can send warning signals to the
protected objects when a debris flow is over a given stage. These warning systems are working in many
debris flow areas, and they have successfully sent warning signals to railways, bridges, and towns and
have saved people's lives and property.
The UJ-2 type ground sound wave probe and warning system was developed by the Chengdu Institute
of Mountain Hazards Studies in 1984. The system can automatically work for 3 months powered by
batteries. The system was tested in 1984-1985 and successfully sent warning signals of 12 debris flows
to the protected areas 2.8 km downstream. The signals arrived at the protected object 8 minutes ahead of
the debris flow. The system worked well but the warning time is not long. The warning devices may be
installed on rock walls of debris flow gullies, concrete levees, dams or dykes, and buried in the channel
bed. Figure 4.70 shows a ground sound wave probe installed on a dam.
Fig. 4.70 A ground sound wave probe installed on a dam
VI-1 type and DFT-3 type debris flow stage probes and warning systems were developed by Shanghai
Changning Science Association. The systems sent signals to the protected objects when a debris flow was
over a critical stage. The signals arrived at the protected object 8 minutes ahead of the debris flow head.
4.4.3 Debris Flow Control Engineering
Debris flow-control engineering has a long history in China. People dwelling in debris flow areas built dams