Table 2.

Principal sensors for debris-flow event warning systems.

Sensors	Operation	Advantages	Limitations
Ultrasonic, radar and laser sensors.	Measurement of the flow stage.	Easy to set warning thresholds.	Ultrasonic sensors have to be hung over the channel; installation can prove difficult if the channel banks are unstable.
Geophones and seismometers.	Measurement of ground vibrations caused by debris flow.	Easy and safe installation (the sensors are buried in safe places on stream banks).	Setting warning thresholds can be quite complicated. Risk of false alarms due to other sources of ground vibration (passage of trains or trucks, rockfalls, etc.). The need to filter the signal may increase system complexity.
Pendulums.	Detection of the debris-flow from the tilting of the pendulum.	Simple and robust device.	The pendulum must be hung over the channel; installation can prove difficult if the channel banks are unstable.
Wire sensors.	Detection of the debris-flow from wire breaking.	Simple and robust device.	Need for restoration after activation. Risk of false alarms due to accidental circumstances (passage of animals, falling trees, etc.).
Photocells (infrared photobeams, etc.).	Detection of debris-flow passage.	Non-contact detectors: do not need restoration after activation.	A careful installation is needed to avoid having the sensors come into contact with the flow.
CCD camera for machine-vision detection.	Recognition of debris flows.	Safe installation (the camera can be placed beside the channel).	The presence of fog or the occurrence of debris flow at night may complicate the use of the system and its workability.