Warning Systems and Instrumentation

In the last two decades, sensors and techniques for debris-flow monitoring and their application for warning purposes have progressed significantly and many operational warning systems are working worldwide. Such advances motivate presenting an updated overview of the state of the art of early warning systems tailored towards debris-flow risk reduction. In this chapter, we classify debris-flow warning systems into advance warning systems, which are based on the monitoring of meteorological factors and issue a warning before a debris flow is triggered, and event warning systems, which detect debris flows after they have initiated. Advance warning systems typically rely on rainfall thresholds that are based on rain gauge data related to past debris flows. We discuss the main issues related to the rainfall thresholds and the prospects for the use of radar-based rainfall estimates. Several sensors and devices can be used in event warning systems. In this work, we focus on the most widespread ones: (i) radar and ultrasonic sensors, which detect debris flows by measuring changes in the flow level, (ii) impact sensors such as trip-wires and pendulums, (iii) seismic and infrasound sensors, whose operation is based on the detection of ground vibrations and air pressure waves induced by debris flows, and (iv) video cameras. We outline some basic issues related to the implementation of protocols for debrisflow warning, emphasizing the importance of focusing the warning on end-users with a different technical background and the need to inform the population about debris-flow risks, the expected performance of the warning system, and the action following a warning. In the concluding remarks, we stress that debris flows are often concurrent with other meteorological-induced hazards and risks, such as flash floods. This implies that early warning systems for debris flows need to be developed in a multi-risk framework.