A debris flow is a dense mass movement constituted by a mixture of water and rock sediment, with a wide range of grain sizes, from clay up to boulders, and a highly variable water content. Debris flows can reach high velocity and constitute a significant hazard in mountainous regions, in particular in the European Alps. They occur typically in high gradient torrent channels in catchments with steep slopes and abundant sedimentary debris. They are particularly dangerous to life and property because their high density and speed produce a sheer destructive momentum of the flow. Debris flows are capable of destroying homes, washing out roads and bridges, sweeping away vehicles, knocking down trees, and obstructing streams and roadways with thick deposits of mud and rocks. In European Alpine regions, debris flows generally occur in occasion of intense summer rainfall or rapid snow melt.
They usually start on steep hillsides as shallow landslides that liquefy and accelerate to speeds that can be around 5 m/s, but can even exceed 20 m/s. Once started, debris flows continue flowing down hills and through channels, growing in volume with the addition of water, sand, mud, boulders, trees, and other materials; due to their variable composition they can travel even over gently sloping ground. The consistency of debris flows ranges from watery mud to thick, rocky mud that can carry large items such as boulders, trees, and cars. When the flows reach the mouths of gullies, the debris spreads over a broad area, typically an alluvial fan, that can wreak havoc on populated areas. Additionally, debris flow deposition may greatly reduce the water-carrying capacity of the channel, leading to flood damage caused by subsequent flood waves.
Given the debris-flows dangerousness, to help reduce the risk and design mitigation measures and concepts, a good understanding of the process is necessary. Debris flow research is directed at improving the understanding of the process in a way that is directly relevant for practical applications. Studies span the range from observations of the debris flow process at full-scale automated monitoring stations, to the investigation of mitigation measures, to the evaluation of existing debris flow runout models.
Early warning systems are being developed for use in areas where debris flow risk is especially high. Traditional methods use sensitive seismographs or geophones to detect debris flows that have started moving, ultrasonic sensors to measure the flow depth, geophones to determine flow speed. Other use radar precipitation estimates and established rainfall intensity-duration threshold values to determine when meteorological conditions are right for flows to occur.
The fiber-optic distributed and quasi-distributed sensors are the most innovative technologies to monitor and characterize debris flows and related protective structures.