A FMCW-Doppler radar detects debris flows upstream in the Spreitgraben torrent system in Switzerland by measuring flow velocities. Since 2009, several extremely erosive debris flows have occurred that were initially triggered by rockfalls but became larger, more destructive events over time. The debris flows have deposited a total of 650,000 cubic meters of sediment in the receiving Aare River, threatening important infrastructure like roads, pipelines, and buildings. A comprehensive monitoring system using radars, sensors, and webcams has been established to detect events and provide early warning.